Botulism – A Deadly Killer

Botulism is a neurotoxin produced by the clostridium botulinum bacterium. The bacterium itself is widely found in soils and in the intestinal tract of animals. On its own it is not dangerous, but when exposed to the right environmental conditions it sporulates and releases the botulin toxin. Horses are particularly sensitive to botulism.

In order for sporulation to occur, the bacterium needs to be exposed to a wet, humid environment devoid of oxygen (anaerobic) and rich in decaying organic matter. Most often, baled hay is the culprit of botulism poisoning in horses. Large round bales present the biggest risk, but small square bales can also contain the toxin.

Botulin contamination in hay occurs under two circumstances: when improperly dried forage materials are baled and stored while still damp, or when an animal carcass (mouse, mole, rabbit or snake) is accidently picked up by the baler.

In the case of damp hay, small amounts of soil incorporated into the bale during raking contain the bacterium, and the tightly baled, damp hay provides the moisture and decaying matter needed for sporulation. When a carcass is the cause, sporulation occurs around the decaying dead animal packed tightly in the bale. The decaying carcass provides the bacterium and moisture required for toxin production.

Since larger round bales are fed outside and “whole,” it is less likely that the owner will notice contamination. Small bales are more often fed by the flake, so wet spots and dead animals are noticed more readily. It is important to note that even if a carcass is well desiccated (dried) within a bale of hay, the botulin toxin can still be present and dangerous. Suspect hay, be it from a round bale or square bale, should not be fed to horses. Since it is hard to determine the extent of the contamination it is best to discard the entire bale.

Horses can also contract botulism from water contaminated with animal feces or a dead animal carcass. Keep all water sources clean, and regularly scrub water tanks and buckets. If you find a dead creature or evidence of fecal material in your horse’s water bucket or water tank, empty it and clean it well before refilling.

Once a horse ingests the botulin toxin it quickly passes out of the digestive tract into the bloodstream and disperses throughout the nerve cells. Once the toxin enters the nerve cells it prevents the transmission of impulses that stimulate muscle movement. It sets up a neuromuscular blockade that leads to generalized weakness and eventual paralysis.

The early symptoms of botulism might be easily missed: some horses spend extra time lying down, or push their feed around in the feed tub, seemingly uninterested in eating. Horses exhibiting such behaviors should be monitored closely.

As the disease worsens, the horse may pace anxiously or stand with all four feet placed close together under his body (like standing on a ball) and you may notice muscle tremors in the shoulder of flank area. The horse may walk stiffly with short strides or seem weak and stumble. The tail might lose its tone. The horse may be unable to eat and will drool because he can no longer swallow properly. Horses can lose control of their tongue completely, causing it to hang out of the mouth. A green, feed tinged discharge is often seen coming from the nostril.

At some point the horse may go down and be unable to get up. Horses will often collapse when they go to lie down. Eventually, the toxin causes paralysis and inhibits respiratory and other critical functions.

Botulism poisoning in horses is rare and the central nervous symptoms it causes can mimic those of other diseases such as rabies, equine viral encephalitis or the nervous form of equine rhinopneumonitis. The prognosis of the disease is directly related to the amount of toxin the horse has ingested. While botulism is difficult to treat, if caught early the horse has a much better chance of recovery. Discuss your horse’s risk factors with your veterinarian. If there have been cases in your area or if you feed round bales, it might be wise to have your horse vaccinated. If you observe your horse exhibiting central nervous system symptoms, contact your vet immediately.


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