sealPurdue News

May 17, 2002

Wet weather may spawn bumper crop of mosquitoes and disease

WEST LAFAYETTE, Ind. – The mild winter and the record rainfall this year likely will spawn high numbers of disease-carrying mosquitoes, according to Purdue University and Indiana state experts.

One of the biggest concerns once the blood-sucking insects start biting is whether they will bring steeply increased cases of West Nile virus. Purdue veterinarians and Indiana state health officials are urging people to take steps to eradicate mosquitoes so neither they nor their horses are infected by the deadly disease. The mosquito-borne illness has been responsible for the deaths of 18 people and 187 horses in the United States over the past three years, including one horse in Indiana last October, according to statistics from the U.S. Department of Agriculture Animal and Plant Health Inspection Service and Centers for Disease Control and Prevention.

Though the wet weather ultimately is expected to aid in further spread of West Nile, at least temporarily it has delayed the onslaught of its insect carriers, said Michael Sinsko, Indiana State Department of Health senior medical entomologist.

"We haven't had any (West Nile) cases yet this year," Sinsko said. "There are lots of mosquitoes developing, but the cool, wet weather has held them in the larval and pupa stages. As it warms up, we'll see lot of adult, biting mosquitoes."

Veterinarians at Purdue University are vaccinating horses against West Nile and have given the shots to more than 1,100 of the animals in the central Indiana area since last fall.

"We recommend that everyone have their horses vaccinated," said Amanda Spencer, a Purdue Large Animal Ambulatory Clinic veterinarian. "Even if they had the first two shots last fall, they should have another booster this spring."

The manufacturers of the drug recommend two shots three to six weeks apart the first time the vaccine is administered, and yearly boosters thereafter. No vaccine currently exists to give people immunity against West Nile.

Although the USDA conditionally approved the vaccine last August as an annual shot for horses, its effectiveness is uncertain. However there are no reports of an animal developing the disease after having the vaccination, even those in high-risk areas such as Florida.

"We know that it's a good vaccine, but we don't know how good," Spencer said. "Time will tell. We're keeping our fingers crossed that it will do what it's supposed to do."

She said it's possible that a horse could have an allergic reaction to the vaccine but can't catch West Nile from the vaccination because it's produced with a killed virus. Of all the horses vaccinated locally, only one has experienced a reaction, which was easily treated.

"We're trying to educate people all over the Midwest; it's important to get the shots," Spencer said. "We're stressing that if horses come down with the disease, there is no specific treatment. We can only do supportive treatment – give them fluids, keep them quiet and hope that their immune system kicks in. This disease can come on fast and kill fast."

Sinsko and Spencer said mosquito control is important to prevent West Nile virus in both animals and humans. The preventative measures also are important because mosquitoes carry other diseases, including Eastern encephalitis, Western encephalitis, St. Louis encephalitis and Venezuelan encephalitis. The CDC annually receives reports of 1,000 cases of mosquito-borne encephalitis in humans.

"It doesn't take a lot of effort to check around your property to find standing water," Sinsko said. "People tend to think that all mosquitoes come from swamps. But they can breed and hatch in any standing water. The species that are the primary vectors of West Nile breed mainly in containers and in septic effluent.

"We need to motivate people to clean out watering troughs, old tires, anything that can be a potential mosquito breeding ground."

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

"With the inundation of rain this year, we're concerned about standing water throughout the Midwest," said Timothy Gibb, insect diagnostician in the Purdue Department of Entomology. "Anytime you have standing water, it can become stagnant and a breeding ground for mosquitoes.

"Right now we have a lot of flooded farmland, so a lot of standing water. It should be drained wherever possible."

Gibb advises disposing of any water-holding containers; cleaning clogged roof gutters annually; turning over wading pools, buckets and wheelbarrows when not in use; cleaning out bird baths and livestock watering troughs at least weekly; not letting water collect on swimming pool covers; cleaning and chlorinating swimming pools; and aerating ornamental pools or stocking them with bluegills or sunfish that feed on mosquito larvae and pupae.

Insecticides that are effective on the mosquito larvae and pupae can be used in some water containers, but never in livestock buckets and troughs, Gibb said. Those must be dumped and cleaned on a regular basis.

Adult mosquitoes are difficult to eradicate, he said. Spraying for adults only gives relief for a day. "It's a bandage treatment," he said.

West Nile 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. Equine cases now have been confirmed as far west as southwestern Louisiana. Though human cases have not been confirmed in every state, incidents have occurred as far west as western Arkansas.

"At the beginning of last summer, I thought West Nile would work its way around the Gulf Coast first," Sinsko said. "The way it spread first into the Midwest was surprising."

The disease's advance across the country has been so rapid and widespread that USDA Animal and Plant Health Inspection Service veterinarian Randy Crom said the agency no longer will track the cases state by state. Instead it will only report when big outbreaks strike one area; it will be up to the states to provide statistics.

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.

No evidence exists that the disease can be passed from horse to horse, horse to human or vice versa, or whether other mammals can contract West Nile. However, Sinsko and Spencer said the medical and scientific community is still learning about the disease.

"We don't have all the answers," Sinsko said. "We do know that we will see it again this year, and most likely it will spread even further and we'll have more cases."

Doctors and veterinarians can detect exposure to West Nile through a blood test, making it important to get a prompt and proper diagnosis, he 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, 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 the United States 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 the CDC. However using screens and insect repellents can help.

Screened areas should be kept free of adult mosquitoes. The most effective and safe repellents seem to be those with diethyl-meta-toluamide (DEET), Sinsko said. However medical attention should be obtained immediately if someone experiences an adverse reaction to any repellent.

West Nile virus first appeared in the United States in late 1999, although it 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.

States in which horses have died due to West Nile virus include Connecticut, Delaware, Florida, Georgia, Illinois, Indiana, Kentucky, Louisiana, Massachusetts, Maryland, Mississippi, New Hampshire, New Jersey, New York, North Carolina, Pennsylvania, Tennessee and Virginia, according to the Animal and Plant Health Inspection Service. In addition to the above states, from 1999 through December 2001 equine cases of the disease have been confirmed in Alabama, Maine and Rhode Island.

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

Sources: Amanda Spencer, (765) 494-8548,

Timothy Gibb, (765) 494-4570,

Michael Sinsko, Indiana State Dept. of Health, 317-233-7397

Randy Crom, USDA, Animal and Plant Health Inspection Service, (800) 940-6524

Related Web sites:
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

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

Purdue News Service: (765) 494-2096;

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