Equine influenza can strike a horse regardless of time of year. Competitive horses which attend events where other horses are congregated or horses which live in large boarding barns are particularly at risk. Not every horse which contracts a viral infection will display overt clinical signs, but such a horse might then be a carrier capable of infecting less immune-competent individuals. In this way, a horse which has never left your property could become infected from a non-symptomatic carrier.

Horses at greatest risk of developing influenza are those which are less than three years old. In an outbreak, about 75% of these youngsters get sick. Older horses might have developed some natural immunity over the years that protects them.

Any respiratory inflammation sets up the potential for that horse to develop chronic obstructive pulmonary disease (COPD), also known as heaves. Moldy hay is one means of starting this chronic disease, but damage from respiratory viruses is another common cause of this airway disability.

In a National Animal Health Monitoring System (NAHMS) Equine Study of 1998, it was found that respiratory problems were the third leading cause of days of lost work in the horse population. In general, prevention is the key to good airway health, but even in the best of circumstances, horses do develop respiratory infections