The majority of human alphavirus infections are asymptomatic, the only evidence of infection being seroconversion. Clinical manifestations, when they occur, often take the form of a nondescript fever. However, three alphaviruses sometimes cause potentially lethal encephalitis, and five others cause painful and sometimes persistent arthritis (Table 25-2).
In a small proportion of persons infected with eastern, western, or Venezuelan encephalitis viruses, a few days after the onset of lever the patient develops drowsiness, often accompanied by neck rigidity, and may progress to confusion, paralysis, convulsions, and coma. Case-fatality rates average 10-20% but may be considerably higher with eastern equine encephalitis. Survivors from encephalitis caused by any of these viruses are often left with permanent neurologic sequelae such as mental retardation, epilepsy, paralysis, deafness, and blindness.
The fever/rash/arthritis triad of clinical features defines the disease caused by chikungunya, o'nyong-nyong, Ross River, Mayaro, and Sindbis viruses. Indeed, chikungunya and o'nyong nyong are colorful African terms describing the agony of the affected joints' Typically, after a short incubation period of 2-3 days, there is an abrupt onset of fever, chills, myalgia, and severe polv-arthralgia affecting mainly the small joints; a rash, generally maculopapular, then appears. Other constitutional symptoms such as nausea, headache, backache,photophobia, and retroorbital pain may also be present but are not so diagnostic. The fever is characteristically high and sometimes of the "saddle-back" (biphasic) variety in chikungunya, but it is not at all prominent in Ross River virus infection. The arthritis usually resolves in a few weeks but may persist for months or even years in chikungunya and Ross River virus infections.
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