1348 AJVR, Vol 66, No. 8, August 2005 R ecurrent airway obstruction (RAO) is a common respiratory disease that affects horses and is char- acterized by intermittent periods of airway obstruc- tion. Clinical signs include wheezing, coughing, exer- cise intolerance, tachypnea, and increased respiratory effort. 1,2 This disease has no breed or sex predilection and commonly affects middle-age to older horses (> 7 years old). It is believed that RAO is an allergic hyper- sensitivity disease that develops when horses are exposed to various antigens (eg, Micropolyspora faeni, Aspergillus fumigatus, and Thermoactinomyces vulgaris) located in a horse’s environment and fodder. 3-5 Diagnosis of RAO is based on clinical signs and evalu- ation of bronchoalveolar lavage fluid. Although these diagnostic tests may confirm RAO, they do not specif- ically identify causative antigens involved in airway hypersensitivity. The pathophysiologic characteristics and treatment of RAO are thoroughly discussed else- where. 1,2 Interestingly, most current treatment modali- ties involve palliation of clinical signs rather than iden- tification of causative antigens and treatment of the underlying disease process. Intradermal testing (IDT) is a diagnostic tool in humans and small animals that allows more precise identification of causative antigens in allergic patients. 6-8 Intradermal testing is useful because it exploits cutaneous mast cells as a reflection of mast cell reactivity in other body systems (eg, the respiratory tract). Therefore, when a particular antigen causes degranulation of cutaneous mast cells, the same anti- gen may also cause degranulation of mast cells in other organ systems. Once causative antigens are identified, selected patients can be treated with allergen-specific immunotherapeutics in an attempt to decrease the patient’s dependence on corticosteroids and ameliorate the frequency and severity of clinical signs. Techniques used during IDT in people and small animals have been extrapolated to horses in an attempt to identify causative antigens involved in horses with RAO. Allergen-specific immunotherapeutic protocols have also been extrapolated for use in equine patients, but results of IDT and allergen-specific immunothera- py in horses have been inconsistent. 4,5,9-16 The exact rea- sons for the ambiguous results of IDT in horses have not been defined. Some of the inconsistencies in other studies of IDT in horses may be attributable to the fact that appropriate antigens have not been identified, appropriate dosages of antigens have not been deter- mined, appropriate evaluation time points after ID injections have not been established, and differences in response to IDT antigens between clinically normal and RAO-affected horses have not been adequately defined. Received September 8, 2004. Accepted November 29, 2004. From the Departments of Large Animal Clinical Sciences (Wong, Buechner-Maxwell) and Small Animal Clinical Sciences (Manning), and the Research Service Laboratory (Ward), Virginia- Maryland Regional College of Veterinary Medicine, Virginia Polytechnic and State University, Blacksburg, VA 24061-0442. Dr. Wong’s present address is the Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Iowa State University, Ames, IA 50010. Supported by the Veterinary Medical Fund and Clinical Research Grant, Virginia Polytechnic and State University. The authors thank Dr. Robert Esch for providing materials and Dr. Zorana Ristic for technical assistance. Address correspondence to Dr. Wong. Comparison of results for intradermal testing between clinically normal horses and horses affected with recurrent airway obstruction David M. Wong, DVM, MS; Virginia A. Buechner-Maxwell, DVM, MS; Thomas O. Manning, DVM, MS; Daniel L. Ward, PhD Objective—To evaluate differences in response to ID injection of histamine, phytohemagglutinin (PHA), and Aspergillus organisms between clinically normal hors- es and horses with recurrent airway obstruction (RAO). Animals—5 healthy adult horses and 5 adult horses with RAO. Procedure—Intradermal testing (IDT) was performed on the neck with 2 positive control substances (hista- mine and PHA) and a mixture comprising 5 Aspergillus species. Four concentrations of each test substance plus a negative control substance were used. Equal volumes (0.1 mL) of each test substance were prepared to yield 15 syringes ([4 concentrations of each test substance plus 1 negative control sub- stance] times 3 test substances) for each side of each horse (ie, 30 syringes/horse). Intradermal injections were administered; diameter of wheals was recorded 0.5, 4, and 24 hours after injection. Results—Hypersensitive responses to ID injection of histamine were detected 0.5 hours after injection, and a delay in wheal formation after ID injection of Aspergillus mixture 24 hours after injection was detect- ed in RAO-affected horses but was not observed in clin- ically normal horses. No differences were detected between the 2 groups after ID injection of PHA. Conclusions and Clinical Relevance—RAO-affected horses are hypersensitive to histamine, suggesting that RAO is associated with a heightened vascular response to histamine. Higher concentrations of Aspergillus mix- ture may be needed to detect horses that are sensitive to this group of antigens. Wheal reactions to Aspergillus may be a delayed response, suggesting that IDT results should be evaluated 0.5, 4, and 24 hours after ID injec- tion. (Am J Vet Res 2005;66:1348–1355) Unauthenticated | Downloaded 08/13/22 10:38 AM UTC