Monensin poi­son­ing: Rare but deadly

In­gest­ing even a small amount of this an­tibi­otic meant for cows and other live­stock can rapidly kill a horse. Own­ers need to know what to look for and how to pro­tect their horses.

EQUUS - - Contents - By Peter J. Sa­cop­u­los

In­gest­ing even a small amount of this an­tibi­otic meant for cows and other live­stock can rapidly kill a horse. Own­ers need to know what to look for and how to pro­tect their horses.

In Oc­to­ber 2014, three horses at the Mas­ter­piece Eques­trian Cen­ter in Davie, Florida, fell se­ri­ously ill with fever, el­e­vated heart rates and gen­eral weakness that ad­vanced rapidly to­ward paral­y­sis. All three died within a week. Soon oth­ers in the barn be­gan show­ing the same signs of ill­ness.

The cause? All the horses had con­sumed monensin, an an­tibi­otic that is com­monly fed to cattle and poul­try but is deadly to horses. In­de­pen­dent lab­o­ra­tory test­ing, re­quested by the horses’ own­ers, iden­ti­fied the source of the poi­son­ing: The toxic drug was present in the horses’ feed, which had been pur­chased from Florida-based Lake­land An­i­mal Nu­tri­tion (now called Hub­bard Feeds) in Lake­land, Florida.

Lake­land’s own internal tests con­firmed that two pro­duc­tion runs of its horse feeds con­tained monensin. Equilete Pel­let For­mula, the prod­uct pur­chased by Mas­ter­piece Eques­trian Cen­ter, con­tained 1.48 grams per ton of monensin as well as 1.20 grams per ton of lasa­locid, an­other an­tibac­te­rial drug that is toxic to horses.

The com­pany quickly launched a prod­uct re­call and halted the pro­duc­tion of all equine feeds. No other ill­nesses or deaths were at­trib­uted to the Lake­land feed.

But at Mas­ter­piece Eques­trian Cen­ter, the dam­age was done. In to­tal, 22 horses died or were left per­ma­nently dis­abled by con­sum­ing the tainted feed. The story made head­lines around the world, caus­ing the eques­trian com­mu­nity to worry about the health and safety of their horses.

The poi­son­ings at Mas­ter­piece were ac­ci­den­tal, re­sult­ing from a man­u­fac­tur­ing er­ror. But it was not an iso­lated in­ci­dent. Al­though the threat that monensin poses to horses is well estab­lished, mis­takes get made, and feed laced with toxic drugs can still oc­ca­sion­ally reach our horses.

It’s a good idea for all horse own­ers to be aware of this threat. You also need to know what to look for---and what to do---if you sus­pect a new batch of feed is mak­ing one or more of your horses ill.


Also known by the trade names Coban and Ru­mensin, monensin is used to treat and pre­vent coc­cid­io­sis, a par­a­sitic in­testi­nal in­fec­tion com­mon in beef and dairy cattle, chick­ens and other live­stock. The drug be­longs to a fam­ily of an­tibi­otics known as ionophores, which are also used to help pre­vent bloat­ing and stim­u­late weight gain and milk pro­duc­tion. Ionophores in wide agri­cul­tural use in­clude laid­lomycin (Cat­tlyst), lasa­locid (Avatec, Bo­vatec), madu­ram­icin (Cy­gro), narasin (Mon­te­ban, Max­iban), sali­no­mycin (Bio­cox, Sa­cox) and sem­du­ram­icin (Aviax).

High doses of monensin can be toxic

in cattle and poul­try---a lethal dose is about 20 to 34 mil­ligrams per kilo­gram (mg/kg) of body weight for cattle and 90 to 200 mg/kg in poul­try---but these an­i­mals gen­er­ally tol­er­ate the drug well. Horses, as well as rab­bits, humans and dogs, do not. Horses have a par­tic­u­larly low tol­er­ance for monensin---a lethal dose is 2 or 3 mg/kg.

Monensin dam­ages the mi­to­chon­dria in the mus­cle cells through­out a horse’s body, in­clud­ing the heart. The mi­to­chon­dria are struc­tures within the cells that play a role in me­tab­o­lism, res­pi­ra­tion and the pro­duc­tion of en­ergy. In­jury to the mi­to­chon­dria ul­ti­mately leads to the death of the cells, and be­cause the mus­cles of the heart con­tain high num­bers of mi­to­chon­dria, monensin tox­i­c­ity of­ten leads to con­ges­tive heart fail­ure.

Signs that a horse has con­sumed a lethal dose of monensin in­clude colic, lethargy, sweat­ing, mus­cle weakness, in­co­or­di­na­tion, a fast pulse with ar­rhyth­mias, and re­cum­bency. These may ap­pear within hours of in­ges­tion of the drug, and death may oc­cur in less than 24 hours.

Horses who con­sume less than a lethal dose may not show out­ward signs of ill­ness, but the ef­fects are cu­mu­la­tive. Be­cause the drug can­not be seen, felt or smelled in the feed, an un­wit­ting owner may con­tinue of­fer­ing the tainted prod­uct to the horse. Over time, the horse may lose weight and be­come un­thrifty, with gen­er­al­ized weakness, poor per­for­mance and a de­pressed at­ti­tude. The horse may sim­ply seem tired or col­icky or go off his feed.

As dam­aged mus­cle cells con­tinue to die they re­lease toxic byprod­ucts, which ac­cu­mu­late in the horse’s blood and in­jure the kid­neys and liver, lead­ing even­tu­ally to mul­ti­ple or­gan fail­ure. As the horse’s con­di­tion wors­ens, signs may in­clude deep­en­ing de­pres­sion, poor ap­petite, di­ar­rhea, el­e­vated tem­per­a­ture, in­ter­mit­tent sweat­ing, in­abil­ity to stand, ir­reg­u­lar heart­beat, and mus­cle weakness that pro­gresses to ab­nor­mal gaits and then paral­y­sis, even­tu­ally lead­ing to death.

There is no an­ti­dote for monensin poi­son­ing. If the prob­lem is iden­ti­fied early and the tainted food is re­moved, a horse may sur­vive with sup­port­ive care. In­tra­venous flu­ids, for ex­am­ple, may re­duce the dam­age to the liver and kid­neys. But the ir­re­versible dam­age to the horse’s heart and other organs are likely to shorten his life and leave him un­able to work.

If monensin tox­i­c­ity is sus­pected in a barn, and other horses have con­sumed the same feed but are not show­ing signs of ill­ness, a vet­eri­nar­ian can per­form blood tests to look for en­zymes that would in­di­cate dam­age to the heart mus­cles. If the re­sults are pos­i­tive, the af­fected horses need to be im­me­di­ately switched to a dif­fer­ent feed and placed on ap­pro­pri­ate sup­port­ive care.


Horses who share a barn or pas­ture with cattle may in­gest monensin or other ionophores if they ac­ci­den­tally gain ac­cess to cattle feed or sup­ple­ments con­tain­ing the drugs. (Scaveng­ing dogs may also be at risk.)

But the most in­sid­i­ous equine cases oc­cur when horse feeds are tainted with one or more of these drugs. While con­tam­i­na­tion can hap­pen dur­ing stor­age, ship­ping or other sit­u­a­tions, the most ob­vi­ous po­ten­tial danger comes from al­ter­nat­ing the milling of equine feeds and med­i­cated cattle feeds on the same equipment at the same pro­duc­tion fa­cil­ity.

The Food and Drug Ad­min­is­tra­tion man­dates that man­u­fac­tur­ers per­form proper clean-outs be­fore non-med­i­cated equine feeds can be pro­duced on the same equipment that was just used to make med­i­cated an­i­mal feeds. Ac­cept­able clean­ing pro­ce­dures in­clude run­ning peanut shells through the mill to ab­sorb and flush out any resid­ual med­i­ca­tions. In most mod­ern man­u­fac­tur­ing plants, all these pro­cesses are highly au­to­mated, and com­put­ers and soft­ware pro­grams play ma­jor roles in the op­er­a­tion of feed pro­duc­tion fa­cil­i­ties. Still, ac­ci­dents can hap­pen. At Lake­land An­i­mal Nu­tri­tion, the Equilete Pel­let For­mula that went to Mas­ter­piece Eques­trian Cen­ter was milled the day af­ter a batch of med­i­cated cattle feed was pro­duced, with­out a proper clean-out of the equipment. The feed com­pany laid the blame for the ac­ci­dent on a soft­ware pro­gram, which failed to gen­er­ate a lock­out that should have flagged the need to clean af­ter the milling of the med­i­cated cattle feed.

Lake­land An­i­mal Nu­tri­tion paid a fine to­tal­ing $4,000 to the Florida Depart­ment of Agri­cul­ture for mul­ti­ple vi­o­la­tions of feed pro­duc­tion

and dis­tri­bu­tion reg­u­la­tions, and in Jan­uary 2015, the com­pany reached a set­tle­ment with the own­ers of the Mas­ter­piece Eques­trian Cen­ter as well as with the own­ers of the af­fected horses. The de­tails of the set­tle­ment were con­fi­den­tial, but news sources de­scribed the com­pen­sa­tion as “con­sid­er­able.” Un­der­stand­able, given that the horses lost were re­ported as hav­ing val­ues rang­ing from $25,000 to hun­dreds of thou­sands of dol­lars.

Mas­ter­piece Eques­trian Cen­ter was a high-pro­file case, but it was not an iso­lated in­ci­dent. In 2017 a com­pany called Western Milling agreed to pay fines in re­la­tion to monensin-re­lated deaths of 13 horses at Black Fence Farm in Clo­vis, Cal­i­for­nia, in 2015. In Novem­ber 2016, six horses died and an­other 25 were af­fected at TMC Per­for­mance Horses in Cuba, New York, af­ter con­sum­ing al­legedly toxic feed man­u­fac­tured by Reis­dorf Brothers in North Java, New York; as of early 2018, le­gal ac­tion was still pend­ing.


It may be fright­en­ing to think that an in­vis­i­ble toxin could ap­pear in your horse’s feed, but there are pre­cau­tions you can take to min­i­mize the risk and help to en­sure his safety.

The best pro­tec­tion against ionophore poi­son­ing is to pur­chase grain from a com­pany that man­u­fac­tures only horse feeds. The next best al­ter­na­tive is to pur­chase from a com­pany that specif­i­cally states that none of its feeds, for any live­stock, con­tain ionophores.

If you do choose to pur­chase horse feeds from a man­u­fac­turer that also pro­duces med­i­cated feeds for other live­stock, call the com­pany and ask what pre­cau­tions are taken to pro­tect the equine prod­ucts. Ask how the ad­di­tives are mixed and how the mill is cleaned be­tween med­i­cated and non-med­i­cated pro­duc­tion runs. If the an­swers are not pro­vided im­me­di­ately and with­out re­sis­tance, find an­other sup­plier.

It’s also a good idea to keep a feed log---a writ­ten record that in­cludes the dates feed is pur­chased, the spe­cific lot or batch num­bers, ex­pi­ra­tion dates, the seller and the price paid. The in­for­ma­tion could prove help­ful in a case of monensin poi­son­ing, but it is also use­ful for other pur­poses, such as bud­get­ing, tax prepa­ra­tion and manag­ing your horse’s weight.

Knowl­edge is power. Con­sumer habits drive business de­ci­sions. If in­creas­ing num­bers of peo­ple in­sist on equine feed that is man­u­fac­tured in fa­cil­i­ties that do not also pro­duce med­i­cated feeds, savvy mar­keters will note that the horse com­mu­nity is “vot­ing with their wal­lets.” This will lead to sig­nif­i­cant pos­i­tive changes within the in­dus­try.

About the au­thor: Peter J. Sa­cop­u­los is an at­tor­ney with Sa­cop­u­los, John­son & Sa­cop­u­los in Terre Haute, In­di­ana. A sig­nif­i­cant por­tion of his prac­tice in­volves equine law, and he has rep­re­sented eques­trian clients on a range of mat­ters in­clud­ing liv­ery­man liens, foal share agree­ments, syndication of stal­lions and in­sur­ance and con­tract dis­putes.

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