The most ef­fec­tive deworming strate­gies are cus­tom­ized for spe­cific sit­u­a­tions and tar­get par­tic­u­lar par­a­sites.

EQUUS - - Equus - By Lau­rie Bon­ner

Par­a­site con­trol: The most ef­fec­tive deworming strate­gies are cus­tom­ized for spe­cific sit­u­a­tions and tar­get par­tic­u­lar par­a­sites.

It’s not of­ten that a tra­di­tional horse-keep­ing rou­tine gets com­pletely up­ended, but that’s ex­actly what has hap­pened with equine par­a­site con­trol over the last two decades. Af­ter years of faith­fully deworming horses ev­ery eight to twelve weeks, ro­tat­ing among prod­ucts of dif­fer­ent chem­i­cal classes, horse own­ers in the mid-1990s were told that ap­proach was mis­guided and even po­ten­tially harm­ful. Here’s the prob­lem: Each time an an­thelmintic (worm-killing) chem­i­cal is used, most of the tar­geted pests in a horse’s gut die, but a few---those least sus­cep­ti­ble to the chem­i­cals---sur­vive. Those re­main­ing par­a­sites go on to re­pro­duce, and when they do, they can pass on their abil­ity to with­stand the deworming agent to the next gen­er­a­tion. Over time, with each suc­ces­sive dose, a pop­u­la­tion of par­a­sites that no longer re­sponds to deworming chem­i­cals grows. It’s like a ge­netic arms race, but for the time be­ing we’re stuck with the weapons we al­ready have. No new classes of de­worm­ers are on the hori­zon right now.

We have not yet reached the stage where we can­not con­trol par­a­sites in horses. But to put that day off as long as pos­si­ble, it’s im­por­tant to use de­worm­ers more ju­di­ciously than be­fore. That means adopt­ing a three-pronged ap­proach to deworming: targeting the horses most affected by par­a­sites, tim­ing treat­ments ap­pro­pri­ately, and tak­ing measures to limit the ex­po­sure of all horses to in­ter­nal par­a­sites.

Your vet­eri­nar­ian can help you de­velop an in­di­vid­u­al­ized par­a­site con­trol pro­gram for your horses. Still, it’s good to know which or­gan­isms you are targeting and the best ways to pro­tect against them.



Many par­a­sitic worms and lar­vae can af­fect horses, but four species pose the most se­ri­ous threats to equine health:

• Small strongyles (cy­athos­tomins). More than 40 species of these worms may af­fect horses. They spend a por­tion of their life span en­cysted in the in­testi­nal wall. Small strongyles may do lit­tle harm, but in very large num­bers, they can cause enough cu­mu­la­tive dam­age to dis­rupt a horse’s di­ges­tion and ab­sorp­tion of nu­tri­ents, caus­ing him to lose weight and be gen­er­ally un­thrifty. When large num­bers of en­cysted lar­vae emerge si­mul­ta­ne­ously, they may cause colic and, rarely, death.

• As­carids (Paras­caris equo­rum) af­fect mainly foals. In­testi­nal in­fec­tions may cause un­thrifti­ness, slowed growth, a dull coat and low en­ergy. Heav­ier in­fec­tions may block the in­testines, caus­ing life-threat­en­ing rup­tures and col­ics. Adult horses tend to de­velop im­mu­nity to as­carids as they ma­ture.

• Tape­worm in­fec­tion can cause ane­mia and gen­eral un­thrifti­ness. In some cases, tape­worms may lead to se­ri­ous col­ics and is­sues such as per­fo­ra­tions of the in­testines and peri­toni­tis 0 .

• Large strongyles, also called blood­worms, are rare in horses to­day but can cause sig­nif­i­cant harm. Sev­eral species can ap­pear in horses, but the most dam­ag­ing, Strongy­lus vul­garis, mi­grates through the walls of the ab­dom­i­nal ar­ter­ies, weak­en­ing them and leav­ing them prone to rup­ture. Their ac­tiv­ity may also cre­ate blood clots, which may in­hibit cir­cu­la­tion to the in­testines. Other species of large strongyles may dam­age the liver or other in­ter­nal or­gans.


These par­a­sites tend to be less of a worry---be­cause they are rare, they do less harm, or be­cause horses de­velop im­mu­nity as they ma­ture. Some, how­ever, may cause prob­lems:

• Bot­flies (Gas­terophilus spp.) are not worms, but the lar­vae of a species of fly that lays its eggs on horses. When horses lick the eggs, the lar­vae hatch and em­bed them­selves in the tis­sues of the mouth, be­fore they emerge and are swal­lowed. Then they at­tach them­selves to the wall of the stom­ach, be­fore turn­ing into grubs that pass out with the fe­ces. The lar­vae may cause dam­age where they at­tach to the stom­ach wall.

• Fi­lar­ids (On­chocerca spp.) are not in­testi­nal par­a­sites. The lar­val stage of the worm, trans­mit­ted by bit­ing midges, mi­grates un­der the skin and may cause itchy der­mati­tis on the face, neck, chest, with­ers, forelegs and ab­domen. The adults bur­row into the neck (nuchal) lig­a­ment, where they may cause in­flam­ma­tory re­ac­tions.

• Pin­worms (Oxyuris equi) do lit­tle harm to the in­tes­tine, but when the fe­males lay their eggs near the anus, they pro­duce se­vere itch­ing that causes the horse to rub bare patches on the rump and tail. Most horses seem to de­velop im­mu­nity to pin­worms as they ma­ture.

• Thread­worms (Strongy­loides wes­t­eri) mainly af­fect foals and may cause se­vere di­ar­rhea. Most horses de­velop im­mu­nity to thread­worms by the time they are 1 year old.


The prod­ucts most com­monly used to de­worm horses fall into three ma­jor cat­e­gories.

Aver­mectins and milbe­mycins are still largely ef­fec­tive against a wide range of par­a­sites, but re­sis­tance, espe­cially in as­carids, is grow­ing. This class in­cludes:

• Iver­mectin, which works against most of the com­mon equine par­a­sites ex­cept for tape­worms. How­ever, it does not work against small strongyles while they are en­cysted.

• Mox­idectin, which is sim­i­lar to iver­mectin but also acts against en­cysted small strongyles.

Ben­z­im­i­da­zoles are ef­fec­tive against many adult par­a­sites, but their ef­fi­cacy against lar­val stages and all small strongyles is de­clin­ing. This class in­cludes:

• Fen­ben­da­zole, which kills large strongyles, pin­worms and as­carids.

• Ox­iben­da­zole, which works against large strongyles, pin­worms, as­carids and thread­worms.

• Oxfenda­zole, which kills large strongyles, round­worms and pin­worms.

Pyrim­idines, also called pyran­tel salts, have sev­eral ap­pli­ca­tions. Re­sis­tance to chem­i­cals in this class is grow­ing among small strongyles. This class in­cludes:

• Pyran­tel pamoate, which works against large strongyles, pin­worms, as­carids and tape­worms in a stan­dard dose. When used at a dou­ble dose it kills 85 to 95 per­cent of tape­worms.

• Pyran­tel tar­trate, which is used for daily feed-through de-de­worm­ers It con­trols large ongyles and as­carids.

• Praz­i­quan­tel, which works specif­i­cally against tape­worms


To help keep our chem­i­cal weapons func­tion­ing longer, it’s im­por­tant to cre­ate in­di­vid­u­al­ized treat­ment plans for each horse in your care. Here are the guid­ing prin­ci­ples of your new strat­egy:

• Drop the idea of elim­i­nat­ing par­a­sites. Re­mem­ber that your over­all goal is to keep all your horses healthy---but that doesn’t mean par­a­site free. Few of the worms your horse will pick up will do him se­ri­ous harm in small num­bers.

• Identify the high shed­ders. Re­search has shown that adult horses shed strongyle par­a­site eggs at dif­fer­ent rates. In fact, in some herds, only a small num­ber of horses may be re­spon­si­ble for most of the strongyle eggs that are shed onto the pas­ture.

The only way to identify which horses are high shed­ders is to con­duct fe­cal egg counts-tests that an­a­lyze the ma­nure to look for par­a­site eggs. If you identify high shed­ders in your herd, targeting your deworming treat­ments to those in­di­vid­u­als will greatly re­duce the num­ber of eggs on your pas­ture. Low-shed­ding horses re­quire no more than twice-an­nual treat­ments to con­trol large strongyles.

• Choose the right agent. An­other ben­e­fit of fe­cal egg counts is that they will identify the types of par­a­sites present on your farm. Then you can choose chem­i­cals that are proven to work against those worms.

Fe­cal egg counts can also help you determine whether the par­a­sites on your farm are de­vel­op­ing re­sis­tance. An ini­tial count is per­formed be­fore a horse is de­wormed and then the test is re­peated 10 to 14 days later. Ide­ally, the num­ber of eggs would be re­duced by 90 per­cent or more. If the count is re­duced by less than 80 per­cent, then you need to in­ves­ti­gate. First make sure that you are us­ing the an­thelmintic ac­cord­ing to in­struc­tions: Check the ex­pi­ra­tion date, make sure you have stored it prop­erly and ad­min­is­tered the cor­rect dosage. If all of those re­quire­ments have been met, the par­a­sites on your farm are be­com­ing re­sis­tant to that chem­i­cal, which means you’ll need to switch to a dif­fer­ent chem­i­cal, both to slow the progress of re­sis­tance and to ef­fec­tively treat your horse.

Note that be­cause tape­worms shed eggs spo­rad­i­cally, they may not ap­pear in a rou­tine fe­cal egg count. Tape­worm in­fec­tion can also be iden­ti­fied us­ing a blood test to look for an­ti­bod­ies. A sim­i­lar test that looks for tape­worm-spe­cific an­ti­bod­ies in the horse’s saliva is now avail­able in Europe but not yet in the United States.

• De­liver the right dosage. Giv­ing a horse too small a dose of a de­wormer is one of the risk fac­tors that can in­crease re­sis­tance. You can cal­cu­late your horse’s weight in pounds with the help of a tai­lor’s tape: Mea­sure his girth

cir­cum­fer­ence in inches, square it, then mul­ti­ply by his body length (mea­sured from the point of his shoul­der to the point of his but­tocks), then di­vide that num­ber by 330.

• Time treat­ments for best ef­fect. Iron­i­cally, the greater the per­cent­age of worms you kill in one deworming on your farm, the faster re­sis­tance will de­velop. That’s why it’s a good idea to time your treat­ments for when a higher per­cent­age of par­a­sites are in “refu­gia” or “out of reach.” That is, while the de­wormer is work­ing in­side one horse, the chem­i­cals would have no ef­fect on eggs al­ready on the pas­ture or in other low-shed­ding horses. Those eggs, for par­a­sites that might be sus­cep­ti­ble to the chem­i­cals, could then be picked up and would “di­lute” the num­ber of re­sis­tance genes found in the next gen­er­a­tion of worms. How­ever, ad­min­is­ter­ing de­worm­ers when the refu­gia pop­u­la­tion is low re­duces this ef­fect---for ex­am­ple, eggs do not sur­vive long on

the ground dur­ing very cold win­ters or in hot, dry sum­mer weather. Fre­quent treat­ment of the whole herd at once also re­duces refu­gia.

Gen­er­ally, the ex­perts rec­om­mend that each horse be given two an­nual deworming treat­ments, with high shed­ders re­ceiv­ing an ex­tra treat­ment. Your vet­eri­nar­ian can help you de­velop a deworming plan that will work best for your horses.


Killing worms and lar­vae that have in­fected your horse is an im­por­tant goal, but it’s a bet­ter idea to limit the num­bers of par­a­sites your horse picks up in the first place:

• Al­low am­ple turnout space. When horses have enough room, they tend to seg­re­gate their pas­tures into eat­ing ar­eas and toi­let­ing ar­eas, called “roughs.” This way, they are less likely to graze over the ar­eas when the par­a­site eggs have been dropped with the ma­nure. Ide­ally, you’d have about two acres per horse.

• Pick up ma­nure reg­u­larly from turnouts. When al­low­ing two acres per horse isn’t pos­si­ble, it’s im­por­tant to pick up ma­nure at least weekly. Com­post ma­nure thor­oughly be­fore spread­ing it on fields; the high heat will kill any eggs.

• Har­row only dur­ing hot, dry weather. Break­ing up ma­nure balls and spread­ing them thin can help kill par­a­site eggs faster, but do this chore only on hot, dry days when the sun will dry it out more quickly.

• Ro­tate graz­ing spa­ces. Pe­ri­od­i­cally mov­ing horses into dif­fer­ent en­clo­sures will al­low for par­a­site eggs to die off be­fore they can be picked up by graz­ers. If you have only one large pas­ture, con­sider us­ing tem­po­rary fenc­ing to limit the horses to one sec­tion at a time.

• Pro­tect feeds from ma­nure. Ground-level graz­ing is best for a horse’s res­pi­ra­tory health, but drop­ping hay on the ground in crowded turnouts may ex­pose him to more par­a­sites. Use ground-level feed­ers to keep hay clean.

• Quar­an­tine new ar­rivals. Keep­ing a new horse sep­a­rate from the main herd for a time is wise for many rea­sons, such as iden­ti­fy­ing any ill­nesses he may be car­ry­ing. While the new­comer is in iso­la­tion, it’s a good idea to con­duct a fe­cal egg count. If he turns out to be a high shed­der, you will want to ad­min­is­ter a de­wormer be­fore he is turned out with the herd.

When al­low­ing two acres per horse isn’t pos­si­ble, it’s im­por­tant to pick up ma­nure at least weekly.

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