Different Origin of Insect Hypersensitivity in Icelandics?

Insect bite dermal hypersensitivity (IBDH) might have a different origin in Icelandic horses than in horses of other breeds, according to a study completed by researchers at the University of Bristol's school of Clinical Veterinary Science in the United Kingdom. The study was published in the August issue of Veterinary Immunology and Immunopathology.


Insect bite dermal hypersensitivity is a type I hypersensitivity (allergic) response to the bites of certain gnats (those in the genus Culicoides). In Iceland, IBDH does not occur because gnats are not present, however researchers note an exceptionally high incidence of this condition in Icelandic horses that have been imported to mainland Europe.


The researchers in this study investigated two groups of horses. The first group contained 122 adult horses that averaged around 12 years of age. These 122 horses consisted of 73 Icelandic horses born in Iceland and imported to Switzerland (31 of which were normal and 42 with IBDH), 24 Icelandic horses born in Switzerland (17 of which were normal and seven with IBDH), and 25 horses of various other breeds including Shetland ponies, Arabians, Franches-Montagnes, and Warmblood horses (12 of which were normal and 13 with IBDH).


The diagnosis of IBDH was based on the clinical signs and histories of the horses. All horses affected with IBDH had pruritus (itchiness), excoriations (areas where the skin is worn off), and thickening of the skin along the dorsal midline, localized mainly at the base of the mane and tail. The horses diagnosed with IBDH also had a history of pruritus that was seasonal (occurring between April and October) and recurrent. The normal horses had no previous history of skin problems and showed no clinical signs of pruritus or cutaneous (affecting the skin) lesions at the time of sampling.


In the first group horses, researchers compared the serum levels of immunoglobulin E (IgE) in 60 healthy horses to levels in 62 IBDH horses. While IgE is a minor component of serum antibody in mammals, it is capable of triggering the most powerful immune reactions, and it has a central role in immunity to parasites and the development of allergies.


In this group, total IgE levels were significantly higher in Icelandic horses with IBDH as compared to normal Icelandic horses for both those born in Switzerland (with an average of 35,000 nanograms/ml for those affected with IBDH and 10,000 ng/ml for normal Icelandic horses) and those born in Iceland (with an average of 30,000 ng/ml for IBDH affected horses and 15,000 ng/ml for normal horses). The IgE levels of other breeds of horses did not vary significantly between normal horses and horses affected with IBDH.


The second group of samples was taken from a group of 31 Icelandic horses before they were imported to Switzerland and in the three years after importation.


High levels of IgE (with an average of 121,970 ng/ml) were present in all horses at the beginning of the study, possibly to due to parasite burden, since deworming is not performed regularly in Iceland. However, levels dropped to normal (100 ng/ml) within the first year after arriving in Switzerland. The IgE levels then began to rise again significantly in the horses that developed IBDH, but they remained normal in horses that stayed healthy.


Icelandic horses have a known high incidence of IBDH, particularly in mature horses imported from Iceland, where gnats are not found. In this study, the time of year at which the horses were imported played a big role in the incidence of the disease. Horses that arrived in Sweden in the winter months were twice as likely to develop IBDH as those that arrived in the summer because the horses arriving in winter had time for their total IgE levels to decrease (from the levels reflecting parasite burden) before they were exposed to the gnats, and that effectively lowered their immunity to parasites and allergies.


Scientists on this study hypothesize that controlled exposure of Icelandic horses to a Culicoides antigen (any substance that evokes an immune response) prior to export from Iceland could have a protective effect against subsequent onset of IBDH.


Researchers conclude that the pathogenesis of IBDH in non-Icelandic horses is more consistent with a compartmentalized allergic disease.


About the Author

Rachael C. Turner

Rachael Turner is the former Photo and Newsletter Editor for The Horse. She is an avid event rider. Rachael's main focus is dressage and on training young horses with the proper foundation for success. She is also a member of the United States Dressage Federation and the United States Equestrian Federation. Her website is avonleaequestrian.com.

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