equine influenza
The recent increased incidence of equine influenza in the United States and abroad necessitates implementation of prevention and control strategies to protect the health of the equine population.

Although vaccination plays an important role, it should not be relied on as the sole preventive measure. Similar to other influenza viruses, equine influenza virus mutates over time and vaccines must be periodically updated to keep current with circulating viruses.

Horses that have been vaccinated according to with manufacturer’s label instructions and the American Association of Equine Practitioners’ vaccination guidelines can be protected against equine influenza. Studies suggest that using the modified live intranasal (administered through the nose) vaccine in previously unvaccinated horses can provide protection within five days of the first administration. However, no vaccine is 100% effective. Investigations into recent cases indicate that vaccinated horses can develop mild clinical signs for a short duration, whereas their unvaccinated counterparts are more severely affected and more likely to develop secondary pneumonia and pleuritis (inflammation of the pleura, the thin transparent membrane covering the lungs and lining the chest cavity).

Implementing biosecurity measures is essential to protecting horse health during influenza season. Because influenza virus is spread most readily through nose-to-nose contact, it is critical to limit horse-to-horse contact and require strict isolation of new arrivals and sick horses. Infected animals should be kept at a minimum distance of 50 yards from healthy horses.

If there’s no suitable permanent isolation stable is available, designate an area on the property where you can erect a temporary isolation structure. When no appropriate onsite isolation areas are identified, consider pursuing appropriate off-site isolation facilities such as vacant barns, empty fair or event grounds, or veterinary clinics.

To prevent virus transmission, designate personnel and equipment to work in and only be used in the isolation area. Personnel should use disposable coveralls, gloves, and boot covers when handling horses in isolation. If they must care for animals both in and out of isolation, they should handle healthy animals first and isolated animals last. Finally, personnel should utilize alcohol-based hand sanitizers after handling all horses—infected, exposed, or uninfected.

The influenza virus can be inactivated by exposure to sunlight or by using disinfectants. The virus can remain viable for up to two days on contaminated surfaces, specifically solid surfaces, including stall door latches. Thus, any shared equipment should be thoroughly cleaned and disinfected between uses.

A critical factor in controlling and preventing influenza virus spread is monitoring the health of all equids on affected properties. This includes taking body temperatures twice daily and recording all clinical observations. Only healthy horses, which have been monitored, should be permitted to leave the premises.

Any horse showing clinical signs of respiratory disease should be immediately isolated and examined by a veterinarian. Horses should remain isolated until they’re clinically healthy and no longer shedding virus. As respiratory influenza virus shedding can persist for seven to 10 days post-infection in a naive animal, horses should remain isolated for a minimum of 14 days.

Appropriate use of vaccination and implementing biosecurity measures are critical to protecting the horse industry from outbreaks of highly contagious viruses, including equine influenza.


CONTACT—Katie Flynn, BVMS—katherine.flynn@cdfa.ca.gov—916/900-5039—California Department of Food and Agriculture, Animal Health Branch, Sacramento

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