Vaccinate Early for Maximum Protection

February 7, 2002 — Despite blustery days now and again, spring is just around the corner, and animal health officials are urging owners to protect their equids, including horses, mules and donkeys against several mosquito-borne diseases before “bug” season starts.

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“Vaccines are available to protect equids against both equine encephalitis, better known as “sleeping sickness” and West Nile Virus (WNV). However, the shots are no value if they aren’t given prior to disease exposure,” said veterinarian Terry Conger, state epidemiologist for the Texas Animal Health Commission (TAHC), the state’s livestock health regulatory agency. “These vaccines require two doses, administered several weeks apart, and full protection doesn’t develop until four to six weeks after the second dose. Realistically, then, it can take up to 10 weeks for the horse to be disease-resistant. That’s why it’s so important to start the round of vaccinations now.”

Dr. Conger noted that cases of “sleeping sickness,” or equine encephalitis, occur sporadically in Texas equids, and on rare occasions, humans contract the mosquito-borne infection. Transmitted by mosquitoes that have fed on diseased birds, Eastern Equine Encephalitis (EEE) and Western Equine Encephalitis (WEE) causes swelling of the brain, resulting in staggering,
convulsions, and fever. Although horses do not transmit the disease to humans, equids are “sentinel animals,” or indicators that infected mosquitoes are in a specific area.

“Veterinarians can administer an effective ‘two-way’ vaccine to protect equids against EEE and WEE,” said Dr. Irby, TAHC equine specialist. “Two injections, 30 days apart, are required, and the animal will need a ‘booster shot’ annually or biannually.”

“Another form of sleeping sickness, Venezuelan Equine Encephalitis (VEE), is a foreign animal disease that originated in South America. In l971, a VEE outbreak swept northward through Mexico, threatening horses in the southern U.S. In two years, due to an all-out effort, more than 2.8 million animals in l9 states were vaccinated against VEE,” Dr. Irby said. “On rare occasions, VEE cases still creep into southern Mexico, and if it should move north, it would threaten American animals. Therefore, as an added precaution, equids in South Texas should receive a three-way vaccine that protects not only against EEE and WEE, but also VEE.”

Although it has yet to be diagnosed in Texas, another mosquito-borne, encephalitic disease causing concern is West Nile Virus (WNV). Dr. Irby said WNV was unheard of in the US until l999 in New York, where seven persons died from the infection and at least 25 equids were infected. In 2000, the virus had spread to 12 states, and by year’s end 2001, the disease impacted 27 states, including Louisiana and Arkansas. At least 564 equids tested positive for the disease in 2001, reported Dr. Irby.

Epidemiologists are still uncertain how or when an infected person, bird or mosquito entered the country, initiating the bird-mosquito WNV disease cycle in the US. First isolated in1937 in the West Nile District of Uganda, the disease now has widespread distribution, including Africa, Europe, the Middle East, and West and Central Asia, As the virus has moved across the U.S., the U.S. Department of Agriculture changed WNV’s designation from a “foreign animal disease,” to an “emerging disease.”

“Texas is particularly vulnerable to the introduction of WNV, because major migratory routes for birds pass over a large portion of east Texas. This summer, we may find that the disease is brought into the state by infected birds,” commented Dr. Irby. He pointed out that at least 17 species of mosquitoes can be involved in the disease cycle.

“Many horses infected with WNV won’t appear to be ill, while, others become debilitated and too weak to rise,” he said. “About 20 percent of the horses infected with WNV in 2001 either died or were euthanized because they were so ill.”

“Late last summer, a WNV vaccine was approved for equids, and like many other encephalitic vaccines, two injections are needed. For WNV vaccine, the injections must be three to six weeks apart. The timing of vaccination is crucial, as demonstrated in Florida last year, where 267,000 doses of vaccine were made available. Eighty-six equine that became infected had received only one dose of vaccine. Seventeen other stricken animals received their second dose of vaccine less than three weeks prior to becoming infected,” he explained. “Since immunity may not develop for four to six weeks after the second dose, owners should have their animals vaccinated now, so they’ll be protected by the time mosquito season begins this spring.”

Dr. Irby reminded owners to reduce mosquito populations by draining stagnant water from birdbaths, flowerpots, troughs and other containers, and by keeping pools properly chlorinated. Insect repellents and insecticides should be used only according to directions, he said, and owners may want to stable their animals at night, to provide added protection.

“WNV is a ‘zoonotic disease,’ one that also can affect human health,” commented Dr. Conger. “Species other than horses, humans and birds are susceptible to the disease, and it has been demonstrated in other states that canines can contract the infection. No vaccine has been approved for dogs, but this information may be useful in an epidemiological investigation of a potential WNV introduction.”

The Texas Department of Health (TDH) also is conducting disease surveillance, for testing mosquitoes, dead crows, blue jays and hawks. To submit a dead crow, blue jay or hawk for laboratory examination, call your regional Texas Department of Health office or the Texas Department of Health in Austin at 1-512-458-7255. TAHC field personnel are collecting blood samples from equids involved in regulatory disease programs, and the agency operates a 24-hour hotline at 1-800-550-8242 for taking reports of unusual signs of disease in livestock, such as:

  1. staggering, falling, or inability to rise
  2. illness affecting a large percentage of animals
  3. sudden death loss
  4. blistering around an animal’s lips, teats or hooves
  5. unusual ticks or maggots

“Only the first three of the disease signs apply to encephalitic diseases, but the other two–blistering and ticks–should be kept in mind, as they could be signs of other very serious disease situations,” said Dr. Irby. “TAHC and USDA veterinarians will work with private veterinary practitioners and producers at no charge to help diagnose foreign or emerging diseases. The earlier we can make a diagnosis, the sooner we can take appropriate measures to control or stop the spread of a disease.”

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