Association between equine leucocyte antigens (ELA) and equine sarcoid tumors in the population of Swedish halfbreds and some of their families

The distribution of equine leucocyte antigens (ELA) in Swedish Halfbreds affected by sarcoid tumors was determined and compared with that of control horses of the same breed. ELA-haplotype A3W13 appeared more frequently in affected horses, resulting in a chi 2 value of 4.45 (P = 0.034) for A3 and 9.05 (P = 0.0026) for W13, respectively. The relative risk factor (RR) could be estimated to 2.13 and 3.00 for A3 and W13, respectively. The etiology fraction (EF) was calculated to 28% and 37% for A3 and W13, respectively. Thus, in the population of Swedish Halfbreds approximately 40% (at least) of the disease appeared to be associated with the genetic background of the affected horse. Family studies established that ELA are codominantly expressed and inherited as simple Mendelian traits and that sarcoids among offspring are significantly associated with one of the parental haplotypes (P = 0.00942). This parental haplotype does not always include A3W13. These results confirm and extend previous results from other breeds and strongly suggest the existence of a predisposition for sarcoids among horses, that is due to an autosomal, dominant, ELA-linked gene with incomplete penetrance. In extension, this indicates a multifactorial etiology of equine sarcoids (additional non-MHC gene(s) and/or environmental factors).