Maryland Department of Agriculture

Encephalitis is a viral disease transmitted by infected mosquitoes. It affects the central nervous system causing fever and can often be fatal. The youngest and oldest segments of the population are most susceptible. Many viruses cause encephalitis and occur in the United States every year. Two different encephalitides occur in Maryland: Eastern equine encephalitis (EEE) and St. Louis encephalitis (SLE). The former is fatal in 70% of infections while SLE has much higher survivorship. These viral diseases have three cycles. The first is the maintenance cycle in the wild host (birds); the second, an amplifying cycle in a susceptible domestic or wild host (domestic animals or birds); and the third may affect the human population. Usually, humans do not have enough of the virus in their blood to infect other mosquitoes or people.

St. Louis Encephalitis

This form of encephalitis is restricted to the Americas and is the most important mosquito-borne encephalitis in the USA. It is common in the urban areas of the Southeastern and Midwest U.S. with periodic epidemics occuring in the Eastern U.S. Symptoms include fever, headache and confusion. Periodic spells of vomiting, diarrhea and neck stiffness may be present. Drugs can be used to relief symptoms but cannot cure the disease. The mortality rate is about 3 to 20 percent and large epidemics occur. In 1933 there were approximately 1,000 cases in St. Louis with more than 200 deaths. From 1975 to 1976 more than 2,000 cases occurred in the U.S. affecting more than 30 states. In 1990, a Florida outbreak of 223 cases caused eleven deaths. The largest outbreak of St. Louis encephalitis in Maryland was in 1975 in the suburbs of Baltimore and Washington, D.C. with ten cases. No deaths occurred. The mosquito that transmits this disease in Maryland is the northern house mosquito, Culex pipiens. It breeds in polluted standing water and is common throughout the state. The songbird population is the reservoir for the disease and it is amplified by mosquito transmission from bird to bird. Many mosquitoes become infected from feeding on these birds and they can transmit SLE to humans. Sparrow populations are monitored for viremia. Transmission to humans is predicted by such surveillance techniques.

Eastern Equine Encephalitis

This form of encephalitis is the most deadly with mortality rates of up to 70%. We poorly understand the ecology of this disease. Outbreaks are unpredictable and small in human populations. A vaccine was developed but worked poorly. We do not know where the virus overwinters but we know that song birds become infected by mosquito bites and are part of the amplification cycle. Unlike St. Louis encephalitis, the mosquito infecting wild birds does not bite humans. Instead, salt marsh mosquitoes or those associated with freshwater cattail marshes become infected by taking blood from infected birds. These mosquitoes bite humans and the virus enters the blood stream. Humans are a "dead end" host because they do not develop a high viremia and cannot infect biting mosquitoes.

We derive the name of the virus from the range of its occurrence - in the Eastern U.S. - and because of equine (horses) susceptibility to the disease. Horse cases are common in most eastern states and the mortality rate is nearly 100% in unvaccinated animals. This virus has become problematic for emu farms becoming established in Maryland. (The emu is a large bird valued for its meat.) These birds are very sensitive to EEE and no vaccine exists for them. Many of these farms are on the Eastern Shore where EEE occurs nearly every year. Pheasant farming is an important industry in Maryland and these birds are highly susceptible. Large epidemics can occur among pheasants because they "peck" one another transmitting virus bird to bird. Such epidemics increase the potential for human infection. Infected flocks must be killed.

In Maryland, two distinct foci exist for EEE. One includes the lower Eastern Shore counties, the second is found at Fort Meade, Maryland along the swamps of the Patuxent River. These areas maintain populations of the bird to bird and bird to human vectors of this disease. Human cases occur infrequently and when they do they are almost exclusively on the Eastern Shore. The last human death in Maryland was in 1989 in the Dorchester County Town of Hurlock. However, horse, pheasant and emu cases are common but the number of cases yearly varies. The Maryland Department of Agriculture conducts extensive statewide surveys for EEE in mosquito populations. Using the collected data we concentrate control efforts in areas with infected mosquito populations to prevent EEE transmission to humans and domestic animals. EEE is found most often in mosquitoes from July to October.

E-Mail Jeannine Dorothy, Maryland Department of Agriculture Mosquito Control