seal  Purdue News

September 5, 2003

Purdue experts say vigilance still needed for West Nile virus

WEST LAFAYETTE, Ind. – Early fall is prime time for West Nile virus cases, according to Purdue University experts who advise continued vigilance in avoiding mosquito bites.

vaccination is important
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The mosquitoes that transmit the illness are most prevalent from mid-August through mid-October when it's still warm and fairly dry. Last year Indiana and surrounding states recorded the most West Nile virus cases in people and horses during this period. The Midwest also recorded the most cases in the country during all of 2002.

"The highest number of cases always occur this time of year, and because we had so many cases last year in Indiana, we're taking an extra precaution by recommending that horses be vaccinated twice this year," said Bill Hope, a Purdue equine community practice veterinarian. "It remains to be seen whether we will have as many cases this year because our horse population is now highly vaccinated against West Nile."

Hope and Michel Levy, an equine clinic veterinarian, said the annual shot, which was first approved for use in the summer of 2001, seems to be effective in Indiana.

"I don't know of any horses that received the initial two shots and the annual booster the following year that have contracted West Nile here in Indiana," Levy said.

The first and second shots are given three to six weeks apart. It takes about two weeks after the second shot before the vaccine is fully effective. After that horses must receive at least one booster shot annually depending on climate, mosquito population and the horse's health. In both horses and humans, those affected most severely by the virus are the elderly and those with compromised immune systems.

"Most of the Indiana horse population has had four or five shots of the vaccine by now, including the two initial shots," Hope said. "This means they have built up a high level of immunity.

"No vaccine is 100 percent effective because every individual has a different immune system. But West Nile vaccine still seems at least 90 percent effective."

As of Sept. 3, Indiana had three confirmed equine cases of the illness and six human cases, according to state agencies. Last year at this time, Indiana's West Nile virus cases totaled 76 confirmed cases in horses and nine probable human cases, including one death. By Dec. 31, 2002, the state had confirmed 722 equine cases with an approximate fatality rate of 25 percent, and 293 human cases with 11 deaths.

The United States recorded 14,717 equine cases and 4,156 human cases from 44 states in 2002. This year, as of Sept. 3 there have been 1,856 human cases and 37 deaths, the majority in the western Midwest and the Rocky Mountain states.

Purdue entomologist Ralph Williams said states such as Indiana may have escaped an onslaught of West Nile cases so far because the summer has been wetter and cooler than normal. Additionally, people have heeded the warnings to vaccinate their horses and protect themselves with insect repellants and proper clothing.

"This is the time of year when the Culex mosquito is around," Williams said. "But time will tell how many West Nile cases we'll have this year. The weekend rains flushed out the Culex breeding grounds. If it gets warm again and areas where the insects normally breed still have water, it's possible the number of cases could jump."

Williams and Hope said people should remember that mosquitoes carry other zoonotic diseases – illnesses that both humans and animals can catch.

One of those diseases, eastern equine encephalitis (EEE), has recently shown up in both horses and people, causing three human deaths. Human cases have been confirmed in Illinois, Florida, Georgia, Alabama and South Carolina this year, and cases in horses have been confirmed throughout the south and also in Illinois and Wisconsin.

"Eastern equine encephalitis is severe, and it's important to vaccinate for it," Hope said. "Vaccinating for West Nile doesn't protect a horse from eastern equine encephalitis."

In its most severe form, West Nile virus can cause swelling of the brain and spinal cord, called encephalitis. While most people show no visible signs of the disease, some will experience flulike symptoms. About 3 percent of those who contract the illness will die.

However EEE is fatal in 35 percent to 50 percent of human cases and in 90 percent or more of the equine cases, according to the Centers for Disease Control and Prevention and the U.S. Department of Agriculture Animal and Plant Health Inspection Service.

Horses with West Nile virus 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.

Cases of EEE initially may appear much like West Nile virus, however, they progress much more rapidly. In horses this can include a fever and paralysis within a few days.

Horse owners should vaccinate their horses in the spring before the mosquitoes lay their eggs. West Nile currently is a separate shot, while the shot against EEE is usually given with the vaccines for western equine encephalitis, tetanus, flu and equine rhinopneumonitis.

No treatment for West Nile virus in people currently exists except supportive therapies. There are no approved vaccines against West Nile or other mosquito-borne encephalitic diseases for people.

However, the USDA recently issued a conditional license for a West Nile virus treatment for horses. The medicine was found safe in initial tests on horses, and the agency approved its use for one year while continued testing on the safety and efficacy continue.

In addition, the National Institute of Allergy and Infectious Diseases of the National Institutes of Health announced it has developed a vaccine against West Nile virus in humans. However, the vaccine only has been tested in laboratory animals and is not yet approved for testing on humans.

Preventative measures are important to protect against mosquito 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 label instructions, 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.

People and pets should stay away from mosquito-infested areas as much as possible. Although mosquitoes can bite anytime, staying inside at dawn and dusk, when they're most likely to bite, is a good preventative measure.

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

Source: Bill Hope, (765) 494-8548,

Ralph Williams, (765) 494-4560,

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

Related Web sites:
Purdue School of Veterinary Medicine
Purdue Department of Entomology
Purdue Extension resources for West Nile virus
Indiana Animal Disease Diagnostic Laboratory
Purdue University Department of Veterinary Pathobiology
Indiana Board of Animal Health
Animal and Plant Health Inspection Service
Centers for Disease Control and Prevention West Nile virus information
U.S. Geological Survey West Nile virus information


Sarah Anderson of Bloomington, Ind., leads a Trakehner gelding to the barn at a West Lafayette, Ind., stable to receive its vaccinations, which include ones for West Nile virus and eastern equine encephalitis. (Purdue University photo)

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