Spread Of Rabies Research Articles

Pattern formation triggered by rare events: lessons from the spread of rabies.

Understanding of large-scale spatial pattern formation is a key to successful management in ecology and epidemiology. Neighbourhood interactions between local units are known to contribute to large-scale patterns, but how much do they contribute and what is the role of regional interactions caused by long-distance processes? How much long-distance dispersal do we need to explain the patterns that we observe in nature? There seems to be no way to answer these questions empirically. Therefore, we present a modelling approach that is a combination of a grid-based model describing local interactions and an individual-based model describing dispersal. Applying our approach to the spread of rabies, we show that in addition to local rabies dynamics, one long-distance infection per 14000 km2 per year is sufficient to reproduce the wave-like spread of this disease. We conclude that even rare ecological events that couple local dynamics on a regional scale may have profound impacts on large-scale patterns and, in turn, dynamics. Furthermore, the following results emerge: (i) Both neighbourhood infection and long-distance infection are needed to generate the wave-like dispersal pattern of rabies; (ii) randomly walking rabid foxes are not sufficient to generate the wave pattern; and (iii) on a scale of less than 100 km x 100 km, temporal oscillations emerge that are independent from long-distance dispersal.

Pattern-oriented modelling in population ecology

Ecological modelling should take its orientation more from real patterns observed in nature, than has been the case up to now, to overcome the deficiencies of the present strategies. Firstly, the orientation towards patterns provides guidelines about the manner and extent of the aggregation of biological information in the model. Modelling thereby loses much of its arbitrariness; secondly, pattern-oriented models are not ‘scale-free’, i.e. they relate explicitly to spatial and temporal scales; and finally, they produce comparative predictions which are better suited for testing than the predictions achieved by models, which are only either complex or generalizing. To demonstrate the strategy of pattern-oriented modelling and its advantages, three examples from population ecology are presented: (1) In a model concerning density dependence and individual variability, the orientation towards a pattern in weight distributions requires explicit inclusion of the within-generation time scale in the model. (2) Orientation towards a pattern in the dispersal capabilities of small organisms leads to a metapopulation model, where environmental correlations about certain distances are taken into account. The survival of the metapopulation depends mainly on the ratio of the correlation distance to the range of dispersal of the organisms. (3) The wave-like pattern of the spread of rabies is reproduced by an extremely simple one-dimensional model, which is based on an extended cellular automaton approach. From this basic model, a description on finer spatial and temporal scales can be developed with the aim of constructing a model which allows for the investigation of spatial barriers against the spread of rabies. A comparison of the three example models shows that the main features of pattern-oriented models are generic. In producing comparative predictions which are related explicitly to scales, pattern-oriented modelling seems to be a strategy well suited to the ‘scaling up’ from population ecology to community and ecosystem ecology.

Fox Contact Behaviour and Rabies Spread: A Model for the Estimation of Contact Probabilities Between Urban Foxes at Different Population Densities and Its Implications for Rabies Control in Britain

1. Contacts between susceptible and infectious individuals determine how fast a disease will spread. However, they are difficult to measure in the wild, and most models of disease spread have been forced to use estimated or derived values. 2. In this paper, a mathematical relationship between fox home range size and population density is derived from the literature and combined with behavioural data on movement patterns and encounters from urban Bristol, UK to formulate a predictive model of inter-group contact probabilities over a range of population densities. 3. This contact probability-population density relationship is used to formulate a revised version of the rabies model of Smith & Harris (1991), which is the model used by the Ministry of Agriculture, Fisheries and Food as the basis of its rabies control policy for an outbreak of the disease in an urban area of Britain. 4. The original and revised versions of the model are compared in terms of their predictions for the rate of rabies spread and their implications for rabies control in specific urban areas. 5. An equivalent probability of successful rabies control is achieved by 5-15% lower fox control for the revised model compared with the original one. The greatest differences between the probability of successful rabies control for a given level of fox control under the two versions of the model occur in winter. 6. The revised model predicts that the rate of rabies spread will be slower than predicted by the original model. This is due to the lower inter-group contact probabilities used in the revised model. Thus, there will be a greater chance of containment and elimination of the disease within a specified control area than was suggested by the original model. 7. The heterogeneous nature of the urban landscape probably results in a lower frequency of contacts than would be observed between foxes living at the same densities in rural environments.

Specificity of rabies virus as a transneuronal tracer of motor networks: transfer from hypoglossal motoneurons to connected second-order and higher order central nervous system cell groups.

The specificity of transneuronal transfer of rabies virus [challenge virus standard (CVS) strain] was evaluated in a well-characterized neuronal network, i.e., retrograde infection of hypoglossal motoneurons and transneuronal transfer to connected (second-order) brainstem neurons. The distribution of the virus in the central nervous system was studied immunohistochemically at sequential intervals after unilateral inoculation into the hypoglossal nerve. The extent of transneuronal transfer of rabies virus was strictly time dependent and was distinguished in five stages. At 1 day postinoculation, labelling involved only hypoglossal motoneurons (stage 1). Retrograde transneuronal transfer occurred from 2.0-2.5 days postinoculation (stage 2). In stages 2-4, different groups of second-order neurons were labelled sequentially, depending on the strength of their input to the hypoglossal nucleus. In stages 4 and 5, labelling extended to several cortical and subcortical cell groups, which can be regarded as higher order because they are known to control tongue movements and/or to provide input to hypoglossal-projecting cell groups. The pattern of transneuronal transfer of rabies virus resembles that of alpha-herpesviruses with regard to the nonsynchronous labelling of different groups of second-order neurons and the transfer to higher order neurons. In striking contrast to alpha-herpesviruses, the transneuronal transfer of rabies is not accompanied by neuronal degeneration. Moreover, local spread of rabies from infected neurons and axons to adjoining glial cells, neurons, or fibers of passage does not occur. The results show that rabies virus is a very efficient transneuronal tracer. Results also provide a new insight into the organization of cortical and subcortical higher order neurons that mediate descending control of tongue movements indirectly via hypoglossal-projecting neurons.

Rabies surveillance in the United States during 1993

Summary In 1993, 49 states, the District of Columbia, and Puerto Rico reported 9,495 cases of rabies in nonhuman animals and 3 cases in human beings to the Centers for Disease Control and Prevention. Greater than 93% (8,889 cases) were wild animals, whereas 6.4% (606 cases) were domestic species. The total number of reported cases increased 9.9% over that of 1992 (8,645 cases), with most of the increase resulting from continued spread of rabies in raccoons (37.1% increase in reported cases over 1992). The 2 epizootics of rabies in raccoons (Northeastern/mid-Atlantic and Southeastern regions) approach convergence in North Carolina (106 cases of rabies in 1993, compared with 49 in 1992). Maine, Rhode Island, and Vetmont remained the only New England states without reported cases associated with the raccoon variant of the rabies virus. New York reported 2,747 cases of rabies, the largest number of cases ever reported during a single year by any state. Increases in reported cases of rabies in Texas and 8 other geographically dispersed states were attributed mainly to larger numbers of reported cases of rabies in bats. Texas reported 71 of the 74 cases in coyotes during 1993 (70 of 75 cases in 1992). Nationally, reported cases of rabies in dogs (130) and cattle (130) each decreased by 29% in 1993, whereas cats (291 cases in 1993, compared with 290 in 1992) continued to be the domestic animal most frequently reported rabid. Twenty-two states and Puerto Rico reported decreases in rabies in animals in 1993, compared with 20 states, the District of Columbia, and Puerto Rico in 1992. Hawaii was the only state that did not report a case of rabies in 1993.

Raccoon rabies: the re-emergence of an epizootic in a densely populated area

Investigation of sporadic or isolated cases of rabies was encouraged in the 1950s by the implementation of a national surveillance system for reporting of rabies cases. Low level enzootic disease was recognized in several wild species during this period, including a small focus of rabies in raccoons in Florida. Phylogenetic analysis of the virus transmitted by rabid raccoons and reconstruction of the early history of raccoon rabies in the United States suggests that rabies was present in these animals long before its recognition by the public health community and that the emergence of raccoon rabies as a major threat to public health was the result of several natural and human-mediated events. In recent years, reports of raccoon-human contact and rabies in raccoons have increased as changes in human demographics and land development have concentrated raccoon populations in urban and suburban settings. In addition to the natural spread of rabies from Florida to raccoon populations in Georgia, Alabama, South Carolina and North Carolina, a separate focus in the northeastern states is believed to have been caused by the translocation of raccoons from the southeast to Virginia and West Virginia. Oral vaccination of wild species may be an important adjunct to traditional rabies control measures in the management of this disease in the future.

Control of rabies in foxes: what are the appropriate measures?

Until recent years, the depopulation of reservoir species was the only feasible measure to break the transmission chain of rabies in wildlife. In Europe at the beginning of this century, when applied with compulsory muzzling and restrictions on movement, dog depopulation was shown to be effective before any large scale vaccination of dogs. Some examples of the success of the application of depopulation measures on wild carnivores in the past are described. But in the case of enzootics of rabies in the fox, the control measures applied to fox populations sometimes decreased the prevalence or postponed the invasion of an area but did not free or protect an area permanently. Some examples taken from France illustrate the difficulties of drawing clear conclusions from the results of control measures. Conversely, the oral vaccination of foxes was demonstrably successful in preventing the spread of rabies. However, mathematical modelling, historical considerations and first observations in areas in which foxes have been vaccinated strongly suggest that the 'traditional' methods of controlling fox populations, at least by hunting, should not be restricted because they complement the effects of vaccination.

Rabies surveillance in the United States during 1992

Summary In 1992, 49 states, the District of Columbia, and Puerto Rico reported 8,644 cases of rabies in nonhuman animals and 1 case in a human being to the Centers for Disease Control and Prevention. Almost 92% (7,912 cases) were wild animals, the largest number of wild animals ever reported, whereas 8.5% (732 cases) were domestic species. The total number of reported cases increased 23.9% over that of 1991 (6,975 cases), with most of the increase resulting from continued spread of rabies in raccoons. The 2 epizootics of rabies in raccoons (Northeastern/mid-Atlantic region and Southeastern region) are now approachig convergence in North Carolina (49 reported cases of rabies in 1992). Massachusetts (57 cases), New York City (41 cases), and New Hampshire (10 cases) became new additions to the epizootic in the Northeast, with Maine, Rhode Island, and Vermont the only states in the region without cases associated with the raccoon strain of rabies. The state of New York (including New York City) reported 1,761 cases (79% in raccoons) of rabies, the largest number ever recorded for any state. Increases attributable to epizootics of rabies in other species were reported by Alaska (25 cases in 1992, compared with 12 in 1991 mainly attributable to rabies in foxes) and Kansas (374 cases in 1992, compared with 63 in 1991, mainly attributable to rabies in skunks). Reported cases of rabies in coyotes (75) increased 50% over those for 1991 (50 cases). In the southern portion of Texas (reporting 70 of the 75 cases in coyotes), there was a similar increase (55%) in reported cases of rabies in dogs, whereas nationally, reported cases of rabies in dogs (182) increased 17%. Twenty states, the District of Columbia, and Puerto Rico reported decreases in rabies in animals in 1992, compared with 16 states in 1991. Hawaii was the only state that did not report a case of rabies in 1992.

On the spatial spread of rabies among foxes with immunity

We consider a model for the spatial spread of rabies in foxes, where a certain proportion of the foxes develop a natural immunity to the disease. We modify the model of Murray et al. (1986, Proc. R. Soc. Lond. B 229 , 111–150) to include a population of immune foxes and find that, with realistic estimates for the size of this class, this aspect has little effect on the propagation speed of the initial wave of the rabies epidemic but it does affect the behavior of the periodic outbreaks associated with the oscillating tail of the wave. We use the modified model to estimate the width of a rabies break which would be required to contain the epidemic. We also investigate the effect of spatial dispersal of susceptible and infected foxes. Previous work hypothesized that the required break width might depend on whether the break was created by killing foxes or by vaccinating them against rabies. With our model we consider this difference and find that the break width does not change significantly unless the rate of spatial dispersal is large.

Discussion on the Meeting on Chaos

Discussion on the Meeting on Chaos

Simulations of a Fox-Rabies Epidemic on an Island Using Space-Time Finite Elements

The two-dimensional reaction diffusion equations for the spread of rabies in a logistically growing fox population are solved numerically. The method, based upon Galerkin's approach, uses space-time finite elements. The numerical model is shown, quantitatively, to possess the essential features of earlier one and two-dimensional models and to reproduce the values of field data accurately. A more realistic illustration of the use of the model, a study of the spread of rabies over the Isle of Anglesey, is then discussed.

Ecology and epidemiology of fox rabies.

Although fox rabies has been reported in every country that has foxes, current foci of fox rabies are limited to Europe and North America. The fox rabies virus has unique characteristics that seem to be the result of adaptation through successive passages, i.e., high pathogenicity for the fox, excretion in saliva of a high proportion of infected animals, and low frequency of postinfection immunity. The spread of fox rabies is greatly favored by the characteristics of the genus Vulpes--ubiquity, broad diet, prolific nature, and its particular ethology and ecology. Animals may be immunized by oral vaccination, but natural mechanisms that also can terminate outbreaks are discussed.

Rabies and rabies control in striped skunks (Mephitis mephitis) in three prairie regions of western North America.

The number and geographic distribution of rabies cases in striped skunks (Mephitis mephitis) from Saskatchewan (n = 2,506 cases), Montana (n = 1,142), and Alberta (n = 199) since 1963 were reviewed. In Saskatchewan the number of cases increased steadily for 5 yr and then fluctuated consistently in a 4 yr cyclic pattern. Similarly an initial sweep across the province was followed by a cyclic pattern of geographic expansion (3 to 4 yr) and reduction (1 to 2 yr). No organized control efforts were conducted in Saskatchewan. Similar cyclic pattern were not seen in data from Montana or Alberta. In the latter areas, the number and distribution of rabies cases in skunks appeared to reflect efforts to reduce the population of skunks. An integrated program of skunk removal using poison and live-traps in association with research and public education successfully contributed to limiting the spread and establishment of rabies in striped skunks within prairie habitats. Rabies did not persist in skunks in other habitats.

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.

A simple model for the spatial spread and control of rabies

A simple mathematical model for the spatial spread of rabies is presented. It models the dynamics of the front of an epizootic wave. We show how the model can be used to estimate the minimum width (in kilometers) of a break, that is, a region in which a control scheme is employed in order to stop the spatial progression of the rabies wave front. A simple expression is derived for the surviving fox population, after the passage of the epizootic, in terms of measurable parameters of the model.

Différences dans le pouvoir pathogène de souches de virus rabique adaptées au renard ou au chien: Change in rabies virus pathogenicity with adaptation to fox or dog

Summary Dogs were inoculated with various amounts of fox rabies virus and foxes with various amounts of dog rabies virus. Dogs were much more resistant than foxes to inoculation of fox virus, and vice-versa. Paradoxically, foxes showed increased resistance with increasing doses. Most of the foxes which survived these inoculations subsequently resisted inoculation with a vulpine virus, whereas this virus was fatal to the control animals. In foxes mortally contaminated by the canine virus, only five out of eight re-excreted this virus. Possible consequences of these observations on the epizootiology and spread of fox and canine rabies are discussed.

A profile of reported skunk rabies in Arkansas: 1977-1979.

Skunk rabies in Arkansas increased from 90 laboratory confirmed cases in 1977 to 140 in 1978 and 297 in 1979. In 1979, the Arkansas Department of Health declared a skunk rabies epizootic in Arkansas. Skunk rabies accounted for 90% of the total rabies cases in 1978 and 1979. March, April and May had the highest incidence in each year. SYMVU computer analysis illustrated the distribution and spread of rabies. In general, rabies was confined to the upland areas of the state. Regression analysis of 16 density-independent variables revealed no correlation with the distribution of rabies, indicating the primary determinants to be biotic.

Habitat and the spread of rabies

Habitat and the spread of rabies

Evidence for an intraaxonal transport of fixed and street rabies virus.

Colchicine was used to inhibit axonal transport and to demonstrate that rabies virus spread from the peripheral inoculation site to the CNS by the retrograde axoplasmic flow. Colchicine was applied by the mean of elastomer implants around the sciatic nerve of young rats in order to obtain higher local concentrations of the drug. This procedure avoided the systemic effects of colchicine encountered with the usual treatment. To confirm the efficiency of the axoplasmic flow inhibition by colchicine, 125I-tetanus toxin was used as a marker. Uptake of colchicine by the sciatic nerve was monitored by the use of 3H-labelled colchicine. Interruption of the retrograde axoplasmic flow resulted in prevention of fixed and street rabies virus propagation. Moreover, the centrifugal spread of rabies could be inhibited using this experimental procedure.

The landscape epidemiology of rabies in Virginia.

Rabies virus, the gray fox, and the Alleghenian Biotic Region form a pathobiocenose; rabies in Virginia exists in a diffuse nidus, the Alleghenian Biotic Region. Physiographic features may serve as barriers or as passageways for epidemic spread of rabies. Epidemic spread is characterized by a radial pattern and a linear northeast-southwest pattern reflecting physiographic features. Incidence of rabies in the fox varies according to seasons. Present control efforts are generally ineffective; under certain conditions, population reduction for control of rabies may be detrimental instead of beneficial.