Intradermal testing of horses with and without insect hypersensitivity, using an extract of Culicoides species trapped in the Netherlands

Abstract

Abstract

Equine insect hypersensitivity is currently diagnosed on the basis of characteristic clinical symptoms and a compatible history. Intradermal allergy tests using a Culicoides nubeculosus extract (Greer Laboratories) proved not to be reliable in identifying horses with insect hypersensitivity in the Netherlands (Sloet 2006). Using tent traps covered with mosquito netting, C. obsoletus and C. pulicaris were identified as the most common species in the vicinity of horses in the Netherlands (94.1 % and 5.8% respectively) (Van der Rijt et al. 2007). Therefore, it was the aim of the present study to evaluate the usefulness of an extract of these ‘Dutch’ Culicoides species for the diagnosis of equine insect hypersensitivity. Thirteen pairs of horses were tested: one horse from each pair was known to suffer from Culicoides hypersensitivity and the other had never shown any compatible symptoms. Three concentrations of the ‘Dutch’ Culicoides whole body extract (1:1000 w/v, 1:10.000 w/v and 1:25.000 w/v) were tested together with a histamine solution (10 mg/ml) and phosphate buffered saline as control substances. Five 0.1 ml injections were given intradermally on the mid region of the neck. Skin test reactions were evaluated after 30 minutes and 1, 4 and 24 hours. At all time points the absolute wheal diameter to the Culicoides antigen 1:1000 w/v was significantly different (p< 0.01) between affected and unaffected horses. For Culicoides antigen 1:10.000 w/v such difference was observed at 1 and 4 hours (p<0.05) and at 24 hours (p<0.01), and for the 1:25.000 w/v solution only at 24 hours (p< 0.05). Similar results were obtained at 30 min, 1 and 4 hours if ‘positive’ skin test reactions were defined as those wheals having a diameter equal to or greater than the average of the diameters of the wheals produced by the histamine and phosphate buffer controls. In conclusion: This study indicates that the intradermal allergy test can support the clinical diagnosis of equine insect hypersensitivity by using environment-related species of Culicoides at a 1:1000 w/v concentration.