Beat Botulism

Foals with botulism generally have a very good survival rate with very aggressive therapy: Their smaller size makes treatment and costs more manageable.

Botulism is largely avoidable, but should your horse encounter this disease, immediate veterinary treatment could save his life.

Sometimes a feeding decision might seem harmless in itself, and even economical. But its result could be deadly: "The person in question poured bags of horse feed into a big bin, then parceled out the feed from the bin. After many years of doing this, he decided to feed all of the feed, some of which had been at the bottom of the bin for years," recalls George McCommon, DVM, of an incident that occurred when he was practicing as a racetrack veterinarian in Delaware.

Shortly thereafter, two of the horses fell ill. "The owner thought the horses were colicking--they were depressed, just standing and staring at the feed. It was when I came and started to pass a nasogastric tube that we realized that the horses could not swallow or eat because of paralyzed throat muscles," says McCommon, who is now an associate professor in Veterinary Science at Fort Valley State University in Fort Valley, Ga.

The combination of clinical signs--two horses becoming sick simultaneously and the discovery that the affected horses had been fed years-old feed--quickly led to a diagnosis of botulism caused by contaminated feed. "When we realized what had happened, we told the owner to dispose of the feed," McCommon says, "so he dumped the feed out in the pasture where another horse got sick from it!"

Despite the errors in judgment, the three horses were fortunate and made complete recoveries. Many other horses and foals are not so lucky, as botulism can be fatal.

A Living Nightmare

Botulism is caused by toxins produced by Clostridium botulinum, a bacterial organism. Botulism toxin is the most deadly toxin in the world. "The toxin binds to nerve cell endings and interrupts nerve impulses to the muscles," explains Kim A. Sprayberry, DVM, Dipl. ACVIM, an internist at Hagyard Equine Medical Institute in Lexington, Ky. "As a result, the affected horse suffers generalized progressive weakness and, in severe cases, paralysis of the muscles, including those that control movement and breathing. Bladder function and intestinal tract motility are often also impaired."

A severely affected horse can't stand, move, blink, eat, drink, or swallow. In a prey species like the horse, this is particularly insidious because botulism only affects muscle movement, not sensory nerves. So the incapacitated horse continues to experience hunger, thirst, stiffness, pain, anxiety, and all other sensations.

Although incidence is low, botulism is of concern because:

  • Horses are exquisitely sensitive to botulism (it takes more botulism to kill a mouse than a horse);
  • It results in a high mortality rate if not treated promptly;
  • It's easy to misdiagnose (and, thus, true incidence might be underreported, particularly in cases involving a single horse); and
  • Treating serious cases can be very involved and expensive.

Horses contract botulism primarily by ingesting contaminated food. Horses can also acquire botulism by drinking contami-nated water or by absorbing toxin through wound sites. Foals and the occasional adult can pick up the bacterium when grazing.

Food and Water Poisoning

"Spoiled hay and silage are the most common sources of forage poisoning," states David R. Hodgson, BVSc, PhD, Dipl. ACVIM, FACSM, professor and head of the Department of Large Animal Clinical Sciences at the Virginia-Maryland Regional College of Veterinary Medicine in Blacksburg.

Silage is made by packaging forage while it's still moist in airtight plastic, thereby setting up a fermentation process that ultimately preserves the forage. During the process, a high-acid, low-pH anaerobic environment develops, which prevents clostridial and bacterial growth. But C. botulinum likes an anaerobic environment, so if something goes wrong in the processing and the acidity does not increase fast enough to inhibit the bacteria, the silage can become a toxin factory. This can happen if:

  • The excessively moist grass is wrapped too hastily. The necessary acidification and pH decrease won't occur, allowing C. botulinum spores to sporulate under anaerobic--but pH neutral--circumstances, says Lutz S. Goehring, DVM, MS, PhD, Dipl. ACVIM, assistant professor in equine internal medicine at Colorado State University.
  • The silage contains soil that's contaminated with C. botulinum spores.
  • The plastic tears and air gets in, causing the hay to spoil where oxygen is present, says Sprayberry. "The pH stops its decline, and when it does, the survivability of bacteria (including C. botulinum) goes back up."

A round bale can go bad if left uncovered in the field and rain penetrates the bale's center, says Goehring. "This causes rot, decay, and an anaerobic environment ideal for sporulation of C. botulinum already present because of a small cadaver or spore-contaminated soil mixed with the hay."

The small cadavers he's talking about are those of animals killed and swept into the hay during the mowing process. If undetected, says Sprayberry, these bits of decomposing carcasses are incorporated into and contaminate chopped or baled hay and even processed cubes and pellets.

Other botulism sources include:

  • Mowed grass clippings, or wilted or spoiled produce, which form ideal conditions for clostridial growth, Sprayberry says.
  • Feed bins containing old, crusty feed, fecal droppings, or carrion carried in by birds.
  • Water sources with decaying carcasses.

When a contaminated forage or feed product is distributed among a number of horses at once, the consequences can be horrifying. Hodgson reports that several multiple-horse botulism poisonings have occurred in the last two decades in the United States, Europe, and Australia from contaminated silage, grass clippings, oaten chaff, and alfalfa cubes. At a Florida farm in 2008, about 100 horses died from contaminated haylage. Several years ago Hodgson dealt with an outbreak at a yearling sale in Australia, where 33 horses died or were euthanized after eating contaminated chopped hay.

Contamination Through Grazing

In the mid-Atlantic States, the moisture content and pH levels in the soil combine to provide a favorable soil habitat for C. botulinum spores, which grazing horses can ingest.

"This form of botulism, known as toxicoinfectious botulism, usually occurs in foals (resulting in a form of botulism known as shaker foal syndrome)," explains Suzanne Sewell, DVM, CVA, CVC, vice president of the North Carolina Veterinary Medical Association and owner of Crestview Veterinary Hospital in Marion, N.C. "The normal gastrointestinal flora in a healthy adult horse typically prevent the growth and multiplication of the Clostridium organism, but the immature GI tract in foals allows the bacteria to multiply and to produce further toxin in the GI tract."

"Although foals of any age can get botulism," notes Sprayberry, "the peak age for affected foals is 4 weeks, with about 70% of cases arising in foals 2 to 6 weeks of age."

Wound Botulism

In rare situations a wound can become infected with C. botulinum. Wounds that can become infected include gastric ulcers, castration incisions, abscesses in the navel or lungs, wounds in skin and muscle, and surgery incisions. The foal's umbilicus can also become contaminated.


"Occurrence depends primarily on the density of botulinum spores in the environment, and on the area feeding practices," says Goehring. "We recognize about one to three clusters each year of botulism cases due to poor-quality hay or large round bale feeding on farms in Wyoming, Colorado, and New Mexico--a 'cluster' typically including several cases on a farm."

In the West, outbreaks are mostly caused by decaying carcasses or toxin in the feed or feeding area, says Sprayberry. Because of the rarity of pasture, most horses in the arid West are fed baled hay or processed feed, cubes, or pellets. Spores don't live in the soil there.

"In the East, most horses are on pasture and, therefore, do not consume dead animal parts," Sprayberry notes. But in the mid-Atlantic states, the soil contains spores which can be ingested through grazing. "Referral practices in endemic areas see at least a handful of cases every year."

Regardless of where a horse lives, if he consumes botulism-contaminated silage, feed, or water, he'll likely suffer the conse-quences. "Most people in Saskatchewan don't feed haylage, and botulism isn't very common around here," says Katharina L. Lohmann, MedVet, PhD, Dipl. ACVIM, an associate professor in large animal medicine at Saskatchewan's Western College of Veterinary Medicine. Nevertheless, botulism does occur: "A recently reported outbreak was attributed to feed trough dirt."

Clinical Signs

Early signs usually include difficulty swallowing (dysphagia), eating, and/or drinking, along with eyelid, tongue, and tail weakness. "Affected animals become increasingly recumbent (unable to rise) due to progressive muscle weakness," says Hodgson. "Muscle tremors, carpal (knee) buckling, and ataxia (incoordination) can occur. Pharyngeal and lingual (tongue) paralysis causes marked dysphagia and predisposes to aspiration pneumonia. Animals tend to quid (chew and drop) their food. Paralysis of the diaphragm and intercostal muscles (those along or between the ribs) results in an increased respiratory rate and decreased chest wall expansion. Severely affected animals die from respiratory paralysis and cardiac failure."

Goehring adds, "After the onset of initial clinical signs, but before the horse becomes recumbent, the muscles that are responsible for standing--the so-called anti-gravity muscles of the limbs--show signs of fatigue and fasciculations (involuntary muscle twitching). A standing horse with clinical signs of botulism shows a characteristic low head and neck carriage, and it brings its forelimbs close together to its hind limbs. This gives them the appearance of a 'horse on a ball.'

"With an affected parasympathetic nervous system, the frequency of passing feces decreases and some horses end up with a paralyzed bladder that is unable to contract," Goehring notes. "Urine leaks out of the penis or vulva."

Clinical signs in "shaker foals" include the inability to nurse (drooling milk) and muscle trembling (thus the name shaker foal), followed by abrupt collapsing.

The speed of onset and the severity of clinical signs depend upon the amount of toxin absorbed: Small amounts could take up to two weeks to produce signs, whereas large amounts can lead to recumbency and death within 24 hours--sometimes so quickly that few or no clinical signs are noticed. "I have seen horses in the afternoon with some mild muscle fasciculations and standing, and they were found dead the next morning," states Goehring.


University of Pennsylvania researchers are working on a test to quickly detect C. botulinum neurotoxin type B in equine samples, says Hodgson. He notes that fecal samples are mainly used and serum might also be useful, but concentrations are often too low to detect in the latter sample type. But for now, current tests for botulism are too inaccurate and take longer than the affected patient could survive if treatment were delayed until results were available.

Therefore, diagnosis is based on clinical signs, ruling out of other disorders, and history, including feeding practices and risk factor analysis. Says Goehring, "When more than two to three horses show similar clinical signs within a short period of time, we think of (botulism) intoxication."

Single horses can be trickier to diagnose, as many of the signs of botulism could be attributed to other disorders, such as choke or colic. "Veterinarians can miss out on early and effective treatment just because botulism didn't make the list early enough," notes Lohmann. "That's the scary thing about rare diseases."


The goal of treatment is to keep the horse alive long enough to allow the damaged nerves to spontaneously regenerate. This is done with antitoxin (this doesn't reverse damage, but it halts progression), antibiotics (to decrease complications caused by aspiration pneumonia or pressure sores), discontinuing use of and discarding suspect feed, removing foals from pasture, cleaning up contaminated water sources, stabling (to preserve an affected horse's strength), and supportive therapy.

Supporting a seriously affected horse for the 10-14 days it takes for nerve regeneration can be the most daunting aspect of treatment. Says Sprayberry, "Nursing care could include catheterization of the bladder, evacuation of the rectum, application of eye ointment to replace the lost function of blinking, application of a shade over the eyes to provide for the lost function of pupil constriction, feeding and watering through an indwelling nasogastric tube, and frequent rolling of a recumbent horse from side to side. Horses can colic, aspirate feed material into the trachea and lungs, develop extensive areas of myositis (muscle inflammation), and suffer severe abrasions on the head and limbs from struggling and paddling."

Horses that are mellow accept recumbency and handling, and those without respiratory paralysis can be assisted with a sling for a short period once daily. "If the horse does well, we increase the duration of standing gradually," Goehring states.

But Sprayberry warns that slinging is contraindicated when chest muscles are severely weakened, visible as very shallow breathing excursions. "The weight of the horse in the sling will cause compression of the thoracic wall and kill the horse."

Horses with breathing problems might respond to oxygen therapy. If breathing muscles are seriously impaired, horses will need mechanical respiration to survive. "This support is only feasible for young or small animals," she says. "Ventilators can only support breathing in animals weighing no more than about 400 pounds."

Treatment can be costly: Antitoxin can cost $1,000-$3,000 per dose, and ventilating can add up to $300-$500 per day. On top of that, there are also the costs of antibiotics, nutritional support, fluids, and human resources.

A botulism vaccine--an inactivated toxoid, like the tetanus vaccine--is available and is considered very effective. However, the only botulism toxoid readily available in North America protects only against the B strain (see sidebar on page 32). Farms considering feeding haylage, silage products, or round hay bales should consider protecting their horses from botulism, especially broodmares, who can pass their antibodies on to their nursing foals.


Prediction for case outcome depends on the amount of toxin exposure and the promptness of treatment, says Sewell.

Describes Sprayberry, "Horses which are fine one day and recumbent the next have ingested a larger and likely lethal dose of toxin and will have a more fulminant (sudden, severe, and intense) course of disease. In this instance, so many neurons will be destroyed before the problem is noticed and antitoxin given that the life-sustaining respiratory muscles (diaphragm and inter-costals) will fail, leading to death."

Adult horses that progress to respiratory paralysis usually die or require euthanasia. Those that do not develop respiratory paralysis can make a complete recovery without lingering effects, says Lohmann, although some can have prolonged problems stemming from complications (pneumonia, pressure sores, etc.).

Foals generally have a very good survival rate with very aggressive therapy: Their smaller size makes treatment and costs more manageable.

Take-Home Message

Botulism is largely an avoidable problem. You can reduce risks by:

  • Being aware of the disease prevalence in your area and recognizing early clinical signs of botulism.
  • Vaccinating horses, pregnant mares, and foals that reside in areas where type B botulism is endemic (see sidebar on page 32).
  • Providing high-quality hay and feed.
  • Avoiding or using caution when feeding haylage and big-bag bales. "Smaller square bales are preferred over large round bales," states Sewell. "Round bales are more likely to spoil if fed in pasture in entirety and have a higher chance of contamination." There is no significant increased risk if the round bale is of good quality and is fed one feeding portion at a time and the rest kept in the barn.
  • Inspecting water sources for dying waterfowl and wildlife, suggests Lohmann.
  • Not spreading poultry litter on pasture which often contains bits of dead birds, notes Hodgson.

About the Author

Marcia King

Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.

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