EQUUS - - Eq Medicalfro­nt -

High-tech di­ag­nos­tics may seem like overkill when deal­ing with com­mon equine skin con­di­tions, but re­searchers say poly­merase chain re­ac­tion (PCR) tech­nol­ogy can be in­dis­pens­able for in­ves­ti­gat­ing dif­fi­cult cases of rain­rot.

A DNA-am­pli­fi­ca­tion tech­nique, PCR is com­monly used in gene se­quenc­ing, foren­sic anal­y­sis and the di­ag­no­sis of in­fec­tious dis­eases.

Rain­rot, a crust­ing skin in­fec­tion caused by Der­matophilus con­golen­sis bac­te­ria, is of­ten seen dur­ing rainy sea­sons. D. con­golen­sis is an op­por­tunis­tic or­gan­ism that re­quires the right set of en­vi­ron­men­tal con­di­tions to be­come in­fec­tive. When ac­ti­vated by wet, hu­mid con­di­tions, the bac­te­ria mul­ti­ply rapidly, ir­ri­tat­ing the skin of some horses. The re­sult is crust­ing, tight scabs and hair loss that fol­lows a “drip pat­tern” along the horse’s back, rump and flanks. It can also af­fect ar­eas of the horse in con­tact with moist pas­ture such as the lower legs and muz­zle.

Sus­cep­ti­bil­ity to rain­rot varies, with older horses and those with com­pro­mised im­mu­nity at the high­est risk of more se­vere in­fec­tion.

Peo­ple of­ten iden­tify rain­rot, which is tech­ni­cally known as der­matophilo­sis, on their own and treat their horses with­out a ve­teri­nar­ian’s guid­ance. “I think [rain­rot] is over-di­ag­nosed by horse own­ers since not all crusts are rain­rot,” says Linda Frank, MS, DVM, DACVD, of the Univer­sity of Ten­nessee. “But it is a very com­mon dis­or­der and I do feel that if own­ers think that is what the horse has, then it is worth try­ing their fa­vorite treat­ment first. Of course, I rec­om­mend us­ing prod­ucts that are tested in horses and not nec­es­sar­ily recipes they find on the In­ter­net.”

A stub­born case of rain­rot, how­ever, is cause for a vet­eri­nary visit and pos­si­ble diagnostic test­ing. “Ve­teri­nar­i­ans are called in specif­i­cally for this con­di­tion when it is quite se­vere and gen­er­al­ized, ap­pear­ing atyp­i­cal with large ar­eas of scale and hair loss, and/or not re­spond­ing to treat­ment,” says Frank.

To di­ag­nose rain­rot, most ve­teri­nar­i­ans ex­am­ine crust sam­ples un­der a mi­cro­scope, look­ing for D. con­golen­sis. But, says Frank, this re­quires time and pro­fi­ciency. “In chronic le­sions very few or­gan­isms may be seen, so it

can take a lot of time to find them or you get a false neg­a­tive re­sult "she ex­plain, "If you do not per­form this test on a reg­u­lar ba­sis you may not be as good at iden­ti­fy­ing the or­gan­isms. It is a skill that needs to be main­tained.”

PCR is a vi­able al­ter­na­tive, Frank says. “[PCR] has be­come a rou­tine diagnostic test in most lab­o­ra­to­ries. It is not more ex­pen­sive than sub­mit­ting sam­ples for cy­tol­ogy, in fact it can be less ex­pen­sive than that.” She adds that the test is very sen­si­tive, which means it is un­likely to re­turn false neg­a­tive re­sults.

In a study of sam­ples col­lected from 14 horses with sus­pected rain­rot and 12 horses with other crust­ing skin con­di­tions, cy­tol­ogy iden­ti­fied rain­rot in nine (64.3 per­cent) of the sus­pected cases, while PCR found the or­gan­ism in 11 (78.6 per­cent.) Nei­ther cy­tol­ogy nor PCR iden­ti­fied the or­gan­ism in crusts taken from horses sus­pected of hav­ing dif­fer­ent skin con­di­tions.

Based on the re­sults of this test, Frank says she sees a role for PCR in the di­ag­no­sis of rain­rot or dif­fer­en­ti­at­ing it from other con­di­tions, “I feel that the place for this test is when the le­sions are se­vere enough that the owner wants the horse ex­am­ined by a ve­teri­nar­ian,” she says. “The ve­teri­nar­ian can sim­ply col­lect some crusts and sub­mit them to the lab. If the horse’s le­sions are se­vere, then the next step would be to biopsy the le­sions. There are other causes of crust­ing than just der­matophilus, and this would be a diagnostic step to start point­ing us in a dif­fer­ent di­rec­tion, prior to a tak­ing a skin biopsy.”

Ref­er­ence: “RT-qPCR for the di­ag­no­sis of der­matophilo­sis in horses,” Vet­eri­nary Der­ma­tol­ogy, Oc­to­ber 2016


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