Eastern equine encephalomyelitis (EEE)
Description, Causes and Risk Factors:
A form of mosquito-borne equine encephalomyelitis seen in the eastern U.S. and caused by the eastern equine encephalomyelitis virus, a species of Alphavirus, which belongs to the family Togaviridae; initial fever and viremia are followed by signs of central nervous system involvement (excitement, then somnolence, paralysis, and death); the incidence of clinical infection in humans is low but case fatality may be high.
Eastern equine encephalomyelitis occurs in nature in a wide variety of songbirds. Blood samples from New Jersey birds, where most of the research on EEE has been done, indicate that the blue jay, tufted titmouse, chickadee, catbird and cardinal are most often infected. Although these birds do not develop the disease, they maintain high levels of the Eastern equine encephalomyelitis virus. They are considered reservoirs in the
Culiseta melanura is the primary mosquito species that transmits the virus from bird to bird. This mosquito inhabits marshes and wooded wetlands and rarely feeds on people or horses. Consequently, the Eastern equine encephalomyelitis virus normally exists in a rather secluded environment where it does not pose an immediate threat to horses or people.
These diseases are transmitted to horses and humans by mosquitoes that have fed on infected wild birds. The Eastern equine encephalomyelitis virus was first identified in mosquitoes in Louisiana in 1951 near Ponchatoula in Tangipahoa Parish. The virus is active in horses to some degree every year.
EEE virus attacks the central nervous system of its host. Unvaccinated horses are particularly susceptible to the infection. The disease appears within five days after mosquitoes transmit the virus to the horse. Onset of clinical signs of EEE are abrupt, and affected horses die within three days. Signs of Eastern equine encephalomyelitis in horses include fever; a sleepy appearance; some muscle twitches of the head, neck, shoulder and flank; and a weak, staggering gait. Affected animals are soon down, unable to stand.
Most persons infected with EEEV have no apparent illness. Severe cases of Eastern equine encephalomyelitis (involving encephalitis, an inflammation of the brain) begin with the sudden onset of headache
, high fever, chills, and vomiting. The illness may then progress into disorientation, seizures, or coma.
Eastern equine encephalomyelitis viruses can be isolated in a number of cell culture systems. The most commonly used cell cultures are primary chicken or duck embryo fibroblasts, continuous cell lines of African green monkey kidney (Vero), rabbit kidney (RK-13), or baby hamster kidney (BHK-21). Isolation is usually attempted in 25 cm2
cell culture flasks. Confluent cells are inoculated with 1.0 ml of tissue suspension. Following a 1-2-hour absorption period, maintenance medium is added. Cultures are incubated for 6-8 days, and one blind passage is made. EEE viruses will produce a cytopathic change in cell culture. Cultures that appear to be infected are frozen. The fluid from the thawed cultures is used for virus identification.
Antigen-capture ELISA has been developed for Eastern equine encephalomyelitis surveillance in mosquitoes. This can be used in countries that do not have facilities for virus isolation or RT-PCR. Immunohistochemical (IHC) procedures are very useful for diagnosis of EEE as they are carried out on fixed tissues. The envelope protein of EEEV is targeted in IHC. Necrotic and inflamed areas of the brain are examined. In EEEV infected horses, positive staining is observed most notably in neurons and associated dendritic processes.
Treatment consists of supportive care. Mechanical ventilation, as well as other measures, may be necessary in some cases. The efficacy of antiviral drugs such as ribavirin is currently unknown. Supportive care including fluid therapy, electrolytes, anticonvulsants, anti-inflammatory therapy and good nursing can be helpful.
Measures to prevent mosquito bites, including the use of repellants and protective clothing (i.e. long pants and long-sleeved shirts) can reduce the risk of infection. Permethrin can also be applied to clothing to discourage bites. Outdoor exposure should be limited at times when mosquitoes are active, especially during outbreaks.
NOTE: The above information is for educational purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition.
DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.