Intradermal and in Vitro Serological Testing - Do They Tell Us Anything in the Horse?

Intradermal testing (IDT) and in vitro serological testing are intended to identify antigen specific IgE, the major mediator of immediate hypersensitivity reactions associated with equine atopic dermatitis and respiratory disease. The tests are performed to choose antigens for purposes of allergen specific immunotherapy (ASIT) and/or to avoid allergen exposure. Significant questions exist regarding test sensitivity, specificity and overall utility. This is in large part due to the lack of an appropriate ‘gold standard’ for test assessment (e.g. challenge with individual allergens and assess response).

Intradermal testing is available for a wide variety of pollens, moulds, mites, insects and environmental allergens. IDT is considered to produce more specific data than in vitro serological testing. Following intradermal injection, evaluation of reactivity (wheal formation) is done at 15–30 min. Delayed reactions (wheal formation) may be seen at 4 h and 24 h; some not preceded by immediate reactions. Because a significant number may be seen at 4 h, it has been suggested that IDT also be read at 4 h (Lorchet al. 2001a) but because of the erratic responses seen at 4 h, others recommend only the 15–30 min reading (Baxter and Vogelnest 2008). It has been shown that horses with atopic dermatitis, recurrent pruritic urticaria (Lorch et al. 2001a; Jose- Cunnilleras et al. 2001) and chronic obstructive pulmonary disease (Jose-Cunnilleras et al. 2001) have a significantly greater number of positive reactions compared to horses without atopy. However, it is important to emphasise that apparently normal horses may have a significant number of positive reactions (i.e. ‘false’ positives). ‘False’ positives may be associated with: nonspecific irritant reactions and/or the use of higher concentrations of allergens (e.g. concentrations of dust mite antigens should be lower than conventionally used concentrations for IDT: Baxter and Vogelnest 2008); some individuals have irritable skin or dermatographism (pressure related urticaria); cross reactivity with other allergens; and prior clinical or present sub-clinical sensitivity. It is important to note that the presence of a positive reaction supports the presence of sensitising antibody. It does not mean that this reactivity is clinically relevant. Because of this relatively high incidence of ‘false’ positives, IDT data should not be used to diagnose atopy. This is a diagnosis that should be made by history, physical examination and rule out. However, a pattern of positive reactions to multiple allergens within an allergen group, along with a compatible history and physical examination supports the fact that clinically relevant hypersensitivity exists. [...]