Beware of botulism

Management measures and vaccination can help protect your horse against this devastating illness.


Mention of botulism strikes fear in people’s hearts, and rightly so. The illness requires intensive supportive care and the outcome is uncertain.

Botulism causes progressive paralysis, starting at the head and spreading throughout the body. Once a horse goes down, his condition deteriorates rapidly, and if his body can’t clear the causal toxin quickly enough, the muscles that control respiration stop functioning. The good news is that botulism is rare and almost entirely preventable through vaccination.

The bacterium that causes botulism, Clostridium botulinum, is ubiquitous in the environment but usually in a harmless, dormant state. It’s only under certain conditions---moisture and a lack of oxygen being the most important---that the bacterial spores become active and multiply quickly. When the active bacteria die, they release a toxin known as botulin.

There are seven types of botulin, but only three are likely to cause illness in horses. Type B is the most common, found east of the Mississippi River and most often in the mid-Atlantic states and Kentucky. It accounts for about 85 percent of all cases. Type A is found on the West Coast and is less common. The third type, C, is found in carcasses of dead animals and bird droppings and is by far the least common.

In mature horses, one of the most common causes of botulism is feeding silage, a type of forage that is some-times packaged in airtight containers while still moist. Commonly fed to cattle and sheep, which aren’t as susceptible to botulism, silage is sometimes fed to horses by owners looking for a less dusty alternative to hay or when conventional hay isn’t available. However, this is risky because a slight change in the internal conditions of the bag can cause C. botulinum to flourish. The same conditions can occur in grass clippings left in piles for more than a few minutes. For safety’s sake, avoid feeding silage and grass clippings to your horse.

Wet and spoiled hay can also harbor botulin toxin. The anaerobic environment deep inside a wet round bale or square bale is hospitable to the botulism spores. This is yet one more reason to be to be vigilant about hay quality. Inspect all bales for signs of spoilage and be particularly cautious with round bales that are left outdoors for long periods of time.

The best defense against botulism, however, is vaccination. The approved equine botulism vaccine is effective only against type B, so consult with your veterinarian to make sure it’s appropriate for your geographic location. The initial vaccine is given as a series of three doses one month apart, followed by an annual booster.

This article first appeared in EQUUS issue #467, August 2016.