Читать книгу Basic Virology - Martinez J. Hewlett - Страница 78

Many of the viruses that cause encephalitis have RNA genomes and are carried by arthropod vectors from zoonoses, and human involvement is often incidental. Such viruses are often termed arboviruses, although this is an imprecise classification that includes two groups of viruses not closely related by other criteria.

The symptoms of encephalitis in wild animals can be difficult to measure, but several equine encephalitis viruses are known to cause serious disease in horses. Often the symptoms of viral encephalitis in humans are drowsiness, mild malaise, and sometimes coma. These mosquito‐borne encephalitis viruses do not usually directly invade neural tissue itself, but rather infect supporting tissue. The host response to this infection and resulting inflammation lead to the observed neurological symptoms.

Since tissue at the periphery of neural tissue is the primary target for such encephalitis virus infections, the infection can be resolved and complete recovery will ensue, provided that the host's immune defenses work properly. During the disease's symptomatic period, lethargy and malaise of infected individuals make them vulnerable to other environmental hazards, including infection with other pathogens. But provided these risks are avoided by means of proper care, the disease generally resolves.

While humans are often accidental targets for encephalitis viruses, it is not clear that symptoms of the disease in humans have any major role in virus spread. As with all arthropod‐borne diseases, transmission is by arthropod ingestion of blood‐associated virus found during the viremic stage of animal infection, and the behavioral effects are incidental. Still, it may be that the lethargy manifested during active disease makes infected animals more easily bitten by arthropods, and perhaps this is a factor in natural transmission.