TRANSMISSION OF TERRESTRIAL AND BAT RABIES
The abiotic environment is unlikely to be important in the transmission of the virus, as ultraviolet light, pH extremes, desiccation, organic solvents, excessive temperatures and putrefaction all result in rapid disintegration of the rabies virus, unless the virus is conserved in frozen carcasses.
Transmission of lyssaviruses through intact skin does not occur, and therefore is not considered an exposure route for rabies. Oral and intranasal exposure, as well as aerosol transmission, theoretically may be possible under certain circumstances but do not play a role under field conditions. Therefore, based on the pathogenesis of rabies virus, the most reliable route of rabies transmission leading to an infection is via a bite or scratch from an infected animal, or less frequently, by licking the mucosa, or by virus deposition into breaks in the skin continuity, whereby the intact skin must be penetrated. For EBLVs repeated multiple low virus dose exposure is being discussed as a modified version of this bite inoculation.Intra- and Inter-Species Transmission
Transmission of classical rabies and bat lyssaviruses predominantly occurs among conspecifics of the reservoir species and occasionally to spillover hosts. The transmission of lyssaviruses is promoted by the social ecology and behaviour of the reservoir species. Annual population dynamics such as mating (sexual activity), birth, adolescence and dispersal lead to increased contact rates among conspecifics, enhancing the chance of virus transmission.
In addition, proximity to humans greatly influences the probability of contact with infected animals; consequently anthropophilic animal species such as the red fox or feral cat represent a higher public health risk.
European bats are solitary foragers but gregarious when roosting, forming colonies that may vary in size from a few individuals up to several thousand bats (e.g.
Miniopt- erus schreibersii or Tadarida teniotis). Most species form transient groups during certain periods of the year, e.g. at hibernation roosts, maternity roosts, and mating roosts. Intra-species contact among members of these colonies are likely to be common(2). A typical annual pattern of rabies peaks is seen with fewer cases during winter, as in terrestrial rabies. A better understanding of the social structure within and between bat colonies will improve our understanding of the EBLV transmission dynamics.Several studies have investigated whether healthy animals could be potential carriers of rabies virus. So far this assumption remains highly controversial and should not lead to any modification of the control measures and quarantine periods of surveillance of suspect animals.
Rabid Behaviour and Virus Transmission
The incidence of rabies in a particular species is dependent upon its susceptibility and the probability of potentially infectious encounters. How the virus from the shedding animal comes in contact with the nerves of the receiving animal in natural conditions remains poorly documented and, for the most part, speculative.
Radio-collared animals have subsequently become rabid and been followed at a distance during the course of the disease(5). Results were based on the follow-up of a small number of individuals and are difficult to generalize, but they suggest that a rabid fox loses its normal daily rhythm of activity, can express hyperactivity a few days before death, and finally expire within or just outside its original home range, unless it had started dispersal movements during the incubation period. These observations suggest that disease-induced paresis can induce antagonistic reactions from healthy congeners, favouring the transmission of the virus.
Virus transmission from diseased individuals to susceptible congeners drives the spatial expansion of rabies at a speed that relies on the latency period and activity range of animals, e.g.
during the years 1960—1970, the disease wave front extended some 30—40 km per year.I n gregarious animals (e.g. fallow deer (Dama dama), greater kudu (Tragelephus strepsiceros)~) or social mammals (e.g. hunting dog (Lycaon pictus), simian wolf (Canis simensis)), the virus can be transported through contact with infective saliva left on branches, or by mutual licking, and regurgitation. This ‘natural’ behaviour presents a route for passive virus passage from an infected shedder to a receptive congener.
Temporal variation of rabies incidence is mediated by the changing host behaviour of space use, circadian rhythms and social interactions. As a consequence, the incidence of fox rabies increases in March following an increase in the contact rate between foxes during the vulpine breeding season. By contrast, transmission of raccoon-dog- or raccoon-mediated rabies might be influenced by dormancy during the winter season.
Active aggression, the most spectacular clinical sign of rabies, is relatively rare and is not viewed as the most common way for virus transmission within species. However, it can be of major importance in lyssavirus evolution as an alternative method of transmission, allowing the virus to leap from one species to another (dog to fox, fox to raccoon dog).
Animals (or people) that inspect and investigate, or attack, a paralysed infected fox, or roe deer, cattle and other domestic ruminants, are more likely to be infected, and this is also the case with regard to people and bat rabies.