Inbred Horses Help Scientists Identify IBH Gene Locations
Researchers have gotten closer to understanding the genomics of insect bite hypersensitivity (IBH, also known as “sweet itch”) in horses. By studying a population of inbred horses, scientists have honed in on areas of equine chromosomes that could carry the genes for IBH.

Two “regions of interest” in Chromosome 8 could contribute to the condition that causes itchy skin reactions to insect bites, based on findings from a group of Exmoor ponies, said Brandon Velie, BSc, MSc, PhD, researcher in the Department of Animal Breeding and Genetics at the Swedish University of Agricultural Sciences, in Uppsala.

However, the findings are only a contributing link to solving the complex puzzle of equine IBH, Velie said.

“IBH is a very complex disease that is most likely the result of multiple genes across multiple chromosomes,” he told The Horse. “Our study of Exmoor ponies is just a small step toward truly understanding the complex genetic basis for this disease. Only through additional research in other breeds will we be able to combat and, with any luck, prevent this disease long term.”

Previous research has confirmed the link between IBH and specific genes on Chromosome 20 across multiple studies and breeds, Velie said. But in this new study, exploring the genotypes of 110 IBH-affected horses and 170 controls in an inbred population of Exmoor ponies, Chromosome 8 also appears to be related to the disease. Two distinct regions of the chromosome seem to have a connection with the presence—or absence—of IBH in individual horses.

Research Benefits of ‘Closed’ Herds

Studying an inbred population has significant benefits in genomic research, because the similarities among the genotypes help amplify differences, Velie said. They also help confirm areas of interest, because of the consistent presence of similar findings.

While considered “inbred,” the study ponies did not live in a single herd. On the contrary, they belonged to various owners on multiple farms throughout the British Isles. The scientists recruited the ponies through open calls via the Exmoor pony society and online postings, so it might seem that the ponies weren’t actually “related.” But the nature of the breed is such that, even across an entire country, the ponies still result from inbreeding—which makes them particularly interesting from a genomic research point of view.

“Thanks to selection for specific characteristics in horse breeds, certain breeds, like Exmoor ponies, are very similar to each other on a genetic level,” Velie said. “Exmoor ponies, in particular, are even more genetically similar as a result of a severe genetic bottleneck (narrow breeding base) during the second world war. As a result, there are fewer variations of genes that can have an impact on the performance or appearance of each animal. This means that any genetic cause(s) for a difference in IBH severity from one Exmoor pony to another (especially in the case of siblings) is much easier to identify.”

Combined with other studies, the findings from Velie’s team’s recent work could contribute to a better understanding of the biology of sweet itch reactions in horses as well as other species, he said.