Botulism is a form of paralysis that can be highly fatal if not recognized and treated early. The neuromuscular paralysis is due to interference with the release of the neurotransmitter acetylcholine (ACh) at motor endplates. The interference is induced by a toxin from the bacteria Clostridium botulinum. There are 8 different types of C. botulinum but types B, C, and D are the only ones thought to cause disease in horses. The bacteria survives and thrives in decaying vegetable or plant matter and most cases occur through feeding of spoiled feeds or feeds contaminated with animal parts.
There are 3 well-defined forms of the disease and one speculative:
- Ingestion of preformed toxin (forage poisoning; most common form in adult horses)
- Ingestion of spores with subsequent production of toxin within the intestinal tract (Toxicoinfectious form, “shaker foals”; most common form in foals)
- Contamination of a wound with bacterial spores (Wound botulism)
- Speculation that “Grass Sickness”, an equine dysautonomia syndrome common in England, Wales, and Scotland is a form of botulism (C. botulinum type C1)
Clinical Signs
Botulism is described as a flaccid neuromuscular paralysis. Signs are variable and dependent on amount of toxin present. In foals, the first signs may include a stiff and stilted gait, followed by generalized weakness, muscle tremors, and progressing to recumbency. There is often milk from the nostrils due to regurgitation. Other signs may include poor tail, tongue, and eyelid tone. Breathing becomes laboured and the foals may die due to paralysis of the respiratory muscles.
In adult horses the first signs are usually loss of tongue, tail, and eyelid tone. Most owners become alerted to a problem when the horse develops problems with ingestion of feed. These may include dropping of feed; feed material in the nostrils (dysphagia); excess salivation; or an inability to chew hay. Horses may spend a considerable amount of time playing in their water but have difficulty in drinking. The gait may be stiff, short-strided, or shuffling. Muscle tremors become apparent. There may be an upper airway noise (stridor) due to paralysis of the pharynx/larynx. Bladder paralysis, gastrointestinal stasis (ileus) and constipation can occur in advanced cases; these are further complicated by persistent recumbency.
Diagnosis
The diagnosis is typically made on the basis of signs. In some adults and most foals Clostridium botulinum can be cultured from the manure.
Treatment
Hyperimmune antiserum greatly improves the chances of survival, but it must be administered early in the disease course and does not guarantee a positive outcome. It will not reverse clinical signs and affected horses may continue to worsen after administration. The antitoxin will only bind circulating toxin and does not neutralize that already bound to the motor endplate. Nursing care and nutritional support are critical to a successful outcome.
In areas where botulism is a problem in foals the disease can be prevented by vaccinating the mare prior to foaling.
Tags: Neurology; Muscle; Infectious disease