Dog saliva – an important source of dog allergens

Allergy to dog (Canis familiaris) is a worldwide problem that affects 5–10% of the adult population 1–3 and serves as a triggering factor in children and adults who suffer from asthma and allergic rhinitis 1, 2. It has been estimated that sensitisation to dog, confirmed by skin prick test, occurs in children with a physician-diagnosed asthma, rhinitis or eczema in up to 34%, 33% and 21%, respectively, in Sweden 4. Commercial dander extracts are routinely used for diagnosis of allergy to dog, both in vitro and in vivo. Dander is the preferable source for dog allergen extract preparations 5, 6. However, the outcome of in vitro IgE determinations and skin tests largely depends on the quality of the extracts 6. Clinical experience reveals that tests with commercially available dog allergen extracts occasionally show only slightly positive or even negative results, although the tested patients clearly exhibit dog-related symptoms 7, 8. Furthermore, dog dander extracts might be contaminated with mite allergens that potentially cause false positive skin prick test results 6, 9. Animal saliva is known as a common source of allergens. The major cat allergen, Fel d 1, is a tear, skin and salivary protein. Fel d 1 is produced by lacrimal and sublingual glands and subsequently transferred to fur by licking 10. Rat 11 and rabbit 12 saliva are reported as sources of numerous allergens that differ in SDS-PAGE profiles from analogous dander extract allergens. Five major allergens in rat saliva of molecular weights of 21.5 kDa or less have been identified by immunoblot 11. In rabbit saliva, 12 IgE-binding proteins have been identified. Two of them have by N-terminal sequencing shown to belong to the lipocalin family, while one was identified as the Fel d 1 homologue uteroglobin 12. Can f 1, Can f 2 together with serum albumin (Can f 3) and Can f 4 are known allergens from dog, although other possible candidates have been observed in immunoblot 13, 14. The two allergens Can f 1 and Can f 2 that belong to the lipocalin protein family are recognised as important allergens and are both found in dander and saliva 15, 16. Can f 1 binds more than 50% and Can f 2 one-third of IgE from dog-allergic patients 17. Can f 3 and Can f 4 are less important allergens that cross-react with allergens from other furry animals. In 2009, prostatic kallikrein (Can f 5) derived from dog urine was identified as a major allergen 14. A homologous protein was also detected in dog dander. Recently, the lipocalin Can f 6 was reported as an allergen cross-reactive with cat and horse 18. Despite being a major allergen, Can f 1 alone is not sufficient for diagnosis of dog allergy 16, 17. The aim of this study was to evaluate dog saliva as a potential source of allergens for improved diagnosis of allergy to dog.