Pasture-Associated Heaves

Pasture-Associated Heaves

Bring severely affected horses indoors, not just off of pasture, as allergens might travel into dry paddocks.

Photo: Anne M. Eberhardt/The Horse

Question: It's summer and your coughing horse has been diagnosed with recurrent airway obstruction (RAO). As part of your horse's management plan, should you:

  • A: Keep your horse out of the barn and turned out to pasture, or
  • B: Keep your horse away from pasture and stabled indoors?

Although this may come as a surprise to owners of horses who show signs of heaves when stalled in the winter barn, the answer is B: Keep your horse inside.

Summertime Blues

Most horse owners--at least, those in the north, Midwest, parts of the west, and the northern tier of southern states--see the onset of RAO in their horses occurring during the winter when their horses spend a lot of time stabled. An inflammatory obstructive airway disorder sometimes referred to as heaves or COPD (chronic obstructive pulmonary disease), RAO-affected horses have an allergic reaction to something in their environment, causing constricted airways and excess mucus production. But where summers are hot and humid, a similar syndrome occurs known as summer pasture-associated obstructive pulmonary disease (SPAOD).

Explains Carla Sommardahl, DVM, PhD, Dipl. ACVIM, assistant professor of large animal medicine at the University of Tennessee's College of Veterinary Medicine, "SPAOD is a form of recurrent airway obstruction very similar to heaves--but the triggers that cause the hyper-reactions are different."

Although the exact conditions that trigger SPAOD are unknown, heat and humidity contribute to the hyper-reactions. "The horses may be fine when the weather is nice and cool, but as soon as the heat and humidity go up, affected pastured horses have respiratory attacks," Sommardahl states. "Horses can experience problems throughout the entire summer or seasonally, for example, just in early spring, mid-summer, or late summer, depending on what allergens they are sensitive to."

One major contributor is pollen, although the specific pollens aren't known. "Heat and humidity increase the pollen count because there's more growth in the warm, humid air, and wet air holds more particles," says Sommardahl. "Dust and heat-loving molds may also play a part."

Because heat and humidity are instrumental in provoking attacks, SPAOD occurs geographically and seasonally; it does not strike horses in dry, cool, or desert areas, or during the winter months.

Why some horses are susceptible and others aren't is another mystery. Says Susan C. Eades, DVM, PhD, Dipl. ACVIM, professor of equine medicine in the Equine Health Studies Program at Louisiana State University, "You can have 10 horses managed the same way, and maybe one of them will get the condition. The fact that most horses don't get SPAOD suggests that it's not managerially related. It's something specific in that horse. Whether it's genetic or just something that happens is not known."

Horses that live in areas where winters are cold and summers are muggy might or might not be more susceptible to the winter-related RAO condition. "A horse can have both, but not necessarily," Sommardahl says. "Most horses have one or the other. However, a SPAOD horse with a hyper-reactive airway, especially with ongoing inflammation, could be more susceptible to problems with dust and molds in winter barns, due to more sensitive airways."

As with heaves, age of onset is usually five years or older.

Identifying Disease

The clinical signs of SPAOD and heaves are the same, reports Eades. "In milder stages, affected horses only exhibit coughing and exercise intolerance," she says. "In more serious cases, horses can exhibit nostril flaring, significant abdominal lift, wheezing, and sometimes nasal discharge."

Diagnosing SPAOD is similar to diagnosing heaves. "Clinical signs that are seasonal help us," Sommardahl notes. "We first look for an infectious problem; to rule those out, we do a complete blood count (CBC), and sometimes radiographs to measure how severely the lungs have been affected. Radiographs can also be helpful in determining how much inflammation is going on. We'll do transtracheal washes and collect fluid from the trachea--these horses have increased fluid from their tracheas--and look for signs of infection either on the cells that we see or on culture, as a lot of these horses also develop a secondary infection on top of their allergy problems."


As with heaves, treatment is a three-principled approach consisting of environmental changes, medications to reduce inflammation, and medications to relieve respiratory distress. Desensitization is another method that is also sometimes used.

Environmental changes--"The best treatment is removal [of the horse] from the offending environment," says Eades. "Most of the time, that alone is sufficient." In the case of more severely affected horses, that means moving them indoors, not just off of pasture, as allergens might travel into dry paddocks. However, some horses do okay in a dry paddock or even on grass clipped very short. Remission usually occurs within 48 hours. However, a few horses won't achieve complete remission even in the barn, possibly because some of the offending triggers are airborne and/or the horse is also hyper-reactive to environmental factors inside the barn.

When and how long you keep your horse indoors depends on the severity of your horse's illness. At the very least, bring your horses indoors during the hot, humid times of the day, Sommardahl recommends. "If you want to turn them out, turn them out in the evening or early morning when it's not as hot." This might not be sufficient for some horses.

Many horses should be stabled 24 hours a day until the triggering conditions pass. "Most horses remain in remission for as long as they stay in the barn environment, but they have to stay in the barn environment all the time," Eades warns. "Just an hour in a pasture may be enough to move them out of remission and into exacerbation, then it will take another 48 hours before they'll go back into remission."

Additionally, reduce irritation of sensitive airways by keeping the barn well-ventilated, well-managed, and as dust- and mold-free as possible. Sommardahl suggests doing the following:

  • Stall horses away from indoor arenas, which tend to be dusty, and not under or near where bedding or hay is stored. Remove horses from the area when stalls and aisles are being cleaned or swept.
  • Avoid round bale hay, which usually contains a lot of molds in the center.
  • Reduce inhalation of dust in the hay by feeding hay on the ground or in a low manger so hay doesn't fall into the horse's face. Soaking or lightly wetting down hay also reduces dust.
  • If necessary, switch to less-dusty bedding during periods when the horse displays clinical signs of SPAOD. Large shavings that aren't dusty, shredded paper or cardboard bedding, and rubber mats are good choices. Do not use sawdust, which is dusty.

Pasture management might help delay or lessen the onset of clinical signs. "There is nothing you can do to change the grass, but you can manage the pasture," states Sommardahl. "Keep the pasture mowed so there's not a lot of tall, blooming growth and not as much pollen being produced. Avoid over-crowding and over-grazing, as pastures that are overeaten are more dusty because they don't have enough grass."

Reducing inflammation--Paramount for inhibiting inflammation and improving lung function are anti-inflammatory medications. Corticosteroids are the drug of choice.

"Corticosteroids are highly effective," says Eades. "However, they also produce the most side effects, predisposing the horse to other infections and laminitis." Thus, corticosteroids are generally used early in the treatment process on a short-term basis. They can be administered systemically (oral tablet, oral liquid forms, or injectables) or topically (inhalant delivered through an inhaler). Systemics are easier to administer; inhalants put the drug where it's needed the most, with less risk of side effects, but are more labor-intensive. Inhalants aren't recommended as an initial treatment for severe cases, as severe airway obstruction prevents deposition of the inhaled drug into the airways.

Oral or injectable dexamethasone is the most commonly used corticosteroid. "It rapidly improves lung function, usually within two or three days, with the horse returning to almost normal within a week," reports Sommardahl.

"Another corticosteroid choice," she says, "is oral prednisolone, the active and more absorbable form of prednisone. Prednisolone is a shorter-acting steroid, so it doesn't hang around in the system as long, has fewer side effects than dexamethasone, and is recommended for chronic and longer-term treatments."

Eades notes that antihistamines carry the fewest side effects, but are not very effective. Non-steroidal anti-inflammatory drugs are also reported to have no value and could be contraindicated.

Respiratory relief--Bronchodilator drugs relax constricted airways, relieve airway distress, and decrease mucus production (which can be beneficial as some obstruction results from mucus plugs); they do not relieve inflammation. "Because airway constriction is a serious problem in SPAOD, treatment with a bronchodilator is very valuable," Eades states. These drugs are available in oral, intravenous, or inhaled forms.

The most commonly used bronchodilator drugs are clenbuterol and albuterol. "When effective, they work in one to three days and rarely cause side effects," notes Eades. "The most common side effect is sweating."

Another class of bronchodilators includes glycopyrrolate. States Eades, "These are better bronchodilators, but often cause colic when given systemically."

A fourth option--Although not commonly used, a fourth treatment component is desensitization--allergy shots.

Affected horses are first tested for specific allergies either via blood samples or skin testing. Serum testing looks for antibodies against specific allergens, while skin testing examines the reactions to allergens injected under the skin, Sommardahl explains. "Getting an idea of what these horses are allergic to is helpful because we can try to eliminate some of those allergens from the environment (for example, the type of hay or bedding) and we can give appropriate allergy shots--desensitize them--to specific allergens."

Desensitizing the horse is a six-month commitment of administering allergens subcutaneously. "You start off by giving an allergy shot daily, then every other day, then eventually monthly," Sommardahl says. Usually this is done just once in the horse's lifetime, although horses with persistent problems could remain on monthly shots or be given a shorter round of shots a second year.

Sommardahl cautions that allergy shots are not curative, and environmental management/medical approaches must still be used. "During bad times of the year, these horses could still have flare-ups and need short courses of corticosteroids and bronchodilators, but we definitely see less flare-ups on some horses once they've had the allergy shots."

Sommardahl also notes that allergy testing and allergy shots are controversial: "Serum testing is not very accurate. Also, you can get different results from different companies that perform serum testing. Skin testing is more accurate, but researchers have not been able to correlate through control studies the allergic responses to respiratory problems.

"But we've done the skin testing here for 15 years, and we've received very positive feedback from clients that have given their horses allergy shots. About 50% of the horses have a lot less problems the next season."

What You Can Expect

Unfortunately, SPAOD is a life-long disease without a cure, although proper management and medical therapies can minimize its clinical signs. Says Eades, "Whatever is causing the horse to be hyper-reactive is still there; it is not eliminated through those management solutions."

The ability of the SPAOD-affected horse to work or compete depends upon the severity of the disease. "With management and medical treatment, mildly affected horses can remain athletic throughout the summer," notes Sommardahl. "However, more seriously affected horses with pronounced clinical signs will be less capable during that certain time of year when they are having problems."

Although SPAOD can't be prevented, initiating prompt treatment at the onset of clinical signs could help keep the disease from worsening and offers the best prognosis. "The more inflammation the horse experiences over time, the more likely that scarring and permanent damage to the lungs will occur," Eades warns.

So, when summer heat and humidity rolls around and your SPAOD horse begins to cough, don't delay in providing the management he needs.

About the Author

Marcia King

Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.

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