by Michael Barnaby

It’s that time again – to arm ourselves and our families with mosquito repellant and information. Time to be aware of the unfortunate fact that, more than merely being pesky insects, mosquitoes can harbor serious, sometimes fatal illnesses. Among others, these include Eastern Equine and St. Louis encephalitis (the term refers to an acute inflammatory disease involving the brain and spinal cord).

Both of these forms of encephalitis are "arboviruses", meaning that the infecting agents, Eastern Equine encephalitis virus and St. Louis encephalitis virus, are transmitted by insects. The disease naturally occurs in song birds, normally not harming them. The mourning dove, blue jay, wood thrush, titmouse, chickadee, catbird and cardinal all have been known to have high incidences of the virus. Basically, the virus’s route to human infection can be stated as "mosquito bites infected birds, then bites man." But it can be more complicated - and interesting - than that.

Eastern equine encephalitis, or EEE, is a comparatively rare disease in humans. For the most part, its victims are horses; yet the virus must be transferred to the animal (and in rare cases to man) by a "secondary" mosquito, one which has bitten a song bird already infected by another mosquito species.

The primary culprit is an insect named Culiseta melanura, or black-swamp mosquito, whose larvae can only develop in the underground root systems of certain trees and cattails growing in swamps. As adults, they blood feed only on birds, not humans, flying no further than five miles from their breeding sites. Within this space, bitten birds can become infected with encephalitis virus. And only during a very short period – one or two days – is the bird’s viral concentration high enough to be transferred to a secondary type of mosquito that may come to dine. A further caveat – this secondary mosquito must be of a type which feeds on more than one species, for instance birds plus domestic or wild animals, and man. Human infection thus occurs when a black-swamp mosquito infects a bird, which during a very limited time period is bitten by a second type of mosquito, which then bites a human.

Horses infected with EEE almost always die within 48 to 72 hours after the onset of symptoms, but there is a veterinary vaccine available to prevent the disease. In man, most individuals infected with EEE have no symptoms and never even know they’ve been infected; others get only a mild flu-like illness with fever, headache, and sore throat. For people with infection of the central nervous system, a sudden fever and severe headache can be followed quickly by seizures and coma. About half of these patients die from the disease. Of those who survive, many suffer permanent brain damage and require lifetime institutional care.

Most years fewer than 10 cases in people are reported nationwide. First reported in Florida in the 1930’s, statistics show that between 1964 and 1996, a total of 153 human EEE cases were reported (2 cases in Florida in 1996). From Florida to New England, EEE grows in birds living in fresh water swamps,

Especially in the fall, SLE, or St. Louis encephalitis, becomes a concern. The culprit in SLE is Culex nigripalpus. First isolated in 1933 in St. Louis, Missouri, this variety of mosquito lays its eggs in standing water, such as in roadside ditches, old tires, and anywhere water can collect and become polluted. A night feeder, this species will feed on mammals (including humans), birds and reptiles. First biting an infected bird, culex then passes it to humans. The young and the active, generally, are at low risk for being bitten by culex nigripalpus, since this mosquito prefers a slow or non-moving target. The elderly, and those of us who like to sit outdoors in the evening are at much higher risk. Symptoms are usually mild and include fever, headache, tiredness, and dizziness. However, in its more severe form, the disease can cause convulsions and death.

Lee County Mosquito Control has 18 flocks of chickens distributed throughout Lee County, each consisting of 6 birds, referred to as "sentinel flocks." Chickens are used for many reasons – they’re inexpensive, easy to observe and test, and they themselves are unaffected by the illnesses. Their purpose is to monitor the present prevalence of EEE and SLE in the bird population. At weekly intervals blood samples are taken and analyzed, and from the weekly and cumulative information obtained, health officials can attempt to determine the risk of illness to the public.  Visit Lee County Mosquito Control at http://www.lcmcd.org.