Horses are the most sensitive of the domesticated animals to botulism.

Botulism is a disease that occurs when toxins produced by the bacterium, Clostridium botulinum, enter the horse's body causing weakness which may progress to paralysis. The botulism bacterium is a spore-forming, anaerobic bacteria (grows in the absence of oxygen) which is found world wide. It is commonly present in soil and in decaying animal carcasses. It occurs less often in decaying plant material. Hay, and especially haylage, can be contaminated with the botulism bacterium during the raking and baling process. The higher moisture levels (in comparison to dry hay), the anaerobic conditions and a pH above 4.5 in some haylage are ideal conditions for the growth of this bacterium. When the bacterium grows, it produces one or more toxins. These toxins block the connection between the nerves and muscles. As a result, affected horses often exhibit signs varying from muscle weakness to paralysis (1).

Affected horses:

• Usually have muscle tremors.
• May be so weak that they cannot stand up.
• Lose control of their tongue so it may hang from their mouth.
• Can't eat and they drool because they can't swallow.
• May walk stiffly with a short stride or they may be weak and stumble. Their tail may lose its tone.

Eventually they die because their respiratory muscles become paralysed or because they get other health problems from being down. These clinical signs can occur within several hours or up to 7-10 days post ingestion of the contaminated feed.

Horses can get botulism in any of three ways. In foals up to 8 months of age, botulism can occur if the bacterium grows in the foal's intestines (1). Foals that are growing well and are being fed grain are most likely to get into trouble. These foals are called "shaker foals" because the muscle weakness from the toxin makes them tremble. Occasionally a wound can become infected with the bacterium and cause botulism in adults. Fortunately, this is rare. Far more commonly, botulism occurs when horses eat feed or water which contains preformed toxin. Clostridia grow on substrates (food sources) which are above a pH of 4.5 and are in an anaerobic (non oxygen) environment. Here they produce toxins. Improperly preserved haylage can be an ideal environment for clostridial growth. Water and feed can also be contaminated with the carcass of a dead animal. When several horses develop botulism, toxin in feed or water is usually the reason.

Several incidents of botulism occur each year after horses eat wrapped or bagged round bale haylage. In some of these outbreaks, the haylage looked and smelled spoiled. In others, the bales didn't look as if they were spoiled but horses eating them developed botulism.

Horse Health: Botulism in Horses - Feeding Your Horse - Hay Quality and Horse Nutrition - Feeding Horses Free Choice - Choose the right hay for your horse

Feed Optoins: Straight Grass Hay - Grass or Alfalfa Hay? - Identifying Quality Hay - Contaminated Hay - Economical Horse Hay - Hay News