ARBOVIRUS DISEASE: SPECIFIC ARBOVIRUSES
The agent that causes western equine encephalitis is prevalent in the central valley of California, eastern Washington (Yakima valley), Colorado, and Texas. It has also been re- sponsible for outbreaks in midwestern states (Minnesota, Wisconsin, Illinois, Missouri, and Kansas) and as far east as New Jersey. Horses and humans represent blind-end hosts;
both are susceptible to infection and illness, commonly manifested as encephalitis. Although human infection in endemic areas is commonplace, overall only 1 of 1000 in-fections causes clinical symptoms. However, in young infants, 1 of every 25 infections may produce severe illness. The attack rates are therefore far higher in young infants than in other groups. The disease spectrum may range from mild, nonspecific febrile illness to aseptic meningitis or severe, overwhelming encephalitis. Mortality is estimated at 5% for cases of encephalitis. It is a very serious disease in infants less than 1 year of age; as many as 60% of survivors have permanent neurologic impairment.
The eastern equine encephalitis virus is largely confined to the Atlantic Seaboard states from New England down the coasts of Central America and South America. The mosquito vector (principally Culiseta melanura) generally restricts its feeding to horses and birds, al-though occasional outbreaks among humans have occurred. The virus can cause severe en-cephalitis in horses and also in wild birds. The mortality among humans is estimated at 50% for individuals of all ages, and the incidence of severe sequelae among survivors is high.
The St. Louis encephalitis virus is a major cause of arbovirus encephalitis in the United States. Its geographic distribution and major mosquito vector (Culex tarsalis) are similar to those of western equine encephalitis, but has been much more prevalent in eastern states and in Texas, Mississippi, and Florida. It infects but causes no disease in horses. The disease spectrum in humans is similar to that of western equine encephalitis, but the major morbidity and mortality, as well as the highest attack rates, are among adults more than 40 years of age. Infants and young children are relatively spared.
During the summer of 1999 in the northeastern United States, human West Nile virus infec-tions appeared for the first time in the Western Hemisphere. A subsequent outbreak occurred again in 2000. Together, these outbreaks resulted in 78 hospitalized patients and 9 deaths, mostly among the elderly. More widespread activity was observed in 2001 (66 human cases); then in 2002, a dramatic increase in virus spread was seen across the United States, with activ-ity in 46 states and four Canadian provinces. There were at least 3600 human cases reported in the United States, with 212 deaths. Prior to 1999, outbreaks of human infections were primar-ily confined to eastern Africa, the Middle East, eastern Europe, west Asia and Australia.
The virus is antigenically related to St. Louis encephalitis and Japanese encephalitis. Transmission is from infected mosquitoes to birds, humans and horses, and clinical ill-ness leading to death can result from infections in any of these hosts. Transmission among humans via blood transfusions, breast milk, or organ transplants is also possible. Crows are particularly affected; virus has been detected in dead crows found as far south as Florida, and more recently in the midwestern United States. Clinical illness in the United States has often included muscle weakness and flaccid paralysis, suggesting an axonal polyneuropathy in addition to encephalitis.
Although California virus was first isolated in that state, its major distribution in the United States has been in the Midwest; outbreaks due to the LaCrosse subtype are partic-ularly prevalent in Wisconsin, Ohio, Minnesota, Indiana and West Virginia. In Wisconsin and Minnesota, California virus is considered the most important cause of encephalitis. However, studies elsewhere in North America and throughout the world, indicate that California virus or closely related agents are present nearly everywhere. The primary mosquito vector (Aedes triseriatus) is commonly encountered in suburban or rural environments. Unlike western equine, eastern equine, and St. Louis encephalitis viruses, the highest attack rates are seen in those aged 5 to 18 years. Infection is often character-ized by abrupt onset of encephalitis, frequently with seizures.
Geographically, yellow fever is distributed throughout the Caribbean and Central America, the Amazon valley in South America, and a broad central zone in Africa from the Atlantic Coast to the Sudan and Ethiopia. It continues to be a potential threat to the southeastern United States because of an urban vector (Aedes aegypti) in that area. The clinical disease is characterized by abrupt onset of fever, chills, headache, and hemorrhage. It may progress to severe vomiting (sometimes with gastric hemorrhage), bradycardia, jaundice, and shock. If the patient recovers from the acute episode, there are no long-term sequelae.
There are four related serotypes of dengue, any of which may exist concurrently in a given endemic area. These agents are widespread throughout the world, particularly in the Middle East, Africa, the Far East, and the Caribbean Islands, and they have invaded the United States in the past. The vector (Aedes aegypti) is the same as the domestic vector of yellow fever. The known transmission cycle is human–mosquito–human, although a syl-vatic cycle involving monkeys may also exist.
The characteristic clinical illness usually results in fever, an erythematous rash, and severe pain in the back, head, muscles, and joints. Especially in the Far East (Philippines, Thailand, and India), the disease has periodically assumed a severe form characterized by shock, pleural effusion, and hemorrhage often followed by death.
The flavivirus species that causes Japanese B encephalitis is prevalent on the eastern coast of Asia, on its offshore islands (Japan, Taiwan, and Indonesia), and in India. Its transmis-sion cycle resembles that of the St. Louis encephalitis and western equine encephalitis viruses. A high proportion of human infections are subclinical, especially in children; when encephalitis does develop it is severe and often fatal.
Powassan virus is the only known tick-borne Flavivirus species of North America. First isolated in Ontario from a fatal human case of encephalitis, it has been found in infected ticks in Ontario, British Columbia, and Colorado. Its significance to humans is not yet es-tablished; only a few patients with encephalitis proved to be caused by this agent have been described. However, serologic evidence suggests that the virus is prevalent in many areas of North America.
The tick-borne Orbivirus species that causes Colorado tick fever has been found through-out the western United States, including Washington, Oregon, Colorado, and Idaho, and also Long Island. It is frequently found in Dermacentor andersoni, which are also vectors for Rickettsia rickettsii. The typical illness, which occurs 3 to 6 days after the tick bite, is characterized by a sudden onset with headache, muscle pains, fever, and occasionally en-cephalitis. Leukopenia is a consistent feature of infection. It is estimated that no more than one clinical illness occurs for every 100 infections with this agent.