Evaluation of the precision of intradermal injection of control substances for intradermal testing in clinically normal horses.

The research is on the accuracy of intradermal testing (IDT) – a method used to test allergic reactions in horses. The findings suggest that reliable results can be obtained with histamine and phytohemagglutinin (PHA) as control substances.

Objective and Methodology

• The study aims to assess for precision or reproducibility in the application of IDT on horses. The IDT is an allergy testing method typically applied to the horse’s neck skin to reveal whether it has an immune response to specific injected allergens or substances.
• The methodology involved a total of 12 healthy adult horses. Each horse had IDT performed on the neck using 2 positive control substances (histamine and phytohemagglutinin [PHA]) and one negative control substance. A total of 20 syringes were prepared for each side of the horse’s neck.
• For each horse, a hair from both sides of the neck was clipped first, after which an equal volume (0.1 mL) of the substances were injected. Diameter of the resulting skin reactions (wheals) were measured at intervals: 0.5, 4, and 24 hours post-injection.

Results

• Skin reactions from injections of histamine and PHA on the horses were uniform in size at 0.5 and 4 hours after injection, respectively. This reveals intra- and interhorse consistencies in the wheal sizes, which signifies repeatability and precision in the results.
• However, significant variation in wheal sizes was observed after 24 hours following PHA injection. This signifies that the reaction to PHA had varied within and between the horses after this duration.

Conclusion

• The conclusion drawn from the study is that ID injections of histamine and PHA respectively at 0.5 and 4 hours results in repeatable and precise outcomes. Therefore, concentrations of 0.005 mg of histamine/mL and 0.1 mg of PHA/mL are recommended as positive control substances during IDT in horses.
• Moreover, the study concludes that consistent wheal size is evident with ID injection of control substances, and any variation observed in wheal sizes is due to the individual horse’s immune response to the particular injected antigen or allergen.