St. Louis Encephalitis

St. Louis encephalitis is a viral disease that is spread to humans through the bite of infected mosquitoes. The virus sometimes causes a severe febrile illness that may affect the brain. Symptoms include severe headache, muscle weakness, tremors and altered mental status. St. Louis encephalitis is found throughout much of the United States, as well as parts of Canada, the Caribbean, and South America. The virus was named for the city where it was discovered in 1933.

Electron micrograph of the SLE virus inside a mosquito salivary gland. Source: Wikimedia Commons.

Types

St. Louis encephalitis (SLE) is caused by a Flavivirus, which is a family of viruses that contain Ribonucleic acid (RNA), as opposed to Deoxyribonucleic acid (DNA), as genetic material. West Nile virus, Murray Valley encephalitis virus, Japanese encephalitis virus, dengue virus and yellow fever virus are other types of flaviviruses. Several of these viruses have similar structures and cause similar symptoms of infection. The illnesses can be hard to distinguish from one another because of this structural similarity. Unlike some other similar viral encephalitides in the United States, SLE does not affect horses.

St. Louis encephalitis virus is found throughout the United States. Symptoms and outcomes of infection vary from region to region. The SLE virus east of the Mississippi river has caused severe epidemics, and historically, of all the SLE outbreaks, it has caused the most deaths.

Symptoms

Infection is usually subtle, and rarely (less than 1 in every 100 cases) causes symptoms. The incubation period (the period between exposure to virus and developing symptoms) is between 5 and 15 days. ^[1].

St. Louis encephalitis usually occurs in two stages: the initial viral prodrome phase and the encephalitis stage.

Prodromal SLE

In the former stage, the illness affects several body systems. These are some symptoms of prodromal SLE:

• Fever (may be very high, up to 41 degrees Celsius or 104 degrees Fahrenheit)
• Headache (may be severe and associated with neck stiffness, confusion, pain in response to light)
• Generalized malaise (non-specific feeling of being unwell)
• Muscle and joint aches
• Cough, sore throat or urinary symptoms

Encephalitis

In this phase the virus affects the brain. In adults and children, often only the surface of the brain is affected. In this case, headache, neck stiffness and confusion may be the worst features.

Other symptoms may include:

• Confusion, coma, and other signs of brain dysfunction in more severe cases (typically in older people) where the whole brain is affected.
• Tremor in the face, mouth and arms and legs, which can last for weeks after the fever has stopped
• Bell's palsy. This illness is characterized by weakness of the face muscles.
• Inability to speak or move some body parts
• Unsteady walking
• Seizures
• Loss of smell (rare)

Similar Diseases

St. Louis encephalitis often occurs in clusters, which are the hallmark of an outbreak. Physicians in the region of the outbreak are often alerted after the prodromal phase. The first cases, or index cases, are difficult to diagnose because infection can mimic other illnesses, such as the following:

• Other viral encephalitis (particularly West Nile virus)
• Stroke
• Heat stroke (the high fever and altered mental status may appear to be heat induced, particularly as the outbreaks are often in mid-summer)
• Drug overdose
• Viral or TB meningitis
• Herpes simplex virus encephalitis (although this is more likely to cause seizures).

Causes

St. Louis encephalitis is a mosquito-borne illness. The SLE virus is transmitted when a mosquito injects saliva during a bite (see Figure of SLE virus within mosquito salivary gland). The virus then spreads throughout the body by way of the lymph node system and the bloodstream. The virus divides actively throughout most of the body before spreading to the brain.

Infection spreads rapidly after the virus enters the brain. Weakness or faults in the blood brain barrier may make the elderly more susceptible to infections in the brain.

Diagnosis

St. Louis encephalitis is rare. Many tests are used to diagnosis SLE. Often many different tests are necessary because SLE mimics other illnesses.

Laboratory tests

• Blood tests are critical to diagnosing SLE. Blood tests such as blood count and liver and kidney function tests may show mild abnormalities. Proteins, small amounts of red blood cells and white cells may also be seen in the urine during urine tests.

• Cerebrospinal fluid analysis. The cerebrospinal fluid (CSF) is the fluid that surrounds the brain, and it carries the virus. It is sampled by way of a lumbar puncture. The characteristic findings are an increased pressure and a slightly elevated protein count. Cerebrospinal fluid sampling may distinguish SLE from bacterial meningitis.

Other tests

Electroencephalography

Electroencephalography (EEG) is a test for brain wave patterns, which detect abnormalities in brain function such as slowing of the brain waves and abnormal activity.

Imaging tests

Imaging tests may be performed. There is no obvious abnormality on imaging tests but tests such as a Computed Tomography (CT) scan and Magnetic resonance imaging (MRI) of the brain may be performed. These tests can often detect other illnesses that appear similar to SLE.

Treatment

No treatments exist that target the SLE virus. This means that all medical treatment is designed to support the patient through the illness and prevent complications. One drug under investigation to treat the virus is interferon alfa-2b, which modulates the immune system.^[2] A preliminary trial showed that paralysis and breathing difficulty lasted more than 1 week in 2 of 15 patients treated with the drug compared to 11 of 17 patients not treated. Symptoms persisted for more than two weeks in only one of the treated patients and in five of the non-treated patients. Much more study is needed, however, before interferon alpha-2b is ready for use.

Supportive treatments include electrolyte and fluid replacement, with water restriction, if necessary to prevent swelling of the brain. Care is taken to reduce the risk of developing pneumonia, pressure ulcers, deep vein thrombosis, stomach ulcers, and other infections. Mechanical ventilation is used if breathing stops.

Prevention

The Centers for Disease Control and Prevention has outlined several preventive measures ^[3]

• Use of insect repellent containing DEET or another EPA-registered active ingredient on exposed skin.
• Long sleeves and pants.
• Avoidance of outdoor activity when mosquitoes are active (Culex species mosquitoes are most active between dusk and dawn)
• Secure screens on windows and doors to keep mosquitoes out.

Removal of standing water in such places as wading pools, birdbaths, tires, flower pots, buckets and barrels eliminate mosquito breeding grounds.

There is no available vaccine against SLE virus. In the USA, some states and counties perform surveillance for the SLE virus using chickens or mosquito collections for virus testing. This is to watch for patterns of spread that suggest an outbreak is imminent.

Risks

St. Louis encephalitis is prevalent throughout almost all of the 48 contiguous states in the USA (see Figure). The most important risk factor for the development of symptomatic encephalitis is age, with elderly persons at highest risk. St. Louis encephalitis develops in about 1 in 50 people over the age of 70 years who are infected, and in about 1 in 1,000 children. Consequently, SLE is very rare in children.

Source:Centers for Disease Control

Prognosis

Death occurs in 5%-15% of SLE cases ^[4]. Usually death occurs near the start of the illness, often within the first two weeks of infection. Typically death follows coma and respiratory depression. Case-fatality rates adjusted for age are higher in the eastern than in the western United States, maybe because of different viral strains in the two regions.

Death in the first two weeks of infection is caused by direct injury to the brain. Deaths occurring later are due to complications of hospitalization, especially bronchopneumonia and pulmonary embolism (clot in the lungs). A very high fever, convulsions, old age, or depression worsens the outcome of infection.

Most patients, particularly the elderly, take a long time to recover from SLE and may never return to their previous level of functioning. The recovery period can be complicated by persistent headaches, anxiety, irritable behavior, poor memory, tremor, and dizziness.

References

1. ↑ Centers for Disease Control and Prevention: St. Louis Encephalitis
2. ↑ Rahal JJ, Anderson J, Rosenberg C, Reagan T, Thompson LL. Effect of interferon-alpha2b therapy on St. Louis viral meningoencephalitis: clinical and laboratory results of a pilot study. Abstract. J Infect Dis. 2004 Sep 15;190(6):1084-7.
3. ↑ Centers for Disease Control and Prevention: St. Louis Encephalitis
4. ↑ Centers for Disease Control and Prevention: St. Louis Encephalitis