EQUUS - - Eiv Equine Influenza Virus -

elec­tron mi­cro­graph of an in­fluenza virus par­ti­cle

World­wide equine flu out­breaks are mon­i­tored by the Ex­pert Sur­veil­lance Panel on Equine In­fluenza Vac­cine Com­po­si­tion, which is sup­ported by the World Or­ga­ni­za­tion for An­i­mal Health (which uses its his­toric French acro­nym, OIE). Virus sam­ples are col­lected dur­ing each out­break and an­a­lyzed—with a pri­mary goal of help­ing to keep vac­cines up to date.

“The panel meets ev­ery year in Fe­bru­ary or March to re­view the sit­u­a­tion for the pre­vi­ous year,” says Thomas Cham­bers, PhD, of the Maxwell H. Gluck Equine Re­search Cen­ter of the Univer­sity of Ken­tucky, who sits on the OIE panel.

One of the panel’s more in­ter­est­ing find­ings is how vac­ci­na­tion rates seem to in­flu­ence the fre­quency and sever­ity of equine in­fluenza out­breaks. “In the United States, there is a cer­tain level of vac­ci­na­tion, and the dis­ease is en­zootic, which means it is al­ways around at some low level,” Cham­bers says. “As a re­sult, we see a few out­breaks at race­tracks af­fect­ing maybe 20 or so horses, or an out­break at a vet­eri­nary hos­pi­tal, like we saw in Ore­gon in Oc­to­ber 2015, that af­fects a rel­a­tively small num­ber of horses be­fore they get it un­der con­trol.”

By com­par­i­son, he says, “In­dia has had huge out­breaks in­volv­ing many thou­sands of horses, and af­ter­ward the dis­ease will dis­ap­pear for about 10 years or so. Then they’ll have an­other huge out­break, fol­lowed by an­other long pe­riod of quiet. They don’t rou­tinely vac­ci­nate in In­dia, so we think that when they have a huge out­break, just about ev­ery horse is ex­posed and de­vel­ops some im­mu­nity. This high level of ‘herd im­mu­nity’ ap­par­ently stays in place for some years af­ter. But, of course, it even­tu­ally wears off and then they ex­pe­ri­ence the next big out­break.”

So many Amer­i­can horses are vac­ci­nated against EIV that a large out­break is un­likely, but the sud­den ap­pear­ance of a for­eign strain of the virus could cause trou­ble.

“At some point there will prob­a­bly be a rein­tro­duc­tion of Florida Clade 2 strain [seen pri­mar­ily in Europe], which makes it de­sir­able to use vac­cines with up­dated virus strains,” says Cham­bers.

Cur­rent U.S. vac­cines prob­a­bly of­fer some pro­tec­tion against for­eign strains. “All cur­rent flu strains cir­cu­lat­ing in the world, whether Clade 1 or Clade 2, trace their lin­eage back to the same ori­gins—they are all H3N8 viruses—con­sist­ing of Amer­i­can lin­eage, Florida sub-lin­eage,” says Bryant Craig, DVM, tech­ni­cal ser­vices vet­eri­nar­ian with Merck An­i­mal Health. “If an out­break of Clade 2 EIV were to oc­cur on U.S. soil, there would likely be some cross-pro­tec­tion from cur­rently vac­ci­nated horses that would lessen the im­pact of the dis­ease.”

How­ever, says Cham­bers, “Sooner or later cur­rent vac­cines will need up­dat­ing, and the only way we can tell how EIV is chang­ing is by study­ing di­ag­nos­tic sam­ples from in­fected horses.” He urges horse own­ers to help: “If your horse has a flu­like res­pi­ra­tory in­fec­tion, have your vet­eri­nar­ian take nasal swabs—or ideally na­sopha­ryn­geal swabs—and sub­mit them to a vet­eri­nary di­ag­nos­tic lab­o­ra­tory along with a vac­ci­na­tion his­tory in­clud­ing the spe­cific vac­cine prod­ucts and the dates they were given.”

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