Infectious Disease. Implementing Pasteur's vision for rabies elimination.

Introduction Rabies has been one of the most feared diseases throughout human history and has the highest human case-fatality proportion of any infectious disease [1,2]. Every year over 7 million people receive post-exposure prophylaxis, and an estimated 55,000 people die from rabies [3] (more than yellow fever, dengue fever, or Japanese encephalitis [4]). Over 99% of these deaths occur in developing countries where rabies is endemic in domestic dog populations [5]. However, the impacts of canine rabies are often overlooked, largely because human rabies deaths are now extremely rare in Western Europe and North America, where mass vaccination successfully eliminated the disease from domestic dog populations [6]. Increasing incidence of canine rabies in Africa and Asia has prompted concerns that similar strategies may not be effective in these areas [7,8]. The critical question now is whether global elimination of domestic dog rabies is achievable. Keys to answering this question include: a quantitative understanding of the transmission dynamics of rabies in domestic dog populations, particularly the basic reproductive number, R0; a quantitative understanding of domestic dog demography; and information about the practicality and effectiveness of various vaccination strategies. While recent data support the feasibility and practicality of domestic dog vaccination strategies [9–11], there are very little quantitative data on rabies transmission dynamics [12] and the underlying demographic processes. Transmission is the most important process underlying infectious disease dynamics [13], but it is also the least understood. Rates of transmission are usually inferred from population patterns of disease incidence, but population-level analyses do not capture between-individual variation in transmission resulting from differences in behaviour, genetics, immune status, and environmental and stochastic factors, which play an important role in determining disease dynamics [14,15]. Contact tracing has been used to directly measure case-to-case transmission, and applications of the technique to emerging infections such as SARS have generated important insights into disease transmission and control in human populations [16,17], but transmission processes for diseases circulating in animal populations are much harder to study. Rabies is an acute viral encephalitis that is spread through the saliva of infected hosts [2]. Clinical manifestations vary, but the neurological phase often includes increased aggression and the tendency to bite and thereby transmit infection; rapid progression to death is inevitable [4]. These distinctive signs make transmission of rabies easier to track than that of most other diseases and provide an unusual opportunity to explore epidemiological patterns at the scale of the individual. Here, we present data on rabies transmission in two districts of rural Tanzania, Serengeti and Ngorongoro (Figure 1). We were able to monitor the spread of infection using contact-tracing methods, which were feasible due to the discrete and memorable nature of transmission events. We recorded >3,000 potential transmission events between 2002 and 2006 and reconstructed case histories of over 1,000 suspect rabid animals that illustrate heterogeneity in several aspects of transmission, including the latency, movement patterns, and biting propensity of infected individuals. Although these districts border the Serengeti ecosystem, we have argued that domestic dogs are the sole maintenance population of rabies in this community: they make up over 90% of our observations of rabid animals, and the >70 isolates that have been sequenced (from 13 host species) are all consistent with the Africa 1b canid strain [18,19]. This is one of the most extensive datasets on individual transmission events assembled in an animal population; it has potential to shed light on critical, but often elusive, details of infectious disease transmission. We also analyze data from rabies outbreaks around the world, which provide a global and historical context for the Tanzania dataset. Figure 1 The Location and Timing of Animal Rabies Cases in Serengeti and Ngorongoro Districts, Northwest Tanzania (A) Rabies cases in Serengeti (blue) and Ngorongoro (red) districts from January 2002 until December 2006. LGCA = Loliondo Game Controlled Area, NCA = Ngorongoro Conservation Area. Dark gray lines show village boundaries. Populations of humans and domestic dogs are denser in Serengeti district than Ngorongoro (Table 3). (B) Biweekly time series of rabies cases in each district. Results Epidemiological Parameters and Transmission Analyses of the contact-tracing data generated robust estimates of epidemiological parameters that have important implications for rabies control (Table 1, Figures 2 and 3, and Figure S1) and provide insight into how infectious disease transmission scales from individual behaviour to population-level dynamics. We estimated R0 for rabies in Serengeti and Ngorongoro districts directly from infectious histories, from reconstructed epidemic trees based on the spatiotemporal proximity of cases, and from the exponential rate of increase in cases at the beginning of an epidemic. Biting behaviour of rabid dogs during the course of infectious periods was highly variable (mean bites per rabid dog = 2.15, 95% confidence interval (CI) from fitting a negative binomial distribution: 1.95–2.37; variance = 5.61, CI: 4.63–6.92; shape parameter k = 1.33; CI: 1.23–1.42) (Figure 3A). The probability that an unvaccinated dog developed rabies after being bitten by an infectious animal was high (P rabies|bite = 0.49, CI: 0.45–0.52) (Table 1) if the bitten dog was not vaccinated or killed immediately after exposure. Multiplying the average number of dogs bitten per rabid dog by the probability of developing rabies following exposure gave an R0 estimate of 1.05 (CI: 0.96–1.14) (Figure 3A and Table 1). These estimates should be regarded as lower bounds, because not all transmission events were observed (this calculation excludes rabid dogs that were killed before biting other animals or that disappeared and likely corresponded to unknown or unobserved rabid dogs in other areas; see Materials and Methods). Detailed data on the timing and location of transmission events and infections allowed us to estimate the spatial infection kernel and generation interval (distances and times between source cases and their resulting infections, respectively) (Figure 2) and probabilistically reconstruct transmission networks (Videos S1 and S2). Calculating the average number of secondary cases per rabid dog during the period of exponential epidemic growth (before vaccinations were implemented) from these reconstructions gave similar R0 estimates of 1.1 in Serengeti district and 1.3 in Ngorongoro (CIs: 1.04–1.10 and 1.26–1.42, respectively) (Table 1). The more traditional approach of estimating R0, by fitting a curve to incidence data over the same interval of exponential epidemic growth, also produced similar estimates of 1.2 in Serengeti and 1.1 in Ngorongoro (CIs: 1.12–1.41 and 0.94–1.32, respectively) (Table 1 and Figure 3B). This approach is robust to underreporting (Text S1 and Figure S2) but should likewise be considered a lower bound, because some local control measures were instituted (such as tying or killing). We also estimated R0 from the intrinsic growth rate of outbreaks of domestic dog rabies elsewhere in the world (Table 2) and obtained values between 1.05 and 1.85, which are consistent with our estimates from northwest Tanzania. Table 1 Epidemiological Parameter Estimates Figure 2 Observed Frequency Distributions of Important Epidemiological Parameters (A) The incubation period, (B) the infectious period, and (C) the spatial infection kernel. The best fitting gamma distributions to the data are shown by black lines (see Materials and Methods). Figure 3 Transmission of Rabies (A) The distribution of dogs bitten per rabid dog (fitted by a negative binomial distribution with mean = 2.15 [95% CI: 1.95–2.37]; variance = 5.61 [95% CI: 4.63–6.92]; shape parameter k = 1.33 [95% CI: 1.23–1.42]; R0 ∼ 1.1). To calculate R0, we excluded dogs that were killed, tied, or those that disappeared before biting any other dogs. Variability in biting behaviour means that a small number of individuals disproportionately affect transmission and can potentially spark an epidemic, but since most individuals cause few, if any, infections, R0 is low and most introductions quickly die out (Figure 4C). (B) Exponential epidemic growth in Serengeti (blue, R0 ∼ 1.2) and Ngorongoro (red, R0 ∼ 1.1) districts. The R0 estimates from the epidemic trajectories were relatively insensitive to the period used for fitting the exponential curve. The inset shows the distribution of R0 estimates based on fitting to different regions of the time series. (C) The effective reproductive number, R, (averaged over three-month intervals) for Serengeti (blue) and Ngorongoro (red) districts measured from reconstructed epidemic trees that incorporate prior knowledge on who infected whom. Dots indicate the number of secondary cases resulting from each primary case (inferred from the composite tree of most likely links, with random jitter to avoid superposition on the y-axis). R0 estimated from these reconstructions (during the period of exponential epidemic growth) was ∼1.1 and ∼1.3 for Serengeti and Ngorongoro, respectively. Table 2 Estimates of R0 for Outbreaks of Rabies in Domestic Dog Populations around the World Table 3 Demographic Parameters and Population Attributes Estimated from Domestic Dog Populations in Northwest Tanzania Figure 4 The Impact of Vaccination on Transmission (A) The size of village-level outbreaks (defined as at least two cases not separated by more than one month, isolated cases are assumed to be non-persistent introductions) in Serengeti (blue, n = 138) and Ngorongoro (red, n = 20) districts plotted against village-specific vaccination coverage at the outbreak onset. Coverage was extrapolated from a demographic model initialized with village-specific dog population estimates and incorporating village-specific vaccination data. Gray shading and contours correspond to the probability of observing an outbreak of a particular size or less, generated from 10,000 stochastic simulations of rabies transmission for every initial vaccination coverage (contours were calculated conditional upon >1 secondary case occurring). The inset illustrates a village-level example of the susceptible reconstruction used to calculate instantaneous vaccination coverage plotted beside rabies cases in that village. (B) The distribution of secondary cases per infectious dog as inferred from reconstructed epidemic trees in Serengeti (blue) and Ngorongoro (red) districts, plotted against vaccination coverage in the village where the primary case occurred. Random jitter was added to prevent superposition on the y-axis. (C) Probability of an outbreak being seeded by an introduced case under different levels of vaccination coverage. Due to heterogeneity in the transmission process outbreaks rarely occur when coverage is maintained above P crit. However if infections are frequently imported from outside the vaccinated region, at least 40% coverage would need to be maintained to reduce the probability of subsequent outbreaks (of at least ten cases) to 70%. Small outbreaks occurred in villages with lower coverage and the largest (and longest) outbreaks only occurred in villages with 0.5 IU/ml) in response to vaccination [32], suggesting that these factors do not impair the efficacy of dog vaccination in rural Tanzania. In addition, numerous practicalities—such as occasional failures in the cold chain, improper vaccination of animals, mistaken registrations, etc.—will all reduce the level of population immunity below the estimated vaccination coverage. Furthermore, our observations and simulations confirm that small outbreaks may occur simply by chance even when coverage exceeds P crit [33], and these are particularly likely when there is individual variation in transmission (Figure 4). Higher levels of coverage are therefore necessary to reduce the chance of outbreaks with greater certainty; especially where the risk from imported infections is highest (Figure 4C). This could be a concern if canine rabies were to be eliminated from domestic dog populations but continued to circulate in sympatric wildlife; however, canine rabies was successfully eliminated in Western Europe and North America despite the presence of wildlife hosts capable of transmission. Thousands of people die every year from this horrific and preventable disease, because the control of canine rabies has been severely neglected in developing countries [2]. Inherent inter-annual periodicity of epidemics exacerbates the situation, with rabies only intermittently perceived as problematic [6], as illustrated by the recent outbreak in China [34]. The problem of canine rabies has often been considered intractable in rural Africa, because of poor infrastructure, limited capacity, and the misperception that large populations of wild carnivores are responsible for disease persistence. Our analyses show that global control of canine rabies is entirely feasible and that successful elimination of canine rabies in many parts of the world has likely been achieved precisely because R0 is so low and institutional commitment to maintain high levels of vaccination coverage has been sustained [6]. Achieving vaccination coverage of 60% or more in dog populations in Africa is both logistically and economically feasible through annual vaccination campaigns [9–11,29]. The resultant reduction in costs of human post-exposure prophylaxis suggest that vaccination interventions targeted at domestic dog populations could translate into appreciable savings for the public health sector [3,8,29]. Furthermore, the inherently low R0 and the tractability of rabies contact-tracing indicates that once endemic rabies is controlled, elimination could be achieved through active case detection in remnant foci of infection (much like the strategy used to eradicate smallpox [35]); similar measures are proving effective in programmes to eliminate canine rabies in the Americas [36]. However, the most crucial step towards global elimination of canine rabies will be sustained commitment and coordinated efforts to maintain sufficient vaccination coverage in domestic dog populations. Materials and Methods Study areas. We collected data from two districts in northwest Tanzania: Serengeti, inhabited by multi-ethnic, agro-pastoralist communities and high-density dog populations, and Ngorongoro, a multiple-use controlled wildlife area, inhabited by low-density pastoralist communities, predominantly Maasai, and lower-density dog populations (Figure 1). Attributes of the dog populations in these districts are presented in Table 3. Wildlife populations also differ in the two districts, but domestic dogs are the focus of this study because they are the only maintenance population of rabies in the area [18]. Incidence data. Data on patients with animal-bite injuries from hospitals and dispensaries, case reports of rabid animals from livestock offices, and community-based surveillance activities were used as primary sources [18]. Visits were made to investigate incidents reported in 2002 to 2006 involving suspected rabid animals. Cases were mapped at the site of the incident (wherever possible) and villagers interviewed to evaluate the status of the biting animal, determine its case history, and identify its source of exposure and subsequent contacts (if known). The same procedure was exhaustively followed for all associated exposures/cases. Interviews were conducted with veterinary officers, local community leaders, and livestock field officers in attendance, resulting in an active reporting network. Cases were diagnosed on epidemiological and clinical criteria, adapting the “six-step” method through retrospective interviews with witnesses [37]. Rabies was suspected if an animal displayed clinical signs [37] and either (a) disappeared or died within 10 days, or (b) was killed, but had a history of a bite by another animal or was of unknown origin. Additional clinical criteria for wild carnivores (∼10% of human exposures were caused by wild animals and ∼10% of inferred transmission events involved rabid wildlife) included tameness, loss of fear of humans, diurnal activity (for nocturnal species), and unprovoked biting of objects and animals without feeding. When multiple incidents involving suspected rabid wildlife were reported on the same/consecutive days within neighbouring homesteads, we assumed a single animal was involved. Brain samples were collected and tested for confirmation wherever possible, but despite efforts to obtain diagnostic samples, most cases reported here were suspected rather than confirmed. Inadequate sample preservation such as storage at room temperature and long intervals between sample collection and testing (during which samples underwent repeated freeze-thaw cycles) probably caused specimens to deteriorate. Composite samples of each brain necessary to achieve the highest test reliability were also rarely available. Nevertheless, a high percentage of samples from suspected cases of rabies were confirmed by laboratory diagnosis (∼75%) suggesting that use of epidemiological and clinical criteria is justified and reliable [18]. Researchers are encouraged to contact the authors regarding data availability. Vaccination data. Dog vaccination campaigns in Serengeti district in 2000 resulted in low and patchy vaccination coverage (35–40% estimated from post-vaccination household surveys). Annual campaigns conducted from 2003 onwards in a 10-km zone adjacent to the western border of Serengeti National Park achieved higher coverage levels of between 40 and 80%. In 2004, the Tanzanian government conducted vaccinations in villages in Serengeti district beyond the 10-km zone reaching 55% coverage across the remainder of the district, but in subsequent years, campaigns were less systematic and conducted in fewer villages. Vaccination in Ngorongoro was restricted to small-scale localised campaigns in the district town centre until 2004, whereupon widespread annual vaccinations were implemented with overall coverage exceeding 80% [9]. Data on the number of dogs vaccinated in each village and on each campaign date were collected from 2003 onwards. Parameter estimation. The incubation period and duration of infectiousness were estimated for rabies in domestic dogs from records of when individual dogs were bitten, developed clinical signs, and were killed or died. Gamma distributions were fitted to these data using maximum likelihood with interval censoring to account for cases where the relevant data were only approximately known (Figure 2 and Table 1). To estimate the probability distribution of the generation interval, G(t), an incubation and an infectious period were drawn from their respective distributions, a “time-to-bite” deviate was drawn from a uniform distribution over the interval of the infectious period, and the two intervals were summed. There was a significant correlation between the length of the infectious and incubation periods, but significance was entirely due to a single data point; we therefore treated the distributions as independent. The spatial infection kernel K(d) was estimated by fitting a gamma distribution to the distances between known source cases and animals that they contacted. Many contacts occurred within the same, or neighbouring, homesteads. In these cases, the precise distance was not always recorded, but we assumed it was less than 100 m. We therefore replaced the probability of a contact within 100 m by the probability distribution averaged over the range 0–100 m. The basic reproductive number R0. (1) Direct estimates from infectious histories. Using maximum likelihood, we fitted a negative binomial distribution to data on biting behaviour of rabid dogs (Figure 3A). The probability of developing rabies following a bite (P rabies|bite) was estimated, excluding bitten animals that had previously been vaccinated, or that were either killed or vaccinated immediately after the bite, and binomial confidence intervals were calculated. R0 was estimated as the probability P rabies|bite multiplied by the average number of bites per rabid dog and confidence intervals were calculated using a resampling procedure. Dogs that were removed (killed or tied up) before causing secondary cases in other dogs (even if they bit people) were excluded from this calculation, as were suspect rabid dogs that either disappeared before biting other dogs or that were of unknown origin and were killed before being observed to bite other dogs (Figure 3A). We pooled data from both districts for this estimate because insufficient complete case-histories of rabid dogs (after excluding cases with interventions) were traced to accurately estimate R0 for Ngorongoro (35 versus 477 in Serengeti). We also estimated R0 directly from the distribution of secondary cases per rabid dog. Dogs that were bitten by rabid animals but did not develop rabies because of interventions (previous vaccination or being killed/vaccinated immediately after the bite) were multiplied by P rabies|bite and added to observed secondary cases, giving an expected number of secondary cases per rabid dog in the absence of intervention and a similar estimate of R0 (1.14, CI: 1.03–1.25) (Figure S1). (2) Epidemic tree reconstruction. We used an algorithm for probabilistically constructing epidemic trees based on the location of cases in space and time [38]. For each suspected case (i), we chose a progenitor (j) at random with probability pij from all n cases preceding that case, where: G is the distribution of generation times, tij is the length of time (in days) between the occurrence of case i and its potential progenitor j (G(t) = 0 for t 1 secondary case (Figure 4A). Demographic parameters were incorporated, and 10,000 runs were completed for each starting condition. We also calculated the probability of an outbreak of a particular size or larger being seeded by one infectious case to evaluate the coverage needed to prevent outbreaks with different degrees of certainty (Figure 4C and Figure S4). If V and N denote numbers of vaccinated individuals and the total population size respectively, then vaccination coverage can be expressed as a proportion P = V/N. The number of vaccinated dogs declines following a campaign as individuals die and as vaccine-induced immunity wanes (V t = V0 e – (d+ν)t , where d is the death rate and 1/ν is the duration of vaccine-induced immunity), whereas the total population grows at the rate of population increase (N t = N 0e rt ). To prevent sustained endemic transmission, vaccination coverage must be maintained above P crit (such that R is held below 1). From our estimates of demographic parameters and R0, we calculated the proportion of the population that needs to be vaccinated, P target, to prevent vaccination coverage falling below P crit during the interval, T, between campaigns: P target = e(ν+d+r)T P crit. This formulation for estimating the coverage needed to interrupt endemic transmission given turnover in the domestic dog population assumes that immunity from vaccination lasts an average of 1/ν time units and declines exponentially. In reality, vaccine-induced immunity is likely to be closer to a fixed duration, and thus fewer dogs would be expected to lose immunity during the 1-y interval between campaigns than under the exponential model. This indicates that our estimate of P target may be slightly overestimated, although this is an important area for further investigation. Supporting Information Figure S1 R0 Estimated from Secondary Case Distributions Observed numbers of secondary cases are shown in gray. We extrapolated additional cases (using the probability of developing rabies following a bite, 0.49) that would have occurred had there been no intervention (black). The inset shows the estimated secondary case distributions (∼1.1-1.3) for dogs in Serengeti (black) and Ngorongoro (red) districts based on the reconstructed trees during the early stages of the epidemics. (3.92 MB EPS) Click here for additional data file. Figure S2 Accuracy of R0 Estimates Derived from Epidemic Trajectories (A) Estimates of R0 from fitting to trajectories of simulated epidemics plotted against the underlying R0 used in the simulations (biting behaviour was modelled using a negative binomial distribution, varying the mean number of bites per dog whilst keeping the shape parameter constant). The median R0 estimate from 1,000 realizations is shown by the solid black line and 95 percentiles are indicated by gray shading. R0 was estimated accurately across a range of underlying R0 values apart from at very low values (R0 25% from monthly time series and >800% from weekly time series, upper 95% prediction intervals of 1.71 and 2.65, respectively, versus 1.65 and 1.69, respectively). (5.58 MB EPS) Click here for additional data file. Text S1 Impacts of Under-Reporting and Incomplete Tracing on Estimation of R0 (24 KB DOC) Click here for additional data file. Text S2 Effects of Heterogeneity in Transmission Behaviour (22 KB DOC) Click here for additional data file. Video S1 Rabies Transmission in Serengeti District Inferred from Detailed Spatiotemporal Incidence Data and Estimated Epidemiological Parameters Rabies cases appear as red dots. Incubating animals appear as black dots, which turn red when clinical signs start (only animals that went on to develop rabies are shown). When a rabid animal bites another animal that will subsequently develop rabies, a black line connects the two individuals. The video is on a weekly timescale and the red arrow on the time series of rabies incidence corresponds to infectious cases during that week. (2.20 MB AVI) Click here for additional data file. Video S2 Rabies Transmission in Ngorongoro District Inferred from Detailed Spatiotemporal Incidence Data and Estimated Epidemiological Parameters Rabies cases appear as red dots. Incubating animals appear as black dots, which turn red when clinical signs start (only animals that went on to develop rabies are shown). When a rabid animal bites another animal that will subsequently develop rabies a black line connects the two individuals. The video is on a weekly timescale and the red arrow on the time series of rabies incidence corresponds to infectious cases during that week. (409 KB AVI) Click here for additional data file.

Introduction Rabies is a viral zoonosis caused by negative-stranded RNA viruses from the Lyssavirus genus. Genetic variants of the genotype 1 Lyssavirus (the cause of classical rabies) are maintained in different parts of the world by different reservoir hosts within ‘host-adaptive landscapes’ [1]. Although rabies can infect and be transmitted by a wide range of mammals, reservoirs comprise only mammalian species within the Orders Carnivora (e.g. dogs, raccoons, skunks, foxes, jackals) and Chiroptera (bats). From the perspective of human rabies, the vast majority of human cases (>90%) result from the bites of rabid domestic dogs [2] and occur in regions where domestic dogs are the principal maintenance host [3]. Over the past three decades, there have been marked differences in efforts to control canine rabies. Recent successes have been demonstrated in many parts of central and South America, where canine rabies has been brought under control through large-scale, synchronized mass dog vaccination campaigns [4]. As a result, not only has dog rabies declined, but human rabies deaths have also been eliminated, or cases remain highly localized [5]. The contrast with the situation in Africa and Asia is striking; here, the incidence of dog rabies and human rabies deaths continue to escalate, and new outbreaks have been occurring in areas previously free of the disease (e.g. the islands of Flores and Bali in Indonesia – [6]; http://wwwn.cdc.gov/travel/contentRabiesBaliIndonesia2008.aspx). In this paper, we identify four major reasons commonly given for the lack of effective domestic dog rabies control including (1) low prioritisation, (2) epidemiological constraints, (3) operational constraints and (4) lack of resources (Table 1), focussing on the situation in Africa. We address each of these issues in turn, using outputs from modelling approaches and data from field studies to demonstrate that there are no insurmountable logistic, practical, epidemiological, ecological or economic obstacles. As a result, we conclude that the elimination of canine rabies is a feasible objective for much of Africa and there should be no reasons for further delay in preventing the unnecessary tragedy of human rabies deaths. 10.1371/journal.pntd.0000626.t001 Table 1 Reasons commonly given for the lack of effective dog rabies control. Reason Explanation Oral evidence Published evidence LOW PRIORITISATION Lack of accurate data on the disease burden and low recognition among public health practitioners and policy makers; lack of inclusion of rabies in global surveys of disease burden; only recent recognition of rabies as a neglected tropical disease; statements of rabies as an ‘insignificant human disease’ Ministries of Health; statements by doctors and health workers; WHO (up until 2007) I-VI* EPIDEMIOLOGICAL CONSTRAINTS Abundance of wild animals and uncertainties about the required levels of vaccination coverage SEARG meetings, scientific meetings, national veterinary meetings; statements from district veterinary officers and local communities; draft rabies control policies VII-XIX OPERATIONAL CONSTRAINTS Perception of existence of many inaccessible stray/ownerless dogs SEARG meetings, inter-ministerial meetings, national veterinary meetings; statements from district veterinary and medical officers, and livestock officers; draft rabies control policies; international organizations XX-XXVIII Owners unwilling or unable to bring dogs for vaccination SEARG meetings, inter-ministerial meetings, national veterinary meetings, scientific meetings; statements by veterinary and livestock officers XXIX,XXX Insufficient knowledge of dog population size and ecology SEARG meetings, inter-ministerial meetings, scientific meetings; statements from veterinary and livestock officers and wildlife authorities; draft rabies control policies; international organizations XIV,XXIV,XXXI LACK OF RESOURCES Weak surveillance and diagnostic capacity SEARG meetings, inter-ministerial meetings; international and national reference laboratories; international organizations VI,XXIII,XXIV,XXXII-XXXVIII Insufficient resources available to veterinary services SEARG meetings, inter-ministerial meetings, scientific meetings, national veterinary meetings; statements from politicians, veterinary authorities, local communities, wildlife authorities; international organizations; media XXVI,XXXIV,XXXVII,XXXIX,XL-XLIII SEARG = Southern and Eastern Africa Rabies Group. *Including indirect evidence (e.g. absence of any mention of rabies in published literature indicating lack of priority). See Appendix S1 for references. Methods This paper compiles previously published data (see references below) and additional analyses of those data, but we present a brief summary of the data collection methods below. Hospital records of animal-bite injuries compiled from northwest Tanzania were used as primary data sources. These data informed a probability decision tree model for a national disease burden evaluation [7], which has since been adapted for global estimates of human rabies deaths and Disability-Adjusted Life Years (DALYs) lost due to rabies [3], a standardized measure for assessing disease burden [8],[9]. Hospital records were also used to initiate contact tracing studies [10]–[12], whereby bite-victims were interviewed to obtain more detail on the source and severity of exposure and actions taken, allowing subsequent interviews with other affected individuals (not documented in hospital records) including owners of implicated animals. Statistical techniques applied to these data for estimating epidemiological parameters and inferring transmission links are described elsewhere [10],[12]. Rabies monitoring operations including passive and active surveillance involving veterinarians, village livestock field officers, paravets, rangers and scientists were used to collect samples from carcasses (domestic dogs and wildlife whenever found), which were subsequently tested and viral isolates were sequenced [10], [13]–[16], with results being used to inform estimates of rabies-recognition probabilities [7] and for phylogenetic analyses [10],[16]. Operational research on domestic dog vaccination strategies was carried out in a variety of settings [14],[17]. Household interviews were also used for socio-economic surveys and to evaluate human:domestic dog ratios, levels of vaccination coverage achieved and reasons for not bringing animals to vaccination stations [17],[18]. The study was approved by the Tanzania Commission for Science and Technology with ethical review from the National Institute for Medical Research (NIMR). This retrospective study involved collection of interview data only, without clinical intervention or sampling, therefore we considered that informed verbal consent was appropriate and this was approved by NIMR. Permission to conduct interviews was obtained from district officials, village and sub-village leaders in all study locations. At each household visited, the head of the household was informed about the purpose of the study and interviews were conducted with verbal consent from both the head of the household and the bite victim (documented in a spreadsheet). Approval for animal work was obtained from the Institutional Animal Care and Use Committee (IACUC permit #0107A04903). Results/Discussion (a) There is not enough evidence to define rabies control as a priority A principal factor contributing to a low prioritization of rabies control has been the lack of information about the burden and impact of the disease [19],[20]. Data on human rabies deaths, submitted from Ministries of Health to the World Health Organization (WHO), are published in the annual World Surveys of Rabies and through the WHO Rabnet site (www.who.int/rabies/rabnet/en). For the WHO African region (AFRO) comprising 37 countries, these surveys report an average of 162 human deaths per year between 1988 and 2006. It is therefore unsurprising that for national and international policy-makers, rabies pails into insignificance in comparison with other major disease problems. This perceived lack of significance of human rabies is reflected in the absence of any mention of rabies in either of the two published Global Burden of Disease Surveys [21],[22], which assessed more than 100 major diseases. These surveys adopted the metric of the DALY which is widely used as the principal tool for providing consistent, comparative information on disease burden for policy-making. Until recently no estimates of the DALY burden were available for rabies. Official data on human rabies deaths submitted to WHO from Africa are widely recognized to greatly under-estimate the true incidence of disease. The reasons for this are manifold: (1) rabies victims are often too ill to travel to hospital or die before arrival, (2) families recognize the futility of medical treatment for rabies, (3) patients are considered to be the victims of bewitchment rather than disease, (4) clinically recognized cases at hospitals may go unreported to central authorities, and (5) misdiagnosis is not uncommon. The problems of misdiagnosis were highlighted by a study of childhood encephalitis in Malawi, in which 3/26 (11.5%) cases initially diagnosed as cerebral malaria were confirmed as rabies through post-mortem tests [23]. Several recent studies have contributed information that consistently demonstrates that the burden of canine rabies is not insubstantial. Human rabies deaths Estimates of human rabies cases from modeling approaches, using the incidence of dog-bite injuries and availability of rabies post-exposure prophylaxis (PEP), indicate that incidence in Africa is about 100 times higher than officially reported, with ∼24,000 deaths in Africa each year [3],[7]. Consistent figures have subsequently been generated from detailed contact-tracing data: in rural Tanzanian communities with sporadic availability of PEP (a typical scenario in developing countries), human rabies deaths occur at an incidence of ∼1–5 cases/100,000/year (equivalent to 380–1,900 deaths per year for Tanzania) [11]. Similarly, a multi-centric study from India reported 18,500 human rabies deaths per year [24], consistent with model outputs of 19,700 deaths for India [3]. A crude comparison of annual human deaths for a range of zoonotic diseases is shown in Figure 1 (top). While diseases such as Severe Acute Respiratory Syndrome (SARS), Rift Valley Fever and highly pathogenic avian influenza cause major concerns as a result of pandemic potential and economic losses, these figures provide a salutary reminder of the recurrent annual mortality of rabies and other neglected zoonoses, such as leishmaniasis and Human African Trypanosomiasis (HAT). Decision-tree models applied to data from East Africa and globally indicate that the DALY burden for rabies exceeds that of most other neglected zoonotic diseases (Figure 1 - bottom) [3],[25],[26]. 10.1371/journal.pntd.0000626.g001 Figure 1 Annual human deaths for a range of zoonoses and global disability-adjusted life years (DALYs) scores for neglected zoonoses. Top figure - Numbers of human deaths per year for rabies compared with peak annual deaths from selected epidemic zoonoses (Severe Acute Respiratory Syndrome, SARS, 2003; H5N1, 2006; Nipah, 1999; and Rift Valley Fever 2007). Data sources: Rabies (LVII), Leishmaniasis, Human African Trypanosomiasis (HAT), Chagas Disease and Japanese Encephalitis (LVIII), SARS (LIX), Influenza A H5N1 (LX), Nipah (LXI), Rift Valley Fever (LXII,LXIII). See Appendix S1 for references. Bottom figure - Global DALY scores for neglected tropical diseases reported in LXIV and LVII and also assuming no post-exposure treatment (dark grey). See Appendix S1 for references. Human animal-bite injuries and morbidity Most of the rabies DALY burden is attributed to deaths, rather than morbidity because of the short duration of clinical disease. The DALY burden for rabies is particularly high, because most deaths occur in children and therefore a greater number of years of life are lost [25],[27]. DALY estimates incorporate non-rabies mortality and morbidity in terms of adverse reactions to nerve-tissue vaccines (NTVs) [3], which are still widely used in some developing countries such as Ethiopia, however rabies also causes substantial ‘morbidity’ as a direct result of injuries inflicted by rabid animals, and this is not included in DALY estimates. Contact-tracing studies suggest an incidence as high as 140/100,000 bites by suspected rabid animals in rural communities of Tanzania [11]. Thus, for every human rabies death there are typically more than ten other rabid animal-bite victims who do not develop signs of rabies, because they obtain PEP (Figure 1 - bottom) or are simply fortunate to remain healthy. The severity of wounds has not yet been quantified, but case-history interviews suggest that injuries often involve multiple, penetrating wounds that require medical treatment. Economic burden The major component of the economic burden of rabies relates to high costs of PEP, which impacts both government and household budgets. With the phasing out of NTVs, many countries spend millions of dollars importing supplies of tissue-culture vaccine (∼$196 million USD pa [3]). At the household level, costs of PEP arise directly from anti-rabies vaccines and from high indirect (patient-borne) costs associated with travel (particularly given the requirement of multiple hospital visits), medical fees and income loss [3],[28]. Indirect losses, represent >50% of total costs (Figure 2). Total costs have been estimated conservatively at $40 US per treatment in Africa and $49 US in Asia accounting respectively for 5.8% and 3.9% of annual per capita gross national income [3]. Poor households face difficulties raising funds which results in considerable financial hardship and substantial delays in PEP delivery [11],[28]. Shortages of PEP, which are frequent in much of Africa, further increase costs as bite victims are forced to travel to multiple centres to obtain treatment, also resulting in risky delays [11]. 10.1371/journal.pntd.0000626.g002 Figure 2 Economic burden of canine rabies (data source: LVII in Appendix S1). PET, Post-exposure treatment. Additional economic losses relate to livestock losses derived from an incidence of 5 deaths/100,000 cattle estimated to cost $12.3 million annually in Africa and Asia [3]. However, substantially higher incidence has been recorded in Tanzania, with 12–25 cases/100,000 cattle reported annually in rural communities (Hampson, unpublished). Canine rabies introduced from sympatric domestic dog populations is also recognized as a major threat to endangered African wild dogs (Lycaon pictus) and Ethiopian wolves (Canis simensis) [29]–[32]. Potential losses of tourism revenue may be substantial; African wild dogs are a major attraction in South Africa National Parks with the value of a single pack estimated at $9,000 per year [33] and Ethiopian wolves are a flagship species for the Bale Mountains National Park. Psychological impact An important, but often under-appreciated component of disease burden is the psychological impact on bite-victims and their families. In rural Tanzania, >87% of households with dog bite victims feared a bite from a suspected rabid animal more than malaria [28] because malaria can be treated whereas clinical rabies is invariably fatal and malaria treatment is generally affordable and available locally in comparison to PEP. When human rabies cases occur, the horrifying symptoms and invariably fatal outcome result in substantial trauma for families, communities and health care workers [34]. (b) Epidemiological constraints Increasing incidence of rabies in Africa has prompted concerns that the epidemiology of the disease may be more complex, involving abundant wildlife carnivores that may sustain infection cycles [13], [35]–[38]. There is also uncertainty about the level of vaccination coverage needed to control rabies particularly in rapidly growing domestic dog populations [39],[40]. To eliminate infection, disease control efforts need to be targeted at the maintenance population [41]. This is clearly demonstrated for fox rabies in Western Europe, whereby control of rabies in foxes (through mass oral vaccination) has led to the disappearance of rabies from all other ‘spill-over’ hosts [42]. Despite the predominance of domestic dog rabies in Africa, the role of wildlife as independent maintenance hosts has been debated, and many perceive the abundance of wildlife as a barrier to elimination of canine rabies on the continent. It has also been argued that the predominance of dog rabies is an artefact of poor surveillance and under-reporting in wildlife populations [43]. In the wildlife-rich Serengeti ecosystem in Tanzania, evidence suggests that domestic dogs are the only population essential for maintenance [10],[13],[16]: (1) phylogenetic data showed only a single southern Africa canid-associated variant (Africa 1b) circulating among different hosts [16]; (2) transmission networks suggested that, for wildlife hosts, within-species transmission cannot be sustained [16]; and (3) statistical inference indicated that cross-species transmission events from domestic dogs resulted in only relatively short-lived chains of transmission in wildlife with no evidence for persistence [10]. The conclusion that domestic dogs are the only maintenance population in such a species-rich community suggests that elimination of canine rabies through domestic dog vaccination is a realistic possibility, and provides grounds for optimism for wider-scale elimination efforts in Africa. In other parts of central and west Africa, transmission of rabies appears to be driven by domestic dogs [44]. An outstanding question relates to southern Africa. Earlier and recent evidence indicate that jackal species (Canis mesomelas and C. adustus) and bat-eared foxes (Otocyon megalotis) may maintain the canid variant in specific geographic loci in South Africa and Zimbabwe [2], [36]–[38], [45]–[50], but it is still not clear whether these cycles can be sustained over large spatial and temporal scales in the absence of dog rabies [13],[51],[52]. Independent wildlife cycles may preclude continent-wide elimination of this variant through dog vaccination alone and wildlife rabies control strategies, in conjunction with dog vaccination, may need to be considered in specific locations [38]. A critical proportion of the population must be protected (Pcrit) to eliminate infection and this threshold can be calculated from the basic reproductive number (R0, defined as the average number of secondary infections caused by an infected individual in a susceptible population) [53]. Vaccinating a large enough proportion of the population to exceed Pcrit will not only protect the vaccinated individuals but will reduce transmission such that, on average, less than one secondary infection will result from each primary case (effective reproductive number, Re 80% coverages can still be achieved through house-to-house delivery strategies or community-based animal health workers [17]. Young pups usually make up a large proportion (>30%) of African dog populations [62] and there is a widespread perception among veterinary authorities and dog owners that they should not be vaccinated, which leads to insufficient coverage [17]. However, rabies vaccines can safely be administered to pups 0.5 IU/ml) of rabies virus neutralizing antibody [64]. The issue of inclusion of pups can effectively be addressed through appropriate advertising before campaigns. Cost-recovery, through charging dog owners for rabies vaccination, is widely promoted for sustainable programmes and to encourage responsible dog ownership. However, charging for a vaccination that represents a public rather than a private good, can be counterproductive, resulting in low turnouts and coverage ( 600,000 PEP courses per year at an estimated cost of ∼$27 million/year [84]. 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 [14]. 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). 10.1371/journal.pntd.0000626.g004 Figure 4 Number of cases of bite injuries reported to hospitals in pastoralist communities to the east of Serengeti National Park (north-western Tanzania). Numbers are recorded as a result of bites from both rabid and normal healthy animals as well as those of unknown status (either the bite victims could not be traced, or insufficient information could be obtained during interviews to make an informed judgement about the health of the biting animal). The arrows mark the end of successive dog vaccination campaigns. 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 [17], Philippines ∼$1.19–4.27 [85], Tunisia ∼$1.3 [86], Thailand ∼$1.3 [86] and Urban Chad ∼$1.8 [87]) 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. Supporting Information Appendix S1 Appendix with additional references. (0.07 MB DOC) Click here for additional data file.

Human rabies in developing countries can be prevented through interventions directed at dogs. Potential cost-savings for the public health sector of interventions aimed at animal-host reservoirs should be assessed. Available deterministic models of rabies transmission between dogs were extended to include dog-to-human rabies transmission. Model parameters were fitted to routine weekly rabid-dog and exposed-human cases reported in N'Djaména, the capital of Chad. The estimated transmission rates between dogs (beta(d)) were 0.0807 km2/(dogs x week) and between dogs and humans (beta(dh)) 0.0002 km2/(dogs x week). The effective reproductive ratio (R(e)) at the onset of our observations was estimated at 1.01, indicating low-level endemic stability of rabies transmission. Human rabies incidence depended critically on dog-related transmission parameters. We simulated the effects of mass dog vaccination and the culling of a percentage of the dog population on human rabies incidence. A single parenteral dog rabies-mass vaccination campaign achieving a coverage of least 70% appears to be sufficient to interrupt transmission of rabies to humans for at least 6 years. The cost-effectiveness of mass dog vaccination was compared to postexposure prophylaxis (PEP), which is the current practice in Chad. PEP does not reduce future human exposure. Its cost-effectiveness is estimated at US $46 per disability adjusted life-years averted. Cost-effectiveness for PEP, together with a dog-vaccination campaign, breaks even with cost-effectiveness of PEP alone after almost 5 years. Beyond a time-frame of 7 years, it appears to be more cost-effective to combine parenteral dog-vaccination campaigns with human PEP compared to human PEP alone.