The common arthropod-borne viruses (arboviruses) that cause encephalitis in the United States and their geographic range have been mentioned earlier. Most of the agents are in the category of Flaviviruses. There are alternating cycles of viral infection in mosquitoes and vertebrate hosts; the mosquito becomes infected by taking a blood meal from a viremic host (horse or bird) and injects virus into the host, including humans. The seasonal incidence of these infections is practically limited to the summer and early fall, when mosquitoes are biting. In the equine encephalitides, regional deaths in horses usually precede human epidemics. For St. Louis encephalitis, the urban bird or animal or possibly the human becomes the intermediate host. West Nile outbreaks are preceded by illness in common birds such as crows and jays. St. Louis, California, and La Crosse agents are endemic in the United States because of the cycle of infection in small rodents. Powassan virus from the deer tick has been added to the list of causes of Flavivirus encephalitis in North America as in the report by Tavakoli and coworkers.
The clinical manifestations of the arbovirus infections are almost indistinguishable from one another, although they do vary with the age of the patient. The incubation period after mosquito or tick bite transmission is 5 to 15 days. There may be a brief prodromal fever with arthralgia or rash (e.g., West Nile fever). In infants, there may be only an abrupt onset of fever and convulsions, whereas in older children and adults the onset is usually less abrupt, with complaints of headache, listlessness, nausea, or vomiting, drowsiness, and fever for several days before medical attention is sought; convulsions, confusion, stupor, and varying degrees of stiff neck then become prominent. Photophobia, diffuse myalgia, and tremor (of either action or intention type) may be observed. Asymmetry of tendon reflexes, hemiparesis, extensor plantar signs, myoclonus, chorea, and sucking and grasping reflexes may also occur. McJunkin and colleagues described the clinical features of 127 patients with La Crosse infection seen at their medical center over a decade, and their descriptions are representative of other arboviral infections. In addition to the typical features of viral encephalitis they emphasize aspects that occur in a proportion of patients: hyponatremia, raised intracranial pressure with cerebral swelling, and, most notable to us, signal changes in the MRI that simulate herpes encephalitis.
A special syndrome of febrile, flaccid, paralytic poliomyelitis resulting from West Nile virus infection is now also well known. It evolves over several days and in a few cases, is accompanied by facial paralysis (Jeha et al). Several cases have had an early extrapyramidal syndrome; any of these features may occur with the other Flaviviruses.
The fever and neurologic signs of arboviral encephalitis subside after 4 to 14 days unless death supervenes or destructive CNS changes have occurred. No antiviral agents are known to be effective; one must rely entirely on supportive measures. On occasion, brain swelling reaches a degree that requires specific therapy, as outlined in “Management of the Acutely Comatose Patient and
Management of Raised Intracranial Pressure” in Chap. 16.