Determination of threshold concentrations of multiple allergenic extracts for equine intradermal testing using normal horses in three seasons

Abstract

Forty-one normal horses were evaluated for reactivity to intradermally injected aqueous allergens to determine allergen threshold concentrations (TC), with potential relevance to equine intradermal testing (IDT). Horses were tested three times over 1 year to assess seasonal variation in reactivity, using three to five serial dilutions of 27 allergens each time. Injection sites were evaluated after 15 min, 1 h, 4 h and 24 h. The highest allergen concentration at which < 10% of horses demonstrated positive reactivity (subjective score of > or = 2, scale of 0 to 4) at 15 min was considered the TC. The TC was determined for nine pollens (2000 to > 6000 PNU mL(-1)), four moulds (4000 to > 6000 PNU mL(-1)), seven insects (ant, horse fly 125 PNU mL(-1); house fly, cockroach 250 PNU mL(-1); moth 60 PNU mL(-1); mosquito 1000 PNU mL(-1); Culicoides nebeculosis 1 : 5000 w v(-1)) and three of four storage mites (1 : 10,000 w v(-1)). The TC was not determined due to excessive reactivity at the lowest concentrations tested for dust mites (Dermatophagoides farinae [< 1 : 12,000 w v(-1)], D. pteronyssinus [< 1 : 30,000 w v(-1)]), and Acarus siro (< 1 : 10,000 w v(-1)). Minor variation in the TC for specific allergens occurred in different seasons. Progressive sensitization with repeat testing occurred for grain mill dust mix. Positive reactivity at 1 h and 4 h occurred in > 10% of horses for nine of 19 allergens (pollens, mosquito, storage mites) at their determined TC. Positive reactivity was rare at 24 h. This study in normal horses suggests that appropriate testing concentrations of allergens for equine IDT in atopic horses may be > or = 1000 PNU mL(-1) for pollens and moulds, 60 to 250 PNU mL(-1) for most insects and < 1 : 12,000 w v(-1) for dust mites; and that reactions at 1-4 h may be insignificant.

Skin Immune System and Allergic Skin Diseases

“…30,42 Allergen selection. However, some studies indicate that different concentrations may be necessary for different antigens (e.g., 60-250 PNU/mL for most insects and dust mites).…”

Equine intradermal test threshold concentrations for house dust mite and storage mite allergens and identification of stable acari fauna

“…Although raising horses in specific pathogen‐free conditions has been done, this is extremely costly. Threshold concentrations have been arbitrarily defined as the concentration where <10% of nondiseased animals have a positive reaction . It is proposed that if a greater percentage of healthy animals react to the allergen concentration used, then the reaction should be considered a false‐positive result because the allergen was acting as an irritant …”

“…Equine studies for TCs are limited, both in number and in scope. Five studies have examined IDT TCs in horses . Three studies examined TCs of insect extracts and the other two reported the seasonal variation of TCs for pollen, insect, mould and mite extracts .…”

“…Five studies have examined IDT TCs in horses . Three studies examined TCs of insect extracts and the other two reported the seasonal variation of TCs for pollen, insect, mould and mite extracts . In one study, the TCs of two HDM and one storage mite SM extract could not be determined owing to excessive reactivity at the lowest TC used …”

Evaluation of the presence of house dust mites in horse rugs

“…One pilot study published as an abstract found house dust mite antigen and mites similar in appearance to house dust mites in a limited number of horse barn dust samples, 1 but this has not been studied further. Despite this, the response to dust mite antigens is frequently assessed by intradermal or serological testing of atopic horses, in a similar fashion to allergy tests in atopic humans, dogs and cats 2–4 . Some atopic horses exhibit positive reactions on intradermal testing with dust mite allergens, although it is controversial whether this represents true allergy to house dust mites, irritant reactions or cross‐reactivity with other true allergens 2–4 …”