St. Louis Encephalitis Virus · What It Is, Transmission, Signs, Symptoms, Diagnosis, and More

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 amplifying 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 or to other animal species. 

Strategies to prevent virus transmission include utilizing mosquito repellent (e.g., DEET, permethrin), wearing protective clothing, and avoiding the outdoors during dawn and dusk. Using screens on windows or mosquito nets over beds can prevent mosquito bites while indoors. Towns can also eliminate standing water sources, which is where mosquitoes tend to breed, and spray insecticides into the air. 

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), with signs including 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 20%, with the highest rate in those 70 years or older. Children and young adults are more likely to have a mild disease course. Between 5 and 10% of individuals show persistent neurological symptoms, including gait and speech disturbances and behavioral problems.   

The incubation period from infection to symptom onset is between 4 to 14 days, though many individuals who are infected may remain asymptomatic. A study indicated that the likely ratio of subclinical (i.e., not presenting detectable signs and symptoms) 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 a clinical evaluation, including a review of signs and symptoms and medical history, with particular attention to risk factors for potential exposure to the virus (e.g., living or traveling to an area where it is known to circulate).  Blood work, such as a complete blood count (CBC), is performed to assess for signs of infection (e.g., high lymphocyte count).  

Brain imaging, such as computed tomography (CT) scan or magnetic resonance imaging (MRI), may be used to assess for signs of neurologic dysfunction. A lumbar puncture, performed to collect a cerebrospinal fluid (CSF) sample, may also be necessary if meningeal signs, such as a stiff neck, are present. If SLE virus is present, CSF analysis may reveal a high opening pressure, increased lymphocytes count (i.e., lymphocytic pleocytosis), and an elevated cerebrospinal fluid  protein. An electroencephalography (EEG), a test recording the brains’ electrical activity, can also be performed and may reveal diffuse slowing. 

Once suspicion is high, targeted testing for the SLE virus may be performed. Serologic testing - used to detect SLE virus-specific antibodies in the blood or CSF - is the main method for diagnosing SLE virus encephalitis. Immunoassays such as enzyme-linked immunosorbent assay (ELISA) or indirect immunofluorescence assay (IFA) are typically employed to detect IgM antibodies. If IgM results are positive, more specific confirmatory tests, such as plaque reduction neutralization tests (PRNT), are performed to identify neutralizing antibodies against SLE virus. In the early stages of infection, real-time polymerase chain reaction (RT-PCR) may also be used on serum or CSF samples to directly detect viral RNA. If IgG 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.