Rabies is a fatal disease in humans and other mammals that is caused by a virus transmitted by animal bites. It is the oldest infectious disease known to man and has been present for more than 3000 years.
The virus responsible is a lyssavirus of which there are 7 genotypes. Only 3 of these genotypes have been isolated in South Africa.
In South Africa rabies is an endemic disease responsible for between 10 and 30 human deaths annually following dog bites. The incubation period can be weeks to months but once clinical signs are observed the disease is fatal.
The major host species for rabies in South Africa are the dog, black-backed jackal, bat-eared fox and yellow mongoose.
Infection cannot occur through intact skin but does occur when an animal (or human) is bitten by a rabid animal carrying virus in its saliva. The virus is transmitted into the bite wound, enters peripheral nerves and makes its way via these nerves to the central nervous system and ultimately localises in the spinal cord and brain. This gives rise to abnormal behaviour that increases the likelihood of further interaction, and virus transmission, with susceptible animals. Once widespread central nervous system infection has occurred death is generally due to respiratory paralysis.
Once infected with rabies wild animals often lose their fear of humans. Animals may show abnormally tame behaviour (mongoose and wild cat) or become excessively aggressive (jackal, duiker and honey badger). Kudus have been known to salivate profusely, may become docile, tame or paralysed and have been known to even enter houses.
The role of wildlife in the maintenance and distribution of rabies virus is interesting. The viverrid genotype virus in yellow mongoose is not modified in any specific way that will restrict spill-over from wildlife into canines. However, species behavioural factors and population densities appear to limit this cross-infection. The black-backed jackal and the bat-eared fox are the only species other than domestic canines that are capable of maintaining the canid rabies genotype. Black-backed jackals are capable of maintaining continuous infection cycles without the involvement of domestic dogs, under specific ecological conditions. Aggressive interactions during territorial defence create an ideal opportunity for transmission of the virus within the species. Furthermore, the wide ranges covered by these animals together with the sharing of resources such as water and large carcasses facilitate close contact between remote individuals in the population.
Theoretically rabies in black-backed jackal and bat-eared foxes could be controlled with oral bait vaccination, as has been done in Europe with the fox population. However, the environmental dynamics, including terrain, sociological issues and the presence of other wild animal species that might consume the bait all make these strategies difficult in an African environment.
JL: Dr Dittberner, for another very informative article.