A genetic analysis of the occurrence of pulmonary haemorrhage in racing thoroughbreds in Southern Africa

[摘要] ENGLISH ABSTRACT:This study was carried out to investigate whether environmental and/or genetic factorshad an effect on the incidence of epistaxis related to exercise-induced pulmonaryhaemorrhage among racehorses in Southern Africa. A further aim was to estimate theheritability of liability to epistaxis in the Southern African Thoroughbred population.For the purpose of the environmental study, the data covering the period 1986-2001and involving a total of778 532-race runs, were analysed. This included the followingrace start information: date of race (day/month/year), age, sex, breeder, trainer,distance, jockey, state of going, weight carried, centre of racing and altitude. Thegenetic part of the data was two-fold in nature and included firstly the analysis of allhorses that suffered epistaxis whilst racing in Southern Africa from 1986 to 2001 andinvolving 1118 individual bleeders. The second genetic analysis included the sameSouthern African population plus those Southern African horses exported to Mauritiusand then being recorded as bleeders in that country (1252 bleeders in total). Pedigreedata covering the period 1960-1986 was used as required to calculate the incidence ofbleeding amongst ancestors of the post 1986 era. Only pedigrees of horses that racedwere included in this study as it was not possible to predict whether non-runnerswould have bled had they raced. Consequently all non-runners and also those thatraced overseas in countries where bleeding occurrence was not recorded wereexcluded.Veterinarians employed by the Jockey Club suspended officially recorded horses thatshowed epistaxis as demonstrated by frank bleeding from the nostrils after racing. Oncourseendoscopy is not employed as a routine on any of the Southern Africanracetracks.In the environmental study epistaxis was identified in 1 287 race starts (0.165%).Epistaxis related to exercise-induced pulmonary haemorrhage was significantly(p<0.001) associated with altitude, age, race year, month and the day of racing. Morehorses demonstrated epistaxis at sea level than at altitude, between the months of May- October than the rest of the year, in older horses than in horses less than three-yearsold, after 1995 than between the years 1986 and 1995, and on Fridays and Sundaysthan on any other week day. No association could be established between epistaxisand breeder, trainer, distance, jockey, state of going, sex and weight carried.The heritability of liability method as described by Falconer (1989) was used toestimate the relative importance of heredity and environment. For the periodinvestigated, the population incidence for epistaxis in Southern African horses was2.1%. The estimation of heritability ofliability showed that first-degree relatives hada figure of 55.4%. The heritability of second- and third degree relatives were 41.3%and 30.4% respectively. The data investigated depicts horses that bled almostexclusively on race days as only a small percentage (- 5%) was reported as havingbled during exercise. Accordingly, the full extent of epistaxis amongst racingThoroughbreds in Southern Africa is difficult to gauge.Pedigree and race run data from Thoroughbreds racing in Southern Africa, coveringthe period 1986-2002 (63 146) horses in pedigree data-set and 778 532 race runs,were further analysed in order to study genetic and environmental factors affecting theincidence of epistaxis as associated with EIPH (exercise-induced pulmonaryhaemorrhage). As fixed effects for the model, variables that were tested significantlyin a preliminary data analyses, were included. Various combinations of suchvariables namely age, weight, altitude, sex, month and going were tested. Fixedeffects that were included in the fmal model were gender, going and altitude. Theheritability estimates from a logit transformed analysis for epistaxis fitting both theanimal and sire generalized mixed models were 0.23 and 0.40 respectively, whichindicated that epistaxis as associated with EIPH in the Southern AfricanThoroughbred sires has a strong genetic basis. Genetic trends indicating an increasein epistaxis were also found.It is concluded that the frequency of epistaxis related to pulmonary haemorrhage isassociated with altitude, winter and spring months and the horse's age. It is suggestedthat racing at a lower altitude may increase the probability of exercise-inducedpulmonary haemorrhage.It is clear that epistaxis in the racing Thoroughbred has a strong genetic basis. It isfurther suggested that horses showing frank bleeding from the nostrils after racing orexercise, be suspended and not used for breeding purposes. This would result inrelatively fast progress being made towards eradicating this costly scourge of themodem Thoroughbred racehorse. Affected stallions and those racing whilst beingtreated with furosemide, should be barred from breeding and not be considered asfuture sires. Estimated breeding values for epistaxis should be used as a tool forselecting against it and be considered in breeding programmes to decrease theincidence thereof.

[发布机构] Stellenbosch University