Getting to the Bottom of Heaves

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Researchers have found that a drug used to relieve inflammation of the airways in horses suffering from heaves is effective at low doses. The drug, beclomethasone diproprionate (BD), is a corticosteroid, or steroid, and can cause side effects at high doses.

BD is commonly used for human asthmatic patients, according to Bonnie R. Rush, DVM, MS, DACVIM, of Kansas State University. Rush and a team of researchers studied the effects of BD on eight horses with naturally occurring heaves over eight days. While BD is already used in horses, the minimal dose required for efficacy had not been established. In this study, the researchers found that 500 mg BD caused improvement in pulmonary function, similar to those achieved with higher doses of BD.

Even after improving the stabling environment for a horse with heaves, drug treatment may still be necessary to ensure the horse’s comfort. Typical treatments are bronchodilators (drugs that relax the smooth muscle of the airways, thereby relieving the airway obstruction) or corticosteroids (drugs that relieve inflammation of the airways).

For mildly affected horses, bronchodilators, such as atropine, clenbuterol, pirbuterol, albuterol or ipratropium bromide, may be used intermittently. Long term use as solo therapy may cause deterioration of lung function, according to Rush. Bronchodilators, however, are inadequate for the horse severely affected with heaves and an anti-inflammatory should be considered.

Only four anti-inflammatory drugs have been studied for use in horses: prednisone, triamcinolone, dexamethasone and beclomethasone diproprionate. Prednisone is ineffective, and triamicinolone and dexamethasone can cause laminitis, and so must be used with caution.

Therefore, an effective anti-inflam­­­matory that can be aerosolized would be good news for horses with severe heaves. BD may prove to be such an anti-inflammatory, but careful attention should be paid to dose.

Rush, who has conducted numerous studies on BD and its use in horses, has found that using large doses of BD results in significant side effects. When given BD doses above 500 mg, adrenal gland suppression develops in both horses with heaves and in horses with normal respiratory function. The adrenal gland produces hormones such as cortisol, epinephrine and norepinephrine.

“Long term use [of bronchodilators] as solo therapy may cause deterioration of lung function...”

“Abuse or overdosing horses with corticosteroids for long periods of time has the potential to cause poor hair coat, poor muscle tone, frequent urination and susceptibility to infection,” she said.

According to Rush, horses are more sensitive to the suppressive effects of aerosolized BD than humans. So she and her team set out to evaluate the horse’s response to incremental doses of BD. The researchers looked at clinical signs, pulmonary function and adrenal gland response in horses with heaves.

The afflicted horses were randomly assigned to one of four treatment groups. Three groups received aerosolized BD at doses of 500, 1,000 or 1,500 mg, while one group received an aerosolized placebo. The researchers measured each horse's pleural pressure each morning before BD or the placebo was given, and 15 minutes after it was given. In addition, each horse was graded for airway obstruction using a scoring system devised by previous researchers. On a scale of one to four, a horse with heaves is rated for the amount of abdominal effort require to expel a breath. The more severe the disease, the more effort required.

Among horses receiving BD, researchers observed relief from heaves 24 hours after treatment began, with all three dose regimens. Pleural pressure improved by 25 to 50 percent of the base-line values, and the scores for airway obstruction were significantly lower than those for the placebo treated group, the researchers said.

“Even after improving the stabling environment...drug treatment may still be necessary.”

There was no significant difference in the magnitude of improvement between the three treatment groups, In fact, the 1,000 mg treatment group did not respond as well as the 500 mg treatment group until 72 hours later.

“I was surprised that they all got better at about the same rate,” Rush said. “Tripling the dose didn’t seem to matter.”

In this study, Rush and her colleagues found that giving more than 1,000 mg BD caused a 35 percent reduction in morning serum cortisol concentration within 24 hours and 80 percent reduction after five days.

In addition, the researchers found “no immediate (15 minute) treatment effect” with BD. So while the drug is an effective treatment for heaves, it should not be used for treatment of horses in severe crisis. In such cases, treatment with a bronchodilator would be appropriate, according to the researchers.

Rush noted, in addition, that distribution of an anti-inflammatory drug such as BD might be assisted by preliminary administration of a bronchodilator.

Clinicians are encouraged to administer the lowest effective does of [BD] for treatment of individual patients with recurrent airway obstruction, said Rush.

This article originally appeared in the April 2001 issue of International Equine Science. (802) 888-6189.