sealPurdue News

March 6, 2003

Vaccinate now to prevent equine West Nile virus

WEST LAFAYETTE, Ind. – Though winter lingers, Purdue University experts say it's time for people to vaccinate horses against West Nile virus and to implement plans to prevent themselves and their animals from being bitten by disease-carrying mosquitoes.

In 2002 West Nile virus spread across the country at an alarming rate, infecting 4,017 people in 40 states and resulting in 274 deaths, according to Centers for Disease Control and Prevention (CDC) statistics. During the same period, 14,717 horses contracted the illness with a fatality rate of about 25 percent, according to the U.S. Department of Agriculture's Animal and Plant Health Inspection Service (APHIS). The agencies are still collecting data on totals for the past calendar year.

"It's important to start vaccinating horses now so that they have full immunity when the first mosquitoes appear," said Michel Levy, associate professor of large animal medicine. "This is especially important if the horse is receiving the vaccine for the first time, because initially it must be given in two doses, three to six weeks apart."

Even for an animal immunized with the initial two shots late last summer or in the fall, it's best to administer the annual booster now to provide as much immunity as possible through the entire mosquito season, said Levy, who headed the Indiana Governor's West Nile Virus Task Force. No vaccine is available for people or other animals.

Ensuring a horse has full protection as long as possible is paramount because the types of mosquitoes most likely to spread West Nile virus are most prevalent from the second half of the summer through the fall, said Ralph Williams, a Purdue entomology professor who also is a member of the governor's task force. Mosquitoes early in the spring usually are rainwater, or nuisance, mosquitoes.

"When the mosquitoes develop depends on temperature and moisture," Williams said. "With very wet conditions followed by drought, mosquitoes still can be a big problem."

This is because some stagnant standing water where mosquitoes breed remains in catch basins, containers, poor drainage areas and creek beds, and not enough rain falls to flush it out, he said. With this winter's near record snowfall, the ground will be water saturated in the spring, indicating a good chance for large numbers of mosquitoes.

Levy said in the Midwest, even with optimum mosquito-breeding weather, only an annual booster is needed after the initial two shots. In other areas that have more mosquitoes for a longer period of time, such as Texas and Florida, a West Nile vaccine booster every six months is probably a good idea. Either way, he said, giving the vaccine twice a year is not harmful.

Last year some horses contracted the illness after they had the first of the initial two-shot series and some even after the second shot, Levy said. It takes a couple of weeks after the second shot is administered for the vaccine to be fully effective. However, he said that no vaccine has a 100 percent guarantee of preventing any illness.

A few horses had a reaction to the West Nile vaccine, but this is unusual, Levy said. Although there is no way to predict which horses will develop problems after the immunization, the ones that usually do have experienced an adverse reaction to other shots in the past.

Veterinarians will stay with a horse after vaccinations to make sure it doesn't have an immediate allergic reaction, which is manifested by restlessness, excitement, sweating, salivation, diarrhea or collapse.

Although West Nile virus cases typically occur later in mosquito season, Williams said it's important to protect both people and animals from the bites of the blood-sucking insect at all times. The mosquitoes that appear early in the season spread other diseases, including heartworm in dogs. The same species that spread West Nile also carry St. Louis encephalitis, while other types carry Eastern, Western and Venezuelan equine encephalitis, and malaria, in various parts of the world.

Preventative measures are important to protect against bites, Williams said. This includes disposing of, emptying and/or cleaning anything that can hold standing water, including livestock watering troughs, ditches, puddles, birdbaths, rain gutters, buckets, old tires, ponds and swimming pools.

People also should use repellents with N,N-diethyl-meta-toluamide (DEET) and carefully follow the instructions on the label, Williams said. When using any repellent, make sure that it's registered with the U.S. Environmental Protection Agency. Always seek medical attention immediately if someone experiences an adverse reaction to any repellent.

Purdue experts recommend that human anti-insect products not be used on animals because it could make them ill. Special repellents are available for horses, but nothing is available for dogs and cats.

Wearing light-colored clothing and long-sleeve shirts, long pants, socks and hats will help prevent mosquito bites on people. Horses can be covered with light-colored, lightweight or netted sheets to help keep bugs away.

House pets should be kept away from mosquito-infested areas as much as possible. Although mosquitoes can bite any time, staying inside during times they're most likely to bite (dawn and dusk) is a good preventative measure for people, house pets and horses.

West Nile Virus is a flavivirus commonly found in Africa, West Asia and the Middle East. Closely related to St. Louis encephalitis virus, West Nile was first identified in the United States in New York City and Long Island in 1999.

Mosquitoes transmit West Nile by biting infected birds, then biting people or horses. More than 110 species of birds have been identified as being susceptible to the disease.

When a mosquito bites, virus-laden saliva flows 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 illness may develop. Some people and horses test positive for the disease but never become ill.

It's important to get prompt and proper diagnosis both for people and horses so the appropriate treatment can begin as early as possible. Doctors and veterinarians can detect exposure to West Nile through a blood test.

Severe cases of West Nile virus cause encephalitis, or swelling of the spinal cord and brain, and can lead to permanent neurological damage or death in people and horses. Most people infected with the virus won't show symptoms, but if symptoms do develop they will include fever, headache, body aches, skin rash and swollen lymph glands, according to the CDC. Horses initially exhibit an abnormal, wobbly, unsteady gait due to loss of muscle control, lethargy, and later, partial paralysis; however, their body temperature usually remains normal.

As with many diseases in people and animals, the elderly, very young and those with compromised immune systems are most like to develop West Nile virus, either in its mild or more severe forms.

At least 14 mosquito species of the 50 found in Indiana have been identified as West Nile virus carriers.

In 2002, Illinois had the highest number of human West Nile cases and deaths, 835 and 54, respectively. Indiana was fifth in number of cases and seventh in number of deaths, with 287 and eight, respectively. The largest number of equine cases for the year was 1,597 in Texas, while Indiana was eighth with 703 diagnosed cases, according to APHIS.

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

Sources Michel Levy, (765) 494-8548,

Ralph Williams, (765) 494-4560,

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
Centers for Disease Control and Prevention West Nile virus information
U.S. Geological Survey West Nile virus information
Indiana Animal Disease Diagnostic Laboratory
Purdue University Department of Veterinary Pathobiology
Indiana Board of Animal Health

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