Page 502 - The Veterinary Care of the Horse

means that it can only live in an environment that has no oxygen.

VetBooks.ir Causes

In an anaerobic environment, these bacteria produce 8 different neurotoxins; types B and C
are the commonest causes of botulism. The disease usually occurs when feedstuffs are

contaminated with decaying plant matter or dead animals, e.g. rodents. Stagnant water
sources contaminated by carcases are another source of the toxin.

There are 3 recognized routes of infection.

1 Ingestion of the toxin. This is often referred to as forage poisoning as the source is often
dead animals accidentally included in big bales of silage, haylage or hay.

2 Ingestion of spores from the environment which develop into toxin-producing bacteria in

areas of tissue damage, e.g. gastric ulcers in foals.
3 Contamination of wounds such as umbilical stumps and castration wounds with

Clostridium botulinum which then produces toxin that circulates around the body.

The toxin interferes with the release of the neurotransmitter acetylcholine at various nerve
endings including neuromuscular junctions. The muscles cannot function properly without

stimulation from the nerves and paralysis is the result. The sensory nerves are not affected
and nor is the central nervous system, so affected animals often remain bright and alert,

despite their weakness or paralysis.

Clinical signs

The severity of the clinical signs varies with the dose of toxin.

FOALS

The disease is seen in foals as young as 1–2 weeks of age. In severe cases, the foal may be
found paralysed and unable to stand, or even dead. If the intoxication is less severe, the first

signs may be a stiff-legged gait and muscle weakness, with more time spent lying down than
usual. Due to loss of muscle tone of the tongue and difficulties with swallowing, milk may

dribble from the mouth or return down the nostrils. The muscle tone of the eyelids and tail
also becomes weak. These foals are still bright and alert. As the disease progresses, they have

difficulty standing and develop muscle tremors. At this stage they are known as ‘shaker
foals’. Eventually they are no longer able to stand and have difficulty breathing. They
ultimately die as the result of paralysis of the respiratory muscles.