seal  Purdue News

March 12, 2004

Now is the time to take a bite out of mosquito season

WEST LAFAYETTE, Ind. - Protective measures against infection by West Nile virus and other mosquito-borne diseases need to start now for horses, according to Purdue University experts.

Bill Hope
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Vaccines currently approved for these diseases, which include West Nile virus, eastern equine encephalitis and western equine encephalitis, are only for horses. Because no treatments exist, it's important to protect horses through preventative measures, including the shots to provide disease immunity.

"Late summer into mid-fall is when we see the most cases of West Nile virus, but vaccinations for horses must start now," said Bill Hope, a Purdue community practice equine veterinarian. "We are recommending two boosters a year of the equine West Nile vaccine, one in March or April and another in August or September. This way the horses will have their highest level of immunity at the times when the virus-carrying mosquitoes are most likely present."

The vaccine manufactured by Fort Dodge Animal Health, a division of Wyeth and approved for use in the summer of 2001, has proven to be 95 percent effective in protecting horses. "That is as good as we can expect," Hope said.

Another equine vaccine, made by Merial, was recently approved and is available, he said. The difference between the two is that the Ft. Dodge product is ready-to-use while the Merial product is a powder that must be mixed with sterile water and used within two hours of liquefying. Although the Merial vaccine was effective in trials and is believed to provide longer-lasting immunity, veterinarians don't yet know how well it will work in the field.

The types of mosquitoes that usually carry West Nile virus are more prevalent from late summer through fall, according to Ralph Williams, Purdue entomologist. However, preventative measures should be taken throughout the year because different mosquitoes carry different diseases, and some of the biting insects live through the winter.

Mosquitoes already are hatching in some areas of the southeastern and southern United States, but Williams said it will probably be a month or more before Indiana experts have an estimate of the possible severity of the mosquito population in the state this year.

Hope said horses exhibiting West Nile virus symptoms have an abnormal, wobbly, unsteady gait due to loss of muscle control. They also are lethargic, and in more severe cases, suffer partial paralysis. Their body temperature usually remains normal.

In its most severe form, West Nile virus can cause swelling of the brain, called encephalitis, or of the brain lining and spinal cord, called meningitis.

Most people who contract the disease show no visible signs, while others develop flulike symptoms. The disease has a fatality rate in humans of about 3 percent.

Horse owners also must vaccinate for other mosquito-borne diseases including eastern equine encephalitis and western equine encephalitis, Hope said. Although symptoms may appear similar to West Nile virus, the illnesses are from a different family of viruses, so different vaccines are required.

People also can become ill from eastern equine encephalitis, and a number of human cases were reported last year and confirmed in Illinois, Florida, Georgia, Alabama and South Carolina. Cases in horses were recorded throughout the south and also in Illinois and Wisconsin. Indiana confirmed that some birds were carrying the zoonotic illness.

Zoonotic diseases are those that can affect both people and animals. Not all of them are carried by mosquitoes, however, West Nile, eastern and western equine encephalitis are mosquito-borne. Mosquitoes transmit these illnesses by biting infected birds, then biting horses or people. This sends virus-laden saliva into the animal's bloodstream. According to the USDA, there is approximately a one- or two-week lag time between when a horse or person is bitten and when the sickness develops. Some evidence also exists that West Nile virus can be transmitted through blood transfusion, organ transplant and in the womb from a mother to the fetus.

In general, eastern equine encephalitis is more severe than West Nile virus. Initially, cases of eastern equine encephalitis may appear similar to West Nile virus, however, they progress more rapidly. In horses, the eastern form of the disease can cause fever and paralysis within a few days.

Eastern equine encephalitis 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 (CDC) and the U.S. Department of Agriculture Animal and Plant Health Inspection Service.

In 2003 the CDC reported 9,186 West Nile virus cases in people with 231 deaths in the United States. In Indiana the numbers were 47 and four, respectively. Equine cases totaled 4,636 nationwide and 45 in Indiana, according to the inspection service.

The national human death rate from West Nile dropped by 22 percent from 2002 to 2003. The death rate in horses is less than 25 percent.

The USDA has issued a conditional license for a West Nile virus treatment for horses. The medicine was found safe in initial tests, and the agency approved its use for one year while tests continue on its safety and effectiveness. In addition, the National Institute of Allergy and Infectious Diseases is testing a West Nile vaccine and treatment for use in humans.

In addition to vaccines, it's also important to take steps to minimize the number of mosquitoes. These measures include 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, Williams said.

People also should use repellents with N,N-diethyl-meta-toluamide (DEET) and carefully follow label instructions, he said. When using any repellent, make sure that it's registered with the U.S. Environmental Protection Agency and check that the concentration is approved for use on children. 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, Williams said.

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

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

Ralph Williams, (765) 494-4560,

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

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

Veterinarians recommend that horses receive a vaccination against West Nile virus in March or April and a second one in August or September. Purdue University community practice equine veterinarian Bill Hope prepares a syringe to administer the shot to a horse at the Clinton County Fairgrounds in Frankfort, Ind. (Purdue Agricultural Communications photo/Mike Kerper)

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