Vaala said owners and veterinarians should be aware that EIV prevalence is on the rise, with adult horses at greatest risk in late winter and early spring. | Photo: The Horse Staff

Equine influenza virus (EIV) cases have increased over the past decade, with some years worse than others. Plus, the demographics of affected horses are changing, and more horses vaccinated against the disease are becoming infected.

To gain a better understanding of EIV’s current prevalence and patterns, Wendy Vaala, VMD, Dipl. ACVIM, director of life-cycle management equine and companion animal at Merck Animal Health, and a team of researchers from Merck and the University of California, Davis, looked at EIV cases identified through Merck’s voluntary biosurveillance study over 11 years. She presented their findings at the 2019 Annual American Association of Equine Practitioners Convention, held Dec. 7-11 in Denver, Colorado.

Vaala’s team reviewed data from 239 clinics in 38 states from March 2008 to February 2019. The study included 7,809 horses showing respiratory signs and fever. Upon testing their nasal swabs using qPCR, which looks for DNA, 9.2% (718 horses) tested positive for EIV, with the number of cases per year increasing throughout the study period