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Vaccination programs important for horse health

By Kylene Scott

Seven horses reported in Colorado with West Nile virus. Several more are infected and die from Eastern equine encephalomyelitis in Texas. Another is quarantined for fear of spreading a reproductive disease in Florida.

All these instances occurred in late 2008 and 2009, and horse owners need to be even more aware of the importance of vaccinating their horses against infectious disease.

Dr. Carolynn MacAllister, Oklahoma State University Cooperative Extension veterinarian, urged owners to be aware of the disease following reports of infected horses in surrounding states.

"It takes a week to 10 days after a vaccination for a horse to develop protective antibodies," MacAllister said in an OSU news release in July 2009. "Booster shots should be administered as directed to maintain the highest level of protection."

Texas state veterinarian and head of the Texas Animal Health Commission Dr. Bob Hillman released similar information after several horses died of Eastern equine encephalomyelitis in Texas, in June and July 2009.

"As part of their routine health care, horses in all parts of the state should be vaccinated to protect against dangerous mosquito-borne diseases, including West Nile virus, and eastern and western equine encephalitis," said Dr. Bob Hillman in an Associated Press article.

Hillman also warned horse owners should be diligent in protecting both themselves and their horses.

"As effective as vaccines are, however, they are not foolproof, and rare instances of disease can occur," Hillman said in the article. "Therefore, it is essential that you protect yourself and your horses against mosquito exposure with a repellent containing DEET."

Most common

The most commonly vaccinated equine diseases include: Tetanus, equine herpes virus (EHV), influenza, strangles, equine viral arteritis (EVA), rabies, Potomac Horse Fever, encephalomyelitis (eastern and western strains), West Nile virus, Rhinopneumonitis, botulism and rotaviris A. Each has a certain protocol as to dosage, booster shots and when to give. Equine veterinarians should be consulted for a specific program to vaccinate horses.

Tetanus is caused by Clostridium tetani found in the soil, and normally enters the body through a wound. It produces a toxin that affects the horse's nervous system. Horses are very susceptible to tetanus and it is fatal in more than 80 percent of affected horses, according to an OSU Cooperative Extension Service fact sheet, "Equine Vaccination Programs." Annual boosters are recommended, but a booster is also normally administered at the time of a penetrating injury or surgery.

Equine herpesvirus is one of five types of equine herpesvirus or EHV. Types one and four are most common, and infections of EHV can cause respiratory disease, neurologic disease and abortion. Most often, pregnant mares and young horses are vaccinated for this, according to OSU.

Influenza viruses are the most common causes of respiratory disease in horses. These types of viruses are extremely contagious, and damage caused by the virus can predispose horses to secondary bacterial infections if left untreated. Although vaccinations help with this virus, they are often short-lived because of the makeup of the vaccine. In adult horses, it is recommended they get boosters every three to four months, but check with veterinarians for local recommendations.

Strangles is a highly contagious upper respiratory disease caused by Streptococcus equi equi. This bacterium is transmitted through direct contact or environmental contamination of discharges from infected horses. Young horses (ages 1 to 5 years) are most susceptible. The bacteria can live for months in the upper respiratory tracts of horses that appear relatively healthy.

Equine viral arteritis or EVA is a viral disease that can cause severe outbreaks of respiratory disease and abortion. EVA is transmitted through inhalation and exposure to the virus on contaminated items or environment. Venereal transmission also occurs. Many stallions are now EVA vaccinated.

Rabies occurrence is very low in horses, but it is fatal. Vaccination is recommended annually.

Potomac horse fever is caused by a parasite, Neorickettsia risticii. It has been most common in the eastern United States, but cases have been reported in many regions of the U.S. In 2009, some cases appeared in Oklahoma and Colorado. It is seasonal, most often occurring in late spring and fall, with the majority of cases happening in summer months.

Encephalomyelitis is caused by either one of two strains--Eastern (EEE) or Western (WEE) equine encephalomyelitis virus. EEE/WEE are most likely transmitted through wild birds, rodents and biting insects. The disease is generally fatal, especially with EEE. Horses should be vaccinated annually, with boosters every six months.

West Nile virus is a mosquito-borne virus. Mosquitoes become infected by feeding on diseased wild birds, and occasionally those same mosquitoes can transmit the virus to people and horses. An annual booster is recommended after the primary series of vaccines. Depending upon risk, more boosters may be needed.

Rhinopneumonitis (EHV-1 and EHV-4) infects the respiratory tract, typically in the first week of a foal's life. EHV-1 causes abortion in mares, weak nonviable foals and sporadic neurological disorders. Vaccination of mares before breeding and at 4 to 6 weeks post partum is recommended. Breeding stallions should also be vaccinated before the breeding season and semi-annually.

Others to be aware of

Botulism is a result of potent toxins produced by the soil-borne, spore-forming bacteria, Clostridium botulism. There are several types: wound botulism, shaker foal syndrome, forage poisoning and equine grass sickness. According to the AAEP website, botulinum toxin is the most potent biological toxin known and acts by blocking transmission of impulses in nerves, resulting in weakness progressing to paralysis, inability to swallow and, frequently, death. Consult a veterinarian for vaccination recommendations.

Rotavirus is a major infectious cause of foal diarrhea and has been documented to cause more than 50 percent or more of foal diarrhea cases in some areas, according to AAEP. Equine rotavirus is transmitted via the fecal-oral route and damages the small intestinal villi resulting in cellular destruction, maldigestion, malabsorption, and diarrhea. Mares need to be vaccinated at 8, 9 and 10 months gestation for each pregnancy. Foals receive antibodies through passive transfer during birth. Check foals to be assured of transfer.

Healthy horses

The American Association of Equine Practitioners offers guidelines for horse owners to ensure the health of their horses on its website: www.aaep.org. AAEP says there are no standard vaccination programs, and each particular situation should be evaluated based upon:

--Risk of disease;

--Consequences of the disease;

--Anticipated effectiveness of selected products;

--Potential adverse reactions to a vaccine;

--Cost of immunization.

Horse owners should be sensible and understand vaccination by itself--without good management practices--will not prevent diseases, according to AAEP. It only serves to minimize the risk of infection, but cannot prevent disease in all situations.


--Properly administered vaccines (primary series and boosters) should be given before exposure.

--Each horse in a population is not protected to an equal degree nor for an equal duration following vaccination.

--Protection is not immediate. In some cases the primary series must be administered initially to induce protective active immunity.

--All horses should be vaccinated at intervals based on the professional opinion of the attending vet.

--A properly administered, licensed product should not be assumed to provide complete protection during any given field epidemic.

--Although rare, there is potential for adverse reactions despite appropriate handling and administration of vaccines.

Local equine veterinarians can answer specific questions regarding what horses need to be vaccinated against in certain geographic areas, as well as individual needs of both horses and owners. Consult with your veterinarian for specific questions and diagnoses.

For more information visit www.aaep.org/vaccination_guidelines.htm.

Kylene Scott can be reached by phone at 620-227-1804 or by e-mail at kscott@hpj.com.

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