VAC­CINES:

EQUUS - - Eiv Equine Influenza Virus -

THE FIRST DE­FENSE

“Vac­ci­na­tion is still the most prac­ti­cal and eco­nom­i­cal means of EIV pro­tec­tion,” says Craig. The Amer­i­can As­so­ci­a­tion of Equine Prac­ti­tion­ers lists equine in­fluenza as a risk-based vac­cine, mean­ing that vet­eri­nar­i­ans may rec­om­mend it based on your horse’s risk of ex­po­sure to the dis­ease.

Gen­er­ally, horses in the high-risk cat­e­gory fre­quently travel to shows or other venues where they en­counter large num­bers of other horses. “Our sur­veil­lance pro­gram re­in­forces the knowl­edge that travel is a sig­nif­i­cant risk fac­tor for res­pi­ra­tory dis­ease in the horse,” says Craig, “high­light­ing the fact that those horses who spend time trai­ler­ing need an op­ti­mal vac­ci­na­tion pro­gram to stay healthy.”

But even horses who leave home only oc­ca­sion­ally have some risk of ex­po­sure. When eval­u­at­ing your horse’s sit­u­a­tion your vet­eri­nar­ian will con­sider sev­eral ques­tions, in­clud­ing, “What is the level of train­ing the horse is in? Train­ing can cause stress on the horse and com­pro­mise his im­mune sys­tem. How fre­quently does the horse travel? And where does the horse travel?” says Meg Green, DVM, man­ager of equine and large an­i­mal vet­eri­nary ser­vices with Me­rial, Inc.

Cur­rently, three gen­eral types of in­fluenza vac­cine are avail­able:

• In­ac­ti­vated (killed virus), given as an in­tra­mus­cu­lar in­jec­tion. If a horse has never been vac­ci­nated, he needs a two- or three-dose se­ries of this for­mula to con­fer full im­mu­nity, the first two at an in­ter­val of three to four weeks, fol­lowed by an in­ter­val of three to six months be­fore the third. Af­ter that, the horse will need boost­ers at six-month or one-year in­ter­vals, de­pend­ing on his de­gree of risk.

“In the killed vac­cine, the whole virus must re­sem­ble the live form of the virus enough to stim­u­late the im­mune re­sponse, but the vac­cine can­not cause in­fec­tion, and the vac­ci­nated horse can­not shed the virus,” says Green. “The draw­back is that this type of vac­cine stim­u­lates only hu­moral im­mu­nity, which al­lows for cir­cu­lat­ing an­ti­bod­ies within the blood, and makes for a slower im­mune re­sponse.”

On the other hand, says Craig, “These vac­cines pro­vide good pro­tec­tion for horses with mod­er­ate ex­po­sure to the dis­ease and are found in many com­bi­na­tion vac­cine prod­ucts.”

• Mod­i­fied live virus (MLV), ad­min­is­tered in­tranasally. These vac­cines con­tain a live virus that has been al­tered so that it can repli­cate enough to stim­u­late an im­mune re­sponse but can­not cause dis­ease. A sin­gle dose con­fers full im­mu­nity for at least six months.

“These in­duce both hu­moral and cell-me­di­ated im­mune re­sponses,” says Green. “Cell-me­di­ated im­mu­nity al­lows for a cell that is in­fected with a virus to be de­stroyed be­fore the virus can rup­ture that cell. As a re­sult, it re­duces the amount of virus that can in­fect other cells, and this lim­its the amount of in­fec­tion and re­duces shed­ding in the en­vi­ron­ment.” How­ever, be­cause these vac­cines con­tain whole viruses that have been mod­i­fied, she adds, “there is a chance that they will re­vert to vir­u­lence, and they may pro­duce ill­ness in im­muno­com­pro­mised an­i­mals.” For this rea­son, some coun­tries do not per­mit MLV vac­cines.

Ad­min­is­ter­ing this vac­cine in­tranasally also means that it in­duces im­mu­nity at the same site where the virus en­ters the body---in the phar­ynx and nasal pas­sages. “This type of vac­cine pro­vides the best im­mune re­sponse

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