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By Nancy S. Loving, DVM


Equine influenza or other respiratory viruses (rhinopneumonitis, rhino virus) or bacteria can strike a horse regardless of the time of year. Horses that attend events where other horses are congregated or horses that live in large boarding barns are particularly at risk. Not all horses that contract a viral infection will display overt clinical signs, but such a horse may then be a carrier capable of infecting less immune-competent individuals as for example, a young or an old horse. In this way, a horse that has never even left the property could become infected.  

Besides viral or bacterial infections, two other inflammatory conditions occur in equine airways: Heaves (also called recurrent airway obstruction or RAO) or inflammatory airway disease (IAD). A horse with heaves shows specific clinical signs, including coughing (productive, wet cough or an unproductive, dry cough), exercise intolerance, an increased respiratory rate, flared nostrils, and a double expiratory effort that creates a “heave line.” A heave line develops from over-development of the external abdominal oblique muscles in the flank due to forced expiratory efforts.  

Heaves is usually caused by an allergic response to inhaled antigens such as “dust” from feed and bedding. The effect of heaves on the lower airways causes spasm and airway obstruction by mucus and chronic changes in the terminal branches of the respiratory tree. Moldy hay is one antigen that elicits this chronic disease, but damage from respiratory viruses is another inciting cause. Endotoxin aerosolized from manure and/or ammonia from urine are irritating to the lungs and result in either form of lung inflammation.

In contrast, the other airway problem, IAD, also adversely affects exercise tolerance but the horse doesn’t experience airway obstruction or make excessive efforts to breathe as with heaves. An IAD horse is normal at rest, with an occasional cough. Mucus accumulates within the trachea rather than in the lower lungs; increased inflammatory cells are identified in tracheal aspirates or with broncho-alveolar or lung washes.

In general, prevention is the key to good airway health, but even in the best of circumstances, horses do develop respiratory infections. How do you discern if your horse has developed a respiratory problem?

Clinical Signs 

Should your horse stand there listless, or have a discharge from the nose, eyes, or start coughing at the slightest provocation, you can do a few things to obtain information to assist in tracking down the source of the problem. First, obtain a rectal temperature to check for fever. Then consider the hay being fed – is it a new batch or have you recently accessed bales from the back of the stack or from an area that got wet from rain or moist ground? Shake out a few flakes to look for dust. Does it have a musty odor as you’d expect with mold? Also consider the barn environment: Are there strong ammonia fumes due to urine-soaked bedding? Is there a riding arena adjacent to the stalls where horses could be exposed to airborne particles and dust?

Any of these situations can stimulate an inflammatory respiratory condition. In turn, compromised lung quality makes the respiratory tract more susceptible to viral attack. Does your horse have nasal discharge? What does it look like? A clear and watery discharge may be normal and not necessarily a problem but could also signal a horse with a viral infection. An opaque, white discharge often indicates mucus and may be related to an allergy problem or airway inflammation. A nasal discharge with color (creamy, yellow, green) is usually indicative of bacterial infection. 

Feel beneath the horse’s jaw – if you find an enlarged lump denoting an inflamed lymph node, move the horse into isolation and monitor him carefully. Take his temperature daily, feed and water him separately, and watch for any cough or nasal discharge. Lymph nodes infected with Streptococcal organisms create a “strangles” infection that is highly infectious to other horses. Your vet can advise you on appropriate therapy and course of action as each case is individual in how it should be handled. 


A coughing horse should be isolated from others to protect against spread of a viral or bacterial infection through the herd. Incubation of respiratory viral infections in horses generally takes 3-7 days after exposure before clinical signs begin to show. Incubation forStreptococcus equi can take up to three weeks to demonstrate clinical signs. A horse with a respiratory infection should be allowed at least three weeks for healing before being put back into full work. This enables the airways to regain full, healthful function without suffering a relapse. 

Clean-Air Strategies 

Clean air is a definite part of the recipe for tending to a horse with respiratory problems. It is also an excellent strategy for maintaining good airway health. For a horse housed indoors, a better strategy is to move him outside into the fresh air. Feed hay off of the ground rather than in chest-high feeders. Head-down feeding enables a horse to clear dirt and dust from his nostrils and airways rather than inhaling irritating particulate matter into the lower bronchioles and lungs. Shake open flakes of hay and soak each thoroughly with water before you feed it. This holds down the dust and spores that would otherwise irritate the airways. Another effective technique for “cleaning” hay is to use a commercial hay steamer to remove dust and mold spores.

 Prevention through Immunization Strategies 

A routine vaccination schedule implemented by your veterinarian helps keep viral respiratory infections (equine influenza and equine rhinopneumonitis) to a minimum within your herd. Influenza and rhinopneumonitis vaccines are typically given twice annually. The more stress, the more rigorous the competition schedule, the more encounters with other horses, the more diligent should be the vaccination plan. In some cases, the intranasal vaccine for strangles may be advisable in an endemic area. Consult your veterinarian about the pros and cons of using this product.

For more in-depth information, please refer to Dr. Loving’s book, All Horse Systems Go: The Horse Owner’s Full-Color Veterinary Care and Conditioning Resource for Modern Performance, Sport and Pleasure Horses.

Reviewed and updated by original author in 2016.