The disease affects the central nervous system, causing inflammation in the brain and spinal cord. It is deadly in both horses and humans; however, only the Venezuelan variety can be transmitted from horses to humans via the bite of a mosquito. With the Eastern and Western varieties, the usual mode of transmission to either horse or human is from an infected bird or rodent via the bite of a mosquito.

The first symptoms appear within a few days after exposure. A sick horse displays a high fever, excessive depression, and a rapid heart rate. He may also refuse to eat — always a sure sign that something is wrong in a horse. As the infection progresses, the horse will begin to display marked behavioral changes, such as unusual aggression, aimless circling, and pressing or leaning against walls. You may see noticeable muscle twitching or paralysis as well. Eventually, the horse starts to have convulsions and collapses. The death rate runs as high as 90 percent, which makes vaccination with regular boostering imperative. Vaccinate annually and sometimes twice annually in areas where mosquito season runs well into the fall.

Similar to EEE, WEE, and VEE, this mosquito-borne viral disease causes swelling of the brain and spinal cord with all its attendant neurological symptoms. Mosquitoes get the West Nile virus from infected birds and pass it on to other animals, including people. Although humans and horses alike may contract the virus, there is no evidence that an infected horse can transmit the disease to a person or to other animals.

Symptoms may include fever, stumbling, muscle incoordination, partial paralysis, appetite loss, head tilting, walking in circles, or walking blindly into things. Fortunately, a vaccine is available to protect horses from this deadly infection. Good mosquito control methods in and around your barn and pastures also help.

If you plan on hauling your horse into an area that has a heavy mosquito season, ask your veterinarian about boostering your horse's vaccine for added insurance, especially if some time has passed since he was last vaccinated.

Also called swamp fever, EIA is another nasty disease transmitted by biting insects, primarily large horse flies. However, there is no vaccine for it, and the disease is incurable and has a high mortality rate. Horses that recover from a bout with the disease can become carriers of it and spread it to other horses. They either must be euthanized or quarantined away from other horses for the rest of their lives. Symptoms may also become chronic. Fortunately, rigorously enforced testing protocols have been instrumental in reducing the incidence of this difficult disease.

Symptoms include fever, loss of appetite, weight loss, anemia, and muscle weakness. However, some horses can carry the virus without showing any signs of sickness. Because of this, strict rules have been put in place to govern the transport of horses from state to state and from country to country.

The so-called Coggins test requires the drawing of blood, which is tested for EIA antibodies. Although proof of a negative Coggins test is required for a horse to cross state lines and is also usually required at all horse shows and other events where horses congregate, this test has drawn a lot of controversy in the past. If a horse tests positive, most states require the horse to be either quarantined or destroyed, as it may be a carrier even though it never exhibits clinical signs of the disease. The controversy stems from this fact: Only a small percentage of all horses in the country get tested. The ones that do — typically those that travel for shows and other events and need to produce a negative Coggins test for admission to the event's grounds — are thought by some to be unfairly penalized.

Carried by ticks, Lyme disease has become a concern in humans, pets, and livestock. The use of tick-controlling fly sprays can help prevent this disease, particularly in spring when ticks are most voracious. The very tiny deer tick is the culprit in the spread of Lyme disease.