It takes less of the botulism toxin to kill a big horse than it does to kill a small mouse.

Horses are extremely sensitive to botulinum toxin, which is produced by the bacteria Clostridium Botulinum. As a matter of fact, it takes more of this toxin to kill a mouse than it does to kill a horse.

The toxin enters nerves and stops their functioning, particularly of the nerves supplying muscles. The toxin is found in carcass-contaminated bales of hay, water troughs and decomposing bedding and haylage (fermented hay). This can have devastating consequences, so be aware of horses eating mouldy bedding and never feed them spoiled roughage.

Symptoms

Botulism causes a flaccid (loose and floppy) paralysis, particularly of the nerves of the head, causing drooling, difficulty in chewing and swallowing, progressing to muscle trembling or sometimes complete paralysis.

• Affected horses often stand for long periods with their heads in their water bucket, but are unable to drink properly.
• They tend to lose weight because they can’t eat properly.
• In some cases horses are able to eat chopped lucerne and hay, but can’t manage pellets and concentrates.
• Choke and lung infection from food getting into the lungs is an unfortunate and potentially fatal complication.
• Death sometimes occurs as a result of paralysis of the muscles involved in breathing.

A definitive diagnosis is difficult, because tests need to demonstrate that the toxin is in the food, water or intestinal contents. The typical clinical signs of flaccid paralysis and a history of feeding haylage or carcass contamination of food and lack of vaccination are highly suspicious. Often the response to treatment is the only way of “diagnosing” botulism.

Treatment

The treatment of choice is the intravenous administration of a single dose of botulism antiserum. The antiserum contains neutralising antibodies that bind to the botulinum toxins circulating in the horse’s bloodstream.

It has no effect on the toxin that has already blocked the junction between the neuron and the muscle cell, so intensive nursing is required for affected horses until new neuromuscular junctions are created (this usually takes up to 10 days).

• Confine affected horses to a padded stall and provide food and water (or milk, for foals) via nasogastric intubation (stomach tube).
• Thick bedding should be provided to prevent pressure sores from forming as a result of constant lying down.
• Administering fluids (both orally and intravenous) is important as botulism horses are of-ten severely dehydrated due to their inability to drink.
• Food and water at chest height help to prevent aspiration pneumonia and make eating and drinking easier.
• Antibiotics can be given to prevent the development of pneumonia.
• Ask your vet which ones to use as certain antibiotics can make the symptoms worse.
• Horses that remain standing often regain the ability to swallow within 3 to 7 days after the antiserum is administered.
• A full recovery for these horses typically occurs in about one month.

The prognosis is poor if the horse lies down and is unable to stand up again without assistance – especially if the horse has been recumbent for 24 hours.

Prevention

• Vaccination is essential to prevent the disease – and it is cheap.
• An initial vaccination is given under the skin, followed by a booster four weeks later.
• Immunity lasts nine to 12 months, so all horses should be vaccinated annually.
• A temporary swelling may form at the injection site, but in our experience this is rare.

The antibody levels in a vaccinated mare’s colostrum are thought to protect the foals against shaker foal syndrome for up to 8 weeks after birth.

High-risk foals can be vaccinated:

• 3 doses administered four weeks apart can be administered as early as when foals are 2 weeks old.

Work with your veterinarian to develop the most appropriate vaccine schedule for your horse(s). Other ways to avoid botulism are to provide high-quality feeds and to remove carcasses, such as dead birds, from water troughs as soon as possible.

Also read:
How to buy a horse
How to feed your horse
Beware of thumps in horses
Dealing with worms in your horses
Being on top of a horse emergency
Treating tendon injuries in horses
What you should know about colic in horses
What is strangles in your horses and how to treat it?

• This article was written by Dr. Marc Walton and first appeared in Farming SA.