Occupational allergy to horse allergens: More than exposure to horses!

Dear Editor, We read with interest the article from Moghtaderi et al. [1] showing that exposure to horses among individuals who ride enhances the risk of developing allergic sensitization to horse allergens in comparison to healthy controls. The topic is of high importance considering the popularity of a horse as a domestic animal for several reasons (work, leisure, pet therapy, etc.), and the relatively scarcity of data on horse-induced allergic sensitization. However, in our opinion, serious limitations to the conclusions of this research have not been included in the list of possible limitations already acknowledged by the authors. First of all, the authors don’t provide any information on the type of contact of horse-riders and healthy controls with common pets such as cats and dogs (Ownership? Indirect exposure? No apparent contact?). The frequency of ownership and the prevalence of allergic sensitization to cats/dogs vary in different countries according to cultural differences and environmental factors. Moreover, it has been widely recognized that cat and dog allergens should be considered as ubiquitous since they are found not only in indoor environments containing those animals but also in other indoor private/public places where cats/dogs have never been kept [2]. The consequence of pet allergen ubiquity is a persistent stimulation of airways similar to that induced by dust mite. This indirect modality of exposure (through clothes or other items) [3,4] is likely to be involved also for other furry animals, including horses [5]. Another important aspect is that allergic sensitization to furry animals may be induced not only by direct/indirect exposure but also by a cross-reaction mechanism involving some families of allergenic proteins. Lipocalins constitute the most important group of mammalian inhalant allergens because they are the major allergenic materials derived from a dog (Can f 1-2), cattle (Bos d 2), a horse (Equ c 1), a rat (Rat n 1), a mouse (Mus m 1), a guinea pig (Cav p 1), a rabbit (Ory c 1), and a hamster (Pho s 21) [6]. Some lipocalins show a very low amino acid identity whereas others – greater homologies and immunoglobulin E (IgE) cross-reactivity (47–67%), such as Fel d 4, Can f 6, Equ c 1, Ory c 4, Mus m 1, and Rat n 1. It has been shown that mammalian serum albumins are also involved in some clinical conditions, such as anaphylactic reactions after artificial insemination, episodes of food allergy and asthmatic reactions [7]. In our geographical area (Naples, Italy), only about 50% of atopic patients sensitized to common pets (cats/dogs) are directly exposed to those animals whereas the other half is indirectly exposed or not exposed. If we consider allergic sensitization and modalities of exposure to other furry animals, such as rabbits, hamsters, rats, horses, cows, guinea pigs and mice, the percentage rate of sensitized individuals exposed directly to those animals ranges between 0–33.3% whereas the percentage rate of the patients sensitized to the same animals with indirect or no contact ranges between 66.7–100% [8]. Finally, we have shown, by using an in vivo model (skin prick test), that exposure and allergic sensitization to common pets increase the risk of developing sensitization to other furry animals (including horses) by about 14-fold, suggesting a possible predisposition to develop multiple