Spring Prevention: West Nile Virus And Parasites

West Nile Virus and parasites are both major management concerns for stable and farm managers. Staying up-to-date on these two important topics helps keep your charges safe.

West Nile Virus

Once West Nile Virus (WNV) was introduced onto the east coast of the U.S. in 1999, it spread westward. According to the Centers for Disease Control (CDC), there were 1,075 equine cases of WNV reported in 36 states in 2005. The incidence of equine WNV cases has decreased each year since 2003, either because horses are developing a natural immunity or because more horse owners vaccinate their horses annually.

Two vaccinations are currently available. Fort Dodge Animal Health released West Nile-Innovator in 2001. Two initial doses are given three to six weeks apart. Studies by Fort Dodge indicate that the vaccination is 95 percent effective six weeks after the initial doses are administered.

In 2004, Merial released RecombiTEK to combat WNV. Two initial doses are given four to six weeks apart. Studies conducted by Merial indicate that eight out of nine horses are protected twenty-six days after the initial vaccination, and 90 percent of horses are protected a year after they receive the first two doses.

Fort Dodge Animal Health plans to release another vaccine sometime in early 2006—West Nile-Innovator DNA. Fort Dodge worked with the CDC to develop this vaccine, which causes the horse’s own cells to begin producing proteins from WNV to trigger natural immunity.

Since approximately one third of horses who become clinically ill with WNV die, vaccinations are important. The vaccines work in different ways. If you vaccinate with RecombiTEK and decide to switch to West Nile-Innovator, your horses need two boosters before being protected. Merial reports that horses which have received West Nile-Innovator only need one dose of RecombiTEK to be adequately protected.

Although both vaccine manufacturers recommend annual boosters, the American Association of Equine Practitioners (AAEP) recommends that horse owners in areas with heavy mosquito infestation vaccinate their horses every four to six months. They also recommend that horses receive their vaccination in the spring prior to the peak of mosquito season.

Dr. Ilka Wagner operates an equine practice near Caldwell, Texas, and says she recommends that her clients vaccinate semiannually. “I’ve seen mosquitoes in the middle of winter—so we’re not completely safe from risk anytime.” When vaccinating semiannually, give one dose in early spring (March-April) and administer the second dose six months later.

And what about risks associated with these vaccines? A group of mare owners blame the WNV vaccines for stillborn foals and spontaneous abortions. West Nile-Innovator is not licensed for use in pregnant mares, but a study conducted at Texas A&M University in 2004 found no link between a mare’s vaccination status and abortion or stillborn births. The AAEP recommends vaccinating mares prior to breeding, but many veterinarians administer the vaccination four to six weeks prior to foaling—to promote passive transfer of protection to the foals—and then vaccinate foals about three to four months after birth. continued


Parasites have been the bane of horsemen for centuries, and deworming agents have been used for decades. Heavy parasite loads can lead to a lackluster coat and lethargic performance; severe infestations can lead to colic and death. Staying up to date on the best deworming strategies is important for any stable manager.

Since parasite threats vary depending on your location, the AAEP suggests discussing your horses’ needs with your veterinarian. Many veterinarians recommend a fecal count for new horses before designing a deworming protocol. Fecal counts performed under a microscope illustrate the horse’s parasite load and allow your veterinarian to determine which product is best for the horse. That knowledge allows you to deworm horses only when necessary instead of administering routine treatment.

Using a strong dewormer can be disastrous for horses with an unknown deworming history or those with a heavy parasite load. Strong dewormers may cause a massive die-off of parasites, which can lead to an impaction and colic. Dr. Wagner recommends that horses with unknown histories first receive Strongid or Safeguard and then receive an ivermectin or moxidectin dewormer 30-60 days later.

You have several options when designing an on-going deworming protocol for horses at your facility:

  • Daily dewormers. These products are added to horses’ grain daily. They do not protect against bots, so you should also administer an ivermectin dewormer at least twice a year, and at least one dose should contain praziquantel to protect against tapeworms.
  • Paste dewormers. The AAEP recommends that you rotate paste deworming products, since no one product protects against all types of parasites. Deworming frequency with these products depends on the horse’s exposure. A horse who is exposed to few new horses may only need to be dewormed every three to four months, while those who are exposed to a variety of horses may need to be dewormed every six to eight weeks.
  • Pelleted dewormers. Some horses are difficult to deworm with a paste dewormer; pelleted dewormers added to a horse’s feed can be an option.

In addition to a good deworming program, the AAEP also emphasizes good pasture management. That includes removing manure from paddocks and turnout areas, mowing and harrowing pastures regularly to break up manure and expose worm eggs, avoiding pasture overcrowding, feeding hay and grain in feeders instead of on the ground, and prompt removal of bot eggs.

Keeping horses on a similar schedule is also important. Schedule a day when all owners at your facility deworm their horses. Or include deworming as part of the services incorporated into board or training to insure that all horses are properly dewormed. Working with your veterinarian and your clients, you can develop a deworming protocol to protect your equine charges.

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