Arthropodborne Viruses

Epidemiology and Risk Factors. The arthropod-borne viruses (arboviruses) are transmitted to humans by the bite

of a mosquito or tick. The epidemiology of arthropod-borne viral encephalitis is influenced by the season, the geographic location, the regional climatic conditions, such as the amount of spring rainfall, and the patient's age. LaCrosse virus is the most important arboviral cause of pediatric encephalitis in the United States. y The midwestern states of the United States report the highest incidence of LaCrosse virus infections. The LaCrosse virus belongs to the California group of encephalitis viruses. It is responsible for nearly all infections due to California group viruses in the United States. Most cases occur in late summer to early fall. The vector, Aedes triseriatus, is a forest-dwelling mosquito, and the risk of infection is associated with outdoor activities in woodland areas. y St. Louis encephalitis (SLE) occurs as periodic focal outbreaks of encephalitis in the midwestern, western, and southeastern United States, followed by years of sporadic cases. St. Louis encephalitis virus has caused large urban epidemics of encephalitis. y , y Western equine encephalitis virus is an alphavirus that infects horses and humans in western North America, with most cases occurring between April and September. Eastern equine encephalitis (EEE) virus is also an alphavirus that infects horses and humans, primarily along the eastern seaboard and the Gulf coast; its peak activity occurs in August and September. Eastern equine encephalitis virus has been isolated from species other than horses including pheasants, piglets, and litters of puppies. y It causes a much more severe encephalitis than western equine encephalitis virus. Venezuelan equine encephalitis virus is endemic in South America and has caused rare cases of encephalitis in Central America and the southwestern United States, particularly in Texas.

Japanese B encephalitis virus is a member of the St. Louis complex of flaviviruses and is the most common cause of arthropod-borne human encephalitis worldwide. Epidemic disease occurs in China and the northern parts of southeast Asia, as well as in areas of India and Sri Lanka. y The principal vector is a rice-field-breeding culicine mosquito. There is no seasonal pattern to Japanese B encephalitis except in some instances when outbreaks are associated with rainfall, irrigation, or floods.y Dengue, a mosquito-borne flavivirus infection that is hyperendemic in Thailand, is a leading cause of hospitalization of children in southeast Asia. y The principal vector is the mosquito Aedes aegypti, which breeds indoors in clean water, mainly in artificial water containers. y

The most important tickborne causes of viral encephalitis in North America are Powassan (POW) virus and Colorado tick fever (CTF) virus. Powassan virus is a flavivirus most closely related to Russian spring-summer and Central European encephalitis viruses. The POW virus is widely distributed in the United States, but human infections with POW virus occur infrequently. In the years 1958 to 1994, a total of 24 confirmed POW cases among humans was reported in North America.

The risk of infection is highest in wooded areas, where the potential for contact with infected ticks is highest. y Colorado tick fever virus is a member of the family Reoviridae. This virus has caused human infections in Colorado, Utah, Montana, California, Wyoming, Idaho, Oregon, South Dakota, Washington, New Mexico, Nevada, and southern Alberta and British Columbia, Canada. The primary risk of acquiring CTF is through exposure to infected Dermacentor andersoni ticks during outdoor recreational activities in mountain areas where the CTF virus is endemic in the months of April through June. y

Pathogenesis and Pathophysiology. Arboviral infections are transmitted to humans by a mosquito or tick bite. After local replication of the organism at a skin site, a viremia occurs with seeding of the reticuloendothelial system, including the liver, spleen, and lymph nodes. With continued viral replication at these sites, a secondary viremia occurs, seeding the CNS. '53! The probability of CNS infection depends on the efficiency of viral replication at the extraneural sites and the degree of the ensuing viremia. The arboviruses can invade the CNS through the cerebral capillary endothelial cells of the cerebral blood vessels or through the choroid plexus. y

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