This year West Nile Virus (WNV) joins St. Louis (SLE) and Eastern Equine Encephalitis (EEE) among Southwest Florida’s potential mosquito-related threats. Statewide, as of the end of August, four cases of West Nile Virus had been reported. After steadily progressing throughout the northern part of Florida, the virus unexpectedly jumped to Monroe County in the southernmost part of the peninsula. But, according to acting Florida State Epidemiologist Dr. Steven Wiersma, it’s not unexpected. "Although it might seem unusual that West Nile virus has now been found in the extreme southern part of Florida, this is consistent with migratory bird patterns."

All of the above forms of encephalitis are "arboviruses," meaning that the infecting agents are transmitted by insects. The disease naturally occurs in song birds, normally not harming them (except in the case of West Nile, which is fatal to crows and bluejays). The mourning dove, bluejay, wood thrush, titmouse, chickadee, catbird and cardinal all have been known to have high incidences of these viruses. Basically, the virus’ route to human infection can be stated as "mosquito bites infected birds, then bites man." An overview of these diseases follows:

Basically, the virus’ route to human infection can be stated as "mosquito bites infected birds, then bites man."

West Nile Encephalitis
West Nile encephalitis was first documented in the United States in 1999, when 62 cases of severe disease, including 7 deaths, occurred in the New York area. In 2000, 21 cases were reported, including 2 deaths in the New York City area. To date in 2001 there has been a single death, in Atlanta, Georgia. The Centers for Disease Control and Prevention (CDC) says that because of the continued expansion of this virus, West Nile is now considered to be permanently established in the United States.

The threat this virus holds for the general public should be kept in proper perspective: According to the CDC, even in areas where mosquitoes do carry the virus, very few mosquitoes—much less than 1%—are infected. And if the mosquito is infected, less than 1% of people who are bitten become infected and become very sick. Thus, the chances that you will become severely ill from any one mosquito bite are extremely small. Further, West Nile virus isn’t transmitted from person-to-person. It should be kept in mind, though, that persons older than 50 years have the highest risk of severe disease.

When the sickness does strike, it follows transmission (bite) by an infected mosquito of the Culex species, which in turn has already bitten an infected bird or animal. The virus multiplies in the person's blood system and crosses the blood-brain barrier to reach the brain, where it interferes with normal central nervous system functioning and causes inflammation of brain tissue, or encephalitis.

Precautions that you can take against West Nile Virus are the same as for all mosquito-borne diseases (see below).

Eastern Equine Encephalitis
"Eastern equine encephalitis, or EEE, is a comparatively rare disease in humans," states Robert South, Ph.D., epidemiologist at the Lee County Health Department. 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.

Lee County Mosquito Control keeps 18 flocks of chickens distributed throughout our county, each consisting of 6 birds, referred to as "sentinel flocks."

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 the 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.

As of late August there had been two confirmed cases of Eastern equine encephalitis (EEE) in Florida.

Saint Louis Encephalitis
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.

Sentinel Chickens
Lee County Mosquito Control keeps 18 flocks of chickens distributed throughout our 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 WNV, 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.