This disease also stands apart from other acute viral infections by virtue of the latent period that follows inoculation with the virus and its stunningly distinctive clinical and pathologic features. Human examples of this disease are rare in the United States; between 1980 and 1997, only 34 instances were known to have occurred and since 1960, there have not been more than 5 or so cases in any 1 year. In some areas (Australia, Hawaii, Great Britain, and the Scandinavian peninsula), no indigenous cases have ever been reported. In contrast, in India there is a high incidence. The importance of this disease derives from the fact that it is almost invariably fatal once the characteristic clinical features appear, making survival of the infected individual dependent on the institution of specific therapeutic measures before the infection becomes clinically evident. Furthermore, each year 20,000 to 30,000 individuals are treated with rabies vaccine, having been bitten by animals that possibly were rabid, and although the incidence of complications with the newer rabies vaccination is much lower than before, a few serious reactions continue to be encountered (see in the following text and also Chap. 35).
Practically all cases of rabies are the result of transdermal viral inoculation by an animal bite. In developing countries, where rabies is relatively common, the most frequent source is the rabid dog. In Western Europe and the United States, the most common rabid species are raccoons, skunks, foxes, and bats among wild animals and dogs and cats among domestic ones. Because rabid animals commonly bite without provocation, the nature of the attack should be determined. Also, the prevalence of animal rabies virus varies widely in the United States, and local presence of the disease is useful in assessing risk. Rare cases have been caused by inhalation of the virus shed by bats; a history of spelunking suggests this mode of acquiring the infection. In a few cases the source of the infection may not be identifiable. The epidemiology and public health aspects of rabies have been reviewed by Fishbein and Robinson.
The incubation period is usually 20 to 60 days but may be as short as 14 days, especially in cases involving multiple deep bites around the face and neck. Tingling or numbness at the site of the bite, even after the wound has healed, is characteristic. This is thought to reflect an inflammatory response that is incited when the virus reaches the sensory ganglion.
The main neurologic symptoms, following a 2to 4-day prodromal period of fever, headache, and malaise consist of apprehension, dysarthria, and psychomotor overactivity, followed by dysphagia (hence salivation and “frothing at the mouth”), spasms of throat muscles induced by attempts to swallow water or in rare cases by the mere sight of water (“hydrophobia”), dysarthria, numbness of the face, diplopia, and spasms of facial muscles. These features indicate the involvement of the tegmental medullary nuclei. Generalized seizures, confusional psychosis, and a state of agitation may follow. A less common paralytic form (“dumb” rabies of older writings, in distinction to the above described “rabid” form) as a result of spinal cord infection may accompany or replace the state of excitement. The paralytic form is most likely to follow bat bites or, in the past, the administration of rabies vaccination. Coma gradually follows the acute encephalitic symptoms and, with rare exceptions as noted below, death ensues within 4 to 10 days, or longer in the paralytic form.