Mosquito Borne Illnesses

woman embracing with her Dalmatian dog outdoors

2010 has been the hottest year on record in the United States so far. The sweltering heat mixed with the wet summer days has increased mosquito activity. As mosquito season is still lurking, there are some important diseases associated with mosquitoes that can be transmitted to humans and pets that everyone needs to be aware of.

Heartworm disease most commonly affects dogs, however cats and humans are sometimes affected. Heartworm disease is caused by heartworms, which live in the blood vessel connecting the heart to the lungs. It is a life-threatening disease for dogs. Individuals are infected with the worm through the bite of a mosquito carrying the larvae of the worm. It can be prevented in dogs and cats with monthly pills or topical treatments. Once an individual is infected, treatment is very difficult and is risky.

“Treatment of heartworm disease is expensive and potentially dangerous,” explains Dr. Mark Stickney, clinical assistant professor at the Texas A&M College of Veterinary Medicine & Biomedical Sciences. “There are numerous side effects to treatments, for example the patient can develop blood clots. The best option is to take preventative measures and develop a monthly heartworm preventative schedule to give to your pets.”

The West Nile virus is another mosquito borne illness that is common among animals. It is spread when a mosquito bites a bird infected with the virus and then in turn bites another individual to spread the disease. It first appeared in the United States in New York City in 1999. Since then, it has spread throughout the United States. Horses are the most commonly affected animals. Humans and dogs are also affected but on much rarer occasions.

“The symptoms of the West Nile virus are similar in horses and humans,” notes Stickney. “Both victims develop neurologic symptoms that include stumbling, seizures, and inability to use limbs. When dogs are exposed to the virus their body normally does not show any outward reactions because their body usually fights it off. When they do develop the disease, dogs also show neurological signs.”

At this point there is no treatment for the West Nile virus. Scientists are currently working on a vaccine for humans.

One of the most dangerous mosquito borne illnesses that affects horses and humans is Eastern Equine Encephalitis (EEE). It affects the central nervous system and causes severe complications that may lead to death. This virus also originates from mosquitoes biting an infected bird and then passing that infection on to their victim.

EEE is also known as the “sleeping sickness” because its onset is very fast and is hard to diagnose. Symptoms of EEE in horses usually break through within five days of the infected mosquito bite. Initially, horses are depressed and quiet. They experience impaired vision, inability to swallow, and aimless wandering. As the virus strengthens the horse will start to exhibit paralysis, convulsions, and ultimately death. Death normally occurs after two to three days of the infected horse showing signs. Vaccines are available for horses and it is recommended that they get them yearly.

Most people who are exposed to EEE do not have any complications. The rare few who are affected incur severe symptoms. Initially they experience headaches, fever, chills, and vomiting. The symptoms may advance to disorientation, seizures, coma, or sometimes even death.

One can take preventive measures to avoid the occurrence of mosquitoes and mosquito borne illnesses.

“Avoid being outside from dusk until dawn during mosquito season when mosquitoes are most active,” explains Stickney. “Get rid of standing water. If you have a pond, lake, or tank on your property put mosquito dunks in the water to prevent mosquito eggs and larvae from developing. Don’t depend on flea and tick labeled repellants to ward off mosquitoes because your pet can still get bitten.”

Prior to mosquito season it is important to do a check up and mosquito proof houses. Fix or install window and door screens so that there are no leaks into the house. Make sure to remove areas or cover containers with standing water where mosquitoes lay eggs.

“Don’t assume that just because your cat or dog has long hair that mosquitoes won’t bite them because they will,” notes Stickney.  “If your pet is an inside animal they are also affected by mosquitoes because mosquitoes can force themselves inside too. The best method is to take preventative measures because with all these mosquito transmitted diseases an ounce of prevention is truly worth a pound of cure.”


Pet Talk is a service of the College of Veterinary Medicine & Biomedical Sciences, Texas A&M University. Stories can be viewed on the Web at Suggestions for future topics may be directed to

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