St. Louis Encephalitis Virus

St. Louis encephalitis virus may be transmitted through the bite of an infected mosquito. The virus has a mosquito-bird-mosquito transmission cycle, in which birds act as hosts, allowing the virus to replicate without producing symptoms. In this cycle, mosquitos of the Culex genus are the vectors which transmit the virus when they feed. Humans and animals are considered dead-end hosts that become infected, meaning they cannot transmit the virus to each other.

St. Louis encephalitis virus infection causes a range of signs and symptoms, including a flu-like prodrome with fever, headache, fatigue, body aches, and nausea. A small subset of people infected will go on to develop signs of central nervous system dysfunction. Those who do can develop encephalitis (i.e., inflammation of the brain), meningitis (i.e., inflammation of the meninges), stiff neck, neurologic dysfunction, flaccid paralysis, confusion, dizziness, agitation, tremors, and coma. Immunocompromised individuals and older adults are more at risk of developing severe complications. The fatality rate can vary anywhere between 5 to 15%, with the highest rate in those 70 years or older. Children and young adults are more likely to have a mild disease course.

The incubation period from infection to symptom onset is between 5 to 15 days, though many individuals who are infected may remain asymptomatic. A study indicated that the likely ratio of subclinical to clinical cases is 300:1, meaning that many people who are infected with the virus have minimal to no symptoms. 

Diagnosis of St. Louis encephalitis virus infection typically begins with blood work, such as a complete blood count (CBC), to assess for signs of infection (e.g., high lymphocyte count). Brain imaging, such as a CT scan or MRI, may be used to assess for signs of neurologic dysfunction. A lumbar puncture may also be necessary if meningeal signs, such as a stiff neck, are present. In cases of the SLE virus, a lumbar puncture with cytology may reveal a high opening pressure, lymphocytic pleocytosis, and an elevated cerebrospinal fluid (CSF) protein. An electroencephalography (EEG) can also be performed, which may reveal delta-wave activity with isolated spikes and diffuse slowing.

Once suspicion is high, targeted testing for the SLE virus may be performed. Serum and CSF testing can show the presence of the SLE virus, antigen, or nucleic acid using either a polymerase chain reaction (PCR) or immunohistochemistry. Immunoglobulin testing showing IgM antibodies using enzyme-linked immunosorbent assay (ELISA) to SLE virus may also indicate an SLE infection. If antibody titers show a four-fold increase, that can also indicate recent infection with the virus. 

There is no antiviral treatment available to treat St. Louis encephalitis. Treatment primarily involves supportive care with intravenous fluids and NSAIDs (e.g., ibuprofen) for pain and fever relief.