Breeding healthy horses

Insect bite hypersensitivity (IBH) is the most common seasonal recurrent allergic skin disorder in horses, caused by bites of certain Culicoides species. IBH causes an intense itch, which results in self-inflicted trauma. The welfare of affected horses is seriously reduced and some affected horses are even unsuitable for riding or showing purposes.

Horse owners encounter economic losses due to veterinary costs and a reduced commercial value of affected horses caused by disfiguration. Currently, no effective treatment for or prevention of IBH is available. The aetiology of IBH is multifactorial in origin and involves environmental and genetic factors. However, the contribution of genetic factors is poorly understood. The Animal Breeding and Genomics Centre (ABGC) has started a project on this topic aiming to unravel the genetic background of IBH.

The first part of this PhD project focused on estimating the heritability of IBH in the Dutch Shetland pony population. During foal inspections in 2003, 2005 and 2006, 7,924 IBH scores on 6,073 mares were collected. Mares were scored for clinical symptoms of IBH from June until November by 16 inspectors. The overall mean IBH prevalence was 8.8% and varied from 8.5% in 2006 to 9.5% in 2005. The observed IBH prevalence in mares descending from affected dams was greater (13.4%) than the prevalence in mares descending from unaffected dams (7.7%). The observed IBH prevalence of progeny groups per sire varied from 0% to 37%. Heritability was 0.08 (SE = 0.02) on the observed binary scale and 0.24 (SE = 0.06) on the underlying continuous scale.

We concluded that IBH, based on clinical symptoms, is a heritable trait in the Dutch Shetland pony population. Therefore, the IBH prevalence in the Dutch Shetland pony population can be decreased by selection.