St. Louis Encephalitis
P K Ghatak, MD
St. Louis encephalitis virus is an indigenous Americas virus. Songbirds, finches, house sparrows and other species of birds harbor this virus and are immune. Birds are reservoirs of the virus and are the source of human infection. The virus is active over a wide territory from Northern Canada to Argentina. But the St. Louis virus (STLV) infection is limited to the warm months of August to early October in the North-East, and Mississippi River water-shed areas. The annual incidence of SLEV infection varies between 3 and 70 cases per 100, 000 population.
Fever starts abruptly 1 to 3 weeks after the mosquito bite. The majority of patients suffer only a few days of sore throat, aches, and fever. Elderly people with underlying diseases like diabetes, cancer, heart disease are susceptible to more serious illnesses. More seriously ill patients continue to be sick and have headaches, confusion, neck pain, and transient cranial nerve palsy. In children, convulsions are common. Severely affected patients lose consciousness and develop coma. Fatality rates among elderly men may run as high as 20 % in some years.
In 1933 SLEV was identified in St. Louis, Missouri. In that year, 1,000 cases of encephalitis were recorded. During 2014 -2015 a limited outbreak in Arizona followed by 2016-2017 in California occurred. Argentina, in 2005, had an outbreak in Córdoba.
SLEV is transmitted by several species of Culex mosquitoes
Detecting viral antigen by PCR in the CSF and or serum is diagnostic. ELISA antibody IgM and IgG test is also an accepted diagnostic method.
The gray matter is mostly affected. And the white matter is spared. The lesions dominated in basal ganglion and other midbrain ganglia and cerebellum, and lesions are also present in the cerebral cortex and spinal cord. About 30 % of encephalitis cases develop hyponatremia from Inappropriate antidiuretic hormone secretion (SIADH); and its complications – cerebral edema and Central pontine myelinolysis
Children when recovered from encephalitis may suffer from poor intellectual development later in life.
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