Wet Weather Increases Botulism Risk; Vaccination Recommended

Wet weather is known to give rise to increases in some equine diseases, including botulism, Potomac horse fever, and mosquito-borne diseases such as West Nile virus, and Eastern, Western, and Venezuelean equine encephalitis.

One veterinarian at the Hagyard Equine Medical Institute in Lexington, Ky., noted that the hospital has seen a higher incidence of adult botulism cases in the hospital this year.

Above average precipitation was recorded in November 2004 in many areas of the country. Wet weather can contribute to higher incidence of adult botulism and other equine diseases.

Veterinarians are attributing this rise to the extremely warm and wet fall and winter that has allowed the causative bacterium, Clostridium botulinum, to flourish in the soil. The bacterium is endemic in certain areas of North America, and in those areas, veterinarians recommend vaccination against the disease. They stress that pregnant mares should be vaccinated to pass antibodies to foals born in botulism-endemic areas to prevent "shaker foal syndrome," as the disease is called in young foals.

"I vaccinate all the weanlings and yearlings and all adult horses, stallions included," stated Dr. Stuart Brown, a field veterinarian with HEMI. "It's such a cheap vaccine, and an ounce of prevention..."

The mortality rate in untreated foals is up to 90%; in adult horses mortality approaches 70%.

Horses are "exquisitely" sensitive to the toxin produced by the bacterium, noted Dr. Neil Williams, of the Kentucky Livestock Disease Diagnostic Lab. There are several strains of botulism, not all of which affect horses. The vaccine for horses protects against the common Type B.

"We need to vaccinate, particularly in this area (Central Kentucky), because of endemic Type B toxin," said Brown. "I worry about horses eating hay out of those big round bales covered in plastic you see in the fields. You want to vaccinate to protect against puncture wound entry of the bacterium, and to protect from contaminated forages."

The toxin that causes botulism interferes with neurotransmitters where the nerve signals the muscles. Botulism usually strikes very swiftly. Affected horses lose the ability to swallow and will drop grain and saliva from their lips. They move in a shuffling fashion or drag their toes, and they can show depression, muscle tremors, a protruding tongue, dilated pupils, constipation, colic, shortness of breath, and violent spasms or seizures.

Within 48 hours, horses affected with botulism often are unable to rise. Respiratory paralysis usually forces euthanasia. The severity of symptoms is largely dependent on the amount of toxin the horse receives (either through ingestion or a wound). Less severely affected horses might decline slowly--a characteristic that can confuse diagnosis.

Horses can contract botulism three ways. The most common is ingestion of the toxin (not the bacterial spores themselves) through contaminated feed or water. Decomposing carcasses of rodents or birds, caught in baled hay, are often blamed, but it is far more common for hay or silage products to be contaminated through improper storage or poor fermentation. The risk increases markedly when horses are fed large round bales, especially the wrapped silage type, which can become infected with C. botulinum. Outbreaks of botulism in several horses on a farm are almost always due to problems with this type of feed.

Horses can contract botulism from contamination of a puncture wound. In young foals, botulism is contracted through the entrance of the bacterium through the tissue of the umbilical stump. The result is called "shaker foal syndrome" because the foal develops violent muscle tremors, a stilted walk, and an inability to swallow. Death usually occurs within 72 hours.

Normally, outbreaks of botulism in large groups of horses are rare. However, they have occurred. In April 1994, Australia's Thoroughbred industry was rocked by an outbreak of botulism at the Easter yearling sales. Some 460 yearlings were consigned to the sale, with 41 of them displaying signs of the disease; 33 of the 41 affected yearlings died or were euthanatized. It is likely that the organism was introduced to the yearlings via contaminated feed.

Horses can recover from botulism with intensive nursing care and often with the help of an antitoxin that binds circulating toxins in the body.