Sensitisation to furry animals seems to be on the increase globally. Davila et al have recently published a consensus document on dog and cat allergy in which they state that measurement of IgE antibodies to animal dander extract is an established diagnostic tool, but has several limitations. In some cases, especially in poly-sensitised patients, molecular diagnosis is strongly recommended.
This will distinguish between co-sensitisation and cross-reactivity between various different animals. It is useful in poly-sensitised patients, in order to provide recommendations for avoidance as well as to identify allergens that are significant components in immunotherapy.
DOG DANDER COMPONENTS
There are six known dog allergens. Four of these, Can f 1, 2, 4 and 6 belong to the lipocalin family and are present in dog dander, saliva and urine.13 Can f 1 is a major allergen and is detected in 50%- 90% of patients sensitised to dog. Sensitisation to Can f 2 and to the more recently characterised Can f 4 and Can f 6 is less common. These allergens can be detected in 15-35% of dog sensitised patients and have been shown to cross-react with cat and horse lipocalins. Can f 3, the dog serum albumin, is a highly cross-reactive minor allergen detected in 15-35 % of dog-sensitised patients.
IgE reactivity to Can f 5, the prostatic kallikrein from a male dog, has been found in up to 70% of dog-sensitised individuals among whom 38% were not sensitised to Can f 1, 2 or 3. This suggests there may be individuals who are allergic specifically to male dogs. A number of studies have now shown the impact of molecular spreading of furry animal components on clinical disease. Käck et al show that polysensitisation to all six dog components conferred the overall highest risk for a positive reaction to dog nasal challenge.
Sensitisation to more than three single lipocalins, prostatic kallikrein and secretoglobin components from furry animals has been associated with asthma severity. Moreover, sensitisation to several components from the same species has shown to be a marker for clinical pet allergy. Käck et al also found that the presence of IgE to Can f 4 and 6, together with the serum albumin Can f 3, conferred the highest risk for a positive nasal challenge.
CAT AND DOG COMPONENTS
The secretoglobin Fel d 1 is the dominant allergen in cat dander extract. Sensitisation to Fel d 1 often starts early in life and is most likely to precede the appearance of allergic symptoms. The first described cat lipocalin, Fel d 4 was reported to bind IgE antibodies in over 60% of cat-allergic subjects. The cat lipocalin Fel d 7 was recently identified and shown to bind IgE in 38% of a cat allergic population. Tsolakis et al found that sensitisation to Fel d 2 and 4 were independently associated with type-2 biomarkers, such as exhaled nitric oxide and blood eosinophils. Uriarte and Sastre found that sensitisation to albumins were associated with severe respiratory symptoms and with high odds ratios for asthma diagnosis.