The genetic basis of equine allergic diseases 1. Chronic hypersensitivity bronchitis

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Abstract
The genetic influence on chronic hypersensitivity bronchitis (CB) was investigated in families at two studs and among half-siblings of three affected and three non-affected sires at several farms. The family members at the two studs were born and raised under the same conditions, whereas the half-siblings were kept individually under very different conditions and were exposed to various environmental factors. The diagnosis was based on long-term observations and multiple clinical examinations at each of the two studs. In the half-sibling group, the diagnosis was based on the individual history and on a thorough clinical examination. The history of all horses suggested the disease was caused by allergies (symptoms provoked by hay). Statistical analysis of the data in the first study showed that a greater percentage of off-spring of two affected parents developed CB (9 of 13) than those with only one affected parent (23 of 48) and those with two healthy parents (5 of 29). The distributions of the affected offspring in these three categories (none, one or both parents affected) differed significantly (P<0.005) from what would have been expected without a genetic effect. The tendency to develop the disease was inherited equally from dams or sires. In the second stud fewer animals (n=42) were included in the study, but the results were similar. Parents without a history of CB produced off-spring with a low incidence of disease (1 of 16) compared with a higher incidence among descendants of one or two affected parents (10 of 26; P=0.02). Half-siblings descending from healthy sires had a lower incidence of disease than horses sired by affected stallions (P=0.02). In this group the quality of stabling also showed an effect on the condition of the horse (P<0.05). These results suggest that hypersensitivity CB in horses is a multifactorial disease with a rather strong genetic basis. There was no association with the major histocompatibility complex.