how to protect your horse from equine influenza
Equine influenza (EI) is considered endemic in both the U.S. and Europe, but the viruses causing EI differ slightly. For many years, the viruses in circulation in the U.S. have been “Florida clade 1” (FC1) whereas in Europe they have been “Florida clade 2” (FC2). These clades split apart in 2003; circulation of FC2 ceased in the U.S. around 2005 and, by 2010, there was no evidence of FC1 in circulation in Europe. Because of the extensive movement of horses between North America and Europe, the international panel of EI experts has recommended for the last 10 years that EI vaccines contain representatives of both FC1 and FC2. Some (not all) available EI vaccines meet this recommendation.

Equine influenza virus activity has recently increased in the U.S., Europe, and Nigeria. Normally, the virus circulates at a variable, but fairly low, level in the United States, but virus activity surged in the last three months of 2018 with outbreaks in 12 states. Additionally, an extensive EI event occurred in a donkey sanctuary in Nigeria. And, for the first time since 2015, multiple outbreaks of EI were reported during January and February of 2019 in France, Belgium, the Netherlands, Germany, Ireland, England, and Scotland.

In England, it resulted in a temporary lockdown of at least 174 premises and cancellation of racing for six days in February. Outbreaks were also reported in California, Arizona, Ohio, Indiana, and Washington State.

Some of the horses in these outbreaks, in both the U.S. and Europe, had been vaccinated against EI, raising the question: Is this a new strain that is not in the vaccines? The answer appears to be no. While the virus causing the Nigerian EI event is still uncharacterized, genetic analysis of isolates from both England and the U.S. confirms these as FC1.

There are two mutations that make them different from the recommended vaccines strains, but are these important? That is still under investigation.

The absence of FC1 from European circulation means their horses have no natural immunity and are dependent on vaccination for protection. The reports from England indicate that the clinical disease is of shorter duration and less severe in vaccinated horses; this would indicate the vaccines are working, at least partially. Milder disease in vaccinated horses may reflect an inadequate level of protective immunity following exposure to unvaccinated horses shedding large quantities of virus.

What should owners and veterinarians do to protect their horses from EI?

Familiarize yourself with the clinical signs of EI.

Often the very first sign is a harsh cough. Other signs include fever and nasal discharge, which is usually watery (serous) at first and then turns thick and yellow (mucopurulent). The horse may show unusually rapid breathing (tachypnea) or lose its appetite (anorexia). Sometimes there may be enlarged submandibular lymph nodes or dependent limb edema. Keep in mind that not all these signs may be present. Horses can be infected and still appear normal (subclinical infection), especially if they have been previously vaccinated. Other infectious agents can produce clinical signs that look like EI but aren’t. Equine herpesvirus-1/4 and strangles (Streptococcus equi) are examples. Have your veterinarian collect a nasal swab, or ideally a nasopharyngeal swab (which goes beyond the nostrils into the back of the throat) and send it to a veterinary diagnostic laboratory to confirm a diagnosis. Information on nasal-swabbing can be found at If your horse does develop clinical signs of EI, then the rule of thumb is that for every day of fever, it should be stall-rested for a week.

Vaccinate your horses using a vaccine that protects against both FC1 and FC2 viruses.

Horses’ antibody responses to vaccination do not last indefinitely, so if your horse has not been vaccinated for six months or more, then he is due for a booster. If it has been three months or less since the last booster, then hopefully your horse’s immunity should be at its peak. Consult your veterinarian and refer to the American Association of Equine Practitioners’ Vaccination Guidelines. In the event of an EI outbreak, where your horse may potentially be exposed, vaccination in the immediate face of the event might help if there is sufficient time—at least a week—for the horse’s immune system to start making antibodies.

Communicate with your veterinarian and with the manager or resident veterinarian at any facility to which you are taking your horse.

Is there an outbreak situation? If so, re-evaluate your horse’s vaccination status, and re-evaluate exposing your horse to flu.

Biosecurity from infectious diseases is best enforced by avoiding exposure whenever possible.

For farms, the best biosecurity is obtained by quarantining newly arriving horses away from the general herd for sufficient time to assure that the new arrivals are not bringing diseases with them. EI is transmitted through the air by coughing and indirectly on hands or clothing/equipment (fomites) that have been in contact with an infected horse.


CONTACT—Thomas Chambers, PhD——859/218-1126—University of Kentucky Maxwell H. Gluck Equine Research Center, Lexington

This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyd’s, London.