Preventing attacks of wheezing, coughing, and labored breathing in a horse with recurrent airway obstruction (RAO) involves eliminating allergens from the horse’s environment. But how do you know which allergens are most problematic for your horse? Should you have him tested? Intradermal skin testing (IDT) identifies the specific substances called antigens that trigger RAO in individual horses. David Wong, DVM, MS, says IDT, as it is currently conducted, might not be optimal for horses with RAO. “The limitations of IDT in horses are numerous,” says Wong.

Wong and a group of researchers from Virginia-Maryland Regional College of Veterinary Medicine and Virginia Polytechnic and State University designed a study to evaluate the responses of horses with and without RAO to intradermal injection of three substances; histamine, PHA, and a mixture of five species of Aspergillus mold. Histamine and PHA are known to induce wheals, or hives, in all horses, but at different times after injection. Histamine induces an immediate immune reaction, causing wheals within four hours, while PHA induces wheals at approximately 24 hours, mimicking a delayed immune reaction.  With the Aspergillus mixture, only RAO horses would be expected to develop wheals, since healthy horses are not allergic to the mold.  “Not every horse that has RAO is sensitive to Aspergillus,” explains Wong, “but Aspergillus is a common environmental antigen, and likely one of the most common antigens to induce signs of heaves in RAO horses.”

As expected, the histamine injections produced wheals that increased in size until four hours after injection. However, in RAO horses, the wheals grew more rapidly, at a rate of 10.8 mm per unit of histamine injected, compared to 5.4 mm per unit for healthy horses. There was no similar difference in growth rate for wheals after PHA injection.  This led to the conclusion that the immediate immune response was abnormal in RAO horses. 

When the Aspergillus injection sites were examined, there were no measurable wheals present in RAO horses until the highest concentration was tested and at the latest time, 24 hours after injection.  The reaction was not seen by four hours, as expected.  To investigate this further, Wong’s group took biopsy tissue from the injection sites of these horses.  What they discovered was cellular evidence of a delayed immune response. This led to the conclusion that RAO horses experience a dual hypersensitivity response to Aspergillus–a weak immediate reaction and a strong delayed reaction.  Horses might, therefore, have unique requirements for IDT