sealPurdue News

August 23, 2002

Horses and humans unlikely to spread West Nile

WEST LAFAYETTE, Ind. – The high number of horses infected with West Nile virus is a concern, but not a human health threat, said a Purdue University disease expert.

No evidence exists that animal-to-animal or animal-to-human transmission of West Nile virus is possible, said Leon Thacker, director of the Indiana Animal Disease Diagnostic Laboratory at Purdue. It takes a mosquito and a bird to spread the disease.

Thacker said a mosquito biting an infected horse is unlikely to give West Nile virus to people or other animals because the amount of viral agent in a horse's bloodstream is too low to cause infection in another horse or human.

"The only way you can contract the disease is to be bitten by a mosquito that has bitten an infected bird or by handling an infected bird," Thacker said. "A horse can't give it to you. You can't give it to a horse. And a horse can't give it to another horse or to a pet.

"The virus doesn't multiply in people and horses the way it does in birds. It multiples rapidly in birds, so they have a very high concentration of the virus. But in people and horses, the virus remains at levels too low to readily pass it on."

Experts recommend that you wear gloves or call your animal control department if you must move a dead bird.

The best preventative for any mosquito-transmitted disease is to avoid the insects by draining standing water and cleaning any receptacles where water collects, such as birdbaths, old tires, water troughs, etc. These could become breeding grounds for the blood-sucking insects, Thacker said.

Experts also suggest using DEET-based repellents according to label instructions and wearing light-colored, long-sleeved shirts and long pants.

"For all practical purposes, the only species that show symptoms of or die from West Nile virus, are horses, humans and birds," Thacker said. As of Wednesday (8/21) four people and at least eight horses in Indiana had been diagnosed with West Nile virus in 2002.

People and horses are infected with West Nile virus when a mosquito sends its virus-laden saliva into the victim's bloodstream. According to the USDA, there is approximately a one- or two-week lag time between a bite by an infected insect and when the sickness may develop. Some people and horses test positive for the disease but never become ill.

Severe cases of the disease cause encephalitis, or swelling of the spinal cord and brain, and can lead to permanent neurological damage or death in people and in horses, making it important to get a prompt and proper diagnosis, Thacker said. A simple blood test can detect exposure to West Nile virus.

As with many diseases in people and animals, the elderly, very young and those with compromised immune systems are most like to develop severe cases of West Nile virus.

If symptoms develop in people they include fever, headache, body aches, skin rash and swollen lymph glands, according to the Centers for Disease Control and Prevention. No vaccine against West Nile virus currently exits for people, but the Food and Drug Administration this week approved drug testing trials on interferon as a possible treatment for the disease.

Although no cure exists for West Nile virus, it can be successfully treated with supportive measures – anti-inflammatory drugs, lots of fluids and keeping the patient quiet.

Disease-carrying mosquitoes can bite dogs and cats, but no evidence exists that they are at risk from West Nile virus, Thacker said. Dogs and cats have not exhibited symptoms of the disease and apparently have a natural immunity to the mosquito-borne illness.

However, owners should take precautions to keep mosquitoes and ticks away from their pets, he said. The insects can cause other health problems, such as heartworm and Lyme disease. Insect repellents designed especially for dogs and cats are available.

Thacker said that because horses can contract West Nile virus, it's vital that owners have their horses vaccinated and use the repellent sprays and wipes designed specifically for the animals.

"My counterpart in Louisiana says the West Nile vaccine is highly effective," Thacker said.

The initial immunization consists of two shots given three to six weeks apart, followed by annual boosters.

Horses stricken with the illness exhibit an abnormal, wobbly, unsteady gait due to loss of muscle control, lethargy, and later, partial paralysis; however, their body temperature usually remains normal, according to the USDA Animal and Plant Health Inspection Service.

Writer: Susan A. Steeves, (765) 496-7481,

Source: Leon Thacker, (765) 494-7460,

Ag Communications: (765) 494-2722; Beth Forbes,;

Related Web sites:
Purdue Department of Entomology
Purdue Extension resources for West Nile virus
Animal and Plant Health Inspection Service
Indiana Department of Health
U.S. Geological Survey

Purdue News Service: (765) 494-2096;

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