ON THE GENETIC-BASIS OF EQUINE ALLERGIC DISEASES .2. INSECT BITE DERMAL HYPERSENSITIVITY

The horses studied were of the Swiss Warmblood breed and most were ELA -typed to assess a possible association of dermal hypersensitivy to in sect bites with the major histocompatibility complex. Firstly, the occ urrence of the condition was examined in 304 half-siblings sired by si x stallions (A to F). Fourteen cases of dermal hypersensitivity were r ecognized and all were in the 153 offspring of Stallions C, E and F. M ost animals of this group were also investigated for chronic hypersens itivity bronchitis: none of the sires displayed clinical signs of derm al hypersensitivity, but Stallions D, E and F were affected by chronic bronchitis. Among the animals investigated for both conditions only o ne horse showed coincidence of the two diseases as can be expected whe n the diseases are not correlated. The frequency of manifest dermal hy persensitivity and/or chronic hypersensitivity bronchitis varied in th e half-sibling groups of individual sires. These findings suggest that the allergic conditions are independent entities. Secondly, the occur rence of dermal hypersensitivity was studied in three generations of h orses at a stud at which Stallion C had exerted a particularly strong influence. A total of 302 animals, all born and raised at this stud, w ere surveyed over a period of 12 years. The descendants of Stallion C showed a significantly higher incidence (P < 0.01) of dermal hypersens itivity (two daughters out of 19; eight second generation offspring ou t of 103; one third generation offspring out of 85) than did the contr ols of the same age classes but unrelated to Stallion C at the same st ud (0 out of 95). In this group the predisposition was not associated with the ELA antigens of Stallion C. Thirdly, the pedigrees and the EL A-haplotypes of eight affected half-siblings by Stallion F were studie d more closely. The predisposition in these horses was associated with the paternal haplotype A3, W23. Four multiple-case families were stud ied separately. One sire (G) produced nine affected offspring out of 2 2 out of four clinically healthy dams. The dams, however, were descend ants from Stallion C and they produced four unaffected foals by other sires. The paternal ELA-haplotype, A15, W23, segregated (with one exce ption) coupled with the predisposition to the affection. Stallion G an d the dams were born, raised and kept at the same stud as their offspr ing. Based on these findings, we suggest that certain animals can tran sfer hereditary susceptibility for dermal hypersensitivity due to inse ct bites to their offspring. In different families the ELA class II sp ecificity W23 segregated with the susceptibility. A gene or genes of t he major histocompatibility complex appeared to play a role in some fa milies as well as a gene or genes outside the MHC. Therefore we sugges t that insect bite dermal hypersensitivity is a multifactorial disease , including hereditary and environmental factors in its pathogenesis.