Intradermal challenge with allergenic Culicoides variipennis (diptera: Ceratopogonidae) extracts in some horses affected by sweet itch from central Italy

Sweet Itch (SI) is a recurrent and seasonal dermatitis of horses caused by allergenic insect species, of which the most involved ones are biting midges of the genus Culicoides. Sweet Itch is being recognised with increasing frequency in many parts of the world, but, until now, just few epidemiological data were available in Italy. Aim of this work was to highlight the presence of SI in Central Italy and to develop an intradermal test, using a purified total body extract of Culicoides variipennis, as allergen. The trial was performed between Spring 2004 and Summer 2005; 15 horses with history of a pruritic seasonal skin disease and 15 healthy horses, farmed in Central Italy, were selected. The 15 affected horses were submitted to routinary examinations, to exclude the presence of endo-ectoparasits, followed by histopathological examination to confirm the involvement of a hypersensitivity in the pathophysiology of SI. Furthermore, both clinically normal and symptomatic horses were submitted to intradermal test. To determine an appropriate concentration for the allergen, the extract was used for intradermal skin testing in 3 different dilutions (0,001, 0,0002, 0,0001 w/v), using 2 doses of inoculum for each concentration (0,1 and 0,05 ml). The occurrence of wheals was evaluated at 20, 60 min and 4-6 hours for the immediate hypersensitivity and 24 and 48 hours for the delayed hypersensitivity. At 20, 60 min and 4-6 hours after inoculation, wheats developed more in hypersensitive than in healthy horses, while after 24 e 48 hours no reactions was evident in any groups. To evaluate the best allergen volume and concentration, the results were statistically analyzed with the Student's T Test. Even if the symptomatic horses exibited the highest positive reactions at the allergen concentrations of 0,001 w/v (for both the volumes) and 0,0002 w/v for the volume of 0,1 ml, these concentrations have to be considered irritant as they evocated significant (even if lower in respect to what evidenced by the symptomatic animals) reactions in the control group. Vice versa, allergen concentration of 0,0002 w/v, tested at the volume of 0,05 ml, evocated positive reactions in 13/15 horses with clinical diagnosis of SI while it was insufficiently reactive for the control group. As a consequence, this concentration was considered appropriate for- the use of the insect extract in intradermal tests. Additional reserch was performed on the distribution of Culicoides on animals and in the environment by means of ligh traps; the insects caught were identified as belonging to the Complex Obsoletus (principally) and Pulicaris.