EQUUS - - Eq Medical Front -

For some horses, an all­hay diet may re­duce the ef­fi­cacy of omepra­zole in treat­ing ul­cers, ac­cord­ing to a new study from Aus­tralia.

Omepra­zole, a so-called pro­ton-pump in­hibitor, re­duces the amount of acid re­leased in the stom­ach; low gas­tric pH is as­so­ci­ated with the de­vel­op­ment of gas­tric ul­cers. De­vel­oped for use in hu­man medicine, omepra­zole (trade name Gas­troGard) is the most com­monly pre­scribed ul­cer med­i­ca­tion for horses.

To see whether diet in­flu­ences how omepra­zole is ab­sorbed by the horse’s body, Ben Sykes, BVMS, PhD, of the Uni­ver­sity of Queens­land, mea­sured changes in stom-ach pH in six healthy horses re­ceiv­ing oral omepra­zole un­der a va­ri­ety of con­di­tions. Two di­ets were stud­ied: an all-hay ra­tion in which horses had con­stant ac­cess to for­age, es ge, and a high-grain, low-fiber ra­tion, in which the horses ef­fec­tively fasted overnight.

Two dos­ing pro­to­cols were an­a­lyzed: 1 mg/kg given once daily (the dose in­di­cated on the drug la­bel for pre­ven­tion of ul­cers) and 4 mg/kg given once daily (the dose in­di­cated on the drug la­bel for treat­ment of ex­ist­ing ul­cers). Each horse was mon­i­tored on each com­bi­na­tion of diet and dose for six con­sec­u­tive days, and its gas­tric pH was con­tin­u­ally mea­sured via gas­tros­tomy tubes. The re­searchers also drew blood on the first and fifth day of the study to mea­sure lev­els of omepra­zole.

The data re­vealed that the horses typ­i­cally had bet­ter ab­sorp­tion of omepra­zole while on the high-grain diet as op­posed to the all-hay ra­tions. The im­pact of an all­hay ra­tion was dra­matic in some horses, with min­i­mal in­crease in pH ob­served, even at the higher dose, in these an­i­mals. The horses also tended to ab­sorb more and re­spond best to the higher dose of the med­i­ca­tion.

Ex­actly how an all-hay diet af­fects drug ab­sorp­tion and ef­fi­cacy in some horses wasn’t ad­dressed by this study, but Sykes says the find­ing isn’t with­out prece­dent. “It is well doc­u­mented in other species that feed­ing prac­tices can im­pair the ab­sorp­tion of this class of drugs, so the ef­fect is not un­ex­pected.”

Sykes ad­mits that the idea that hay may in­ter­fere with omepra­zole treat­ment seems coun­ter­in­tu­itive, given that a pri­mary rec­om­men­da­tion for man­ag­ing a horse with gas­tric ul­cers is to re­duce his grain ra­tion and pro­vide free-choice hay. “The pro­vi­sion of an all-for­age diet re­mains an im­por­tant part of ul­cer pre­ven­tion, es­pe­cially for squa­mous dis­ease,” he says. “How­ever, our find­ings some­what dis­rupt the stan­dard treat­ment rec­om­men­da­tions that we have used for years as, based on the re­cent studies, it does ap­pear that feed­ing such a diet

Horses on a high­grain diet typ­i­cally had bet­ter ab­sorp­tion of omepra­zole than did those on all-hay ra­tions.

dur­ing the treat­ment phase may be coun­ter­pro­duc­tive in some an­i­mals.”

What this means, ex­plains Sykes, is that cer­tain horses may re­quire a dif­fer­ent dos­ing ap­proach. “Some may re­spond bet­ter if they are sub­ject to an overnight fast (as­sum­ing morn­ing med­i­ca­tion) prior to ad­min­is­tra­tion of omepra­zole, with feed­ing oc­cur­ring ap­prox­i­mately 60 to 90 min­utes later---this is the peak time for drug ab­sorp­tion,” he says. “Feed­ing roughage through­out the day re­mains log­i­cal but the night­time feed­ing reg­i­men be­comes more crit­i­cal to drug ef­fi­cacy the fol­low­ing day. Iden­ti­fy­ing the non­re­spond­ing horse is chal­leng­ing, but if one fails to re­spond to treat­ment the role of diet should be con­sid­ered as one pos­si­ble fac­tor.”

Sykes adds that the de­vel­op­ment of in­jectable omepra­zole may even­tu­ally solve these dos­ing chal­lenges. “It will be some time be­fore it is com­mer­cially avail­able and there is a lot of work to do to de­ter­mine its most ap­pro­pri­ate use,” he says. “But it is an ex­cit­ing de­vel­op­ment.”

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