EQUUS - - Eiv Equine Influenza Virus -

small mu­ta­tions new strain which can be com­bined in many ways. The num­bers in the virus des­ig­na­tion re­fer to the spe­cific type of these anti­gens that are present. “Any com­bi­na­tion of these makes each in­di­vid­ual virus in each host species dif­fer­ent,” Wil­son says. For ex­am­ple, the in­fa­mous Spanish flu of 1918 was caused by H1N1. The “bird flu” out­break of 2003 was caused by H5N1. With a few ex­cep­tions, most in­di­vid­ual flu viruses can in­fect only one species of an­i­mal---a healthy per­son can­not catch in­fluenza from a horse, at least not with the cur­rent va­ri­eties of equine in­fluenza virus.

EIV, like its rel­a­tives in the in­fluenza family, has two char­ac­ter­is­tics that make it dif­fi­cult to con­trol. For starters, it spreads quickly from host to host via air­borne droplets. “The in­cu­ba­tion pe­riod is very short---just a cou­ple of days,” says Mark Cris­man, DVM, MS, DACVIM, se­nior vet­eri­nar­ian for Zoetis, who also teaches at Vir­ginia–Maryland Col­lege of Vet­eri­nary Medicine. “We’ve seen out­breaks sweep through a sta­ble or barn, and in a teach­ing hos­pi­tal, where within 48 hours ev­ery horse in the barn was cough­ing.”

The sec­ond no­table char­ac­ter­is­tic of in­fluenza viruses is their abil­ity to change con­stantly. Over time, as the virus spreads from host to host, it un­der­goes small ge­netic changes---a process known as anti­genic drift. At

Both anti­genic shift and anti­genic drift can pro­duce viruses that “jump species” into new pop­u­la­tions with no im­mu­nity. Equine in­fluenza virus jumped to dogs in 2004, when an out­break oc­curred in rac­ing Grey­hounds in Florida.

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