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Botulism in Horses

By: Dr. Melissa Mazan

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Clostridium botulinum is a bacteria that is able to survive indefinitely in the soil in spore form. As a spore, it is inactive and protected. When the clostridium bacterium finds the right conditions, such as rotting vegetation or a puncture wound, it starts to proliferate and produce an extremely potent toxin. Horses, for reasons we do not understand, are more sensitive to this toxin than most other species. This means that it takes a very small amount of toxin to kill a full-grown horse.

The Toxin

All mammals use a chemical called acetylcholine to act as a signal from the nerves to the muscles. Signals pass from the brain, to the peripheral nerves, and then via the action of acetylcholine, to the muscles. The botulism toxin binds to the ends of the nerves, and prevents the acetylcholine from being released – and the end result is that the muscles gradually cease to work.

Since muscle action is important in many different organ systems, the effects of botulism can result in the gastrointestinal system slowing down, defecation becoming difficult and inability to urinate, in addition to many other signs of muscle weakness. Once the toxin has bound to the nerve ending, it cannot be moved.

There are many different subtypes of botulinum toxin – the ones that we worry about in horses are types B and C. We tend to see C.botulinum type B in Kentucky, Pennsylvania, and the mid-Atlantic area, whereas type C has been found primarily in Florida. This means that there tend to be more cases of botulism in these areas. In addition to different subtypes, there are different forms of botulism.

Forage Poisoning

This is the most common type of botulism in adult horses. The bacterium finds a good place to proliferate – often rotting vegetation or carrion. Typical signs include weakness and muscle trembling, difficulty swallowing (dysphagia), drooling, stiff or stilted gait, and recumbency. Oftentimes, these signs will improve temporarily with rest, and then become severe again. Horses may also present with signs of colic, because the gastrointestinal system is no longer working properly.

  • Diagnosis

    Diagnosing botulism can be difficult. Horses are very sensitive to the botulinum toxin and the amount necessary to make a horse very sick, or even kill a horse, may be below the level that can be detected. The best method of diagnosis is to rule out other causes of the signs and a careful clinical examination.

    However, your veterinarian can try to detect toxin in the serum of your horse, in the feed that is suspected of causing the problem, or, in the case of an outbreak when some horses may have already died, in the gut contents of those horses.

  • Treatment

    With forage poisoning, the bacteria have already formed the toxin, which the horse then eats. If the disease is detected early, then antitoxin may help. The antitoxin works by binding the toxin that is floating free in the serum. Once the toxin has bound to nerve terminals, then the antitoxin is ineffective. Because forage poisoning is due to pre-formed toxin, antibiotics are of little use.

    The mainstay of treatment for the chronic stages of forage poisoning is good supportive care. This can be both very expensive and labor-intensive. For instance, the horse that cannot swallow will likely need both intravenous fluids and nutritional support. The down horse may need to be maintained in a special sling apparatus, and is likely to develop multiple pressure sores, despite the caregiver's best efforts. In the case of the horse whose respiratory muscles have failed, it will be necessary to begin mechanical ventilation. For a full-grown, adult horse, this is often impossible even in a referral hospital, due to the size of the horse.

  • Prognosis

    In general, if the signs progress rapidly, the prognosis is poor. However, if the signs progress slowly, then the prognosis is better. This is probably because horses with slower progression of signs have not absorbed as much toxin as horses in which the signs progress rapidly. When horses do recover, it takes many weeks to months. This is because the nerve terminals must all experience their natural turnover in order to rid the body of the botulinum toxin entirely. By the time this happens, many horses have lost a large amount of weight, and have such weak muscles as the result both of muscular inactivity and lack of nervous input, that the survivor needs a long rehabilitation period

    Toxico-infectious Botulism (Shaker Foal)

    Foals up to 8 months of age are very likely to have stomach ulcers. These ulcers are basically a wound in the stomach lining, which can provide an ideal environment for the proliferation of the clostridium bacterium. Foals, with their innately curious natures, are more likely to eat soil than are older horses, thus ingesting the bacterial spores.

    Shaker foals develop much the same signs as do older horses with botulism. One of the most obvious signs, which gives rise to the name of the syndrome, is profound muscle trembling. This muscle trembling often subsides with rest, and reappears as the foal tries to walk again. The foals often appear weaker in the hind limbs. Because the central nervous system is not affected, the foals are alert, and are usually hungry – but they can't suckle properly. For this reason, pneumonia can be a secondary complication. The poor swallowing ability can lead to aspiration of milk into the lungs. Like adults with forage poisoning, these foals can appear colicky due to a dysfunctioning gastrointestinal system, and they can also have difficulties urinating. The ultimate cause of death with these foals is usually paralysis of the respiratory system.

  • Diagnosis

    Shaker foals are equally difficult to diagnose. Ancillary diagnosis should include gastroscopy in order to determine whether the foal does indeed have gastric ulcers.

  • Treatment

    The treatment is much the same as for forage poisoning, although it is considerably easier to ventilate a foal mechanically than it is an adult horse. Antibiotic therapy is more useful with toxico-infectious botulism, as it may be possible to kill the bacteria that are invading the foal's gastric ulcers, and thus decrease the amount of toxin entering the body. Antitoxin is very useful if it is given early enough. It also makes sense to treat the foal aggressively if gastric ulcers are present.

  • Prognosis

    Prior reports have indicated that the prognosis is very poor – less than a 10 percent chance for survival. However, if the facilities exist for mechanical ventilation, antitoxin is given early, appropriate antibiotics are delivered, and the owner is committed to the foal financially, there is a fair chance for recovery.

  • Prevention

    In areas endemic for botulism, a vaccine is available. This vaccine is given to the mare before foaling. The foal then received disease preventing antibodies through the mother's milk.

    Wound Poisoning

    This type of infection is rare in the horse. Similar to tetanus, C.botulinum can grow in wounds, such as puncture wounds.

    Diagnosis, treatment and prognosis are all similar to toxico-infectious botulism. Antibiotic therapy is particularly important in this case, as there is an active infection that can be treated. Often, the veterinarian must be aggressive in debriding, or opening up the wound, in order to let oxygen in. It is only in the absence of oxygen that the bacterium can thrive.

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