Is there an allergic march in horses?

In humans, the development of atopic dermatitis in infancy and subsequent allergic rhinitis and/or asthma in later childhood or adulthood is known as the atopic march.

In certain horse families there is a genetic association between recurrent airway obstruction (RAO; severe equine asthma, EA) and a chromosomal region containing the interleukin-4 receptor α-chain (IL4RA) gene, which is associated with skin allergies and parasite defense in humans and other species. We therefore hypothesized that EA and skin allergies in horses may also have a common immuno-genetic background and/or may be linked to exposure and defense of infectious agents. Indeed, in two independent populations we found that EA is associated with insect bite hypersensitivity (IBH, odds ratio 7.4 and 13.1, respectively). Furthermore, multiple hypersensitivities also included urticaria and were associated with decreased parasite egg shedding. In a subsequent study, horses suffering from IBH were significantly more sensitive to inhaled histamine than healthy controls. Furthermore, horses suffering from IBH had a lower pO2 than healthy controls. These results suggest that IBH is associated with airway hyperreactivity even in the absence of overt respiratory clinical signs. Genetic studies have so far not revealed a common causative variant, preliminary microRNA profiles indicate involvement of a Th17-pathway and investigations of serum IgE against specific allergens and of the microbiome in horses affected with multiple hypersensitivities are underway. Interestingly, in cases of the recently described intestinal gluten hypersensitivity, cutaneous and/or respiratory signs were also observed.

The natural history of IBH and RAO suggests that typically an “allergic march” would lead from the former to the latter. However, further longitudinal studies will have to test such assumptions and may also address other allergic manifestations like urticaria or even food allergies. Presently, we conclude that there is genetic, epidemiological and clinical evidence that in horses, as in other species, different manifestations of hypersensitivity may occur together.