Health Up­date

Practical Horseman - - Special Dressage Issue -

Ge­netic link to mus­cle dis­ease dis­cov­ered; a vac­cine for al­ler­gies

Im­mune-me­di­ated myosi­tis is a mus­cle dis­ease seen in Quar­ter Horses and re­lated breeds such as Paints and Ap­paloosas. It can lead quickly to mus­cle weak­ness and at­ro­phy and can leave your horse feel­ing gen­er­ally ill. Un­for­tu­nately, not a lot has been known about the cause of IMM. New re­search, though, shows there is likely a ge­netic link.

A team led by Car­rie Finno, DVM, PhD, DACVIM, as­sis­tant pro­fes­sor in the depart­ment of pop­u­la­tion health and re­pro­duc­tion at the Univer­sity of Cal­i­for­nia, Davis, set out to ex­am­ine ge­netic as­pects of the dis­ease. The team in­cluded Stephanie J. Val­berg, DVM, PhD, DACVIM, ACVSMR, from the depart­ment of large an­i­mal clin­i­cal sciences at Michi­gan State Univer­sity, and col­leagues from both uni­ver­si­ties as well as from the Univer­sity of Min­nesota.

The re­searchers gath­ered DNA sam­ples from three groups of horses:

36 Quar­ter Horses with a his­tory of IMM

54 Quar­ter Horses with no his­tory of IMM 175 horses from 21 other breeds. Each group was a mix of geld­ings, mares and stal­lions with ap­prox­i­mately the same av­er­age age range. All were housed in the same en­vi­ron­ment. Any horses with polysac­cha­ride stor­age my­opa­thy (an­other mus­cle dis­ease par­tic­u­larly found in Quar­ter Horses) were ex­cluded from the study. None of the horses were re­lated to each other within at least one gen­er­a­tion.

Af­ter ex­ten­sive anal­y­sis of the DNA sam­ples, the team iden­ti­fied a mu­ta­tion in the MYH1 gene. This gene is re­lated to the pro­tein myosin, which plays a role in mus­cle con­trac­tion. The mu­ta­tion af­fected 14 dif­fer­ent amino acids in the pro­tein. Specif­i­cally, it ap­peared to de­stroy Type II (slow-twitch) mus­cle fibers, lead­ing to the dis­ease’s char­ac­ter­is­tic mus­cle at­ro­phy.

No­tably, the mu­ta­tion was not found among the 175 horses from non-Quar­ter Horse breeds. This sup­ports the idea that IMM may be an is­sue only for Quar­ter Horses and re­lated breeds.

The re­searchers also an­a­lyzed the horses’ pedi­grees and found that cer­tain stal­lions were heav­ily rep­re­sented among the horses show­ing the mu­ta­tion, in­di­cat­ing that IMM has a ge­netic com­po­nent.

How­ever, re­searchers do not nec­es­sar­ily be­lieve that the MYH1 mu­ta­tion di­rectly causes IMM. Rather, horses with the mu­ta­tion are more likely to ex­pe­ri­ence ac­tive symp­toms of the dis­ease un­der cer­tain cir­cum­stances. For in­stance, this could in­clude age, as IMM is more likely to af­fect horses un­der age 8 or over age 17. And, ac­cord­ing to past stud­ies, there may also be a con­nec­tion be­tween IMM symp­toms and a re­cent his­tory of cer­tain types of in­fec­tions or vac­ci­na­tions.

On the bright side, IMM-af­fected horses gen­er­ally re­gain full mus­cle mass af­ter one to 10 weeks of cor­ti­cos­teroid treat­ment.

A Vac­cine for Al­ler­gies

As if the dis­tract­ing buzzing of fly­ing in­sects wasn’t enough of a bother, bug bites are also a lead­ing cause of al­ler­gic skin re­ac­tions in horses. In par­tic­u­larly bad cases, horses may be faced with swelling, skin flak­ing or thick­en­ing and even weep­ing, bleed­ing wounds. Now imag­ine if a sim­ple vac­ci­na­tion could pre­vent all that un­pleas­ant­ness.

That’s just what an in­ter­na­tional team of re­searchers be­lieve they’ve de­vel­oped. The par­tic­i­pants in­cluded rep­re­sen­ta­tives from the Uni­ver­si­ties of Bern and Zurich (both in Switzer­land), Eng­land’s Ox­ford Univer­sity, the Lat­vian Biomed­i­cal Re­search & Study Cen­ter and pri­vate-sec­tor part­ners.

An al­ler­gic re­ac­tion can oc­cur when an in­sect, such as a mosquito, in­jects al­ler­gens while feed­ing on your horse. (An al­ler­gen is any sub­stance that causes an al­ler­gic re­sponse.) In a nut­shell, the al­ler­gen trig­gers a type of im­mune re­ac­tion in the horse’s body that in­cludes send­ing an “itch” mes­sage to the brain as well as giv­ing rise to other symp­toms com­monly as­so­ci­ated with an al­lergy.

The re­searchers the­o­rized that if they could block the in­ter­nal trig­gers of the im­mune re­sponse, they could then pre­vent the al­ler­gic out­come. So they cre­ated a vac­cine with two pri­mary func­tions—con­trol both the im­mune re­sponse and the re­lated al­ler­gic re­ac­tion. To do this, the vac­cine works to con­trol T-cells and eosinophils, two types of white blood cells im­por­tant to the im­mune sys­tem and al­ler­gic re­sponses.

The team tested its vac­cine on a herd of 34 Ice­landic horses af­flicted with chronic al­ler­gic re­ac­tions to in­sect bites. Nine­teen of the horses were given the vac­cine while 15 were given a placebo. All of the vac­ci­nated horses showed clin­i­cal im­prove­ments, in­clud­ing re­duced skin le­sions com­pared to what they had ex­pe­ri­enced the pre­vi­ous year and com­pared to the placebo group. In ad­di­tion, none of the horses showed ad­verse re­ac­tions to the vac­cine. Con­sid­er­ably fewer horses in the placebo group showed im­prove­ment in their al­lergy symp­toms.

The re­searchers be­lieve this de­vel­op­ment could po­ten­tially change med­i­cal treat­ment for all pets, as it demon­strates the suc­cess­ful use of an im­munother­apy for a chronic health is­sue. It may even as­sist in asthma ther­apy for hu­mans. — Sushil Du­lai Wen­holz

Sci­en­tists have found a ge­netic link to the cause of im­mune-me­di­ated myosi­tis, a mus­cle dis­ease of­ten seen in Quar­ter Horses.

Re­searchers are work­ing on a vac­cine to al­le­vi­ate al­ler­gic skin re­ac­tions that can be caused by in­sect bites.

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