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August 15, 2002

Purdue experts advise taking precautions against West Nile virus

WEST LAFAYETTE, Ind. – In the wake of Indiana's first confirmed case of West Nile virus in a person and a possible outbreak in horses in the southwestern area of the state, Purdue University experts recommend taking action to minimize exposure to disease-carrying mosquitoes.

Ralph Williams, Purdue entomology professor, said mosquito control is imperative to prevent West Nile virus in both animals and humans. Preventative measures also are important because mosquitoes carry other diseases, including St. Louis encephalitis, and Eastern, Western and Venezuelan equine encephalitis.

"With the first human West Nile virus case and more reports of the disease in horses, we can't stress enough how important it is to eliminate mosquito breeding areas," said Williams, an expert on disease-spreading insects. "Any standing water poses a potential danger concerning mosquitoes."

Repellents should be used by people and also on horses, Williams said. The most effective and safest repellents for people seem to be those with diethyl-meta-toluamide (DEET). Certain repellents are recommended for adults and others for children. Special products are available for horses.

Williams cautioned to always follow label recommendations and seek medical attention immediately if someone experiences an adverse reaction to any repellent.

In addition to these recommendations, Purdue veterinarians urge horse owners to take advantage of available immunization programs.

"We recommend that everyone have their horses vaccinated," said Leon Thacker, of the Purdue Department of Veterinary Pathobiology and director of the state Animal Disease Diagnostic Laboratory. "We want to educate people so they can cut the chances that either they or their horses contract any of the mosquito-carried diseases."

A 46-year-old Wabash County resident recently was hospitalized with West Nile virus for several days before recovering, according to the Indiana Department of Health. On Wednesday (8/14), the state Board of Animal Health reported that preliminary tests indicated four horses in the Daviess-Martin County area had an arbovirus – an insect-borne disease. Final laboratory results to determine if the horses contracted West Nile virus or Eastern or Western equine encephalitis are expected early next week.

No evidence exists that West Nile virus can be passed from horse to horse, horse to human or vice versa, or whether other mammals can contract the disease. But infected mosquitoes can bite anytime, anywhere.

"Mosquitoes can breed in any standing water, not just in swamps," Williams said. "Everyone needs to check their property and empty and clean out watering troughs, old tires, birdbaths, unused swimming pools, gutters. You also should clean and chlorinate swimming pools and aerate ponds or stock them with bluegills or sunfish that feed on mosquito larvae and pupae."

According to the U.S. Department of Agriculture Animal and Plant Health Inspection Service, mosquitoes can fully develop in any water that stands for as little as four days.

The Centers for Disease Control and Prevention annually receives reports of 1,000 cases of mosquito-borne encephalitis in humans, but the most prevalent for the past three years has been West Nile. The illness has spread rapidly in the United States, with both human and equine cases first recorded in 1999 in the New York City and Long Island areas. So far this year, the CDC reports 156 human cases including nine deaths in the United States due to the virus. Seven of the deaths have occurred in Louisiana and two in Mississippi. In addition, two nonfatal cases were reported this week in Ohio. The CDC also reports 139 cases in horses across an area from the East Coast as far west as Texas and North Dakota.

Mosquitoes transmit West Nile by biting infected birds, then biting people or horses. This sends 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.

Doctors and veterinarians can detect exposure to West Nile through a blood test, making it important to get a prompt and proper diagnosis, Thacker said.

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. 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. Initially, horses 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.

At least 14 mosquito species found in Indiana have been identified as carriers of the virus. Because different species bite at different times of the day and night, staying indoors during certain periods may not aid in avoiding the insects, according to the USDA and CDC. However using screens and insect repellents can help.

To prevent infection of horses, the manufacturers of the West Nile virus vaccine recommend two shots three to six weeks apart the first time the vaccine is administered, and yearly boosters thereafter. Vaccines also are available to immunize horses against Eastern, Western and Venezuelan equine encephalitis, but no vaccines currently exist for people.

West Nile virus has been known for hundreds of years in the Middle East and Africa. The name comes from the first time the disease was isolated. That was a 1937 case of an Ugandan woman in the West Nile District. The first equine cases recorded were in Egypt and France in the 1960s.

Writer: Susan A. Steeves, (765) 496-7481, ssteeves@purdue.edu

Sources: Ralph Williams, (765) 494-4560, rew@purdue.edu

Leon Thacker, (765) 494-7460, thackerl@purdue.edu

Ag Communications: (765) 494-2722; Beth Forbes, bforbes@aes.purdue.edu; https://www.agriculture.purdue.edu/AgComm/public/agnews/

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

Purdue News Service: (765) 494-2096; purduenews@purdue.edu


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