The agents of the rabies virus are enveloped, with a bullet-shaped RNA virus that usually measures 75 x 180 nm in length. The single strand of nonsegmented RNA which is consider negative sense, encodes five structural proteins: a nucleocapsid protein, a phosphoprotein, a matrix protein, a glycoprotein, and a RNA-dependent RNA polymerase. These viruses have been isolated worldwide and are considered to have antigenic and genetic differences or variants among various isolates from major wildlife and domestic animal hosts within a given geographic region.
Transmission and Hosts
The disease is nearly always caused by the bite of an infected animal that has rabies virus in its saliva. Other modes of transmission are infrequently involved in infections of the dog and cat but may serve to maintain infection in wildlife. Transmission from exhaled or excreted virus has been suggested for spread between animals in extremely large colonies of cave-dwelling bats, however, such airborne infections probably involve large quantities of aerosolized virus under conditions of poor ventilation and a susceptible exposed host. Rabies can potentially result from ingesting infected tissues or secretions.
Although all mammals are susceptible to rabies virus, members of Canidae, Viverridae, and chiropteran species are the most capable vectors of the disease. Throughout the world, in most of the Northern Hemisphere, rabies is predominately a disease of wildlife, whereas in the Southern Hemisphere, the feral dog in urban areas of the primarily species involved in the transmission of the disease. Despite the fact that all warm-blooded animals are susceptible, rabies virus is a given enzootic area is usually a distinct variant that adapts itself to a single dominant reservoir host. Therefore, independent host-specific enzootic cycles of infection exist among individual host species. For example, wildlife reservoir species in various geographic areas of the United States are racoons, skunks, foxes, coyotes, and insectivorous bats. However, in Europe and parts of Asia, the primary species are foxes and racoon dogs, whereas in South Africa and certain Caribbean nations jackals and mongooses predominate, respectively.
Rabies in enzootic areas appears to be cyclic. It spreads into unexposed, susceptible wildlife populations in a region; subsequent decreases and increases in the prevalence of disease are caused by population mortality and immunity, which periodically cycles in the wildlife populations. These wild animals serve as maintenance hosts for virus transmission to dogs, cats, cattle, and horses. Most human exposures result from contact with these domestic species.