Though rare, botulism can still cause problems in horses

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Few horse owners are familiar with botulism in horses. But if the infrequent, deadly disease strikes their herd, they’ll never forget it.

In late January 2012, Shiloh Acres Horse Rescue owner Amber Herrell found 10-year-old Wren down and unable to completely rise. The black Percheron/Friesian cross mare wasn’t sweaty or presenting other colic symptoms. Herrell had owned the horse nearly six years and Wren had been perfectly healthy the entire time.

While awaiting the arrival of veterinarian Ed Wydallis, Herrell comforted the mare, who was shaky and had little muscle strength or control. The other horses in the same pen seemed fine, eagerly eating from their large bale of hay. They took turns coming over to urge Wren to her feet, so much so that Herrell turned them out into the pasture.

Wydallis thoroughly examined Wren and found all her vital signs within the normal range. Before leaving, he gave her a cocktail of steroids and an anti-inflammatory pain reliever. He left instructions with Herrell to transport the mare to an equine hospital when she rose if all symptoms didn’t abate.

The situation quickly worsened, however, and Wren laid flat. Herrell’s family worked together with ropes to turn the huge mare several times. Late that night, the horse couldn’t lift her head and experienced difficulty breathing. Wydallis returned and the decision was made to euthanize Wren.

The following day, Herrell hauled the corpse to Colorado State University for a necropsy. The vet there immediately suspected botulism due to the mare’s lack of apparent symptoms before going down. But since botulism occurs in outbreaks and Wren’s penmates seemed fine, post-mortem tests were conducted for rabies and equine herpes virus. Stomach samples were retained in case Herrell chose to have them sent to one of only a couple U.S. labs that test for botulism.

Herrell left for work but a knowledgeable friend checked her other horses in the pen, including monitoring temperatures. After work, all horses seemed well when Herrell returned to feed.

The next morning, however, struck her a dreadful blow. Lucas, her beloved black Percheron gelding in his early teens, was down and sicker than Wren had initially been. Paddle marks in the dirt documented his failed efforts to rise. When she first approached in her truck and saw Lucas on the ground, the horse whinnied to her but could not raise his head. When Lucas realized Herrell was unable to help him, his eyes glazed over and he gave up, she said.

She summoned the vet who, upon arrival, immediately euthanized the suffering animal. Herrell then called the chief veterinarian at the CSU necropsy department. He advised her what symptoms to look for in the remaining five horses in the pen. To her dismay, Herrell found four of them displaying early, subtle indications of botulism: droopy eyelids, tail paralysis, and two were eating clumsily.

After online research and consultation with vets, she ordered anti-toxin serum from a New York laboratory. (The botulism toxin is so deadly that only a few U.S. labs are authorized to work with it.) At $500 per dose, two were overnighted to Herrell who had to make the cruelly difficult decision which horses to receive them.

She chose Tosca, a blue roan Brabant mare, and Sula, a buckskin Clydesdale or Shire cross. That left Vienna, a flea-bitten gray Shire mare, Belgian mare Lyra, and quarter horse mare Mia. Tosca was the only one not showing symptoms and, therefore, had the best chance of survival if infected. A tense two weeks of waiting followed.

No other horses died. After three weeks, Tosca remained asymptomatic. Sula continued wobbly and displayed tail and lip paralysis for months. The other three mares’ symptoms never worsened. All five have since fully recovered.

Herrell removed and disposed of all suspect hay. She found a dried-up antelope leg at the pen’s far end. Had it been the culprit, hidden in the anaerobic recesses of the large, round bale that originated in a Longmont river valley? Herrell was unable to afford the high, out-of-state testing costs.

Veterinarians told her she was lucky that only two out of seven horses had died. Huge outbreaks are notorious and some owners lose whole herds. In 1994, 33 thoroughbred yearlings died or were euthanized out of 41 symptomatic when botulism struck at a Easter Yearling Sale in Newmarket, Australia.

Karen Riedlinger, MS, DVM of Animal Medical Services of the Rockies, is familiar with a few botulism cases via another vet in the practice. That 2010 outbreak killed horses in a Wellington area herd.

Riedlinger advised that a preventive vaccine is available. Three doses must be administered the first year and one booster dose annually thereafter. Her clinic currently charges $21.50 per dose. As with many other vaccines, there are no guarantees. Riedlinger cautions horse owners to never feed large bales, regardless of cost savings. The perfect conditions to produce the botulism toxin are more likely to occur in anaerobic large bales than in small ones.

Owners can’t realistically inspect every flake of hay, and even a tiny mouse foot can cause the toxin. Any hay in which a dead animal or part is found should be disposed of, as should any bale touching it.

The other best preventive measure is vigilance for initial symptoms, and early veterinary intervention.

Botulism 101

• Botulism displays subtle early symptoms and a generally lethal outcome.
• The causative agent is Clostridium botulinum, unseen by the naked eye.
• Eight distinct types are known. In Colorado, Type C is the most common, caused by consumption of feed, especially hay, containing carcasses or parts thereof.
• Early clinical signs include lip and tail paralysis, droopy eyelids, tongue hanging out, mild depression, wobbly gait, difficulty rising from prone position.
• Treatment is limited to administration of antiserum, which must match to the distinct toxin (i.e. Type A, B, Ca, Cb,etc.)
• Occurs worldwide in outbreaks. If one horse is sick, others in a herd should be closely monitored for symptoms for up to two weeks.
• Call a veterinarian immediately if any symptom is present.
• Surviving horses return to normal. No residual problems remain.