On the spatial spread of rabies among foxes

We present a simple model for the spatial spread of rabies among foxes and use it to quantify its progress in England if rabies were introduced. The model is based on the known ecology of fox behaviour and on the assumption that the main vector for the spread of the disease is the rabid fox. Known data and facts are used to determine real parameter values involved in the model. We calculate the speed of propagation of the epizootic front, the threshold for the existence of an epidemic, the period and distance apart of the subsequent cyclical epidemics which follow the main front, and finally we quantify a means for control of the spatial spread of the disease. By way of illustration we use the model to determine the progress of rabies up through the southern part of England if it were introduced near Southampton. Estimates for the current fox density in England were used in the simulations. These suggest that the disease would reach Manchester within about 3.5 years, moving at speeds as high as 100 km per year in the central region. The model further indicates that although it might seem that the disease had disappeared after the wave had passed it would reappear in the south of England after just over 6 years and at periodic times after that. We consider the possibility of stopping the spread of the disease by creating a rabies 'break' ahead of the front through vaccination to reduce the population to a level below the threshold for an epidemic to exist. Based on parameter values relevant to England, we estimate its minimum width to be about 15 km. The model suggests that vaccination has considerable advantages over severe culling.     

Environmental and human demographic features associated with epizootic raccoon rabies in Maryland, Pennsylvania, and Virginia

We assessed land use and demographic data as predictors discriminating between counties experiencing large or small first epizootics of rabies among raccoons (Procyon lotor). Monthly county reports of raccoons testing positive for rabies were obtained from rabies surveillance databases from Maryland, Pennsylvania, and Virginia (USA). Environmental and demographic data for the three states were obtained from public sources. On the basis of total reports of raccoon rabies during the first defined epizootic period, the 203 counties were dichotomized at the 75th percentile as having a large epizootic (> or = 24 rabid raccoons in the first epizootic) (51 counties) or a small epizootic or no epizootic (152 counties). A high percentage of agricultural land use [OR = 9.1, 95% CI (3.6-23.1)], high water coverage in combination with low human population density [OR = 8.8, 95% CI (2.9-27.0)], and low water coverage with high human population density [OR = 11.7, 95% CI (4.0-34.1)] were positively associated with large rabies epizootics. Counties with more than 15% of mixed forest were less likely to experience large epizootics than were counties with < or = 15% of mixed forest [OR = 0.3, 95% CI (0.1, 0.9)]. A combination of land use and human population density measures provided the best model for determining epizootic size and may be important predictors of epizootic behavior and risk of exposure to this reservoir species.     

Epizootiology of skunk rabies in North America

Public health surveillance data from the United States and Canada (1961 to 1982) were analyzed to determine if consistent temporal and spatial patterns in skunk rabies could be identified. Enzootic/epizootic rabies was recognized in 18 states (enzootic states) based on the criteria of greater than or equal to 20 yr of reported skunk rabies and at least 1 yr with a minimum of 50 reported rabid skunks. In other wildlife species, epizootics have been demonstrated to expand along a wave-like front. We hypothesized: if skunk rabies behaved in a similar fashion, states reporting rabid skunks would change over time. No such change was noted. During epizootics the number of counties reporting increased but not the number of states. Within Illinois certain counties were demonstrated to have persistent rabies histories and likely served as enzootic foci. Enzootic states combined prevalence indicated a 6 to 8 yr cycle for epizootics. Data on monthly percent rabies positive (number rabid/total number tested) were available from six states and Canada. Mean distributions were bimodal with winter and spring peaks. The patterns identified for skunk rabies differ from those of other major wildlife vectors and have significance for potential vaccination control regimes.     

Spatial dynamics and molecular ecology of North American rabies

Rabies, caused by a single-stranded RNA virus, is arguably the most important viral zoonotic disease worldwide. Although endemic throughout many regions for millennia, rabies is also undergoing epidemic expansion, often quite rapid, among wildlife populations across regions of Europe and North America. A current rabies epizootic in North America is largely attributable to the accidental introduction of a particularly well-adapted virus variant into a naive raccoon population along the Virginia/West Virginia border in the mid-1970s. We have used the extant database on the spatial and temporal occurrence of rabid raccoons across the eastern United States to construct predictive models of disease spread and have tied patterns of emergence to local environmental variables, genetic heterogeneity, and host specificity. Rabies will continue to be a remarkable model system for exploring basic issues in the temporal and spatial dynamics of expanding infectious diseases and examining ties between disease population ecology and evolutionary genetics at both micro- and macro-evolutionary time scales.     

Traveling wave solutions of a reaction diffusion model for competing pioneer and climax species

Presented is a reaction-diffusion model for the interaction of pioneer and climax species. For certain parameters the system exhibits bistability and traveling wave solutions. Specifically, we show that when the climax species diffuses at a slow rate there are traveling wave solutions which correspond to extinction waves of either the pioneer or climax species. A leading order analysis is used in the one-dimensional spatial case to estimate the wave speed sign that determines which species becomes extinct. Results of these analyses are then compared to numerical simulations of wave front propagation for the model on one and two-dimensional spatial domains. A simple mechanism for harvesting is also introduced.     

Landscape-Genetic Analysis of Population Structure in the Texas Gray Fox Oral Rabies Vaccination Zone

ABSTRACT In west-central Texas, USA, abatement efforts for the gray fox (Urocyon cinereoargenteus) rabies epizootic illustrate the difficulties inherent in large-scale management of wildlife disease. The rabies epizootic has been managed through a cooperative oral rabies vaccination program (ORV) since 1996. Millions of edible baits containing a rabies vaccine have been distributed annually in a 16-km to 24-km zone around the perimeter of the epizootic, which encompasses a geographic area >4 × 10⁵ km². The ORV program successfully halted expansion of the epizootic into metropolitan areas but has not achieved the ultimate goal of eradication. Rabies activity in gray fox continues to occur periodically outside the ORV zone, preventing ORV zone contraction and dissipation of the epizootic. We employed a landscape-genetic approach to assess gray fox population structure and dispersal in the affected area, with the aim of assisting rabies management efforts. No unique genetic clusters or population boundaries were detected. Instead, foxes were weakly structured over the entire region in an isolation by distance pattern. Local subpopulations appeared to be genetically non-independent over distances >30 km, implying that long-distance movements or dispersal may have been common in the region. We concluded that gray foxes in west-central Texas have a high potential for long-distance rabies virus trafficking. Thus, a 16-km to 24-km ORV zone may be too narrow to contain the fox rabies epizootic. Continued expansion of the ORV zone, although costly, may be critical to the long-term goal of eliminating the Texas fox rabies virus variant from the United States.     

Wave solutions for the deterministic non-reducible n-type epidemic

A model is formulated to describe the spatial spread of an epidemic involving n types of individual. This encompasses the measles, host-vector and carrier-borne epidemics, and in addition rabies involving several species of animal. The existence, uniqueness and non-existence of wave solutions for different speeds are established for this model.     

Combining direct and indirect genetic methods to estimate dispersal for informing wildlife disease management decisions

Epidemiological models are useful tools for management to predict and control wildlife disease outbreaks. Dispersal behaviours of the vector are critical in determining patterns of disease spread, and key variables in epidemiological models, yet they are difficult to measure. Raccoon rabies is enzootic over the eastern seaboard of North America and management actions to control its spread are costly. Understanding dispersal behaviours of raccoons can contribute to refining management protocols to reduce economic impacts. Here, estimates of dispersal were obtained through parentage and spatial genetic analyses of raccoons in two areas at the front of the raccoon rabies epizootic in Ontario; Niagara (N = 296) and St Lawrence (N = 593). Parentage analysis indicated the dispersal distance distribution is highly positively skewed with 85% of raccoons, both male and female, moving < 3 km. The tail of this distribution indicated a small proportion (< 4%) moves more than 20 km. Analysis of spatial genetic structure provided a similar assessment as the spatial genetic correlation coefficient dropped sharply after 1 km. Directionality of dispersal would have important implications for control actions; however, evidence of directional bias was not found. Separating the data into age and sex classes the spatial genetic analyses detected female philopatry. Dispersal distances differed significantly between juveniles and adults, while juveniles in the Niagara region were significantly more related to each other than adults were to each other. Factors that may contribute to these differences include kin association, and spring dispersal. Changes to the timing and area covered by rabies control operations in Ontario are indicated based on these dispersal data.     

Current rabies epizootiology

Situation in rabies in the Russian Federation (RF) remains to be tense and is characterized by important specific features. Central Russia and the Moscow region have essential differences in the epizootic situation, the epizootological structure of rabies and other indices as compared with the Russian Federation. In the course of the last 25 years the ecological stereotype of rabies has undergone considerable transformations, becoming natural focal infection with the circulation of the infective agent among wild carnivores, which is now particularly obvious in the Moscow region. In 1998 a sharp rise in rabies morbidity occurred in Central Russia: peak values exceeded average annual values 2 times for the RF, 4 times for Central Russia and more than 10 times for the Moscow region. The situation in rabies in the Moscow region permits to use it as a model in the study of today rabies.     

Rabies in rodents and lagomorphs in the United States, 1971-1984: increased cases in the woodchuck (Marmota monax) in mid-Atlantic states

A review of surveillance data on animal rabies from the Centers for Disease Control revealed 104 cases of rabies in rodents and lagomorphs for 1971 through 1984 in the United States; 80% of these were reported between 1980 and 1984. Woodchucks (Marmota monax) accounted for 64% of the cases. Most of the cases of rabies in woodchucks were associated with an epizootic of rabies in raccoons (Procyon lotor) in the mid-Atlantic states. In rabies endemic areas, humans exposed to woodchucks should receive rabies postexposure prophylaxis if the animal is not available for testing. The decision to administer postexposure prophylaxis to humans exposed to other rodents and lagomorphs should take into consideration the epidemiology of rabies in the exposing species.     

Epizootic characteristic of rabies today

During the 23 year period rabies was registered in the Moscow region in 163 cases among foxes, 22 cases among racoon-like dogs, 92 cases among dogs, 54 cases among cats. In 1991-2000 ten cases of rabies were registered among other wild animal other than foxes and raccoon-like dogs (hares, hedge-hogs, polecats, badgers, hamsters, martens, rats). Under today conditions the generalized epizootological pattern of rabies is characterized by the vector "natural foci-->anthropurgic foci" with wild and domestic animals playing an alternative role in the epizootic process and the circulation of the infective agent. Wild carnivorous animals maintain natural focal infection in time and space, while all domestic animals are a direct or indirect ecological impasse and took no part in the maintenance of the infection. Foxes are the main source of infection for the animals of the anthropurgic cycle: they play a special role in the development of the epizootic situation in the region as the main reservoir and source of the causative agent of rabies as a natural focal infection. Among other wild animals, raccoon-like dogs are involved into epizootic and epidemic chains. Dogs are the main objects to be infected in the anthropurgic cycles, while cats--a progressing group of risk. During the period of 25 years a decrease in the probability of natural rabies was noted.     

Emergency rabies control in a community of two high-density hosts

ABSTRACT: BACKGROUND: Rabies is a fatal viral disease that potentially can affect all mammals. Terrestrial rabies is not present in the United Kingdom and has been eliminated from Western Europe. Nevertheless the possibility remains that rabies could be introduced to England, where it would find two potentially suitable hosts, red foxes and badgers. With the aim to analyse the spread and emergency control of rabies in this two species host community, a simulation model was constructed. Different control strategies involving anti-rabies vaccination and population culling were developed, considering control application rates, spatial extent and timing. These strategies were evaluated for efficacy and feasibility to control rabies in hypothetical rural areas in the South of England immediately after a disease outbreak. RESULTS: The model confirmed that both fox and badger populations, separately, were competent hosts for the spread of rabies. Realistic vaccination levels were not sufficient to control rabies in high-density badger populations. The combined species community was a very strong rabies host. However, disease spread within species appeared to be more important than cross-species infection. Thus, the drivers of epidemiology depend on the potential of separate host species to sustain the disease. To control a rabies outbreak in the two species, both species had to be targeted. Realistic and robust control strategies involved vaccination of foxes and badgers, but also required badger culling. Although fox and badger populations in the UK are exceptionally dense, an outbreak of rabies can be controlled with a higher than 90% chance, if control response is quick and follows a strict regime. This requires surveillance and forceful and repeated control campaigns. In contrast, an uncontrolled rabies outbreak in the South of England would quickly develop into a strong epizootic involving tens of thousands of rabid foxes and badgers. CONCLUSIONS: If populations of both host species are sufficiently large, epizootics are driven by within-species transmission, while cross-species-infection appears to be of minor importance. Thus, the disease control strategy has to target both host populations.     

Epizootic situation and measures for control of wild animals' rabies in Republic of Bashkortostan

Epizootologic characteristics of rabies incidence in the Republic of Bashkortostan for the period 1998 - 2006 are presented. Districts of Bashkortostan are divided on 3 zones according to severity of epizootic situation. Three years-cyclicity of rabies incidence with peak during winter was determined. Foxes, raccoons, wolves, lynxes, badgers, minks and, during recent years, corsacs are involved into the epizootic chain. Main source of rabies in the Republic is foxes with share of 91.9% among sick wild animals. During 1999-2006 8 cases of hydrophobia were registered. Results of analysis of effectiveness of oral immunization and regulation of wild fauna population are presented.     

Predictive spatial dynamics and strategic planning for raccoon rabies emergence in Ohio

Rabies is an important public health concern in North America because of recent epidemics of a rabies virus variant associated with raccoons. The costs associated with surveillance, diagnostic testing, and post-exposure treatment of humans exposed to rabies have fostered coordinated efforts to control rabies spread by distributing an oral rabies vaccine to wild raccoons. Authorities have tried to contain westward expansion of the epidemic front of raccoon-associated rabies via a vaccine corridor established in counties of eastern Ohio, western Pennsylvania, and West Virginia. Although sporadic cases of rabies have been identified in Ohio since oral rabies vaccine distribution in 1998, the first evidence of a significant breach in this vaccine corridor was not detected until 2004 in Lake County, Ohio. Herein, we forecast the spatial spread of rabies in Ohio from this breach using a stochastic spatial model that was first developed for exploratory data analysis in Connecticut and next used to successfully hind-cast wave-front dynamics of rabies spread across New York. The projections, based on expansion from the Lake County breach, are strongly affected by the spread of rabies by rare, but unpredictable long-distance translocation of rabid raccoons; rabies may traverse central Ohio at a rate 2.5-fold greater than previously analyzed wildlife epidemics. Using prior estimates of the impact of local heterogeneities on wave-front propagation and of the time lag between surveillance-based detection of an initial rabies case to full-blown epidemic, specific regions within the state are identified for vaccine delivery and expanded surveillance effort.     

The role of the Badger (Meles meles) in rabies epizootiology and the implications for Great Britain

The occurrence of a wildlife rabies epizootic in Britain remains a very unlikely event, but it is important to examine all the possible consequences of such an event. Here, I examine the possible role of the European Badger (Meles meles) in such an epizootic. The population density of Badgers in Britain is much higher than that in Europe, and appears to have increased substantially over the last decade or so. The population parameters and epizootiology of rabies in the Badger are reviewed in comparison with the Fox (Vulpes vulpes) and other species. Mustelids appear to be very susceptible to rabies, with the smaller mustelids becoming aggressive, although Badgers do not appear to show heightened aggression when infected. Badger populations on the continent become severely reduced when rabies arrives in the area, and circumstantial evidence strongly suggests that Badgers can easily transmit the virus. Preliminary models support the idea that the Badger could be a very significant secondary host, especially in the initial rabies outbreak. The population recovery rate of the Badger suggests that it is unlikely to become a primary host, although short‐term epizootics in the Badger population are likely. The potential for controlling rabies in the Badger is also examined.     

Land use associations and changes in population indices of urban raccoons during a rabies epizootic

Land use associations and changes in population indices were assessed for an urban population of raccoons (Procyon lotor) in Baltimore, Maryland (USA), from January 1984 to December 1987. Records were examined for 1,458 raccoons trapped alive and removed dead from city streets during, and after, the peak of a rabies epizootic. The distribution of raccoons was associated with single-unit residential areas primarily along the northern and western perimeter of the city. Beginning in March 1985 an ending in May 1987, an epizootic of raccoon rabies spread through Baltimore, ultimately resulting in the identification of 95 rabid raccoons. Within the study interval, annual numbers of trapped raccoons remained stable from 1984 to 1986, before showing a marked decline in 1987. The number of raccoons removed as vehicle mortalities (road-kills) varied little from 1984-1985 but declined in the last 2 yr of study. Numbers of other road-killed species did not decrease concurrently, suggesting a specific decrease in the urban raccoon population. The rabies epizootic, in conjunction with the increased city and private control, appears to have contributed to a decline in the number of raccoons in Baltimore.     

Differential permeability of rivers to raccoon gene flow corresponds to rabies incidence in Ontario, Canada

The correlation of landscape features with genetic discontinuities reveals barriers to dispersal that can contribute to understanding present and future spread of wildlife diseases. This knowledge can then be used for targeting control efforts. The impact of natural barriers on raccoon dispersal was assessed through genetic analysis of samples from two regions, Niagara ( N  = 666) and St. Lawrence ( N  = 802). These areas are transected by major rivers and are at the northern front of a raccoon rabies epizootic. Genetic clusters were identified in each region using Bayesian clustering algorithms. In the Niagara region, two clusters were identified corresponding to either side of the Niagara River. For the St. Lawrence region, spatially congruent clusters were not identified, despite the presence of the intervening St. Lawrence River. These genetic data are consistent with raccoon rabies incidence data where rabies has been detected across the St. Lawrence River in Ontario while no cases have been detected in Ontario across the Niagara River. This is despite expectations of rabies incidence in Niagara before the St. Lawrence based on the progression of rabies from New York. The results from the two regions suggest different permeabilities to raccoons between New York and Ontario that may be attributed to the rivers. However, other factors have also been explored that could contribute to this difference between these study sites including the shape of the landscape and resource distribution.     

The Phylogeography and Spatiotemporal Spread of South-Central Skunk Rabies Virus

The south-central skunk rabies virus (SCSK) is the most broadly distributed terrestrial viral lineage in North America. Skunk rabies has not been efficiently targeted by oral vaccination campaigns and represents a natural system of pathogen invasion, yielding insights to rabies emergence. In the present study we reconstructed spatiotemporal spread of SCSK in the whole territory of its circulation using a combination of Bayesian methods. The analysis based on 241 glycoprotein gene sequences demonstrated that SCSK is much more divergent phylogenetically than was appreciated previously. According to our analyses the SCSK originated in the territory of Texas ~170 years ago, and spread geographically during the following decades. The wavefront velocity in the northward direction was significantly greater than in the eastward and westward directions. Rivers (except the Mississippi River and Rio Grande River) did not constitute significant barriers for epizootic spread, in contrast to deserts and mountains. The mean dispersal rate of skunk rabies was lower than that of the raccoon and fox rabies. Viral lineages circulate in their areas with limited evidence of geographic spread during decades. However, spatiotemporal reconstruction shows that after a long period of stability the dispersal rate and wavefront velocity of SCSK are increasing. Our results indicate that there is a need to develop control measures for SCSK, and suggest how such measure can be implemented most efficiently. Our approach can be extrapolated to other rabies reservoirs and used as a tool for investigation of epizootic patterns and planning interventions towards disease elimination.     

Allometric scaling and seasonality in the epidemics of wildlife diseases

We present a susceptibles-exposed-infectives (SEI) model to analyze the effects of seasonality on epidemics, mainly of rabies, in a wide range of wildlife species. Model parameters are cast as simple allometric functions of host body size. Via nonlinear analysis, we investigate the dynamical behavior of the disease for different levels of seasonality in the transmission rate and for different values of the pathogen basic reproduction number (R(0)) over a broad range of body sizes. While the unforced SEI model exhibits long-term epizootic cycles only for large values of R(0), the seasonal model exhibits multiyear periodicity for small values of R(0). The oscillation period predicted by the seasonal model is consistent with those observed in the field for different host species. These conclusions are not affected by alternative assumptions for the shape of seasonality or for the parameters that exhibit seasonal variations. However, the introduction of host immunity (which occurs for rabies in some species and is typical of many other wildlife diseases) significantly modifies the epidemic dynamics; in this case, multiyear cycling requires a large level of seasonal forcing. Our analysis suggests that the explicit inclusion of periodic forcing in models of wildlife disease may be crucial to correctly describe the epidemics of wildlife that live in strongly seasonal environments.