Any Florida horse from Pensacola to Lake Okeechobee that is located near a freshwater swamp or marsh is at greater risk to EEE. On the average there are 50 to 75 horse cases each year in the State. Over 90% of these animals would die if left alone. Mercifully, once a diagnosis is confirmed, most animals are destroyed. There are typically only one or two human cases each year in Florida. Unfortunately, many are fatal; and of those that do survive, nearly all have some residual effects, particularly mental impairment. There is no specific treatment or cure for EEE in man or horses, but there is a vaccine against the disease in horses.

THE CAUSE:

EEE is caused by a virus called Eastern Equine Encephalomyelitis. The virus is normally found only in wild song birds and in mosquitoes that live in and around wooded swamps; not just any swamp, but a swamp where there is a certain species of mosquito. The EEE mosquito or "black-swamp-mosquito" has the scientific name Culiseta melanura (cuela-see-ta mel-ah-nur-ah). The virus does not usually cause disease in wild birds, although it may cause a temperature, and perhaps a minor illness similar to a cold or flu in a human. It does not affect the mosquitoes in any way.

MOSQUITO CULPRITS:

Culiseta melanura, which translates to "curly black hairs", is indeed a dark mosquito that has a very long proboscis or probe that it uses to draw blood from its hosts. C. melanura has very specific breeding requirements. It occurs in most states east and a few states west of the Mississippi River. The larvae are found only in the underwater root systems of deciduous trees that grow in swamps. Fortunately for us, they get their blood from song birds; rarely do they bite humans or other mammals. And, since C. melanura flies no further than five miles from its breeding sites, most cases of EEE occur within five miles of these swamps.

HOW DOES IT HAPPEN?:

Well, if C. melanura doesn't bite mammals, how do horses and humans get the disease? The entire story is unknown, but enough facts have been gathered to construct one Florida Model of the life history:

During warm months when C. melanura breeds, there are usually plenty of small birds around for adult mosquitoes to feed on. When female mosquitoes (males do not bite) feed on an infected bird, they pick up the virus. Later, when the mosquito blood feeds on another bird, the virus is transmitted to the new bird. The mosquito remains infected for life and can transmit the virus to all birds it feeds on.

After the mosquito blood feeds, the bird becomes infected and the virus begins reproducing in the bird. In a few days, and for only one or two days, the blood of the bird contains enough virus to infect other mosquitoes that bite it. The bird quickly recovers from the infection and develops immunity. As far as we know, the immunity keeps the birds from becoming infected again. Only newly infected birds can serve as a source of virus for mosquitoes. Therefore, the mosquito seems the most important host as far as virus survival is concerned.

Since C. melanura does not bite people, the key to human and horse infection is tied to the short period when birds have high concentrations of virus in their blood. When other mosquitoes feed on infected birds, they can become infected as well. It is these "secondary" mosquito species that carry the virus to other vertebrate hosts, including horses and humans. For these secondary mosquitoes to transmit the virus from birds to humans, an individual mosquito must successfully blood feed on both groups of animals. Not all mosquito species do that, C. melanura for example.

There is another species of mosquito that is most often associated with outbreaks of EEE in horses and humans. This mosquito, the "salt and pepper mosquito", has the scientific name Coquillettidia perturbans (Coke-qua-la-tid-e ah purr-tur-bans) or "cokes" for short. This is a large black and white mosquito that looks for blood around dusk. Cokes have a geographic distribution similar to C. melanura, but rather than breed in wooded swamps, they breed in cattail or grassy marshes that have a mucky bottom. These types of marshes are often next to the swamps that produce C. melanura.

There may be other mosquitoes, particularly those in the genus Aedes (a-e-dees), that also feed both on birds and mammals and thus could possibly transmit EEE. The species of Aedes involved differ from area to area. Cokes are the only other mosquitoes found throughout the range of EE in the United States. While cokes and Aedes can fly more than five miles, EEE generally does not occur in areas where there are not C. melanura.

Cokes and Aedes can, and do, take blood meals from a variety of other domestic and wild animals; such as cattle, dogs, cats, squirrels, raccoons, and deer. Fortunately, these animals are resistant to the virus and do not develop EEE. Of course, mosquitoes also can blood feed on birds other than wild song birds. Birds that are not native to the U.S., such as ring-necked pheasants and starlings, and some native birds, such as whooping cranes and sandhill cranes can become ill, and many die. Die-off's of exotic birds, particularly pheasants, often precede outbreaks of EEE in humans and horses. Unlike wild birds, infected horses and humans normally do not develop high enough concentrations of virus in their blood to infect mosquitoes. This means that they are not good hosts for the survival of the virus. Once the animal recovers from the infection, or dies, the virus in that animal also dies. Animals like this are called "dead-end hosts", not because they die, but because the virus cannot be perpetuated. Thus, EEE in non-bird hosts is not part of the virus' normal life cycle. It's a fatal mistake for humans, horses and virus alike.

EEE in Florida

EEE has been reported from sentinel chickens throughout Florida. Mosquito control districts, state and local health departments, and the Centers for Disease Control may test dead birds, sentinel birds, mosquitoes, and horses for EEE. Tests that are confirmed positive are reported by the Florida Department of Health. Authorities will then make decisions about what strategies to use to reduce risk of exposure to EEE based on scientific information about the virus and vectors involved, the risk to humans, and local or regional environmental conditions.

KEEPING A LOOKOUT:

Many mosquito control programs in Florida use "sentinel chickens" to alert them when the threat of EEE is greatest. Chickens are penned in areas where there are mosquitoes. Every two weeks, a blood sample is taken from some of the birds and tested for antibodies to the virus. Chickens seldom become ill when infected with EEE virus. Sentinels were first established throughout Florida in 1978 following a 1977 epidemic of SLE. Though chicken flocks were established to monitor SLE, they also have been used to some extent for EEE, discussed elsewhere.

While sentinels do not forecast SLE or EEE outbreaks, they do provide some limited information on virus activity. Still, the absence of virus activity in chickens does not necessarily mean there is no virus activity. Consequently, sentinel chickens can give a false sense of security regarding EEE or SLE activity. A potential research benefit of sentinel flocks is their abilitabilityy to confirm via statistical correlation that a developing encephalitis epidemic is probably due to SLE or EEE virus infection. Sentinel chickens have provided us with seasonal and annual patterns of virus activity shown below. Since the mosquito that infected a sentinal chicken is usually long dead before the blood test results are received from the testing laboratory, their use in guiding mosquito control operations is limited. Use of sentinel flocks has been on the decline among mosquito control programs since 1983. Even the Florida Department of Health and Rehabilitative Services, which set up the system, has reduced its support in recent years.

The best system to monitor for EEE is to: 1) monitor the population levels of the important mosquitoes, C. melanura, cokes, and man-biting Aedes, 2) test these same species for virus, 3) test native song birds for antibodies to EEE virus, and 4) establish a system to rapidly detect horse cases near the swamps. Since EEE activity in a county during any given year can be limited to a single swamp among many, it is necessary to monitor these factors in all C. melanura breeding swamps.

Florida mosquito control programs typically have a mosquito monitoring system in residential areas. Where there is a potential for EEE, the system should extend to the swamps and marshes. Testing mosquitoes and wild birds for virus activity is more expensive than sentinel chickens, but the test results are more reliable for determining the potential for an EEE outbreak. Unfortunately, today's testing methods are not readily available or cost effective for mosquito control programs. Frequently, horse cases occur before human cases and are used to indicate the spill-over of virus transmission from the bird-mosquito-bird cycle to the bird-mosquito-mammal pathway.

How Can the Risk of Exposure to EEE be Reduced?

There is a vaccine against EEE for horses; and every horse in the state should be vaccinated and have at least twice yearly boosters. Unfortunately, there is no vaccine that can protect humans against an EEE virus infection, and like all virus infections, EEE cannot be cured. Prevention must, therefore, concentrate on minimizing contact between man and transmitting mosquitoes. Pay attention to medical and veterinary alerts and follow the specific recommendations. Medical alerts will be posted on the FMEL Encephalitis Information System at http://eis.ifas.ufl.edu.

Individuals can help themselves the most by avoiding being outdoors at night, especially around sunrise and sunset when mosquitoes seem most active. If they must be out, they should wear long sleeved shirts and long trousers, and apply insect repellent. Mosquito control agencies may help by suppressing mosquito populations with various control methods. This means controlling cokes, some Aedes, and particularly, C. melanura. Without C. melanura, there can be no EEE in horses or humans. It is as simple as that.

Larval control is indicated only for prevention, not as a response to an outbreak. Unfortunately, it is usually not practical to spray swamps frequently for C. melanura and coke larva, a practice normally used to control other mosquito species in more residential habitats. Concerted efforts to reduce C. melanura mosquito populations are usually only made after the State Department of Health and Rehabilitative Services declares an EEE alert. Aborting the threat of an EEE outbreak, or an outbreak itself, is most easily done by an aerial application of insecticide to kill adult C. melanura, cokes, and man-biting Aedes. If done properly, only one or two well-timed aerial applications are needed to control the problem for a year. Where aerial application is not possible, less effective ground-based adulticiding techniques may be used to control immigrating carrier mosquitoes.

Personal Protection

Personal protection against biting arthropods, particularly when they are infected with dangerous pathogens, remains one of the most important ways to avoid disease. Avoid mosquitoes. Make sure screens are in good repair to prevent mosquitoes from entering homes. If you must enter areas where there is a threat of encountering infected mosquitoes, wear protective clothing.

Finally, use a personal insect repellent that provides a reasonable Complete Protection Time (CPT). The CPT is the total time following repellent application that the treated individual will remain bite free. For example, under normal conditions the CPT for a 5% formulation of DEET (diethyl toluamide, presently the most effective insect repellent) is approximately 2 hours. The CPT for a 24% DEET formulation is more than 4 hours. For more information on repellents, refer to the University of Florida/IFAS Fact Sheet ENY-671 "Mosquito Repellents".

• Avoid exposure to mosquitoes - stay indoors during peak biting time.

• If you must be outside during peak biting time, wear long sleeves and pants.

• Wear mosquito repellents when outside during peak biting time. Use mosquito repellents containing DEET. Be sure to follow the directions on the label

• Make sure window and door screens are in good repair to prevent mosquitoes from entering homes.

• Remove unnecessary sources of water outside the home that may provide breeding places for mosquitoes.

• Flush out the water in bird baths and outdoor pet dishes every 3 - 4 days.

• Remove leaf litter, standing water and debris from roof gutters and boat covers.

• For more information see: The Florida Medical Entomology Lab's Fact Sheet on Personal Protection (ENY-671 - Mosquito Repellents)

Public Protection

Fortunately, the USA has some of the best mosquito and arthropod control programs in the world. Vector control and personal protection against vectors and the diseases they carry are the best way to avoid infection with vector-borne pathogens. Strategies that might be effective against Eastern Equine Encephalitis infection include: source reduction of mosquito breeding sites; focal applications of insecticides directed against adult and immature mosquitoes; Public Service Announcements to educate residents about the vector, the disease, and disease avoidance; tips to help prevent home-invasion by infected vectors; and information about the most effective means of personal protection. St. Louis and West Nile encephalitis epidemics and outbreaks in Florida have proved that an effective means of reducing human infection is to widely disseminate accurate information through the media to educate the public. An individual's first line of defense during a vector-borne disease emergency is knowledge and personal protection.

Vaccine for Horses

Florida Agriculture Commissioner Charles H. Bronson urges all equine owners to have their horses vaccinated against EEE and to be vigilant about receiving booster shots at regular intervals. For more information on this vaccine, contact your veterinarian or call the Florida Department of Agriculture and Consumer Services . Horses that have been vaccinated against Eastern Equine Encephalitis (EEE) are not protected from WNV.

Further information on Eastern Equine Encephalitis

For updates on the current situation on West Nile and other mosquito-borne diseases in Florida, visit the Florida Medical Entomology Laboratory's Encephalitis Information System to view current health alerts (http://eis.ifas.ufl.edu). Additionally, information is provided to assist readers in understanding the real risk of exposure to the mosquito-borne viruses in Florida.