The inheritance of liability to epistaxis in the southern African Thoroughbred.

This study was carried out to estimate the heritability of liability to epistaxis in the southern African Thoroughbred population. Data of all horses that suffered epistaxis while racing in southern Africa and Mauritius from 1986 to 2002 and involving 1252 bleeders were analysed. Pedigree data covering the period 1960-1986 was used as required to calculate the incidence of bleeding amongst ancestors of the post-1986 era. Only pedigrees of horses that raced were included in this study as it was not possible to predict whether non-runners would have bled had they raced. Consequently all non-runners and also those that raced overseas in countries where bleeding occurrence was not recorded, were excluded. The heritability of liability method as described by Falconer (1989) was used to estimate the relative importance of heredity and environment. For the period investigated, the population incidence for epistaxis in southern African horses was 2.1%. The estimation of heritability of liability showed that 1st-degree relatives had a figure of 55.4%. Second-degree relatives had a heritability of 41.3% and 3rd-degree relatives came in at 30.4%. The heritability of liability shown in this study could be regarded as being at the lower end of the range but could be appreciably higher. The data depict horses that bled almost exclusively on race days, as only a small percentage (approximately 5%) was reported as having bled during exercise. Accordingly, the full extent of epistaxis amongst racing Thoroughbreds in southern Africa is difficult to gauge. It is clear that epistaxis in the racing Thoroughbred has a strong genetic basis. It is suggested that horses showing frankbleeding from the nostrils after racing or exercise be suspended, and not used for breeding purposes. This should result in relatively fast progress being made towards eradicating this costly scourge of the modern Thoroughbred racehorse.