Ba­sic man­age­ment mea­sures, com­bined with vac­ci­na­tion, will re­duce your horse’s risk of con­tract­ing this deadly form of poi­son­ing.

EQUUS - - Equus - By Lau­rie Bon­ner

Bot­u­lism: Ba­sic man­age­ment mea­sures, com­bined with vac­ci­na­tion, will re­duce your horse’s risk of con­tract­ing this deadly form of poi­son­ing.

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.


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.


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.


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 PRO­TEC­TION: A vac­cine against C. bo­tulinum type B is avail­able for horses. 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.


• 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.

Hay­lage—grass baled with a higher mois­ture con­tent and sealed in plas­tic—is typ­i­cally meant for cat­tle or sheep, which are less sus­cep­ti­ble to bot­u­lism than are horses.

Pro­vide hay in a feeder that keeps the for­age dry.

Foals can de­velop “shaker foal” syn­drome when they in­gest the en­dospores as they nib­ble on grass or other things in their en­vi­ron­ment.

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.

A far less com­mon threat is feed or for­age that has been con­tam­i­nated by bird drop­pings or an an­i­mal car­cass.

Hay that was ei­ther baled while still moist or stored im­prop­erly poses a bot­u­lism risk.


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