Susceptibility to sweet itch investigated

A study of Old Grey Kladruber horses suggests that the heritability of insect bite hypersensitivity (sweet itch) is “medium to high.”

A long-term study from the Czech Republic confirms that horses can inherit a susceptibility to the allergic skin condition known as “sweet itch,” which is technically known as insect bite hypersensitivity (IBH).

A reaction to the saliva of culicoides and other biting midges, IBH often causes itchiness so intense that horses rub affected areas hairless or raw. It has long been suspected that IBH is inherited because the problem is more prevalent in particular breeds, such as Icelandic Horses.

The Czech study focused on Old Grey Kladruber horses, a native warmblood breed, at the National Stud in Kladruby and 10 private breeding facilities. The same person visually inspected the horses over a period of 13 years, from 1996 to 2009, scoring the severity of any signs of IBH at each observation. A total of 1,209 observations were entered in a database, along with information about each horse, including location, age and pedigree. Thereafter, the data were analyzed.

In statistics, the degree of a trait’s heritability is expressed on a 0 to 1 scale, with 0 meaning that none of the variability of the trait among individuals is the result of genetics and 1 meaning that genes are responsible for all the variability. The Czech researchers found the heritability of sweet itch susceptibility among the study horses ranged from 0.305 to 0.626, depending on the factors considered in the calculations. These numbers, the researchers say, indicate the heritability of sweet itch is “medium to high, so there is a possibility to reduce the prevalence of IBH by selection.”

Reference: “Genetic parameters of insect bite hypersensitivity in the Old Grey Kladruber horse,” Journal of Animal Science, January 2017

This article first appeared in the July 2017 issue of EQUUS (Volume #378)

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