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She re­minded horse own­ers that horses can test neg­a­tive for EPM early in the on­set of the dis­ease, so re-test­ing 10-14 days af­ter on­set of clin­i­cal signs is im­por­tant if a neg­a­tive test re­sult is ob­tained within the first 7-10 days of clin­i­cal dis­ease.

Head­shak­ing

Some horses suf­fer from a prob­lem that re­sults in head­shak­ing. This can be from mild, eas­ily con­trolled clin­i­cal signs to de­bil­i­tat­ing head­shak­ing and nose rub­bing that re­quires eu­thana­sia.

John­son said head­shak­ing is a syn­drome for which di­ag­no­sis of the un­der­ly­ing cause can be dif­fi­cult, and clin­i­cal man­age­ment can be even harder. “Re­cent in­ves­ti­ga­tion has pro­vided more in­for­ma­tion re­gard­ing the un­der­ly­ing phys­i­o­logic prob­lem in many cases, and newly de­scribed treat­ment modal­i­ties can help im­prove horse com­fort,” she noted.

Head­shak­ing was first de­scribed in equine texts in 1809. The mean age of on­set is 7.5 to 9 years of age, with geld­ings and Thor­ough­breds more likely to suf­fer from this syn­drome. In 2000, one re­searcher cre­ated a grad­ing sys­tem for head­shak­ers that is still used to­day.

Head­shak­ing Grades

1. In­ter­mit­tent and mild clin­i­cal signs. Fa­cial mus­cle twitch­ing. Ride­able.

2. Mod­er­ate clin­i­cal signs. De­fin­able con­di­tions un­der which they oc­cur/de­velop. Ride­able with some dif­fi­culty.

3. Ride­able, but un­pleas­ant ride, dif­fi­cult to con­trol. 4. Un­ride­able, un­con­trol­lable. 5. Dan­ger­ous with bizarre be­hav­ior pat­terns.

There can be many causes of head­shak­ing, in­clud­ing mites and struc­tural prob­lems (such as si­nus cysts or gut­tural pouch in­fec­tions), but one cause that is fo­cused on is sim­i­lar to trigem­i­nal neu­ral­gia, a de­bil­i­tat­ing con­di­tion in hu­mans. John­son said peo­ple de­scribe their symp­toms as in­ter­mit­tent to per­sis­tent, brief to pro­longed, episodes of “elec­tric shock-like” fa­cial pain that is usu­ally just on one side.

Re­cent re­search in horses has as­so­ci­ated head­shak­ing with trigem­i­nal nerve hy­per­ex­citabil­ity. The re­search showed that head­shak­ers have a much lower thresh­old for nerve ac­ti­va­tion com­pared to clin­i­cally nor­mal horses. This re­search also sup­ports the sup­po­si­tion that head­shak­ing syn­drome is sim­i­lar to id­io­pathic trigem­i­nal neu­ral­gia in peo­ple. Treat­ments can be var­ied and in­clude: nose nets as a counter-stim­u­lant (one study re­ported im­prove­ment in 33% of horses and an­other study re­ported com­plete re­sponse in 27% of horses and par­tial re­sponse for 34% of horses);

eye shad­ing or blind­fold­ing to re­duce sun­light ex­po­sure in one study re­sulted in a “sub­stan­tial dif­fer­ence” in 52% of horses;

var­i­ous med­i­ca­tions (some stud­ies showed up to 76% im­prove­ment), and

sur­gi­cal in­ter­ven­tion. Some of the sur­gi­cal in­ter­ven­tions re­sulted in post-op­er­a­tive in­crease of clin­i­cal signs in about 60% of horses, in­clud­ing those that self-trau­ma­tized their noses or in­creased head­shak­ing. Some horses showed long-term im­prove­ment af­ter this ini­tial set­back. How­ever, 7% post-sur­gi­cal com­pli­ca­tions were so se­vere that they re­sulted in eu­thana­sia. John­son said that horse own­ers who elect for surgery should be pre­pared for a week up to one month of wors­en­ing of clin­i­cal signs be­fore they de­ter­mine if the surgery was suc­cess­ful long-term. Some­times mul­ti­ple surg­eries were re­quired to help clin­i­cal signs. John­son ad­vised that surgery is a last-ditch ef­fort in con­trol­ling this syn­drome.

A pi­lot study that has shown pos­i­tive re­sults, ac­cord­ing to John­son, is the use of per­cu­ta­neous elec­tri­cal nerve stim­u­la­tion (PENS) ther­apy. The thought be­hind this treat­ment is that it might ame­lio­rate the likely hy­per­sen­si­ti­za­tion of the trigem­i­nal nerve in id­io­pathic head­shak­ers. Ini­tial re­sults were pos­i­tive, with five of seven horses demon­strat­ing re­mis­sion for months. “Longert­erm fol­low-up with a larger group of horses showed that ap­prox­i­mately one-third of cases had use­ful re­mis­sion,” noted John­son.

“In­ter­ven­tions not shown to be ben­e­fi­cial in­clude a go­nadotrophi­n-re­leas­ing hor­mone vac­cine, feed sup­ple­ments, and pulsed high-dose dex­am­etha­sone,” she added.

John­son said that three PENS treat­ments were re­quired for max­i­mum du­ra­tion of re­mis­sion, which lasted about three months. Horses can be shown in be­tween treat­ments. He warned that some horses are so “peo­ple-ori­ented” that they won’t show pain or clin­i­cal signs as much when peo­ple are around. Some­times th­ese horses will need to have video ob­ser­va­tion around the clock to de­ter­mine the ex­tent of the syn­drome.

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