ARTHROPOD-BORNE VIRUS ENCEPHALITIS
Arthropod vectors transmit several types of viruses that cause encephalitis. The primary vector in North America is the mos-quito. Arbovirus infection occurs in specific geographic areas during the summer and fall. The four types of arboviral en-cephalitis that occur in North America are LaCrosse encephalitis, St. Louis encephalitis, Western equine encephalitis, and Eastern equine encephalitis (Roos, 1999).
Viral replication occurs at the site of the mosquito bite. If ade-quate virus is inoculated, a viremia ensues. The virus gains access to the central nervous system (CNS) via the cerebral capillaries. It spreads from neuron to neuron, predominantly affecting the cortical gray matter, the brain stem, and the thalamus. Meningeal exudates compound the clinical presentation by irritating the meninges and increasing ICP (Roos, 1999).
All arboviral encephalitis begins with a flu-like prodrome, but specific neurologic manifestations depend on the viral type. LaCrosse encephalitis, for example, may present with focal neuro-logic symptoms and seizures. Mortality is low but residual seizures may occur. A unique clinical feature of St. Louis encephalitis is SIADH with hyponatremia. The mortality rate is 10% to 20%. The clinical manifestations of Eastern equine encephalitis are acute and carry a high mortality rate of 50% to 75% (Roos, 1999). Although the clinical manifestations of Western equine encephalitis are nonspecific, the morbidity rate is high.
Neuroimaging is not useful in diagnosing many types of en-cephalitis. In Eastern equine encephalitis, however, CT scan and MRI may reveal lesions in the basal ganglia and thalamus (Roos, 1999). The CSF analysis shows a normal glucose level, elevated protein level, and polymorphonuclear leukocytic pleocytosis. St. Louis, Eastern equine, and Western equine encephalitis viruses are rarely isolated in the CSF (Roos, 1999).
The age of the patient is important information in making a specific viral diagnosis. La Crosse virus encephalitis is the most common pediatric arboviral encephalitis. St. Louis encephalitis affects adults over 50 years of age; Eastern equine encephalitis is not age-specific (Roos, 1999). Western equine encephalitis can present as pediatric encephalitis but is less prevalent.
There is no specific medication for arboviral encephalitis. Medical management is aimed at controlling seizures and increased ICP (Roos, 1999)
If the patient is very ill, hospitalization may be required. The nurse carefully assesses neurologic status and identifies improve-ment or deterioration in the patient’s condition. Injury preven-tion is key in light of the potential for falls or seizures. Arboviral encephalitis may result in death or life-long residual health issues. The family will need support and teaching to cope with these outcomes.
Public education addressing the prevention of arboviral en-cephalitis is a key nursing role. Clothing that provides coverage and insect repellents should be used in high-risk areas. Community mosquito control is advocated.
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