Comparison of results for intradermal testing between clinically normal horses and horses affected with recurrent airway obstruction.

The research article discusses a study aimed at understanding the difference in response to intradermal injections of certain substances between healthy horses and those suffering from recurrent airway obstruction. The researchers found that horses with RAO were more hypersensitive to histamine, implying that RAO is linked to an increased vascular response to histamine.

Methodology

• The study involved ten horses, five of which were healthy adults and the remaining were adults with Recurrent Airway Obstruction (RAO).
• Participants were subjected to Intradermal Testing, where injections are inserted directly into the dermis of the skin, on the neck region.
• The substances involved in the test were:
  • Two positive control substances: Histamine and Phytohemagglutinin (PHA)
  • A mix consisting of five Aspergillus species
• In total, four concentrations of the above substance were prepared and an additional negative control substance was included for a holistic understanding.
• Each horse, therefore, had 30 syringes prepared for testing – 4 concentrations of each test substance along with 1 negative control substance for each of the three test substances, each for one side of the neck.
• After administering the intradermal injections, the diameter of the wheals (swellings) was measured and recorded at different intervals – 0.5, 4, and 24 hours after the injection. This was done to understand the rate of reaction for each substance, for each individual horse, considering their health status (healthy or RAO-affected).

Results

• An abnormally heightened response was witnessed in the RAO-affected horses to histamine injection, only half an hour after the substance was administered.
• Interestingly, the wheal formation from the injection of the Aspergillus mix was delayed, only surfacing 24 hours after the injection.
• These variations were not observed in the horses that were clinically normal. The response to PHA was observed to be the same in both healthy and RAO-affected horses.

Conclusions

• The results suggested a hypersensitivity to histamine in horses suffering from RAO, indicating a possible correlation between the two, and stated that RAO could be attributed to an escalated vascular response to histamine.
• With the delayed response to the Aspergillus mix, it can be inferred that higher concentrations of this mix of antigens could be needed for a meaningful response in tests designed to detect Aspergillus sensitivity in horses.
• Importantly, the reactions to Aspergillus appeared delayed, which the researchers suggested meant that IDT results should be evaluated over an extended period – at 0.5 hours, 4 hours, and importantly, 24 hours after the intradermal injection is given. This extended investigation period could offer a more precise and comprehensive understanding of the horse’s sensitivity to certain antigens.