What to do about Enterolith­s

Re­searchers are still work­ing to un­der­stand why these in­testi­nal “stones” form. But you can take steps to re­duce your horse’s risk of de­vel­op­ing them.

EQUUS - - In Brief - By Heather Smith Thomas

You might need two hands to hold a large en­terolith, a rock­like mass that can weigh 25 pounds or more and reach the size of a small wa­ter­melon. And it can be dif­fi­cult to imag­ine that such a huge stone may have per­sisted for years un­de­tected in­side a horse’s in­tes­tine.

A horse can carry 100 or more smaller enterolith­s in his gut, or he may de­velop only one or two large ones. Of­ten, the small ones pass un­event­fully with the horse’s ma­nure, or larger ones may re­main dor­mant, do­ing lit­tle ap­par­ent harm to the horse.

But enterolith­s aren’t to be taken lightly. Some­times, these stones can re­sult in re­cur­rent mild col­ics, and in the worst-case sce­nar­ios, they can cause se­ri­ous im­pactions that lead to in­testi­nal rup­ture. Find­ing even one or two tiny peb­bles in your horse's ma­nure is a sign that you need to take ac­tion to pro­tect him. Why some horses de­velop enterolith­s while oth­ers do not isn't fully un­der­stood, and in fact, mul­ti­ple fac­tors prob­a­bly con­trib­ute to the for­ma­tion of the stones. But the good news is that ba­sic man­age­ment changes can go a long way to­ward pre­vent­ing enterolith­s. Here’s what you need to know.


An en­terolith be­gins when a horse swal­lows some­thing in­di­gestible. “It might be a small piece of rock, gravel or some­thing else the horse has in­gested,” says Diana M. Has­sel, DVM, PhD, DACVS, DACVECC, of Colorado State Uni­ver­sity. “It might be a seed, or hair, a coin or a tiny piece of wire or twine.”

Nor­mally, these bits of for­eign mat­ter pass harm­less through the horse’s di­ges­tive sys­tem and go out un­no­ticed in his ma­nure. Un­der cer­tain con­di­tions, how­ever, the ob­ject’s progress comes to a halt in a part of the large colon where the in­testi­nal tract widens and the in­gesta nat­u­rally slows down. “Enterolith­s are formed in the right dor­sal colon,” Has­sel says, “in an area of greater di­men­sion in that lo­ca­tion, called the am­pulla coli.”

While the ob­ject sits in the gut, var­i­ous min­er­als may be­gin to “stick” to it. The con­cre­tion around the ob­ject---now called a "nidus" - con­tin­ues to accu

mu­late, much in the same way a pearl forms around a grain of sands in­side an oys­ter. The min­er­als in an en­terolith con­sist of stru­vite, a com­bi­na­tion of

mainly mag­ne­sium, am­mo­nium and phos­phates. “There are other min­er­als in these stones, in smaller quan­ti­ties,” Has­sel says. “The ac­tual con­tent will de­pend on the horse’s diet and the feed that is present in the gut---which might con­trib­ute some cal­cium and other min­er­als that could be found in these stones as well.”

As the lay­ers of min­eral con­tinue to form, the en­terolith grows in size. A horse may have only one or two stones, or he may pro­duce many, which will shift and rub against each other in his gut. Why some horses form more stones than oth­ers is not fully un­der­stood. “We don’t know why cer­tain horses form many small enterolith­s ver­sus soli­tary large ones,” says Has­sel. “It is not com­mon to see horses that just form lit­tle ones; more of­ten we see big ones mixed with lit­tle ones, but the ma­jor­ity of af­fected horses just form one or two large stones.”

Even­tu­ally, one of three pos­si­ble out­comes will oc­cur:

• While still small---less than the size of a ma­nure ball---the en­terolith may move with the in­gesta and pass harm­lessly onto the ground in ma­nure. “They may get moved along in the tract by feed changes, such as in­creased fiber,” Has­sel says. “If they are fairly small they can pass on through, and you might see them in the horse’s ma­nure.”

But just be­cause a horse’s stones are small does not mean he is not at risk for se­ri­ous col­ics. “All sizes of stones have the pos­si­bil­ity of caus­ing block­age,” says Tyler El­liott, DVM, of San Luis Rey Equine Hospi­tal in Bon­sall, Cal­i­for­nia.

• The stone will re­main in place and may grow very large. “Some of these may get as big as a wa­ter­melon,” says Has­sel. The horse may show in­ter­mit­tent signs of mild colic, as the stone shifts around. The right dor­sal colon nar­rows con­sid­er­ably as it ex­its to the trans­verse colon. “The big­ger stones can be like a hinge valve,” says Has­sel. “If they get pushed up against the en­trance to the trans­verse colon, they may cre­ate an ob­struc­tion, and then if the horse gets a lit­tle treat­ment or is re­hy­drated, the stone may fall back down, no longer block­ing the pas­sage.”

But not all horses with larger enterolith­s will show signs of colic. Many can carry the stones for years with­out any indi­ca­tion of trou­ble---the enterolith­s may be a sur­prise find­ing at necropsy. “In­ter­mit­tent, re­cur­ring colic oc­curs in about 15 per­cent of horses with enterolith­s,” says Has­sel. “So if you have a horse who col­ics a lot, this might be a pos­si­ble cause.”

• Enterolith­s in the mid-range---about the size of a softball---are by far the most dan­ger­ous. After the right dor­sal colon nar­rows into the trans­verse colon, the gas­troin­testi­nal tract nar­rows again to be­come the small colon, which is the last sec­tion be­fore the rec­tum.

“The in­tes­tine goes from a very large di­am­e­ter to a much smaller di­am­e­ter, cre­at­ing a sort of bot­tle­neck,” says Has­sel.

An en­terolith be­gins when a horse swal­lows some­thing in­di­gestible. It might be a small piece of rock, gravel or some­thing else the horse has in­gested.

A stone that is small enough to en­ter the trans­verse colon but too big to pass on through to the rec­tum be­comes lodged in place, cre­at­ing a com­plete block­age that leads to painful im­paction col­ics. “The right dor­sal colon can ac­com­mo­date a stone up to 12 or 14 cen­time­ters in di­am­e­ter, whereas the small colon is only about a quar­ter to a third that size, and a large stone can­not fit through it,” says El­liott.

“If the block­age is not treated sur­gi­cally to re­move the stone, the in­testi­nal tract even­tu­ally rup­tures, caus­ing the death of the horse,” says Has­sel. DI­AG­NOS­TIC DIF­FI­CUL­TIES

De­ter­min­ing that a horse is de­vel­op­ing enterolith­s can be dif­fi­cult. Some may be gen­er­ally un­thrifty or ir­ri­ta­ble, or de­velop re­cur­rent mild col­ics. “There may be in­ter­mit­tent di­ar­rhea,” says El­liott. “These are all very gen­eral signs, how­ever, that don’t point specif­i­cally to stones. There’s no one sign you’d see and know that the horse has stones.”

Find­ing small enterolith­s on the ground would be an ob­vi­ous indi­ca­tion, but they are eas­ily over­looked. If they aren’t vis­i­ble while still em­bed­ded in ma­nure, the rounded struc­tures may re­sem­ble nat­u­rally oc­cur­ring river rocks. And then there is the ques­tion of which horse in the herd pro­duced it.

Still, “I tell peo­ple that if they no­tice stones in the ma­nure, even if they are tiny, it’s a good idea to check the horse,” says Troy Herthel, DVM, DACVS-LA, of Alamo Pin­tado Equine Med­i­cal Cen­ter in Los Olivos, Cal­i­for­nia. “If you see stones while clean­ing your horse’s stall, we def­i­nitely rec­om­mend hav­ing ab­dom­i­nal ra­dio­graphs to de­ter­mine if there are more in­side. If the horse has one, most likely there are more.”

The shape of any stones you might find pro­vides another clue. “Most of the stones we see are round, but if you see some with a flat edge or cuboidal tri­an­gu­lar shape, there are prob­a­bly more

Has­sel. “We can’t see the stones with ul­tra­sound, be­cause the con­tent of the gut looks sim­i­lar to the sur­face of the stone.” Rec­tal ex­ams usu­ally aren’t help­ful ei­ther, be­cause the stones may be too deep in the ab­domen to feel.

But even x-rays have lim­i­ta­tions when it comes to imag­ing struc­tures deep in­side the horse’s ab­domen. “Di­ag­nos­tics with ra­dio­graphs are not 100 per­cent although we are gen­er­ally able to cor­rectly di­ag­nose enterolith­s on ra­dio­graphs more than 80 per­cent of the time, if they are still in the right dor­sal colon,” says Herthel. “The stones that travel down into the small colon are harder to di­ag­nose with ra­dio­graphs, due to the horse’s anatomy. Other con­found­ing fac­tors are the size of the pa­tient and the amount of in­gesta within the colon. The larger-bod­ied horses can be more dif­fi­cult to ad­e­quately pen­e­trate with x-ray beams, and if the colon has a large feed im­paction the stone can be ob­scured from view.”


The only treat­ment op­tion for an en­terolith im­paction is surgery to re­move the stone. At that point, iden­ti­fy­ing the en­terolith be­fore­hand is prac­ti­cally moot. “Usu­ally the horse will need surgery any­way, if there’s a block­age---and then you would find the stone dur­ing the pro­ce­dure,” Has­sel says.

Sur­gi­cal re­moval is also rec­om­mended when a larger stone is iden­ti­fied in x-rays. Even if it’s not cur­rently caus­ing trou­ble, a large en­terolith isn’t go­ing to go away on its own, and the risk of colic will al­ways re­main. “They con­tin­u­ally build in size,” says El­liott. “You may see the horse be­come mildly col­icky, but surgery will even­tu­ally be needed. If the horse has a large stone, the only cu­ra­tive treat­ment is surgery.”

Re­cently, a horse in El­liott’s prac­tice un­der­went surgery to re­move two stones. “One was about nine pounds and the other weighed about 11 pounds,” he says. “Based on the his­tory of re­cur­rent colic and the horse not do­ing very well, though still able to pass a lit­tle bit of ma­nure, we sus­pected a par­tial block­age. We took some ra­dio­graphs and these stones showed up--so our rec­om­men­da­tion to the owner was to take the horse to surgery. That horse has now re­cov­ered and is do­ing very well.”

A horse’s chances for re­cov­ery after surgery to re­move an en­terolith im­paction are usu­ally very good. “If the horse goes to surgery early, be­fore the colon is com­pro­mised or rup­tured, suc­cess rates are about 97 per­cent,” says Has­sel.

Un­like the twists, en­trap­ments and other stran­gu­lat­ing col­ics that squeeze off blood flow to the bowel and cause the tis­sue to die, enterolith­s gen­er­ally do not cause se­ri­ous in­jury to the in­testi­nal wall if acted upon in time.

“These horses are some of the health­i­est col­ics we ever see,” says Has­sel. “It’s a sim­ple ob­struc­tion, with noth­ing twisted or in­jured. The stones are usu­ally fairly smooth and solid, though they can have spic­u­lated sur­faces [with spikes or points]. Some are more por­ous and rough. The stones don’t seem to injure the gut lin­ing, how­ever, un­til they get lodged.”

If an en­terolith im­paction is left in place, the built-up pres­sure can se­ri­ously injure the in­testi­nal wall and in­crease the risk of a rup­ture. “If you wait too long, the horse will die rapidly if the in­tes­tine rup­tures be­cause there is noth­ing we can do to help the horse at that point,” says Has­sel.

How­ever, says Herthel, “The ma­jor­ity of the cases we see have a good prog­no­sis. We have to make at least

one in­ci­sion into the colon, some­times two, de­pend­ing upon the size and lo­ca­tion of the stone or stones. Oth­er­wise, these surg­eries are gen­er­ally fairly straight­for­ward.”

If x-rays show that your horse has a clus­ter of smaller stones rather than one big one---all of which ap­pear to be small enough to pass in the ma­nure---your vet­eri­nar­ian may of­fer al­ter­na­tive treat­ments. “Med­i­cal man­age­ment is some­times an op­tion,” says El­liott. “Surgery to re­move the stones is best, but you can put the horse on pas­ture or grass hay and try ag­gres­sive psyl­lium treat­ment for stones that still have a pos­si­bil­ity of pass­ing safely. Peo­ple of­ten try a month of at-home psyl­lium treat­ment and di­etary man­age­ment, then come back for a re-check ra­dio­graph to see if they’ve cleared. These stones are a tick­ing time bomb, but with luck they can be passed on through.”


Why some horses de­velop enterolith­s while oth­ers do not is still a bit of a mys­tery. “We think these stones form as a re­sult of mul­ti­ple fac­tors work­ing to­gether, rather than just one cause,” says Has­sel. Some of the ma­jor fac­tors in­clude diet, ex­er­cise, ge­og­ra­phy and ge­net­ics---all of which can have an im­pact on con­di­tions such as the acid­ity in the gut, min­eral con­tent of the in­gested feed and slow gut motil­ity, which can en­able enterolith­s to form.

“The perfect storm would be to have a high pH within the right dor­sal colon, a high min­eral con­tent in the feed, slow tran­sit time and a nidus for a stone to build around,” says El­liott. “Some horses, per­haps due to in­di­vid­ual ge­net­ics or breed sus­cep­ti­bil­ity, may have de­creased tran­sit time as well as a nat­u­rally higher pH than other horses.”

Here are some of the fac­tors that can con­trib­ute to the for­ma­tion of enterolith­s:

• HIGH MIN­ERAL IN­TAKE. The risk of de­vel­op­ing enterolith­s in­creases when a horse’s diet in­cludes high lev­els of min­er­als. A diet based pri­mar­ily on al­falfa hay, which is lower in fiber and higher in min­er­als than grass hay, has long been known as a risk fac­tor for enterolith­s.

But other feeds---and even wa­ter, in some re­gions---can also be high in min­er­als. “We know diet plays a huge role, and this may in­clude the wa­ter sup­ply as well as the feed,” says Has­sel. “Per­haps even more im­por­tant is the wa­ter sup­plied for growth of the feed, such as the hay crop.” Still, she adds, “The feed it­self is the pri­mary risk fac­tor, since al­falfa hay has been clearly linked to en­terolith de­vel­op­ment in atrisk horses.”

Brans, such as wheat and rice bran, also have high min­eral lev­els but it is not clear that they are ma­jor con­trib­u­tors to the prob­lem. “Peo­ple used to say that bran was a risk fac­tor be­cause in ear­lier times millers’ horses were fed a lot of bran [as a byprod­uct of the wheat milling] and of­ten formed stones, and bran is high in phos­pho­rus,” says Has­sel. “We’ve never been able to iden­tify bran as a risk fac­tor, how­ever, in all the stud­ies we’ve done. This might be be­cause our horses to­day don’t eat as much bran.”

• LOW ACID­ITY IN THE GUT. An en­vi­ron­ment with a higher pH, which is more al­ka­line, en­cour­ages the crys­tal­liza­tion of min­er­als. A lower, or more acidic, pH en­vi­ron­ment is more cor­ro­sive and dis­solves min­er­als more read­ily. There will al­ways be nat­u­ral vari­a­tion among in­di­vid­u­als, and horses with a higher pH in the con­tents of their colons may be more prone to form­ing enterolith­s. A high-al­falfa


diet can also re­duce acid­ity in the gut. “Al­falfa is an al­ka­lin­iz­ing hay com­pared to most grass hays,” says Has­sel.

• LOW GUT MOTIL­ITY. The speed with which in­gested food moves through the gut will also vary some­what among in­di­vid­u­als, and those with slower rates may of­fer more time for enterolith­s to be­gin form­ing. Man­age­ment fac­tors may play a role. “Stall con­fine­ment may con­trib­ute to for­ma­tion of enterolith­s,” says Has­sel. “The horses we see with enterolith­s are of­ten well taken care of, liv­ing in pam­pered con­di­tions. Things that help pre­vent stone for­ma­tion in­clude reg­u­lar turnout and daily pas­ture ac­cess. Whether it’s the ex­er­cise or the grass, be­ing turned out rather than in a stall seems to be pro­tec­tive.”

The rea­son for this may be that reg­u­lar ex­er­cise stim­u­lates gut motil­ity, so in­gested foods move through faster. “If horses have ad­e­quate ex­er­cise they will be pass­ing their feed on through more read­ily,” ex­plains El­liott. “In the­ory, this would mean less time for the in­gesta to sit in the colon and pro­vide the op­por­tu­nity for stone for­ma­tion.”

• Ge­netic pre­dis­po­si­tion. It’s pos­si­ble that fac­tors such as gut pH and motil­ity are her­i­ta­ble traits---in a 1999 ret­ro­spec­tive study of 900 horses con­ducted by Has­sel at the Uni­ver­sity of Cal­i­for­nia–Davis, about 15 per­cent of the horses with stones also had sib­lings with stones.

“Ge­netic pre­dis­po­si­tion hasn’t been proven yet, but there are cer­tain breeds that tend to have more enterolith­s,” says Has­sel. “The most af­fected breed seems to be the Ara­bian, and Mor­gans are the sec­ond most com­monly af­fected breed.” Other breeds iden­ti­fied in Has­sel’s study in­clude Ara­bian crosses, Ap­paloosas, Sad­dle­breds and Minia­ture Horses. “You can’t just have the breed predilec­tion with­out the di­etary fac­tor, how­ever,” Has­sel says.

• Ge­og­ra­phy. Enterolith­s can ap­pear in horses just about any­where, but they are more com­mon in cer­tain re­gions of the coun­try. “Cal­i­for­nia has high in­ci­dence, but there are ar­eas in Cal­i­for­nia where you don’t see this oc­cur­ring, even though those horses are fed al­falfa. We see a lot of stones in south­ern Cal­i­for­nia,” Has­sel says. “Texas, Ari­zona and Florida are states where we oc­ca­sion­ally see horses with stones, but not as of­ten as in Cal­i­for­nia.” In­ci­dences are much lower in other parts of the coun­try.

The rea­sons for this pat­tern are not clear, but re­searchers sus­pect it may be re­lated to the min­eral con­tent of lo­cal wa­ter sup­plies and soils. “It seems that the al­falfa grown in coastal soils, and es­pe­cially in cer­tain ar­eas of Cal­i­for­nia [as well as Ari­zona, Texas and Florida], tends to have a higher mag­ne­sium con­tent,” says El­liott. “Al­falfa al­ready has six times the daily re­quired mag­ne­sium con­tent. This could be par­tially why there is such a re­gion-spe­cific in­ci­dence of enterolith­s.”

But your horse doesn’t need to live in one of those re­gions to de­velop enterolith­s. “Here in Colorado we see en­terolith prob­lems only spo­rad­i­cally. I might see three in one year and then not see any for the next three years,” Has­sel says. “Some­times the af­fected horses come from Cal­i­for­nia. But we do see some cases just out of the blue. It can hap­pen any­where.”

• His­tory of sus­cep­ti­bil­ity. Man­age­ment changes can re­duce the risk that a horse will de­velop enterolith­s, but if he has had stones be­fore, he may be more sus­cep­ti­ble to de­vel­op­ing new ones. “Some horses seem to be stone fac­to­ries,” says Herthel. “We’ve had re­peat of­fend­ers that have come in for another surgery. Most of these horses have one to three stones, but oc­ca­sion­ally we find more. We had one horse that had about 100 stones, but

The rea­sons for re­gional vari­a­tions in en­terolith in­ci­dence are not clear, but re­searchers sus­pect that they are re­lated to the min­eral con­tent of lo­cal wa­ter sup­plies and soils.

most of them were very small---from the size of pea gravel on up to sil­ver dol­lar size, with a few larger ones.”

Talk to your vet­eri­nar­ian if you sus­pect your horse may have, or be at risk for, enterolith­s---es­pe­cially if you find a stone in his ma­nure. “If you have a horse that ex­pe­ri­ences chronic colic prob­lems and you live in one of those re­gions or the horse is be­ing fed al­falfa hay, this might be a rea­son to x-ray the ab­domen. You are much more likely to see a stone when it is not ob­struct­ing the gut than when it is,” says Has­sel. “The lo­ca­tion where stones form are vis­i­ble with ra­dio­graphs in the lower part of the ab­domen and the stones show up very read­ily when they are sit­ting down there, as com­pared to when they are caus­ing a block­age higher up.”


Horses who are likely to de­velop enterolith­s can be man­aged to re­duce the risks with mea­sures that ap­pear to have worked well over the past decade. “We are a re­fer­ral hospi­tal so we get horses from all over south­ern and cen­tral Cal­i­for­nia,” says Herthel. “Within the last 15 years, how­ever, in­ci­dence of stone surg­eries we do each year

has de­creased sig­nif­i­cantly. Back in the 1980s and early 1990s the hospi­tal was do­ing a stone surgery ev­ery week or two, and now it’s sig­nif­i­cantly less. This de­crease in cases may be due to im­proved man­age­ment and feed­ing plans. Most of our horse own­ers are well ed­u­cated re­gard­ing the pre­dis­pos­ing causes and are proac­tive in try­ing to pre­vent stones.”

Here are some steps you can take if you’re wor­ried about enterolith­s:

• Re­duce al­falfa in the diet. Al­falfa hay is rich in nutri­ents, but for horses sus­cep­ti­ble to enterolith­s, the lev­els of min­er­als it sup­plies may be too high. “With the risk for enterolith­s, we don’t rec­om­mend feed­ing more than half the hay con­tent as al­falfa,” Herthel says. “Most of our horse own­ers now add pas­ture grass or grass hay to the diet.”

• Pro­vide am­ple turnout and/or ex­er­cise. Ac­tiv­ity helps to stim­u­late gut motil­ity, and turnout can be es­pe­cially help­ful: “Grass pas­tures are great, be­cause grasses don’t have as high a min­eral con­tent as al­falfa, and the horse gets some ex­er­cise out graz­ing,” Herthel says.

Grass also has a higher fiber con­tent than al­falfa. “A higher fiber con­tent helps nat­u­rally push things through and keeps the feed mov­ing along the tract,” says El­liott. If un­lim­ited graz­ing is un­healthy for your horse for other rea­sons---such as the risk of lamini­tis or obe­sity, for ex­am­ple---then turnout in a dry lot, com­bined with reg­u­lar ex­er­cise, will also help.

• Use hay feed­ers or mats. Plac­ing hay di­rectly on the ground in­creases the risk that the horse will pick up bits of gravel or other for­eign ob­jects as he eats. Plac­ing hay in nets or some other type of feeder will keep it cleaner and also re­duce waste.

• Add psyl­lium to the ra­tion. Psyl­lium is a high-fiber lax­a­tive made from the seed husks of an herb called Plan­tago ovata. When in­gested, it forms a gelati­nous tex­ture that can cap­ture for­eign ma­te­rial and help it to move on through the di­ges­tive tract. Psyl­lium is of­ten given to horses to pre­vent sand colic be­cause it helps to re­move the grains from the in­tes­tine, and it may help pre­vent enterolith­s, too. “If a horse is at risk for hav­ing stones, adding psyl­lium to the diet may be ben­e­fi­cial to help re­move po­ten­tial niduses from the colon be­fore stone for­ma­tion oc­curs,” Herthel says.

• In­stall a wa­ter soft­ener. If your lo­cal wa­ter is high in min­er­als, you may want to con­sider in­stalling a wa­ter soft­ener to fil­ter the wa­ter that your horses will drink.

• Avoid feed­ing too many min­er­als. If you are us­ing a com­mer­cial feed and adding sup­ple­ments to your horse’s diet, he may be get­ting high lev­els of some vi­ta­mins and min­er­als. Your vet­eri­nar­ian or an equine nu­tri­tion­ist can de­ter­mine if any changes are nec­es­sary. Also, avoid giv­ing your horse mashes made with wheat bran or other in­gre­di­ents high in min­er­als.

Pre­vent­ing ev­ery in­testi­nal stone may not al­ways be pos­si­ble, es­pe­cially if your horse is un­usu­ally sus­cep­ti­ble to them. But with vig­i­lance and at­ten­tive care, you can greatly re­duce the risks that enterolith­s will pose a se­ri­ous threat to your horse’s health.

BIG AND SMALL: The sizes of enterolith­s vary, de­pend­ing on sev­eral fac­tors. But even fairly small stones can cause colic.

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