Genome-wide association study of insect bite hypersensitivity in Dutch Shetland pony mares.

Insect bite hypersensitivity (IBH) is the most common allergic disease present in horses worldwide. It has been shown that IBH is under genetic control, but the knowledge of associated genes is limited. We conducted a genome-wide association study to identify and quantify genomic regions contributing to IBH in the Dutch Shetland pony population. A total of 97 cases and 91 controls were selected and matched on withers height, coat colour and pedigree to minimise the population stratification. A blood sample was collected from participating Shetland pony mares, their IBH phenotype was scored and the owner filled in a questionnaire. A total of 40 021 single-nucleotide polymorphisms (SNPs) were fitted in a univariable logistic model fitting an additive effect. Analysis revealed no effects of population stratification. Significant associations with IBH were detected for 24 SNPs on 12 chromosomes [-log(10) (P-value) > 2.5]. Odds ratios of allele substitution effects of the unfavourable allele were between 1.94 and 5.95. The most significant SNP was found on chromosome 27, with an odds ratio of 2.31 and with an allele frequency of the unfavourable allele of 0.72 in cases and 0.53 in controls. Genome-wide association studies on additional horse populations are desired to validate the identified associations, to identify the genes involved in IBH and to develop genomic tools to decrease IBH prevalence.

Genetic parameters of insect bite hypersensitivity in Dutch Friesian broodmares.

Insect bite hypersensitivity (IBH) is a seasonal allergic skin disease in horses caused by bites of certain Culicoides spp. The aim of our study was to investigate the maternal effect on IBH and to estimate the heritability and repeatability of IBH in the Dutch Friesian horse population. Data consisted of 3,453 Dutch Friesian broodmares with 3,763 visual observations on IBH clinical symptoms scored by 12 inspectors during organized foal inspections in 2004 and 2008. Nine percent of the mares (n = 310) were scored in both years. Mares descended from 144 sires and 2,554 dams and 26.2% of the dams (n = 669) had more than 1 offspring in the data set (range: 2 to 6). Insect bite hypersensitivity was analyzed as a binary trait with a threshold animal model with and without a maternal effect, using a Bayesian approach. Observed IBH prevalence in Dutch Friesian broodmare population was 18.2%. Heritability on the liability scale was 0.16 (SD = 0.06); heritability on the observed scale was 0.07; and repeatability was 0.89 (SD = 0.03). Maternal effect was 0.17 (SD = 0.06) and significantly differed from zero, although the animal model without a maternal effect fitted the data better. These results show that genetic and permanent environmental factors affect IBH in Dutch Friesian horses. The dam affected the IBH development of her offspring through an additive genetic influence but also by being part of their rearing environment.

Genetic variables of various manifestations of osteochondrosis and their correlations between and within joints in Dutch warmblood horses.

Osteochondrosis (OC) is an important orthopedic developmental disorder in many horse populations. A review of the literature revealed widely variable heritability estimates for the disorder. We estimated the genetic variables (heritabilities and genetic correlations) of various manifestations of OC. Femoropatellar, tarsocrural, and metacarpophalangeal and metatarsophalangeal joints of 811 randomly selected yearlings from the Royal Warmblood Studbook of The Netherlands, descending from 32 representative stallions, were scored for OC at 28 predilection sites. At each site, OC was scored in 5 categories, distinguishing between flattened bone contours and fragments. At the animal level, the overall heritability of OC was 0.23, the heritability of flattened bone contours was 0.08, and the heritability of fragments was 0.22. At the joint level, heritability was greatest in the tarsocrural joints, intermediate in the metacarpophalangeal and metatarsophalangeal joints, and least in the femoropatellar joints. The heritability estimates for the contralateral joint homologs were very similar. The genetic correlation between the tarsocrural and femoropatellar joint was strong, whereas correlations between the metacarpophalangeal and metatarsophalangeal and other joints were moderate. The genetic correlation between flattened bone contours and fragments at the animal level was 0.80. Scoring OC on a 5-point categorical scale resulted in greater heritability on the observed scale than when analyzing OC as a binary trait. Our results suggest that selection against OC could best be performed by taking into account the OC status of all 4 joints, the femoropatellar, the tarsocrural, and the metacarpophalangeal and metatarsophalangeal joints, and discerning between flattened bone contours and fragments.

Heritability and repeatability of insect bite hypersensitivity in Dutch Shetland breeding mares.

Insect bite hypersensitivity (IBH) is a seasonal recurrent allergic reaction of horses to the bites of certain Culicoides spp. and is found throughout the world. The aim of our study was to estimate the heritability and repeatability of IBH in the Dutch Shetland pony population. A total of 7,924 IBH scores on 6,073 mares were collected during foal inspections in 2003, 2005, and 2006. Mares were scored for clinical symptoms of IBH from June until February by 16 inspectors. Of all mares, 74.4% (n = 4,520) had a single observation, 20.7% (n = 1,255) had 2 observations, and 4.9% (n = 298) had 3 observations in different years. The overall mean IBH prevalence was 8.8%. Heritability was 0.08 (SE = 0.02) on the observed binary scale and 0.24 (SE = 0.06) on the underlying continuous scale. Repeatability was 0.30 (SE = 0.02) and indicates that including repeated observations of the clinical symptoms of IBH will improve the accuracy of breeding values for IBH. We conclude that IBH, based on clinical symptoms, is a heritable trait in the Dutch Shetland pony population. Therefore, the IBH prevalence in this population can be decreased by selection.