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- Author or Editor: Thomas O. Manning x
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Objective—To evaluate differences in response to ID injection of histamine, phytohemagglutinin (PHA), and Aspergillus organisms between clinically normal horses 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 (histamine 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 substance] 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 detected in RAO-affected horses but was not observed in clinically 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 mixture 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 injection. (Am J Vet Res2005;66:1348–1355)
Objective—To evaluate the precision of intradermal testing (IDT) in horses.
Animals—12 healthy adult horses.
Procedure—IDT was performed on the neck of each horse by use of 2 positive control substances (histamine and phytohemagglutinin [PHA]) and a negative control substance. An equal volume (0.1 mL) for each injection was prepared to yield a total of 20 syringes ([4 concentrations of each positive control substance plus 1 negative control substance] times 2 positive control substances times 2 duplicative tests) for each side of the neck. Both sides of the neck were used for IDT; therefore, 40 syringes were prepared for each horse. Hair was clipped on both sides of the neck, and ID injections were performed. Diameter of the skin wheals was recorded 0.5, 4, and 24 hours after ID injection.
Results—Intra- and interhorse skin reactions to ID injection of histamine and PHA resulted in wheals of uniform size at 0.5 and 4 hours, respectively. Significant intra- and interhorse variation was detected in wheals caused by PHA at 24 hours.
Conclusions and Clinical Relevance—ID injection of histamine and PHA caused repeatable and precise results at 0.5 and 4 hours, respectively. Concentrations of 0.005 mg of histamine/mL and 0.1 mg of PHA/mL are recommended for use as positive control substances for IDT in horses. This information suggests that consistent wheal size is evident for ID injection of control substances, and variation in wheals in response to ID injection of test antigens results from a horse's immune response to specific antigens. (Am J Vet Res 2005;66:1341–1347)