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Aiding the diagnosis of California virus (La Crosse)
California virus (La Crosse) is a member of Bunyaviridae and is 1 of the arthropod-borne encephalitides. It is transmitted by various Aedes and Culex mosquitoes and is found in such intermediate hosts as the rabbit, chipmunk, and field mouse.
California meningoencephalitis is usually mild and occurs in late summer. Ninety percent of infections are seen in children under 15 years of age, usually from rural areas. Incubation period is estimated to be 7 days and acute illness lasts 10 days or less in most instances. Typically, the first symptoms are nonspecific, last 1 to 3 days, and are followed by the appearance of central nervous system signs and symptoms such as stiff neck, lethargy, and seizures, which usually abate within 1 week. Symptomatic infection is almost never recognized in those over 18 years old. The most important sequelae of California virus encephalitis is epilepsy, which occurs in about 10% of children; almost always in patients who have had seizures during the acute illness. A few patients (estimated 2%) have persistent paresis. Learning disabilities or other objective cognitive deficits have been reported in a small proportion (no more than 2%) of patients. Learning performance and behavior of most recovered patients are not distinguishable from comparison groups in these same areas.
In patients infected with these or related viruses, IgG antibody is generally detectable within 1 to 3 weeks of onset, peaking within 1 to 2 months and declining slowly thereafter.
IgM class antibody is also reliably detected within 1 to 3 weeks of onset, peaking and rapidly declining within 3 months.
Single serum specimen IgG > or =1:10 indicates exposure to the virus.
Results from a single serum specimen can differentiate early (acute) infection from past infection with immunity if IgM is positive (suggests acute infection).
A 4-fold or greater rise in IgG antibody titer in acute and convalescent sera indicate recent infection.
Infections with arboviruses can occur at any age. The age distribution depends on the degree of exposure to the particular transmitting arthropod relating to age and sex, as well as occupational, vocational, and recreational habits of the individuals. Once humans have been infected, the severity of the host response may be influenced by age: serious La Crosse infections primarily involve children, especially boys. Adult males exposed to La Crosse have high prevalence rates of antibody but usually show no serious illness. Infection among males is primarily due to working conditions and sports activity taking place where the vector is present.
All results must be correlated with clinical history and other data available to the attending physician.
Specimens drawn within the first 2 weeks after onset are variably negative for IgG antibody and should not be used to exclude the diagnosis of arboviral disease. If arboviral infection is suspected, a second specimen should be drawn and tested 10 to 21 days later.
Since cross-reactivity with dengue fever virus does occur with St. Louis Encephalitis antigens, and, therefore, cannot be differentiated further. The specific virus responsible for such a titer may be deduced by the travel history of the patient, along with available medical and epidemiological data, unless the virus can be isolated.
Usually, when an infection with an arbovirus is suspected, it is too late to isolate the virus or draw serum specimens to detect a rise of antibody titer.
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