Equine Influenza Cases on the Rise, Even in Vaccinated Horses

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.

Vaala said equine herpesvirus-4 (EHV-4) is usually the most common respiratory pathogen identified in horses displaying these signs. “When the incidence of EIV overtakes EHV-4, we know it’s going to be an unusually active year for EIV in general,” she said, adding that 2013 marked the first year EIV outranked EHV-4.

In the study, Vaala’s team found a higher-than-expected percentage of EIV positives in horses ages 1 through 9. “It’s no longer a young horse condition,” she said. Quarter Horses and horses that had recently been transported were also overrepresented.

Forty-one percent of infected horses were competition horses, while 37.9% were pleasure horses, which Vaala said indicates “staying home doesn’t protect you from influenza.” Sixty-one percent were current on vaccination against the virus, meaning vaccination is not always protective. This could be due to antigenic drift or shift—minor and major changes, respectively, to the viral genes over time. Further, there are various types of EIV vaccines, and not all are the same.

“Equine influenza virus is always changing (drifting), and the strains of EIV in the vaccine may affect the level of protection provided,” Vaala explained. “It is not just a question of if the horse has been vaccinated, but did the vaccine used contain EIV strains that provided sufficient cross-protection against the flu strains circulating?”

To combat this challenge, researchers are constantly monitoring currently circulating stains of EIV and comparing them to the strains of flu in currently available vaccine products.

Most cases occurred during winter and spring months. “There is an equine flu season,” said Vaala, “from mid-winter to early spring. This might impact when you choose to vaccinate against EIV.”

The AAEP vaccination guidelines recommend veterinarians vaccinate at-risk horses (e.g., performance and recreational horses) twice a year. “Ideally, horses at increased risk for EIV should receive a booster vaccination prior to peak exposure,” which suggests late fall and late spring might be ideal times for booster vaccinations, said Vaala.

In summary, she 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.

“Discuss vaccination with your veterinarian,” she added. “The vaccine used and the timing both play a role in efficacy.”