Botulism is not normally on the northeastern horse owner’s list of most feared diseases, but horses are amongst the most sensitive of species to the debilitating effects of the toxins produced by Clostridium botulinum bacteria. This anaerobic bacteria forms spores that are commonly distributed in soil, standing water and vegetation throughout the Northeast and Mid-Atlantic regions. Toxins produced by C. botulinum are amongst the most lethal and potent known and can affect humans as well as cats, dogs, cows and other livestock in addition to horses.

While there are seven strains of C. botulinum, not all affect horses. The most common strains that impact equines are Types A, B and C, with Type B responsible for anywhere from 75-90% of botulism cases, depending on your source. Types A and B are most commonly associated with exposure through decaying vegetation or wound contamination while Type C is generally diagnosed after horses consume infected carrion.

Three Means of Exposure

While botulinum spores are common throughout the environment, they are generally inert and are not actively producing toxin. Trouble ensues when the spores find themselves in the right sort of environment for toxin production; generally, this environment is anaerobic and moist, with low levels of acidity and high levels of protein. Examples include damp, decaying vegetation (such as improperly cured or stored hay, whether square or round baled), loose hay that has become mashed into the mud and soil around feeders, or puncture wounds on the body that close over at the opening.

The most common source of infection, particularly in adult horses, is through consumption of the botulinum toxin itself through compromised feed. Sometimes this is called forage poisoning, and Type B botulism is usually the culprit. Hay and silage are the most common types of infected feed; commercial grain is rarely contaminated.

A second cause of infection is the ingestion of C. botulinum spores, which then produce toxins that are absorbed via the gastrointestinal tract. The correct term for this form of infection is toxicoinfectious botulism; this occurs most often in the immature digestive tracts of foals and is also known as Shaker Foal Syndrome.

The third manner in which infection occurs in horses is through spore contamination of a suitable wound, such as a puncture. The low oxygen concentration in these types of wounds is ideal for toxin production, which is then quickly absorbed by the blood stream and causes symptoms to develop. Castration sites and umbilical hernia repairs are also frequent locations for C. botulinum infection.

Symptoms: Foals Versus Adults

In both foals and adults infected by botulinum toxin, horses experience neurologic symptoms due to the toxin’s interference with the nervous system’s ability to signal muscles to contract.

In adults, the muscles that are affected earliest tend to be the ones that the horse uses the most, especially those controlling the eyelids, tail, and tongue. As a result, a common early symptom is dysphasia, or difficulty eating, which occurs because the animal cannot coordinate the movement of hay or grain to the back of his mouth. Reduced tongue strength often causes it to protrude or hang from the horse’s mouth, and if the tongue is tugged on, the animal is slow to retract it. Additional symptoms include symmetrical muscle weakness that progresses, sometimes quite quickly. Affected horses show a shuffling gait, with short strides and frequently a camped under posture as well as muscle tremors, especially in the triceps. The disease progresses until the animal is recumbent (lying down) and has flaccid paralysis, meaning that muscles of the body appear relaxed but the animal has no control over them.

Foals show slightly different symptoms, including drooling, dribbling milk, and increased amounts of time spent lying down. When affected foals are forced to stand up, they develop muscle tremors which then cause them to fall back to the ground; hence the name Shaker Foal Syndrome.

Diagnosis

Diagnosis of botulism can be difficult, because the disease is relatively uncommon and its symptoms can mirror many other problems. Due to the speed with which the disease progresses, diagnosis is usually based on clinical symptoms and ruling out other disorders.

One of the most common basic tests used to diagnose potential botulism is the tongue stress test. This is a subjective assessment of the horse’s tongue strength and their ability to retract the tongue into the mouth. The vet will gently tug on the horse’s tongue; healthy horses will normally retract their tongue with one or two tugs, while a horse with botulism may be unable to resist at all.

Another test vets can use in the field is the grain test. This is an objective assessment of how well the horse is able to eat. The vet will place eight ounces of sweet feed in a large, flat tub and then time how long it takes the horse to eat the sweet feed. Most healthy horses will be able to clean up this amount of grain in two minutes or less; horses affected by the botulinum toxin will drool, push the grain around, drop the grain from their mouths and otherwise be unable to successfully clean the tub within this time span.

While blood tests do exist to confirm the presence of the botulinum toxin within the horse’s body, they are time consuming and results are generally not available before an affected horse has either recovered or died.

Vets will need to rule out other diseases which can cause neurological symptoms, including eastern equine encephalitis, West Nile Virus and rabies, as well as disorders such as colic or choke that can contribute to unusual behavior and postures.

Treatment

For a horse that is suspected of having contracted botulism, fast action is warranted. Whether or not a horse will survive the infection has to do with how much of the toxin has entered the body and how quickly symptoms have progressed. In general, prognosis is poor for animals that have become recumbent. Death is normally caused by suffocation when the control of pulmonary muscles decreases.

Once a vet suspects botulism infection, the only known treatment is administration of a botulinum antitoxin. The antitoxin must be specific for the strain or type of botulism that is infecting the horse; there are a few antitoxins that are multivalent, meaning that they are effective against multiple strains. One dose of antitoxin is administered; the antitoxin only works to neutralize the effects of toxin within the body and cannot repair or undo damage already done.

If the spread of toxin is effectively halted, horses then must receive supportive care to allow them time to grow new nerve cells. Some concerns that arise in caring for these animals include the risk of aspiration pneumonia, preventing the development of pressure sores on recumbent animals, possible ocular ulceration due to loss of eyelid function and also loss of proper gut function due to inactivity, restricted diets and the use of antibiotics. The costs for administration of antitoxin and after care frequently run between $10,000 and $15,000.

Prevention

Without doubt, a horse infected with botulism stands only a slight chance of recovery in most cases, due to the rapid onset of disease symptoms and the high costs of treatment. The good news is that a vaccine has been available for horses since the mid 1980s.

Vaccination of horses in high-risk areas or environments is strongly indicated. Currently, the vaccine is only effective against Type B botulism, the most common strain affecting horses. The vaccine has been proven to be highly efficacious, as there has never been a case of Type B botulism reported in a fully immunized horse.

Broodmares should be boostered for botulism four to six weeks prior to foaling in order ensure that their colostrum provides protection to the foal; this passive transfer of antibodies will last for eight to twelve weeks following parturition. Foals born to unvaccinated mares can be inoculated beginning as early as two weeks of age.

Round bales are often seen as culprits causing botulism infection, but they are blamed for the wrong reasons. While horses can and do contract botulinum spores through the consumption of carrion in cured hay, and round bales tend to have a higher incidence of dead matter within them, this usually leads to infection with Type C botulism. Most botulism cases in horses are due to Type B.

Feeding round bales can be a risk factor, however, and horses who feed on them should be vaccinated. Because spores for C. botulinum are present in most places where hay is cured, they are present in most hay, silage and other forage products. Round bales that are not rapidly consumed or that have been put up incorrectly are more likely to have sections in the interior that provide the moist, anaerobic environment favored by C. botulinum spores.

Take Away Message

Botulism infection in horses is frequently lethal and once the horse is infected, the disease progresses rapidly, killing horses within twelve to forty-eight hours of infection unless the proper antitoxin is administered. Prevention for at risk horses is available through vaccination and careful management of both forage feed and wounds.