Beware of botulism

Knowing how to recognize the earliest signs of this fast-acting form of food poisoning could save your horse’s life.

Here’s one of those facts most of us probably prefer not to spend too much time thinking about: A single teaspoon of soil can contain as many as a billion individual bacteria.

And one type of bacteria in that soil—Clostridium botulinum—has the potential to cause serious harm to your horse’s health. Under the right conditions, C. botulinum produces the most deadly biological toxin known to man—the agent responsible for the fast-acting, often-fatal paralytic form of food poisoning called botulism. Without prompt, appropriate treatment, death can occur within hours or days after a horse ingests the toxin, and a single outbreak can kill every horse in a herd feeding from the same contaminated source.

Fortunately, botulism among horses is fairly rare. Probably no more than 100 to 200 or so horses succumb to this disease per year across the United States. With proper management of your feed sources and a few basic preventive measures, you can reduce the risk of botulism even further. Here’s what you need to know.

When conditions are right

C. botulinum is a rod-shaped, gram-positive anaerobe, meaning that it thrives where little oxygen is present. These bacteria are also capable of forming endospores. This means that when environmental conditions are not favorable to them, each bacterium forms a tough, multilayered outer membrane and goes dormant. That covering protects the endospore, often called just a “spore,” against all manner of threats including desiccation0, freezing temperatures and radiation, almost indefinitely. In fact, in 1995 researchers revived bacterial spores recovered from 25- to 40-million-year-old amber.

When environmental conditions are once again favorable for C. botulinum to thrive, the organisms emerge from their dormant state and begin to feed and multiply. “The most important factor is the lack of oxygen,” says Amy Johnson, DVM, DACVIM, of the University of Pennsylvania. “There are also some pH and moisture constraints, but the big factor is an anaerobic environment with very low oxygen levels.”

In the right anaerobic conditions, C. botulinum multiplies quickly, and as each bacterium matures and dies, it releases a deadly toxin, called either botulinum toxin or botulin. “Clostridium botulinum produces an exotoxin,” says Kim A. Sprayberry, DVM, DACVIM, of Cal Poly University in San Luis Obispo, California. “Unlike endotoxin, which is a part of the cell wall in gram-negative bacteria and which can cause serious disease whether the bacteria are dying or proliferating, the clostridia are gram-positive and only make their exotoxin when conditions are right.”

There are seven distinct botulinum toxins designated by letters, type A through G, but only three—types A, B and C—are likely to produce illness in horses. Types A and B normally reside in soil, but your risk of encountering them depends largely on where you live. “Type B is most frequently seen east of the Mississippi River, especially in the Mid-Atlantic region and Kentucky. On the West Coast we more frequently see type A in the soil,” says Johnson. “Some estimates have stated that up to 85 percent of cases in the U.S. are type B, which means that most of them are in [the Mid-Atlantic] and Kentucky, with smaller numbers of type A in other regions west of the Mississippi.”

Type C is found in the carcasses of dead animals and also in bird droppings. “This type can be seen anywhere, but is not as common,” says Johnson. “In our laboratory here, where we do lots of testing for botulism, type C is the least common that we see in horses.”

Three ways botulism makes horses sickBotulins A, B and C can cause equine illness in three main ways:

• Food-borne botulism. The most significant form of botulism in horses occurs when they consume feed or forage contaminated with botulinum toxins. The risk of this occurring increases when forages are processed or stored in ways that foster the growth of the bacteria—in a moist, warm, anaerobic environment. “The most common type of forage where botulism might be a risk would be hay in big bales that were either harvested too moist or stored improperly. The inner parts have been wet and spoil, creating the ideal conditions of moisture and a low-oxygen environment,” says Johnson. Silage or haylage, fermented forages usually fed to cattle and other ruminants, are also a risk if not processed properly. “Horses historically have generally not been fed silage, haylage or haylage in bales,” she adds, “but sometimes now with the high price of hay, some people are trying these feeds and running into problems.” 

Clumps of grass clippings from lawn mowers have also been implicated in botulism outbreaks. And Sprayberry saw a rash of botulism cases in horses who were turned out on pasture that had been submerged during recent flooding: “They ingested toxin that presumably came from rotted vegetation or the carcasses of rodents or other animals drowned in the floodwater.”

Animal carcasses caught up in the hay bales can also be a source of botulism—but this is rare. “This is the form that people worry about the most but is probably one of the least common routes of infection,” Johnson says. “If you find a dead animal in the hay, however, you shouldn’t feed that bale.” 

• Toxicoinfectious botulism (“shaker foal” syndrome). Foals get a slightly different form of botulism when they ingest C. botulinum spores rather than the toxins. “Foals are often nosing around on the ground or nibbling everything and may pick up spores,” Sprayberry says. “When they start colonizing and growing, they can produce the toxins, which are then absorbed directly from the intestines.”

Botulism in foals also occurs when the bacteria colonize ulcers in the gastric lining. “In this situation the bacteria do enter the tissue and begin making their lethal toxin,” Sprayberry says. “Nearly all foals go through some stage in which they develop erosions in the gastric lining, making them susceptible to botulism in parts of the country where the spores are endemic in the soil.”

• Wound botulism. When spores enter a deep wound that then heals over at the surface, a localized anaerobic environment within the tissue is created and botulism can occur. “The bacteria multiply and grow and produce the toxins that are then absorbed by the body,” says Johnson. This is similar to tetanus, but that disease results from a different species of bacterium, Clostridium tetani. Wound botulism is extremely rare.

Systemic failure

No matter how the botulin enters the bloodstream, its effects are the same: The toxin blocks the transmission of nerve impulses in the body. “The toxin prevents the neurons from releasing the neurotransmitter that tells the muscle to contract,” says Johnson. “So the muscles can’t contract properly because the signal from the neurons isn’t getting there.”

As a result, the muscles controlled by those nerves go flaccid, causing paralysis. The first signs may begin to appear within a few hours or days after exposure to the toxin. They worsen as the toxins spread throughout the body. “As more and more toxin molecules bind to neuromuscular synapses0, more and more muscles simply cease to function,” says Sprayberry.

“Usually the earliest signs include inability to swallow, which is called dysphagia,” says Johnson. “An adult horse may eat slowly, taking longer to finish a meal of grain or hay. Because it’s difficult to swallow the food, some may dribble out of the mouth. Sometimes feed or water will come out the nose, as well. The horse may get the food or water toward the back of the throat but then cannot swallow it, so it comes out the nose.” Most likely, you’ll notice that the horse’s lips are slack and immobile and, says Sprayberry, “the tongue may hang out the side of the mouth. The horse can’t retract it.” 

In foals, the problem is evident as difficulty nursing. “He may start to nurse and then cough because he can’t swallow the milk and it runs down the trachea instead of the esophagus,” says Sprayberry. “The swallowing reflex is complex, and the muscles involved have ceased to function. Those muscles are some of the first to become paralyzed.”

More generalized muscular weakness may develop as well. “The dysphagia is soon followed by obvious signs of weakness in the rest of the body,” says Johnson. The tail may become flaccid, and the horse may move about slowly with short steps. “In foals or adults, you might see muscle tremors,” says Johnson. “The horse has muscle tremors over the body, and as he gets weaker he starts to have balance problems and may show what is called elephant-on-a-ball stance, with all four legs close together under the body. Horses do this simply because they are weak and trying to fight gravity. Eventually they spend more and more time lying down.” A foal will stand quivering, then suddenly collapse.

The extent of the paralysis a horse experiences depends upon the amount of toxin in his system. In a milder case, he may do nothing more than become less active and not eat for a few days. In more severe cases, he may become fully recumbent. “Affected horses lie down frequently and for longer periods than normal,” says Johnson. “As the disease progresses, these horses may lose the ability to get up at all, or they can’t stand for more than a few minutes at a time before they have to lie down again. They become so dysphagic that they can’t swallow their own saliva and it comes out the nose or mouth. They can’t drink, and become dehydrated.”

Finally, says Sprayberry, “what kills the horse in the end is suffocation. The last muscle to weaken and become paralyzed is the diaphragm, and the horse is unable to breathe.”

A tricky diagnosis

Treatment of botulism is more likely to be successful if begun early, but identifying a case right away can be tricky. Many of the first signs are easily confused with more common causes, such as colic or choke0. “It looks like choke because they can’t swallow and there is food coming out of the nose or mouth,” says Johnson. “It can look like colic because they are not eating as they normally would and lie down a lot.” 

Tail flaccidity and generalized weakness, especially in the hindquarters, can look like other neurological diseases. One sign of botulism that is hard to miss is quivering in muscles throughout the body. More subtle signs that might point toward botulism are laxness in the lips and a lower head carriage. “The eyelid tone might be weak, or the tail tone might be weak,” says Johnson.“The best clinical test for botulism is the tongue tone test, because lack of tongue strength is one of the first places where we really see the muscle tone diminish,” says Johnson. 

“With a normal horse, if you pull their tongue gently out of the mouth and hold it to the side, as soon as you let it go, he’ll pull it right back into his mouth. In contrast, a horse with botulism can’t pull its tongue back as effectively. It takes more than a couple of tries to get it back in, and some of them can’t do it at all.”

Call your veterinarian if you notice any of these signs in your horse, however subtle they might be. Even if the problem is more likely colic, choke or another illness, it’s still best to ad-dress it right away. 

One more test a veterinarian can do is to try to pass a nasogastric tube into the horse’s stomach; if no obstruction is encountered, that would rule out choke. The horse’s reaction to the procedure can also be telling: “A clue that it might be botulism is that it is sometimes difficult at first to pass the tube because the horse can’t swallow it,” says Johnson. “If the veterinarian has a hard time getting the horse to swallow, and then the tube goes right down to the stomach with no problems, this can be an indication that it might be botulism.”

A veterinarian needs to rely on observation and history to decide whether to treat a horse for botulism. There is no practical laboratory test to detect the toxins. “The length of time required to perform [laboratory] tests is much longer than the horse can survive if the veterinarian waited for the results before beginning treatment,” says Sprayberry. “So for botulism we need a clinical diagnosis, meaning it is diagnosed solely on the basis of clinical signs. We rarely have an opportunity to confirm it and must be fairly certain of this diagnosis to spend the client’s money to get going on the treatment. This is the tricky part of the diagnosis.”

Sprayberry advises horse owners to watch YouTube videos of horses with signs of botulism: “Watching a video of quivering musculature is more memorable than a still photograph to convey what an affected horse looks like.”

Treatment options

If you suspect your horse may have botulism, even before you call the veterinarian, your first course of action is to remove access to any potentially contaminated feed sources—from every animal on your property, as well as the affected horse. “Botulism often appears in outbreaks,” says Johnson. “If all the horses on a farm are eating the same thing, they are all at risk.” Cattle and other livestock can also be susceptible to botulism.

Keep the horse quiet and still while you wait for the veterinarian to arrive. “You don’t want to stress an affected horse any more than necessary,” says Johnson. “If you exhaust the muscles, the disease will progress more quickly. You don’t want to walk these horses around or make them stand if they want to lie down.”

The only specific treatment for botulism—one that can be lifesaving—is to administer the appropriate antitoxin. But this must be done as quickly as possible after the poisoning occurs. “The antibodies in the antitoxin bind to the toxin molecules that are still circulating in the blood—that haven’t yet bound to and destroyed a neuromuscular junction,” says Sprayberry. “The antitoxin cannot remove the toxins that are already bound. If you don’t get the antitoxin on board soon enough, the number of neuromuscular junctions that are already nonfunctioning may make the horse beyond help.

”Botulism antitoxins are more readily available, and cheaper, than they once were. “Here at the New Bolton Center, we were initially the only source, and it was quite expensive,” says Johnson. “It cost at least $2,500 for one unit to treat one horse. Now there are a couple of commercial companies that produce the antitoxin and the cost has gone down. In certain areas of the country that see the disease more commonly, or areas where they see cases on a somewhat regular basis, we encourage veterinarians to have the antitoxin and keep it on hand.”

Still, even if you need to wait for a shipment of antitoxin to be delivered, it can be worth treating the horse. “Sometimes you get lucky and find someone who can send you the antitoxin by overnight delivery, but if the horse is diagnosed on a Friday night you may not be able to get it fast enough,” Johnson says. “If that’s the only way you can get it, that’s what has to happen, because it’s better to treat the horse late than not treat it, as long as the horse is still able to stand. Once the horse loses the ability to stand, treatment is unlikely to be successful, and efforts should be focused on preventing additional deaths.”

Several antitoxins are available, and it’s important that your veterinarian choose one that matches the type of botulism toxin—A, B or C—that your horse likely has. “The initial product that we produced here was effective against all types, and the commercial antitoxin produced by Lake Immunogenics in New York is also good for types A, B and C. These will be effective, no matter which type is affecting your horse,” says Johnson.

“Another company called PlasVacc in California produces an antitoxin that is only labeled for type B,” she adds. “This is a bit trickier because if you are on the West Coast and have a horse who got it from the soil it will probably be type A. That antitoxin might not be the best choice. If you are in a type B endemic area you’d probably be safe using either, but if you think type A or type C might be a possibility you should be sure to get the product that works against all of them.” 

If you discover signs of botulism in one of your horses, and others on your property have been sharing the same feed sources, your veterinarian may opt to treat the other horses with the antitoxin as well, as a protective measure, even if they’ve shown no signs of illness. “They have eaten the same suspect food but perhaps the toxin hasn’t had time to get to its target in their systems,” says Johnson. “By treating prophylactically you might be able to prevent signs before they start.”

Also, she adds, “It is important for horse owners to know that if they do have a suspected case of botulism, it is crucial to check the other horses at least twice a day, especially if they have been eating the same feeds. Look at their tongue strength and their ability to eat grain.” You’ll want to be ready to treat any other developing cases promptly.

The good news is that the paralysis caused by botulism is fully reversible—the damaged neuromuscular junctions will be replaced and muscle function will be restored over time, assuming that the horse can be kept alive long enough with supportive care. “If you can keep these horses from becoming totally recumbent, support them nutritionally and keep them hydrated for a couple of weeks, they can fully recover,” Johnson says. “It usually takes 10 to 14 days for the horse to regain the ability to eat. He may not have full muscle strength for a month or two, but given enough time he should recover 100 percent from this disease.”

Be aware, though, that it can be expensive to support a horse during the early recovery period. “Even if the horse is able to stand up without help, if he is unable to eat or drink you may need your veterinarian to come out a couple times a day to tube the horse with food and water,” says Johnson, “and this can run up a large bill.”

Preventive measures

• Vaccinate at-risk horses. A vaccine is available that works against botulism toxin type B. The American Association of Equine Practitioners (AAEP) includes botulism on its list of risk-based vaccines—those that are not recommended for every horse but rather only for those most at risk of disease. The AAEP guidelines recommend the vaccine primarily for pregnant mares and foals in areas where type B is prevalent, to prevent shaker foal syndrome.

The vaccine can also be used in adult horses at risk of exposure to botulism type B. It can be effective at preventing the disease in 95 percent of cases, and even if a vaccinated horse does fall ill, his chances of survival are better. Talk to your veterinarian if you would like to consider this option for your horse. No vaccine currently available for horses works against toxin types A or C, and the type B vaccine will not confer any cross protection. 

• Discard damp, moldy or contaminated hay. For any horse, vaccinated or not—but especially when vaccination is not an option—the best way to avoid botulism is to prevent access to tainted hay and feeds.

Sprayberry recommends peeling off each flake of hay to examine it closely before feeding. The toxins themselves do not impart any color or smell to the forage, but the damp conditions in which they develop can leave hay smelling musty or moldy. Also discard any hays that feel moist in the center or that contain dead animal parts. Large round bales are especially susceptible to retaining moisture in their centers. 

• Protect hay from the elements. C. botulinum spores are omnipresent in hays, and the bacteria will begin to grow readily in bales that get wet. Keep stored hay in a dry area, and check the roof periodically for leaks. Hay fed on the ground can easily pick up spores from the soil—instead, provide hay from some sort of commercial or homemade feeder that keeps it up out of the dirt and allows drainage underneath. In wetter climates, enclosed feeders that keep the rain and snow out will keep the forage drier. If your horse’s health needs require you to soak his hay, do not soak more than he can eat in one meal. 

• Avoid high-risk forages. Haylage and silage are not recommended for use in horses. Haylage that has been properly processed and sealed in bags theoretically ought to be safe, and some people do choose to feed it to horses. But outbreaks have occurred among horses fed this product, even when it looked and smelled correct. Definitely avoid feeding horses from any haylage bags that have been torn open or that smell or look spoiled. Also, never feed fresh grass clippings to horses, and make sure well-meaning neighbors know better than to toss “treats” like this over your fence.

• Protect hay and feeds from bird droppings. You can’t, of course, stop birds from pooping everywhere your horse grazes. But do prevent birds from nesting in areas where a large amount of droppings would fall into stored hay or feeds, and avoid spreading poultry droppings as fertilizer on hay fields or pastures. 

With some basic management precautions, you can greatly reduce your horse’s risk of contracting botulism. But if he does, quick and appropriate action can save his life.

This article first appeared in EQUUS issue #442. 




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