This educational article is brought to you by Merck Animal Health, the Official Animal Health Care Provider of the USEA. Runny nose, fever, cough, fatigue. It’s flu season. Winter and early spring is the most active time of year for equine influenza, so it’s an important time to vaccinate your horse. And like human flu vaccines, equine flu vaccines must periodically be updated to protect against the strains currently circulating in the field and threatening horses.