Clostridium botulinum bacteria produce the most deadly biological toxin known to man. When ingested, botulinum toxin causes botulism, a fast-acting, often fatal form of food poisoning. Horses who consume feed tainted with botulinum toxin may die within hours or days unless they receive fast, appropriate treatment.
And then there’s the really bad news: The types of C. botulinum most dangerous to horses are present in the soil and in the grasses and hays that they eat. Especially if you live in or purchase forage grown in a region where C. botulinum is endemic, eliminating the bacteria from a horse’s environment is impossible.
But the news isn’t all bad. C. botulinum proliferates and produces botulinum toxin only under specific conditions, which can be prevented with basic management precautions, and vaccination of at-risk horses offers an additional layer of protection. So botulism is fairly rare in horses, and with a few basic steps to keep your horse’s food and water fresh and clean, you can greatly reduce the risk that he will ever have a problem with this disease. Here’s what you need to know.
Profile of a killer
C. botulinum is an anaerobe, which means it thrives in the absence of oxygen. And, when environmental conditions aren’t right for it---when it is in a dry, oxygen-rich atmosphere, for example---it goes dormant, encasing itself in a tough, protective outer membrane called an endospore. In this form, the bacteria do little harm to a horse.
But when external conditions change in its favor---that is, in anaerobic conditions with the right amount of moisture---C. botulinum emerges from its dormant state and multiplies rapidly. As each individual bacterium matures and dies, it releases its deadly toxin.
Seven distinct types of botulinum toxin have been identified---designated by letters from type A through G---but only types A, B and C are likely to produce illness in horses in the United States. Types A and B both reside in soil, but your risk of encountering them depends largely on where you live. Type A is more common in the West, and type B is seen more frequently east of the Mississippi River, especially in Kentucky and the Mid-Atlantic States. Type C is found in animal carcasses and bird droppings, which can be anywhere. However, up to 85 percent of all cases of equine botulism are caused by type B, which means that the risks are highest for horses in the eastern United States.
Botulinum toxin can cause illness in three ways:
• Food poisoning (botulism). Botulism is most likely to occur in horses who eat forage stored in a moist, anaerobic environment that encourages the proliferation of C. botulinum. This might occur, for example, if hay is baled while still moist or stored improperly; the wetness at the center of the bale causes spoilage and creates the ideal conditions for C. botulinum. Improperly processed haylage or silage--fermented forages normally fed to cattle---may also cause botulism in horses, as can clumps of grass clippings left by mowers. A far less common threat is feed or forage that has been contaminated by bird droppings or an animal carcass.
• Toxicoinfectious botulism (“shaker foal” syndrome). Foals are vulnerable to this form of botulism when they ingest the endospores as they nibble on grass or other things in their environment. The bacteria may activate and form colonies in gastric ulcers or the intestines.
• Wound botulism. Dirt and contaminants can carry endospores into a wound; if the surface heals over, an anaerobic environment may be created that allows the bacteria to gain a foothold within the surrounding tissues. This is more likely to occur with punctures and other deeper wounds.
A deadly threat
No matter how the botulinum toxin gets into the horse’s body, the effects are the same. The toxin binds to the synapses of the nerves that control the muscles, blocking the transmission of nerve impulses to the muscles. With no source of input, the muscles go flaccid, causing paralysis. Signs may appear within hours or days and often begin with the inability to swallow. A foal might have difficulty nursing.
As the toxin spreads, the effects begin to appear throughout the body, with signs such as muscle tremors, generalized weakness, a limp tail and gait issues. The severity and extent of the paralysis depends upon the amount of the toxin that a horse consumes. If he ingested only a little, he may just become less active and eat less before recovering after several days. A large dose of botulinum toxin will likely cause a horse to become recumbent. In the most serious cases, the cause of death is often suffocation, as the toxin paralyzes the muscles that facilitate breathing.
The early signs of botulism---difficulty swallowing, lack of eating, lying down, flaccid muscles---can look like other conditions, such as choke, colic or neurological disorders. Signs more specific to botulism include muscle tremors and weakness in the tongue; if you gently pull the horse’s tongue out of his mouth, he won’t be able to retract it. Even if you’re not sure it’s botulism, it’s best to call your veterinarian right away if you notice any of these signs, however subtle they might be.
If you suspect botulism, remove all food from all animals on your farm, including cattle and other livestock, as you wait for the veterinarian to arrive. Botulism often occurs in outbreaks when multiple animals are fed the same tainted forage. You’ll also want to keep the horse quiet and still to avoid exhausting his weakened muscles.
The only effective treatment for botulism is to administer an antitoxin, which must be done as soon as possible. The antitoxin works by binding with botulinum toxin that is still in circulation in the bloodstream, preventing the toxin molecules from binding with nerve cells and preventing the disease from progressing. Nothing can be done to treat neurons that have already been blocked. If treatment is delayed, the horse may be beyond help. If multiple horses have been fed from the same source, your veterinarian may suggest administering the antitoxin to all of them, in case others have ingested the toxin but are not yet showing signs of illness.
If the affected horse can be kept alive, the damaged nerves will heal within a few weeks, and he can make a full recovery. In the meantime, depending on the severity of his signs, he may require extensive supportive care, including nutrition and fluids via intubation.
Vaccinate “at risk” horses
Currently, only one vaccine against C. botulinum is approved for use in horses in the United States. The vaccine, which works against C. botulinum type B, is about 95 percent effective, and though it may not prevent all cases of botulism, it can reduce the severity of the illness and increase a horse’s chances for survival. The vaccine does not provide cross protection against C. botulinum types A or C.
The American Association of Equine Practitioners (AAEP) includes botulism on its list of “risk-based” vaccines, which means it is recommended for those horses most likely to come in contact with the bacteria or toxin. However, according to the AAEP, “Vaccination is warranted for all horses, as C. botulinum type B can be found in soil samples from many areas of the country and movement of horses or forage from non-endemic to endemic regions occurs frequently.”
What that means, says Amy Johnson, DVM, DACVIM, of the University of Pennsylvania, is that “it would be reasonable to vaccinate any horse for botulism, even though certain areas of the country are very low-risk. Since horses move around so much these days, it is possible that the horse would end up in an area of the country where botulism is more common. Likewise, hay and other forages can be shipped long distances, so it is possible that a horse in a low-risk geographic region could be exposed if fed hay from a high-risk geographic region.”
In Kentucky and the Mid-Atlantic States where botulin type B is most common, veterinarians may recommend the vaccine for all horses. “That is because the organism is so prevalent in the soil that sporadic botulism cases occur even in adult horses who are not fed high-risk feedstuffs, such as fermented feeds or large bale hay,” says Johnson. “Also, any horse fed high-risk feeds should be vaccinated.”
Vaccination is also recommended for pregnant mares, especially in endemic areas, to protect their foals against toxicoinfectious botulism. Foals can receive a three-dose series at four-week intervals, beginning at the age of 2 to 3 months, if the dam was vaccinated, or as early as 2 weeks of age if she was not.
Ask your veterinarian whether vaccinating against botulism might be advisable for your horse. If there’s any doubt, consider vaccinating anyway. “The vaccine is not that expensive and almost never causes adverse effects,” says Johnson.
Other preventive measures
• Discard damp or moldy hay. If a hay bale gets moist, the anaerobic conditions at the center create ideal conditions for the growth of C. botulinum. Large round bales are especially susceptible to retaining moisture at their centers. Even if your hay is dry now, any previous dampness may have harbored bacterial growth, and the toxins left behind will still be present. The toxin itself will not detectable by color or smell, but the damp conditions that fostered the bacteria will leave hay smelling musty or moldy. Examine each flake as you peel it off the bale, and discard any hay that is moist or smells funky.
• Protect stored hay from the elements. Periodically check for leaks in the roof and walls of your hay storage area. Stacking hay on wooden pallets will help air circulate and prevent moisture from accumulating underneath.
• Offer hay in feeders. Hay dropped on the ground can easily become contaminated, and rain and mud will help foster the growth of bacteria. Instead, provide hay in a commercial or homemade feeder that keeps the forage dry. Especially if you live in a wetter climate, consider investing in an enclosed feeder that will keep out the rain and snow. Clean up dropped hay regularly. If your horse has a condition, such as heaves, that requires you to soak his hay, do not soak more than he can eat in one meal.
• Avoid high-risk forages. Haylage ---grass that is baled with a higher moisture content and sealed in plastic---is typically meant for cattle or sheep, which are less susceptible to botulism than horses. Some people do feed haylage to horses, especially if they need a low-dust alternative to dry hays, and haylage that has been properly processed and sealed ought to be safe, but the risk of botulism remains, even when the forage seems fresh. Definitely do not feed horses any haylage from bags that have been torn open or that look or smell spoiled. Also, don’t let your horse graze in areas where clumps of cut grass remain from a recent mowing, and warn your neighbors against tossing grass clippings over the fence as “treats” for your herd.
• Watch out for dead animals and bird droppings. Botulism type C is fairly rare, but you do want to avoid feed or water that has been tainted by carcasses or droppings. Discard any hay or bagged feeds if you discover body parts from dead animals, and routinely check water buckets or troughs for drowning victims. (A mesh escape ramp built into the side of a large trough can help small animals who fall in to climb out safely.) Prevent birds from nesting in areas where a lot of droppings would fall onto feeders or stored hay, and do not use poultry manure as fertilizer on hayfields or pastures.
This article first appeared in the October 2017 issue of EQUUS (Volume #481)