EQUUS - - Eq Medical Front -

New re­search from the Uni­ver­sity of Cal­i­for­nia–Davis con­firms that al­falfa hay can in­duce pri­mary pho­to­sen­si­tive re­ac­tions in horses, but the ex­act mech­a­nism re­mains un­known.

Pri­mary pho­to­sen­si­tive re­ac­tions oc­cur when pho­to­toxic com­pounds are in­gested by a horse and ac­cu­mu­late un­der his skin. (In sec­ondary photosensi­tivity, a horse’s liver can­not prop­erly ex­crete phyl­lo­ery­thrin, a byprod­uct of chloro­phyll degra­da­tion.) When ex­posed to sun­light through pink skin, the com­pounds cause painful blis­ter­ing, crust­ing and in­flam­ma­tion.

St. John’s wort, buck­wheat and sev­eral other plants have been found to con­tain com­pounds that can cause pho­to­sen­si­tive re­ac­tions. Sim­i­lar re­ac­tions have been linked to al­falfa hay anec­do­tally but it has not been found to con­tain the pho­to­toxic sub­stances.

To in­ves­ti­gate fur­ther, the Davis re­searchers per­formed a series of case stud­ies, ex­am­in­ing sam­ples of al­falfa hay as­so­ci­ated with seven out­breaks of equine pho­to­sen­si­ti­za­tion from 2004 to 2014. The tests re­vealed no known pho­to­sen­si­tiz­ing com­pounds, pes­ti­cide residues or fun­gal in­fes­ta­tions.

Af­ter the 2004 out­break, three test horses were fed the same al­falfa hay as­so­ci­ated with the photosensi­tivity re­ac­tions. Two de­vel­oped skin blis­ter­ing, con­firm­ing that the al­falfa was the cul­prit.

The Davis re­searchers also tested seven sam­ples of al­falfa hay as­so­ci­ated with a 2014 pho­to­sen­si­ti­za­tion out­break for two pho­to­toxic agents: chloro­phyll and pheophor­bide. They found that lev­els of both sub­stances were sim­i­lar to those found in or­chard grass hay, as well as con­trol bales of lo­cally grown al­falfa not as­so­ci­ated with any pho­to­sen­si­tive re­ac­tions. This, the re­searchers say, rules out those com­pounds as a causative agent.

The re­searchers ad­vise own­ers who feed al­falfa to re­main alert to any changes in the horse’s skin—par­tic­u­larly af­ter switch­ing to a new batch of hay—and to stop feeding it im­me­di­ately and call a vet­eri­nar­ian if a pho­to­sen­si­tive reaction is sus­pected.

Ref­er­ence: “Al­falfa hay in­duced pri­mary pho­to­sen­si­ti­za­tion in horses,” The Vet­eri­nary Jour­nal, May 2016 which would in­di­cate pe­ri­ods of in­creased stress. The small­est spikes were found in the horses from the les­son group, while the rest­ing horses had the high­est peak lev­els of cor­ti­sol.

The re­searchers con­clude that ex­er­cise re­duces stress lev­els in horses even “in cases where rid­ers are clumsy or lack ap­pro­pri­ate horserid­ing ex­pe­ri­ence” and that “rest­ing with­out any par­tic­u­lar ex­er­cise can also in­crease the stress lev­els in horses.”

Ref­er­ence: “Changes in sali­vary cor­ti­sol con­cen­tra­tion in horses dur­ing dif­fer­ent types of ex­er­cise,” AsianAus­tralasian Jour­nal of An­i­mal Sciences, May 2016

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