Rabies continues to pose a severe burden to public health and is ranked one of the most fatal diseases. It causes tens of thousands of human deaths annually, particularly in developing countries. Currently, dogs remain the main source of rabies and are responsible for almost 99% of fatal rabies cases in humans. Since the first development of the rabies vaccine by Pasteur, human rabies vaccines have been improved and refined. Current cell culture rabies vaccines for humans are highly efficacious, safe, and easily accessible in most parts of the world, which in turn has enabled the control of rabies in these regions. Therefore, in terms of rabies exposure, as long as prompt and correct PEP are administered, there is no need for further development of rabies vaccine for current PEP. In high-risk regions, a shift from PEP to application of routine pre-exposure vaccination, especially for vulnerable children and old people, should be considered. This approach is expected to effectively and dramatically reduce the incidence of rabies cases, although a prompt and simplified PEP is still needed after pre-exposure rabies vaccination for complete protection after rabies exposure. However, rabies remains endemic to many parts of the developing world where the resources of appropriate PEP are limited, the infrastructure and facility are inadequate, and, most importantly, awareness about rabies is lacking. In these cases, inexpensive, safe, and effective vaccines are urgently needed. This situation is even more pronounced given the fact that the most important and probably the only practical way to control rabies globally is the mass vaccination of dogs as well as wildlife reservoirs. To accomplish this goal, efficacious oral vaccines that can be given in reduced or single doses should be developed to be cost-effective.

In contrast to the steady improvement in the availability and quality of cell culture-derived rabies vaccines and RIGs, there has been a consistent lack of interest in developing antiviral therapy for patients who have missed the deadline for valid vaccination or whose symptoms have already manifested. As of now, no effective antiviral therapy is available for these patients and death is inevitable, in most instances accompanied by agonizing pain. Fortunately, recent advances in rabies treatment have generated promising tools for preventing and eradicating rabies, at least in murine and non-human primate models. These developments provide hope for realizing the goal of curing patients who are suffering from rabies.

The rationale for rabies therapy is that RABV may stay at the entry site for days or weeks before reaching the CNS and causing symptoms, providing valuable time to limit and control the virus infection. Of the developments in rabies therapy, live-attenuated virus based vaccines might be the most promising. For a successful PEP, vaccines should rapidly induce potent protective immune responses and ideally would at the same time require less or no RIG. Live-attenuated virus-based vaccines match all these requirements and hold the promise to be curative. Most of the concerns about the use of live-attenuated virus-based vaccines come from the assumption regarding the potential risk of residual virulence and toxicity, instability, reversion to pathogenic wild-types, or recombination in nature. However, with the development of modern technology, these live-attenuated viruses used for vaccines are generally replication-deficient or restricted; their pathogenicity is commonly maximally or completely abolished and, after systemic manipulation of their genome, these viruses are theoretically highly stable and irreversible and virulence restoration could be effectively avoided. Currently, a number of live-attenuated virus based vaccines for human use have been widely used, and current data have firmly demonstrated that rabies vaccines based on live-attenuated RABV or HAdV-5 caused no adverse events or safety concerns. Nevertheless, as with any genetically modified agent, enhanced surveillance and ecological monitoring are necessary to evaluate the relative risks of the environmental release of such viruses, given the possibility of recombination and genetic material exchange in nature, even if this is a very rare event.

Development of other novel non-viral biological treatments for rabies has also been progressing rapidly. Within the near future, there should be an increase in the number of available rabies biologics, especially with our deeper understanding of the pathogenesis of rabies and the interactions between viral and host proteins. These advancements have led to novel modalities based on gene transfer technology such as protein subunit and peptide vaccine, nucleic acid-based vaccines, and small molecules that function to interfere with the replication/spread of the virus. Nevertheless, as these forms of vaccines might result in delayed immune responses because of antigen expression/accumulation and immunogenicity, for efficient PEP, proper adjuvants would probably be needed to increase and accelerate the immune responses elicited by these vaccines. However, given the ability to elicit potent and broad immune responses, the excellent safety profile, and the rapid response to newly emerging pathogens of nucleic acid-based vaccines, especially RNA vaccines, the prospects for their application as therapeutic rabies vaccines are very promising. Moreover, research on siRNAs targeting RABV essential genes has also gained a lot of potential in terms of delaying the process of RABV infection and allowing PEP to take effect. Recent progress on proper delivery systems has potentiated the future use of not only siRNA but also other therapeutic molecules such as small-molecule drugs to cross the BBB for rabies treatment. Furthermore, a lot of interest has been focused on the development of monoclonal antibodies as potential alternatives for the limited and expansive RIG and, given the rapid advances in antibody engineering, it can be envisioned that in the near future some monoclonal antibodies would be licensed as adjuncts for RIG. As a novel form of monoclonal antibodies, BsAb has demonstrated powerful application in cancer immunotherapy and warrant further investigation in rabies treatment given their potentially noninvasive BBB penetration. Ongoing preclinical and clinical studies on these novel approaches would certainly shed light on future rabies treatment.

Antiviral therapy is an important component of future efforts to develop effective therapy for human rabies. We have never been so close to controlling and even curing rabies. Within the next few years, the burden of human and canine rabies will decrease in many developing countries and mortality due to rabies will be effectively brought down. However, it should be noted that there are still many open questions in rabies therapy. If future anti-rabies therapies are developed, a relevant animal model is needed for evaluation. Moreover, the genetic and antigenic diversity of different RABV strains with sequence differences might negatively compromise these therapies. Current rabies interventions are mainly focused on viruses from phylogroup I and future vaccine development should take other members of Lyssavirus into consideration to create a vaccine with broad-spectrum protection efficacy. These pan-lyssavirus vaccines could be realized through recombinant viruses coding for the G protein of several lyssaviruses, RABV coding for additional Gs from different lyssaviruses, or chimeric G combined from different lyssaviruses, and are thus expected to provide protection against other members of the lyssavirus genus. Moreover, as RABV infection causes severe neuronal dysfunction and injury, it is equally important to ameliorate the neuronal damage induced by viral infection as well as clear RABV from the CNS for complete recovery from rabies.

Lastly, it is important to note that while some promising results have been achieved in animal models that PEP (with live-attenuated virus-based rabies vaccines) were effective, even when administered late after challenge infection, these treatments were largely prophylactic. Therefore, these treatments have not been shown to be curative, as the animals were not exhibiting any clinical signs at the time when the treatment was performed. Currently there is no solid, scientific evidence for any curative interventions. More studies are needed to further investigate the potential of curative interventions for clinical use. In the future, appropriate combinations of a rabies vaccine, antiviral drugs, immunotherapy, and neuroprotective therapy could profoundly reshape rabies treatment and benefit more patients suffering from rabies.

No specific medical therapy has proven beneficial once people become ill from bat EIDs (at least of viral origin). For example, although rabies is an ancient disease, effective therapeutic treatment of rabies in humans continues to be very challenging. Rapid early diagnosis in the biting animal is critical, since identification of rabies before its fulminant stage allows for effective prophylaxis. Fulminant rabies continues to carry a very poor prognosis. The first case of the successful experimental treatment of rabies in a naïve patient was a 15-year-old girl bitten by a bat in 2004 (145). However, extension of the ‘Milwaukee Protocol’ (i.e., therapeutic coma, antiviral drugs, intensive medical care) in other patients has been much less successful (see for example Rupprecht (146) and Rubin et al. (147)).

Prophylaxis, after exposure but well in advance of illness, has a much higher success rate. Appropriate post-exposure wound cleansing has been shown to reduce significantly the likelihood of RABV transmission (148). Besides washing the wound with soap and water, unvaccinated persons should receive both rabies immune globulin and four doses of cell-culture vaccine. Globally, more than 12 million persons receive post-exposure prophylaxis each year (149).

Besides rabies, novel treatment strategies are being developed for other bat EIDs. The use of RNA interference has been suggested for the treatment of henipaviruses (150). These currently untreatable infections may be ameliorated by the introduction of small interfering RNA molecules homologous to the RNA in these pathogens. While promising in theory for many agents, this line of treatment is still in its preliminary stages, and issues such as efficacy in humans, delivery, and cost have yet to be addressed.

The potential for filoviruses to be used as bioweapons has spurred research efforts for an effective vaccine that could be used in an outbreak. For example, in a mouse model of hemorrhagic EBOV infection, a vesicular stomatitis virus-based vaccine has been shown to be safe and effective in preventing clinical presentation of disease (151). Furthermore, the possibility that this vaccine may be deliverable through mucosal surfaces offers potential as a rapid vaccination agent during an outbreak.

From July 17 to December 28, 2013, TCDC received requests for rabies PEP from 8,241 individuals. The peak of requests occurred on the week of July 28, two days after rabies was confirmed in a house shrew (Fig 2), and the median number of PEP requests per week was 289 (range, 107–985). PEP was denied in 1,607 (20%). The most common reasons for denial included category II or III wounds inflicted by pet dogs or cats (n = 1,245) and exposure as category I (n = 42). Persons sustained bites requiring PEP concentrated in coastal urban areas. Persons exposed to ferret-badger bites were distributed all over the island, and none in the offshore islands (Fig 3).

Of the 6,634 requests approved for PEP, 6,501 (98%) received at least one dose of vaccination. The median age of the PEP recipients was 42 years (range, 0–94), and 3,642 (56%) were male. The earliest date of animal exposure was February 16, 2011, as determined by retrospective identification of ferret-badger exposures, and the median interval from exposure to PEP application was 0 days (range, 0–896). Among those who received PEP, the most commonly exposed animals were dogs or cats (n = 4,953, 76%), followed by rodents (n = 893, 14%), house shrews (n = 406, 6%) and other wild mammals (n = 223, 3.4%). Category II exposures consisted of 2,855 applicants (43.9%) and category III in 3,613 applicants (55.6%). Ferret-badgers accounted for 59/223 (26%) of the wildlife exposures; among them, 37 (63%) persons had category III exposure, and 32 received RIG. Eight persons, even though they had category III exposure and RIG requests consistent with ACIP recommendations, did not receive RIG following physician evaluation. Physicians who chose to not give RIG were not asked for their reasons for doing so.

By February 24, 2014, there were 6501 persons who received at least one dose of PEP vaccination. Among them, 5,692 persons received at least three doses; 4,861 persons completed the first three doses within 7 days of starting the first dose. There were 4,867 persons who completed 5 doses of PEP vaccination; 3,051 persons completed PEP within 28 days, the recommended schedule for PEP vaccination.

Of the 6,501 PEP recipients, the disposition of animals that bit humans were not available in 5,955 (92%) at the time of vaccine request; among these, 4,534 (76%) were dogs or cats. There were 399 (8%) dogs or cats reportedly healthy at initial evaluation. Rabies testing was conducted in 149 (2%) animals, including 45 dogs or cats, 17 ferret-badgers, and 42 house shrews. Among them, 12 ferret-badgers and 1 house shrew tested positive for rabies.

As of December 28, 2013, no patients who received PEP had developed symptoms suspected to be associated with rabies.

While it is often recommended that a detailed understanding of dog ecology is needed for effective canine rabies control, the consistency of research findings generated over the past 30 years allows us to be confident in concluding that mass dog vaccination is feasible across a wide range of settings and campaigns can and should be initiated without delay. In some cases, more nuanced understanding may be required to improve coverage, but these insights can be often be gained through implementation of control measures and used to progressively improve the design and delivery of subsequent interventions. Key considerations include the nature and degree of community engagement, timing of campaigns, placement of vaccination stations and whether or not to charge owner fees [62–64]. The costs of implementing campaigns free of charge may exceed those readily available to government veterinary services, but many approaches can still be explored to improve affordability, acceptability and cost-effectiveness.

While there is widespread agreement about the central importance of mass dog vaccination in canine rabies control and elimination, the role of dog population management remains the subject of debate. There is a rich literature around fertility control for management of roaming dog and wildlife populations. However, as rabies transmission varies little with dog density, reproductive control measures carried out with the aim of reducing dog density are not likely to be effective for rabies control. In theory, reducing population turnover (e.g. through improving life expectancy and/or reducing fecundity) could help sustain population immunity between campaigns and improve cost-effectiveness. However, there is little empirical evidence that dog population management tools have been able to achieve this. Furthermore, even in populations with a high turnover, achieving a 70% coverage during annual campaigns has been sufficient to sustain population immunity above critical thresholds determined by R0. The relatively high cost of sterilization also means that strategies which combine vaccination and sterilization are less cost-effective in terms of achieving human health outcomes than strategies based on dog vaccination alone, even in populations with a large proportion of roaming dogs. Improved dog population management is undoubtedly a desirable longer-term goal for animal health and welfare and may have important secondary benefits for rabies control, for example by enhancing community or political support. However, a focus on mass dog vaccination currently remains the most pragmatic and cost-effective approach to canine rabies control and elimination.

The limited availability and quality of routine animal rabies surveillance data in LMICs has been an obstacle to the application of the analytical approaches from which we have learned so much about wildlife rabies. ‘Gold standard’ surveillance data based on laboratory-confirmed diagnosis is hampered not only by limited laboratory infrastructure but also by the practical challenges of locating, sampling and submitting specimens. However, pragmatic approaches to improving rabies surveillance have yielded rich insights. In addition to providing a foundation for burden of disease estimates, data on animal-bite injuries have been a used as a reliable indicator of canine rabies incidence, revealing new understanding of rabies metapopulation dynamics, as well as improving detection of animal rabies cases, the management of animal bites and the cost-effectiveness of PEP.

Pragmatic solutions are also being found to improve rabies diagnosis in settings with limited laboratory infrastructure, including techniques to support decentralized laboratory testing (e.g. direct rapid immunohistochemical test, dRIT) [73–76] and field diagnosis (e.g. immunochromatographic tests) [77–79]. These have great potential for empowering field staff to engage in rabies surveillance and respond more effectively to surveillance data, but standardization and quality control of field diagnostic kits still needs improvement. Given the rapid advances in metagenomic sequencing methods, future approaches may include real-time genomic surveillance. However, even simple technologies such as mobile phones can serve as leapfrogging technology that can dramatically improve the extent and resolution of rabies surveillance data.

Surveillance of mammal bites is conducted through a newly established PEP application system. Because of the initial shortage of RIG and rabies vaccines, distribution was tightly controlled by TCDC. Physicians needing to provide patients with RIG or rabies vaccines must send in their requests directly to TCDC by fax or email for review. Physicians must indicate biting animals, whether the animal was wild or owned, bite site on body of the bitten, bite category, according to WHO classification, circumstances of the incident, and the disposition of the animal that bit. Medical officers in TCDC would then review the application, and decide on whether to approve the application for PEP. Approval to provide treatment is given by phone within 30 minutes. After one month, as physicians have become familiar with rabies PEP recommendations, physicians were asked to provide rabies PEP immediately, and have their requests reviews retrospectively, to ensure that all patients needing rabies PEP were given appropriate PEP.

After the reemergence of animal rabies, the Advisory Committee on Immunization Practices (ACIP) in Taiwan established new recommendations of rabies PEP. During July 18–29, ACIP recommended PEP for individuals with category II or III exposure, as defined by the World Health Organization [32, 33], to wild mammals nationwide and stray dogs and cats in affected mountainous townships. PEP recommended include five doses of rabies vaccine given on days 0, 3, 7, 14, and 28, plus 20 IU/kg of human RIG given within 7 days of the first rabies vaccine dose at the wound site. On July 31, ACIP expanded PEP recommendations to individuals exposed to stray dogs and cats nationwide. RIG was initially prioritized for individuals with category III exposure to ferret-badgers nationwide and to house shrews in Taitung. On September 6, with increased availability of RIG, individuals with category II exposure to ferret badgers in which rabies was confirmed by DFA were also recommended for RIG, to ensure that no human rabies occurred, even though this is contrary to WHO recommendation.

BsAb is composed of two binding specificities for two different antigens, or two different epitopes of an antigen, in one antibody molecule. BsAb is a newly emerging area in modern biology and is one of the most promising and exciting advancements in cancer immunotherapy. The purpose of BsAb is to recognize and target two different antigens simultaneously, thus realizing the desired therapeutic effects that cannot be achieved by a combination of two monospecific antibodies. In cancer immunotherapy, one of the most obvious advantages of BsAb is its ability to redirect immune effector cells to the proximity of targeted tumor cells, thus improving the tumor-killing potential of these immune effector cells. This prototypic form of BsAb is generally composed of two linked single-chain fragment variables with one targeting molecule present on immune effector cells, such as the CD3 found on T cells, while the other targets a tumor-specific antigen.

Currently, no studies have been reported that use BsAb to treat rabies. As mentioned above, sufficient levels of VNAs and the enhancement of BBB permeability are critical for the clearance of RABV from the CNS. As live-attenuated viruses pose safety concerns while BBB permeability-enhancing agents can cause unwanted neurological complications due to the disruption of normal BBB integrity, the ideal approach is to deliver VNAs without altering the normal function of the BBB. Endogenous BBB receptors involved in the normal receptor-mediated transcytosis (RMT) pathways that function to transport macromolecules such as insulin and transferrin into the brain are ideal targets that can be utilized to deliver biologics; a case in point are rabies VNAs, which can be delivered to the CNS without interfering with the normal functionality of the BBB. Such receptors include two well-characterized transferrin receptors and an insulin receptor, or the recently identified solute carrier CD98 heavy chain (CD98hc). Therefore, targeting these receptors can act as a molecular Trojan horse to ferry biologics fused to the antibody into the brain via RMT. Consequently, a BsAb for rabies treatment could be constructed with one arm targeting one of the endogenous BBB receptors while the other targets the RABV G protein. Theoretically, this BsAb could pass through the BBB while retaining the ability to neutralize RABV in the CNS, thus potentially indicating a novel therapy for rabies treatment in the future.

The pre-publication history for this paper can be accessed here:

http://www.biomedcentral.com/1471-2334/10/234/prepub

Recent research on rabies has generated a strong body of evidence for the feasibility of elimination of canine rabies through mass vaccination of domestic dogs. Global momentum is now building towards implementation of large-scale programmes to achieve first, the elimination of human deaths mediated by canine rabies, and second, disruption of transmission within the dog population and the elimination of canine rabies entirely. However, time is short to reach these global targets and there is no cause for further delay.

According to the current WHO guidelines, we divided rabies post-exposures into three categories (see Additional file 1 for details). Exposure category I describes the lightest degree of exposure to infection, without any skin injury, while category III describes the most serious situations where single or multiple transdermal bites or scratches occurred that required immediate wound treatment and anti-rabies vaccines.

China banned nervous tissue vaccines (NTVs) in 1981, so different provinces adopted slightly different options for rabies vaccine products. For example, Guangdong provincial CDC recommended using the following products: purified Vero cell rabies vaccine (PVRV, Aventis Pasteur, Lyon, France), purified chick embryo cell vaccine (PCEV, ChengDa Biologicals, Shengyang, China) and hamster kidney cell vaccine (PHKCV, Lanzhou Institute of Biological Products, Lanzhou, China). Nevertheless, there are substantial numbers of vaccine products produced by small companies or institutes in China, thus lacking suitable quality and efficacy control. These low-quality vaccine products not only increased the difficulties in controlling and preventing rabies, but also complicated the public health programmes in other Asian countries that imported these products. The standard post-exposure vaccination schedule was the 'Essen' 5-dose intramuscular regimen on days of 0, 3, 7, 14 and 28. However, five pre-exposure or post-exposure schedules are currently used in China (see Additional file 2 for details).

The pre-publication history for this paper can be accessed here:

Kudus can be vaccinated by the oral route and protected against a subsequent rabies infection, although it seems that they are rather refractory to this route of vaccine administration. In any case, further studies need to be initiated to optimize oral vaccine uptake and delivery of this 3rd generation attenuated oral rabies vaccine. Alternatively, recombinant rabies virus vaccines expressing the RABV glycoprotein could also be considered in future studies57. For the time being the minimum effective titer of both attenuated and recombinant vaccine viruses required to efficiently immunize the animals is not known yet. Hence, further research should investigate how vaccine uptake effectiveness can be improved, for example by increasing vaccine titre, vaccination intervals or adding muco-adhesive substances. Attractive baits for oral vaccination of Kudus have been developed already58, however, bait delivery systems need to be optimized in case vaccine potency can be enhanced in this species. Also, validation of serological assays for Kudus is required to make better informed decisions on the immune status.

Veterinary services in Africa usually report very limited budgets and often have to divert resources during outbreaks of other diseases,[81]. This is clearly the most significant constraint to effective rabies control. However, with increasing human and dog populations, dog rabies incidence, human exposures to rabies and the costs required to prevent human rabies deaths through PEP will invariably continue to rise unless rabies can be controlled at the source, i.e. in domestic dog populations. Many countries in Asia, such as Thailand, Vietnam and Sri Lanka have greatly reduced human rabies deaths through increased PEP use, but at a very high cost. In Vietnam, for example, deaths fell from 285 in 1996 to 82 in 2006 with administration of >600,000 PEP courses per year at an estimated cost of ∼$27 million/year.

Although domestic dog populations need to be targeted for the effective control of rabies, this is usually deemed to be the responsibility of veterinary services even though many of the benefits accrue to the medical sector. In rural Tanzania, dog vaccination campaigns led to a rapid and dramatic decline in demand for costly human PEP. In pastoral communities, vaccination not only reduced rabies incidence, but has now resulted in a complete absence of exposures reported in local hospitals for over two years (Figure 4).

Large-scale campaigns can therefore translate into human lives and economic savings through reduced demand for PEP. Costs per dog vaccinated are generally estimated to be low (rural Tanzania ∼$1.73, Philippines ∼$1.19–4.27, Tunisia ∼$1.3, Thailand ∼$1.3 and Urban Chad ∼$1.8) and preliminary studies suggest that including dog vaccination in human rabies prevention strategies would be a highly cost-effective intervention at ∼US $25/DALY averted (S. Cleaveland, unpublished data; see also 82).

Developing joint financing schemes for rabies prevention and control across medical and veterinary sectors would provide a mechanism to use savings in human PEP to sustain rabies control programs in domestic dogs. Although conceptually simple, the integration of budgets across different Ministries is likely to pose political and administrative challenges. However, given sufficient political will and commitment, developing sustained programmes of dog vaccination that result in canine rabies elimination should be possible.

In conclusion, here we show that a substantial body of epidemiological data have now been gathered through multiple studies demonstrating that: (1) rabies is an important disease that exerts a substantial burden on human and animal health, local and national economies and wildlife conservation, (2) domestic dogs are the sole population responsible for rabies maintenance and main source of infection for humans throughout most of Africa and Asia and therefore control of dog rabies should eliminate the disease, (3) elimination of rabies through domestic dog vaccination is epidemiologically feasible, (4) the vast majority of domestic dog populations across sub-Saharan Africa are accessible for vaccination and the few remaining factors compromising coverage can be addressed by engaging communities through education and awareness programs, (5) new diagnostic and surveillance approaches will help evaluate the impact of interventions and focus efforts towards elimination, and (6) dog rabies control is affordable, but is likely to require intersectoral approaches for sustainable programmes that will be needed to establish rabies-free areas.

The last autochthonous human rabies case identified any French territory was reported in 1924 in continental France. However, the risk remains of humans being exposed to the virus in enzootic countries and not seeking PEP due to ignorance of the rabies risk. Since 1970, 21 human deaths from rabies have been recorded in France: 20 cases were imported and 1 was transmitted by a corneal transplant. The first human rabies case diagnosed in French Guiana, in May 2008, and described herein, confirms that the risk of contracting the rabies virus there indeed exists. It was the first case subjected to molecular biology confirmation by the French National Reference Center for Rabies.

The patient's initial clinical picture was not typical and he consulted the Cayenne Hospital emergency unit 3 times before being admitted, further emphasizing the need to include rabies in the differential diagnosis of unexplained encephalitis in humans. Rabies was diagnosed intravitam based on RT-hnPCR–detection of viral RNA in saliva and a skin biopsy. The rabies virus responsible was similar to those circulating in hematophagous bats in this part of the world and closely related to those previously isolated from animals in French Guiana with <4% nucleotide divergence in the nucleoprotein gene (unpublished data). The origin of the contamination was not formally established, although an unrecognized vampire-bat bite seems by far the most likely route of transmission. However, as some cases reported in other countries, the source of contamination could also have been feline, because a cat reportedly died in March 2008, 2 months after having been severely bitten and wounded by a bat.

After the public was informed of this case, the number of patients consulting the CTAR increased dramatically, a phenomenon that had previously been observed in continental France. Since 2008, no other human rabies case has been reported in French Guiana.

Recent emerging zoonoses, e.g., Ebola or Marburg virus hemorrhagic fevers, Nipah virus encephalitis, severe acute respiratory syndrome (SARS), highlight the potential of bats as vectors for transmission of infectious diseases to humans. This potential was already known for rabies encephalitis, since 10 of the 11 Lyssavirus species are transmitted by bats. Rabies control in bats remains very difficult, even though some encouraging experimental results obtained with D. rotundus bats in captivity demonstrated the immunogenicity of the vaccinia-rabies glycoprotein. However, several effective methods are available to limit the access of the bat population to cattle. Furthermore, some preventive and control measures to limit the number of human deaths attributable to rabies transmitted by vampire bats have been successfully implemented.

Rabies diagnosis is a key issue. It is routinely based on clinical and epidemiological information, especially when the exposure is reported in a rabies-endemic country. Although techniques for postmortem diagnosis of rabies have been well-established for decades, tests for intravitam diagnosis of human rabies were rarely optimal, and depended entirely on the nature and quality of the sample supplied. Over the past 3 decades, molecular biology tools have contributed to the development of these tests, resulting in more rapid detection of the rabies virus. Several molecular methods are now available that can be used to complement conventional tests for human rabies diagnosis. The 21st century challenges for diagnostic test developers are 2-fold: first, to achieve internationally accepted validation of a test that will then lead to its acceptance by international organizations; second, these tests are mainly needed in developing regions the world, where financial and logistical barriers prevent their implementation,. The question is even more important in that rapid diagnosis of rabies in suspected human cases influences PEP for potential case contacts and ensures appropriate patient management.

This first human rabies case in French Guiana means that national and local public health authorities must improve preventive and control measures for the local population and travellers. Rabies prophylaxis requires a multifaceted approach, including health education, PEP, systematic vaccination of dogs and cats, and, sometimes, selective immunization campaigns to control transmission among wild animals, e.g. foxes and hematophagous bats. Since human rabies is almost always fatal if prophylactic measures are not initiated, it is essential to increase awareness of who should receive PEP and when it should be administered.

Pre-exposure prophylaxis entails the administration of the rabies vaccine to individuals at high risk for exposure to rabies viruses, e.g., laboratory workers who handle infected specimens, diagnosticians, veterinarians, animal-control workers, rabies researchers, cave explorers….

PEP consists of a multimodal approach to decrease an individual's likelihood of developing clinical rabies after suspected exposure to the virus. Regimens depend on the victim's vaccination status and involve a combination of wound cleansing, rabies-vaccine inoculation, and administration of human rabies immunoglobulins. When used in a timely and accurate fashion, PEP is nearly 100% effective. However, once clinical rabies manifestations have developed, rabies PEP remains supportive. To date, only 5 well-documented cases of prolonged survival or recovery from rabies have been described and were specifically associated with PEP administration before the onset of symptoms. The recently developed Milwaukee protocol added induction of therapeutic coma to supportive care measures and antivirals, claiming it ensured the recovery of an unvaccinated patient. However, its use has yielded inconsistent outcomes.

The impact of this rabies-virus emergence in French Guiana was dramatic, especially in the context of a Department far from continental France. Despite the enormous pressure placed on the crisis-managing team by the local population, healthcare workers and politicians, the number of PEP remained relatively limited compared with previous cases in continental France and other countries–[32]. Notably, no subsequent case developed.

This case illustrates the need for further preparedness of public health infrastructures in rabies-enzootic areas that have not yet recorded human rabies cases. Pertinently, lessons learned from other countries, informing public health professionals and a multidisciplinary approach were essential to crisis management of our case,. His case history enhanced the perception of the risk and, consequently, a vast campaign to educate and inform the general population about zoonotic diseases acquired from domestic, as well as wild animals, like bats, was undertaken in French Guiana as had been done in neighboring countries. In addition to these measures, rabies is now more systematically included in the differential diagnosis of human encephalitis cases consulting at French Guiana hospitals. Indeed, 2 suspected human cases, subsequently found negative, were subjected to rabies testing during 2008–2010 period. In parallel, active surveillance of bat rabies has been established to learn more about rabies-virus circulation in the local bat populations.

Poor surveillance and diagnosis capacity means that (1) data is insufficient to demonstrate disease burden and motivate policy-makers, and (2) impacts of control efforts cannot be evaluated.

Considerable progress has been made in the development of simple and inexpensive techniques for sample preservation and rapid post-mortem diagnosis suitable for laboratories with limited storage and/or diagnostic resources with potential to increase in-country capabilities for surveillance. A new direct rapid immunohistochemical test (dRIT) requires only light microscopes, which are widely available. The test is simple and can be performed by a range of operators if appropriate training is provided. Field evaluation studies in Africa demonstrated that this assay has characteristics equivalent to those of the direct fluorescent antibody (DFA) test, the global standard for rabies diagnosis, including excellent performance on glycerolated field brain material,[73], the preservative of choice under field conditions,[75]. Other simple field-diagnostics that allow rapid screening, including enzyme immunoassays, dot blot enzyme immunoassays and lateral-flow immunodiagnostic test kits,[79] are being evaluated. These tools offer hope of extending diagnostic capacity in resource-limited settings.

Animal-bite injury data from hospitals are an easily accessible source of epidemiological information and have been verified as reliable indicators of animal rabies incidence and human exposures,[14]. Furthermore, increasing availability of communication infrastructure through mobile phone network access in remote areas could enhance surveillance by allowing real-time reporting.

Serum samples (B) were collected at different time points prior, p.v. and p.i. to investigate the development and kinetics of rabies induced antibodies (Fig. 3). Initial blood sampling was on the day of capture (B0); additional blood samples were taken on day 28 p.v. (B1) and 56 (B2) p.v. (vaccination study) as well as on day 261 p.i. (B1) and day 183 p.i. (B3) from contact and surviving animals of the challenge infection in the transmission and vaccination study, respectively (Fig. 3).

For any manipulation (e.g. sampling, vaccination), animals were always immobilized and sedated with a combination of 6–8 mg of thiafentanil oxalate (10 mg/ml Thianil, Wildlife Pharmaceuticals, Rocky Drift, White River, South Africa) and 100 mg azaparone (Kyron) reversed with 80 mg naltrexone hydrochloride (50 mg/ml Trexonil, Wildlife Pharmaceuticals). Sedation was carried out using a dart gun with X-Caliber CO2 operated dart projector with syringe darts of 2 ml with 14 GA × 25 mm needle (Pneu-Dart Inc., Williamsport, PA, USA).

Human and animal mortality from rabies remains a major burden in many countries around the world. Global estimates reveal about 50,000 human deaths from rabies every year, and higher mortality among the livestock. Diverse species of warm-blooded animals transmit the causative agent, a single stranded RNA virus of the genus Lyssavirus of the viral family Rhabdoviridae. Natural exposure to rabies virus through animal bites is very common in the endemic countries of Asia and Africa where companion animals, mainly dogs, are the main vectors of viral transmission to humans. Rabies is 100% fatal but preventable by timely administration of effective pre- or post-exposure vaccination.

At present, mainly cell-culture derived vaccines are used across the world to provide immunity against rabies. However, these expensive, temperature-sensitive biologicals are often not available in many endemic areas. Successful immunization of about 70% of susceptible canine population could prevent rabies transmission among canines, but this faces technical and logistic challenges. Canines require annual rabies vaccination from 3 months of age, and often do not develop adequate protective immunity due to poor immunogenicity of the vaccines used, or co-existent malnutrition and diseases. Huge numbers and rapid turnover of free-roaming canines and difficulties in retrieving them for booster vaccinations introduce additional layers of complexity into canine vaccination programmes in the developing countries. Clearly, potent, inexpensive vaccines and efficient mass vaccination strategies are needs of the hour in canine rabies control in the rabies-endemic countries.

Plasmid-based vaccination has been explored as an alternative to cell culture-based rabies vaccines for animal prophylaxis. A eukaryotic expression vector encoding full-length glycoprotein gene of rabies virus represents the simplest design of an anti-rabies vaccine, capable of generating protective neutralizing antibodies upon in vivo delivery. Such constructs have been shown to mediate efficient prophylaxis in small animal models, but are observed to be poorly immunogenic in larger animals ([5], and references therein). In general, systemic lability, poor cellular uptake and low immunogenicity remain major hurdles limiting the utility of plasmids for in vivo applications. Attempts to improve plasmid-based vaccination are currently focused on improved vector design, gene modifications and the use of efficient delivery vehicles and molecular adjuvants.

Delivery approaches employing gene gun, electroporation, cationic lipids and microparticles, and nanopolymers have been evaluated in improving plasmid-raised immune responses. Molecular adjuvants in the form of gene fragments coding immunomodulatory molecules have also been employed in plasmid vaccination to enhance its immunogenicity and efficacy. Such attempts have generally employed co-administration of discrete plasmids encoding the immunogenic gene and the adjuvant, or constructs designed as fusions of the two. These molecules could be advantageous in achieving site-specific adjuvanting, and limiting adjuvant toxicity. A variety of cytokine, chemokine, pro-apoptotic and other genes have been reported to be effective adjuvants for plasmid-based vaccines.

Toll-like receptors (TLRs) are a group of evolutionarily conserved pattern recognition receptors expressed on a wide variety of immune and non-immune cells, that sense specific pathogenic ligands and initiate inflammatory and immune signaling cascades. Their ligands and signaling intermediaries hold considerable promise as immunomodulatory agents.

TLR ligands need to be present extracellularly to bind their cognate receptors, a requirement which increases the risk for their non-specific interactions, systemic toxicity and other adverse events. TLR adaptor molecules, however, act within the cell, limiting the possible toxicity, and quickly achieve threshold levels and faster kinetics. Myeloid differentiation factor 88 (MyD88) is an adaptor molecule essential in signaling through all TLRs except TLR3, and also has roles in signaling through interleukin (IL)-1R1, IL-18R1 and interferon-γ receptor 1 pathways. Studies have reported critical roles for signaling through TLR and MyD88 pathways in the generation of vaccine-generated humoral immunity. Takeshita et al. reported the enhancement of immunogenicity and protective efficacy of a plasmid-based influenza vaccine, upon the use of Myd88 as a genetic adjuvant.

TLR adaptor molecules have not been investigated previously for their adjuvanting potential in plasmid vaccines against rabies. In the present work, we evaluated Myd88 as a genetic adjuvant in a candidate plasmid rabies vaccine, and report that its effects on the immunogenicity and protective efficacy of the vaccine in Swiss albino mice.

A 41-item structured questionnaire was developed in English and translated and reviewed by native Thai speakers employed by the office of the U.S. CDC, International Emerging Infections Program in Bangkok. The questionnaire was designed to be administered in Thai via face-to-face interviews, with responses entered in PDAs using GeoAge FAST software. Not all questions provided data used in this study. The questionnaire was developed based on socio-ecological reasoning about gaps in rabies knowledge that potentially translate into failed prevention on the individual level.

Data were collected on demographics; primary bat-associated activity and years of experience; history of rabies vaccination; and type and frequency of bat exposures such as cave entry, direct contact with bats, bites and scratches from bats, and bat consumption. Individuals who reported receiving rabies vaccination were asked to indicate whether it was in direct response to an animal exposure (i.e. PEP) or for pre-exposure immunization (PreP), which is a vaccination series most often administered to people who have a relatively high likelihood of rabies virus exposure due to occupational risks or other factors. Those who reported receiving PreP were asked to describe its administration and only those who indicated receiving a series of injections spaced over multiple days were counted as having PreP.

To assess rabies-related knowledge, participants were asked to rate their understanding of the disease as either “little or none”, “basic”, or “extensive”; explain how humans acquire the disease, and identify animal sources of the disease. Each knowledge question was evaluated independently, and the validity of a participant's self-reported knowledge level was not verified using other responses. Participants were also asked to describe the severity of rabies. Only responses that emphasized death or profound suffering with no suggestion that recovery was likely were considered evidence that the participant recognized rabies as being severe. Awareness of other diseases that humans can get from bats was also elicited.

To assess health-seeking practices following transdermal bat exposures, participants were asked about actions they would take if they were bitten or scratched by a bat. Responses to this open-ended question were compared to a similar question later asked about actions a person should take following a bite from a potentially rabid animal, based on the participant's own understanding of what constitutes a potentially rabid animal. Questions that were specifically asked about bats preceded all questions asked about rabies to minimize reporting bias, and whenever feasible, participants were asked open-ended questions to minimize the interviewer's influence on responses. Participants were also interviewed away from other people. Interviewers were instructed to not ask questions in a leading manner and to allow as much time as necessary for participants to answer.

An epidemiological investigation was conducted to identify people potentially exposed to rabies virus, who would require post-exposure prophylaxis (PEP). To do so, the following criteria of exposure were used. A person was defined as potentially exposed when: 1) he/she was a part of the case's entourage (family, friends, sexual partners, sport team members, colleagues, visitors) during the 15 days preceding the onset of the index case's symptoms; 2) he/she was a healthcare worker who had cared for the case; or 3) he/she had been in contact with animals suspected of being contaminated (based on their behavior, illness, death) that were known to have been in contact with the case. An active search was carried out within the case's familial and professional entourage, and in the Cayenne hospital, where the patient had been admitted. A step-by-step search procedure was implemented, questioning all exposed people able to identify other people suspected of being in contact with the index case. All people suspected of exposure had a medical visit at the IPG CTAR, using Questionnaire 2 (Table S2) for the case's familial and professional entourage.

For the healthcare workers, the exposure level was assessed for those working in 3 of Cayenne Hospital's departments: emergency unit, intensive care unit and biology laboratories. To assess their possible exposure, specific healthcare worker questionnaire 1 (Table S1) was filled out during a medical consultation with the physician responsible for the hospital's hygiene unit.

A healthcare worker was considered to be exposed when: 1) he/she had close contact with the index case (<1 m), took part in his resuscitation or performed an act susceptible of generating aerosolization of body fluids; 2) he/she had close contact with the case's biological fluids (laboratory workers); 3) he/she had been bitten by the case; or 4) he/she failed to comply with general hygiene recommendations.

When exposure corresponded to those criteria, the 4-dose Zagreb immunization protocol (1 of the schedules recommended by the World Health Organization guidelines) against rabies was systematically administered.

Chi-square or Fisher's exact test, with a risk α of 5% was used for simple comparisons of rates. The statistical analyses were run with SAS version 9.1 (SAS Institute Inc., Cary, NC, USA).

At the population level, rabies is the quintessential bat EID that has been studied most intensively. Public health guidelines recommend rabies vaccination for humans in high-risk groups, vaccination of pets as well as animals on public display, isolation of domestic animals from the wildlife reservoirs of rabies, and public health education on appropriate precautions. Current guidelines recommend that pre-exposure prophylaxis be offered to those in high-risk groups including veterinarians, animal handlers, rabies researchers, and some laboratory workers. In addition, the vaccine can be offered to long-term travelers to endemic areas, especially if immediate medical attention will be unavailable (148, 152). Routine vaccination of the general population is currently not recommended, mostly due to cost.

Despite advances in determining best practices for animal vaccination, control of rabies in domestic and wild reservoirs remains challenging in resource-limited settings. Control of rabies in bats has proven challenging. Bat rabies has been reported in every state except Hawaii and 1,806 rabid bats were documented in the United States during 2009 (19). Of all animals, bats in particular pose a serious risk for rabies and should be excluded from structures to prevent contact with humans (148, 152). However, widespread reductions in bat populations to control rabies is neither feasible nor desirable. Instead, some novel methods have been explored to control infection in bat populations. Vampire bats can efficiently digest only coagulated blood and they die if the consumed blood is not coagulated. Application of anticoagulant-containing ointment on the fur of captured vampire bats (with their subsequent release) leads to consumption of the coagulant by several roost mates via mutual grooming. Similarly, anticoagulation of livestock is another useful approach to control vampire bat populations where rabies is a threat (reviewed in Kuzmin and Rupprecht (7)). As another approach, it has been suggested that oral vaccination of wildlife may limit the spread of rabies by bats (153). Finally, we know that some species of moths are able to disrupt bat echolocation using ultrasonic clicks of their own (154, 155). The use of similar, artificially produced, sounds to ward off bats from human and livestock habitats should be explored.

A clear example of the value of One Health interventions is provided by approaches to the prevention of human rabies deaths. Human rabies is 100% preventable through two complementary measures: first, post-exposure prophylaxis (PEP), which involves administration of rabies immunoglobulin and a multi-dose course of rabies vaccination to people bitten by suspected rabid animals; second, mass vaccination of animal reservoirs (primarily domestic dogs, the reservoir in the vast majority of human cases), which reduces the risk of human exposure and can ultimately result in rabies virus elimination.

While PEP is highly effective in preventing deaths in people exposed to the virus, many challenges remain for poor people in remote, rural communities in accessing and completing PEP regimens. Delays in receiving the first dose of vaccine can all result in fatal outcomes, and occur as a result of vaccine being available only in larger clinics, a generally poor transport infrastructure, and/or the need to raise cash to cover medical and transport costs. In rural Tanzania, where most people still live on less than US$2 per day, patients would need to spend over US$100 to complete WHO recommended PEP schedules. These challenges are compounded by intermittent vaccine shortages, particularly at rural health facilities, which further contribute to delays in patients receiving PEP and their inability to complete full schedules.

The realities of current approaches to the management of rabies exposures are revealed by data on the outcome of rabies exposures in 844 people from detailed contact-tracing studies conducted in Tanzania from 1996 to 2016. Eighty individuals were recorded to have died from rabies, 71 (89%) of whom had not received any PEP at all, and none of the remaining nine had received a full course. The critical need for prompt PEP administration is shown by four rabies victims who developed rabies despite a delay of only 1 day in receiving the first vaccine dose. The poignancy of these preventable deaths is highlighted by two of these cases where patients had reported immediately to health facilities, but faced health system delays in receiving the first vaccine dose, with fatal consequences.

Rabies also illustrates the critical importance of connecting human and animal health services in the implementation of cost-effective preventive measures. Human deaths can be prevented by a combination of prompt administration of PEP and mass vaccination of domestic dog reservoirs, but the relative levels of investment in these two arms of prevention are often mismatched. In Asia, for example, the incidence of human rabies cases is much higher than in Latin America, despite the elevated levels of per capita expenditure on human PEP provision in Asia (figure 2). Here, even though health sector expenditure on PEP is very high (with US$ 1.3 billion of direct costs spent annually on PEP in Asia), poor people are still dying from rabies due to lack of access to health services with PEP. Conversely, many doses of PEP are given to animal-bite victims who will have had no rabies exposure, often in relatively affluent urban areas. By contrast, measures to prevent rabies at source (i.e. through mass dog vaccination) protect both the rich and the poor, casting a wider ‘safety net’ than can be achieved by focusing on management of human exposures alone and cost considerably less. In Latin America, for example, even modest investments in mass dog vaccination (US$ 61 million per year, representing approximately 20% total expenditure on rabies prevention) have been highly effective in preventing human rabies deaths, with the region on the brink of eliminating dog rabies as a human health problem.

Rabies is caused by neurotropic viruses in the genus Lyssavirus, family Rhabdoviridae, and is transmissible to all mammals. Dogs are the main hosts responsible for human rabies in Africa, Latin Americas and Asia, especially in China, where rabies is re-emerging as a major public health threat, and its severity is only second to HIV and tuberculosis (TB) among all reportable infectious diseases. From the annual ~3000 human deaths, southeast China counts for most cases, with more than 90% attributed to rabid dog bites. Notably, both human population and dog density are high in the region with low rabies vaccination coverage in dogs. Given that the program of dog rabies elimination has not been listed in the priority of governmental agenda, it is possible that long term dog rabies enzootics will lead to spillover events of dog-associated rabies into wildlife species. In addition to rabies transmitted by rabid dogs, other sources of rabies exposure to humans, such as cats, ferret badgers (FB), and pigs, have been continuously reported in China. Interestingly, in provinces like Zhejiang, Jiangxi and Anhui, the percentage of dog-associated human rabies is relatively low. Meanwhile, up to 80% of the reported human rabies cases were inferred to be caused by FB bites in some districts in Zhejiang province from 1994 to 2004. Although rabies in badgers was previously recorded in other countries, FB-associated human rabies has never been reported except in China. The frequent occurrence of FB-associated human rabies cases in southeast China highlights the lack of laboratory-based surveillance and urges revisiting the potential importance of this animal in rabies transmission. Nevertheless, management of such animal bites in humans needs a clear guideline on post-exposure prophylaxis (PEP) for rabies. Currently, FB trading and its meat consumption are common in the related areas, resulting in a frequent source of FB bite to humans. Similar to severe acute respiratory syndrome (SARS) outbreaks through consumption of civet in south china, the close and frequent contact of FB by humans could be an important factor in human rabies cases in southeast China.

To determine if the FB actually contributes to human and dog rabies cases, and the possible origin of the FB-associated rabies in the region, we conducted an expanded retrospective/prospective epidemiological survey, which encompassed both descriptive and molecular epidemiological approaches.

The study design and consent process was approved by the Institutional Review Board (IRB) at CDC (protocol# 5709). All participants were verbally informed of the study's purpose and assured that their responses would be kept anonymous, even if they engaged in illegal activities. Oral consent was obtained to ensure anonymity and accommodate illiterate participants, and was documented by the interviewer electronically via personal digital assistants (PDA) prior to administering the survey. This method of obtaining informed consent was approved by CDC's IRB.

Vaccination programs are one of the most effective means of controlling infectious diseases and with the development of oral vaccines and bait delivery systems, the elimination of diseases circulating in wildlife populations has become a realistic possibility. The large-scale oral rabies vaccination (ORV) campaigns that have eliminated fox-mediated rabies from Western Europe and North America and substantially reduced disease incidence in central Europe are pre-eminent examples for the success of such control programs.

ORV programs in foxes are aimed at increasing herd immunity in the target population using oral rabies vaccines distributed into the environment. Over the past four decades several oral rabies vaccines, mainly live replication-competent attenuated rabies virus vaccines, have been successfully used in ORV campaigns. In Canada for example, the ERA-BHK21vaccine virus strain, a derivative of the cell culture adapted vaccine virus strain Street Alabama Dufferin (SAD), was the only live attenuated vaccine deployed in fox ORV campaigns. In Europe, with the exception of a recombinant vaccinia virus expressing the rabies virus glycoprotein, all constructs have been based on live attenuated rabies virus strains, derived from the SAD Bern original (SAD Bernorig) vaccine virus strain, a successor of the ERA strain. While all these vaccines have been highly efficient in fox rabies control, the first generation of SAD-derived vaccines demonstrated residual pathogenicity in non-target species particularly in rodents. Although several cases of vaccine virus-induced rabies were observed even in species other than rodents over the course of vaccination campaigns in a number of countries, including Germany, Austria, Slovenia, Romania, Poland, and Canada, such cases were without epidemiological relevance.

While previous analyses using high-throughput sequencing approaches revealed substantial genetic heterogeneity within commercial SAD-derived oral rabies virus vaccines, the sub-consensus genetic heterogeneity of viruses isolated from these vaccine virus-induced rabies cases on the contrary revealed nearly clonal genotypes, indicating the presence of a strong bottleneck during infection.

In this study, we attempted to further analyze and elucidate the mechanisms of genetic selection using a combined deep sequencing, variant, and haplotype analysis of a large set of vaccine-induced rabies cases that included additional SAD- and ERA-induced rabies cases from Poland and Canada alongside vaccine virus batches. With this comprehensive approach, we could demonstrate the utility of this approach for identity and genetic stability (revision to virulence) testing of vaccines with a heterogeneous genetic background. Additionally, we were interested to know whether viruses from vaccine-induced rabies cases differ from virulent field rabies viruses (RABV) at a population level.

Polio’s most visible current-day legacy is the permanently paralyzed victims on the streets of affected countries worldwide. In 1988, the World Health Assembly resolved to eradicate polio and as a result the global incidence of polio associated with wild polioviruses decreased from an estimated 350,000 cases in 1998 to 1,997 cases in 2006, and subsequently to 222 cases reported as of January 22, 2013 (symptom onset during 2012, reported in January 2013) (28,29). The number of countries that continue to have endemic circulation of polio has been reduced to three: Pakistan, Afghanistan, and Nigeria. Although transmission of types 1 and 3 polio continue to be reported, albeit in declining numbers, wild type 2 polio virus circulation was last reported in October 1999 (30) from Aligarh, Western Uttar Pradesh, India (31). The elimination of type 2 polio was a milestone for the Global Polio Eradication Initiative, which allowed strategies to focus on the eradication of poliovirus types 1 and 3 (30,32). In December 2011, the CDC Director activated the CDC Emergency Operations Center for the final push toward eradication. Eradicating the final 0.06% of polio is likely to be the greatest challenge. In the GDD Operations Center we monitor not only countries with endemic circulation, but also countries that report imported cases, which during 2012 was limited to Chad (28). Figure 2 shows CDC’s international responses to requests for assistances by countries experiencing cases or outbreaks of polio from January 2007 to August 2012, as reported to the GDD Operations Center. The importance of monitoring polio infections is critical now and will continue to be paramount in the post-eradication era, as even one case will represent an international public health emergency.

The 2009 A(H1N1)pdm09 influenza pandemic, the SARS epidemic in 2003, and the recent emergence of a novel coronavirus are recent reminders of the global health threat posed by zoonotic viruses. Prior to widespread emergence in human populations, such pathogens can cause occasional infections in sub-populations that have been exposed to reservoir species (common reservoir species include for example bats, birds, swine, non-human primates). Whilst viruses causing such “spill-over” infections are usually poorly adapted for sustained human-to-human transmission, they are under strong selection pressure to increase transmissibility once in humans. If the reproduction number R (i.e., the average number of persons infected by a case) evolves to exceed 1, a large scale epidemic in humans may result. Over the last decade, particular concerns were raised regarding highly pathogenic H5N1 avian influenza, due to the high mortality rate seen in humans and the virus's rapid spread in avian populations. However, as the A(H1N1)pdm09 influenza pandemic demonstrated, H5N1 is not the only influenza virus that may pose a pandemic risk. Recently, a swine-origin triple reassortant influenza A(H3N2) variant virus has emerged in the United States, carrying the matrix gene (M) from the H1N1pdm09 virus (H3N2v-M)–[4]. Studies in animal models have suggested that the presence of the H1N1pdm09 M gene may increase transmissibility of the virus,[6]. From January 2012 to September 2012, 307 laboratory-confirmed H3N2v-M human infections were reported to Centers for Disease Control and Prevention (CDC) as opposed to 12 throughout 2011. The majority of cases have been associated with agricultural fairs but there are documented events of human-to-human transmission. The surge in cases observed in summer 2012 raised public health concerns. Threats from zoonoses are not limited to influenza: more than half of all recent emerging infectious disease events were zoonotic.

For efficient prevention and control, quantitative and rigorous assessment of the risks associated with emerging zoonoses is desirable—in particular the risk that an emerging pathogen evolves to cause sustained human-to-human transmission. One approach to such risk assessment is by monitoring the reproduction number R of zoonoses in humans, with an alarm being raised if R increases or approaches 1–[11]. However, until now, estimating R required detailed outbreak investigations of human clusters,[11] and suffered from three important limitations: (1) the resources, access, and expertise needed to conduct investigations is not always available; (2) the proportion of cases that are missed during outbreak investigations may vary by setting and be difficult to assess; (3) even if the study is complete, the data collection process can be affected by a selection bias whereby larger outbreaks are more likely to be detected so that estimates of transmissibility may be biased upward. Consider for example a scenario where R = 0.7, where each case has the same detection probability ρ = 1%, and assume that once a cluster is detected, detailed outbreak investigation ensures that all cases in the cluster are detected. With an average size of 18.3 and a 21% probability of 1-case cluster, clusters that are detected are substantially larger than normal ones (average size: 3.3; 65% probability of 1-case cluster) (Figure 1A). As expected, this selection bias leads to R being overestimated as illustrated for methods that use the distribution of detected cluster sizes (Figure 1B).

Here, we present a new approach to estimate R during spillover events, aiming to address many of the limitations of existing methods. We apply our approach to assess the human-to-human transmissibility of swine-origin influenza A variant (H1N1v, H1N2v, and H3N2v) virus, in particular that of the H3N2v-M virus, from US surveillance data for the period December 2005–December 2011. We also present applications to another zoonotic virus (Nipah virus in Malaysia and Bangladesh) as well as to a non-zoonotic pathogen (Vibrio Cholerae in the Dominican Republic).