The vast majority of rabies cases reported to the CDC each year occur in wild animals such as raccoons, skunks, bats, and foxes. Domestic animals account for less than 10% of reported rabies cases, with cats, cattle, and dogs most often affected. Although all species of mammals are susceptible to rabies virus infection, only a few species are important as reservoirs for the disease.
In North America, several distinct rabies virus variants have been identified in terrestrial mammals, including raccoons, skunks, foxes, and coyotes. In addition to these terrestrial reservoirs, several species of insectivorous bats are also reservoirs for rabies. Rodents and lagomorphs, like rabbits and hares, are unlikely to have rabies.
Various routes of transmission have been documented and include bites, contamination of mucous membranes, aerosol transmission, and organ and tissue transplants. The most common mode of rabies virus transmission is through a bite wound contaminated with virus-containing saliva from an infected animal. The virus cannot penetrate intact skin. Under refrigeration or during colder winter months, the virus can live from 4 weeks up to several months in dead animals. The virus will no longer be infective after a couple of hours in dried blood or other secretions.
Following infection, the virus enters an eclipse phase, during which it cannot be easily detected within the host. During the eclipse phase, the host’s immune defenses may stimulate a cell-mediated immunity against the rabies virus antigen. About 20% of people exposed to the virus develop rabies when the cell-mediated immunity does not neutralize the virus.
The uptake of virus into peripheral nerves is important for infection to occur. After uptake into peripheral nerves, the rabies virus is transported to the central nervous system. Typically this occurs by way of the sensory and motor nerves at the initial site of infection.
The incubation period may vary, from a few days to several years, but is typically 1 to 3 months. There is a direct relationship between the location of the bite and the length of the incubation period. Bites on the head, neck, and arms progress most quickly, because of the close proximity of the central nervous system to these locations. The amount of virus received is also significant. A bite through clothing may result in some of the saliva being absorbed by the clothing, which could reduce the number of viruses that enter the bite wound.
The virus spreads rapidly within the central nervous system. Active cerebral infection is followed by spread of the virus back to the peripheral nerves. This may lead to viral invasion of highly innervated sites, including the salivary glands.
The clinical course, particularly in dogs, can be divided into three phases: the prodromal, the excitative, and the paralytic. The term furious rabies refers to cases in which the excitative phase is predominant. Dumb or paralytic rabies refers to cases in which the excitative phase is short or absent. The disease progresses quickly to the paralytic phase, characterized by flaccid paralysis that leads to eventual death due to respiratory and/or cardiac failure.
In any animal, the first clinical signs of rabies are seen in the prodromal stage. These signs can include a change in behavior, which may be indistinguishable from a variety of other disorders, such as gastrointestinal disorder, injury, a foreign body in the mouth, poisoning, or an early infectious disease. Body temperature change is not significant, and slobbering may or may not be noted. Animals usually stop eating and drinking and may seek solitude. Frequently, the urogenital tract is irritated or stimulated, resulting in frequent urination, erection in the male, and signs of increased sexual desire.
After the prodromal period of 2 to 4 days, animals either become vicious or show signs of paralysis. Carnivores, pigs, and occasionally horses and mules bite other animals or people at the slightest provocation. Cattle may butt any moving object. Rabid domestic cats and bobcats attack suddenly, biting and scratching viciously. Rabid foxes frequently invade yards or even houses, attacking dogs and people. Rabid foxes and skunks are responsible for most pasture cattle losses from rabies and have even attacked cattle in barns.
A rabid raccoon is characterized by a loss of fear of humans, frequent aggression and incoordination, and abnormal activity during the day (raccoons are predominantly nocturnal animals). In urban areas, rabid skunks and raccoons often attack domestic dogs. Skunks are the leading reservoir of rabies in large areas of the United States. Bats flying in daytime are probably rabid.