By Joe Lyman, DVM, MS Director of Professional Services and Product Development, Neogen 

Equine botulism occurs when horses are exposed to botulinum neurotoxin, most commonly type B, produced by the anaerobic, Gram positive bacterium Clostridium botulinum. This disease is generally seen ranging from Kentucky to the Mid-Atlantic states where soil conditions are favorable for type B C. botulinum spores to be present. The most common route of intoxication is forage poisoning, where toxin is produced by the bacteria found in improperly processed or stored forage products. The forage most commonly associated with equine botulism is round bales, but botulism cases have been reported associated with square bales, hay cubes, pelleted feeds, grain products and even pasture.

Affected horses show signs of inability to swallow; loss of muscle tone in the eyelids, tail and tongue; and incoordination. These signs progress to recumbency and possibly death as additional neuromuscular junctions are damaged by the neurotoxin. The severity and speed of progression are linked to the amount of toxin consumed, but only a microscopic amount of neurotoxin can be fatal.

In recent years, large equine botulism outbreaks have occurred in areas where the disease is not generally considered endemic. One such outbreak occurred in Florida in February 2022, with horses affected at multiple locations after eating contaminated field bales. Florida, although not considered a high-risk area for botulism, has seen some of the largest equine botulism outbreaks, likely due to the need to import feeds from other higher-risk areas. The lesson to be learned from outbreaks in non-endemic areas is that botulism can occur anywhere, regardless of the local soil conditions, because forage is not always produced locally. Initiation of treatment may be delayed in these areas as local veterinarians may not be as familiar with the disease. More importantly, veterinarians in these traditionally non-endemic regions are very unlikely to stock or have quick access to the antitoxin required to treat. The net result is that outbreaks in these regions can have catastrophic impacts with high numbers of fatalities.

Care of forage is an important step in prevention of botulism. Storage in raised, dry locations is important for all forage types. Inspection of hay for carcasses or signs of spoilage should be done at all feedings. It should be remembered that the neurotoxin is very stable once produced by bacteria. This means that bales do not need to be currently hot or wet to be at risk. Conditions for growth of the bacteria only need to have been present at any time in the production and storage of the forage for it to be contaminated. Also, considering that nanogram amounts are lethal, it is not always possible that visual inspection will reveal a contaminated bale.

Prevention for equine botulism type B is available through vaccination. The approved equine vaccine requires a three dose primary series with annual boosters. Broodmares should be boostered 30-45 days prior to foaling to promote high antibody levels in colostrum. While cross-protection against types A and C does not occur, type B remains the most common botulinum neurotoxin type affecting horses.

Article sponsored by Neogen.