What Horses Are Affected?

Any breed or type of horse or pony may suffer from Sweet Itch from Shetland ponies to Shires horses and, although it is rare in Thoroughbreds, there are recorded instances of the affliction in this breed also.

Sweet itch is found throughout the world, wherever warm, humid conditions suit the midges and it is known as Summer Eczema in Europe, or Queensland Itch in Australia.

According to Danny W.Scott and William Howard Miller in the book Equine Dermatology (2003) the incidence of Sweet Itch is as follows:- UK 2.8%, Switzerland 3%, Japan 4.4%, Israel 21.85, Canada 26%, Germany 29% and Australia 32%.
Sweet itch in Icelandic horses

There are no midges in Iceland, so native horses do not suffer from sweet itch. However, when Icelandic horses became popular in Europe and North America, large numbers of the breed were imported and owners noticed an unusually high incidence of sweet itch among these native-born horses.

Reports suggest that up to one in four imported Icelandic horses may be affected. A study by Wilson et al (2006) found differences in the expression of IgE antibodies in imported Icelandic horses compared with other breeds suggesting that sweet itch in Icelandic horses may be subject to a different pathological mechanism.

Source: Horses@suite101.com

Susceptibility to sweet itch may exhibit genetic factors

A study of Swiss warmblood horses by Marti et al (1992) suggested that susceptibility to sweet itch may involve genetic factors. The offspring of one particular stallion suffering from the condition were examined, and researchers found 11 out of 207 offspring over three generations showed signs of the condition, compared with 0 out of 95 for the control group.

Swiss Warmblood

In a much larger study on the Dutch shetland pony population (2009) researchers collected data on over 6,000 mares and applied a statistical model to assess heritability. They concluded that insect bite hypersensitivity is a heritable trait in Dutch shetland ponies, and that its incidence could therefore be reduced by selective breeding. However, Wilson claims that Icelandic horses do not exhibit a higher incidence of the genes conferring susceptibility to sweet itch than other breeds.

Shetland Pony

Early exposure to midge bites may have a protective effect

As discussed in the 3rd Havemeyer workshop on allergic diseases of the horse, although exported Icelandic horses seem to display an unusually high level of susceptibility to sweet itch, Icelandic horses born in Europe or North America are less likely to develop the condition. This suggests that early exposure to midge bites may have some sort of protective effect. Bjornsdottir et al (2006) suggest that exposure of foals to midge bites prior to 6 months of age may program the immune system to ignore midge saliva allergens, thus protecting the horse from developing sweet itch in the future.

Source: Suite101- Sweet Itch in Imported Icelandic Horses