Significant Allergens in the UK

Aeroallergens can be broadly divided into "indoor" and "outdoor" allergens.

Indoor allergens include house dust mites, storage mites and mould. These allergens are present throughout the year although mite population burdens and spore concentrations will vary. Sensitised individuals may exhibit non-seasonal disease with worsening of clinical signs throughout the winter months. Outdoor allergens are mainly pollens (grasses, weeds and trees).

HOUSE DUST MITES

The term house dust mites relates to the large numbers of mites associated with dust in dwellings. Dermatophagoides farinae, the American house dust mite, and Dermatophagoides pteronyssinus, the European house dust mite, are the most commonly encountered species.

Mites feed on shed skin of man and animals and thrive in warm, humid conditions. High numbers are found in mattresses, bedding, carpets, soft furnishings etc. The mite allergens are present in faeces as well as live or dead mite bodies. Several mite allergens are proteolytic enzymes; it is thought that this proteolytic activity directly contributes to the allergenicity of mite proteins by facilitating penetration through mucosal surfaces (Arruda et al 2001).

Acaricidals, used to decrease numbers of viable mites, may not significantly affect the allergenic load. Control measures which are useful include vacuuming mattresses, soft furnishings etc, regularly washing or replacing cushions, quilts and other types of soft pet bedding.

Controlling humidity is of major importance, mite populations stop growing and die out when the relative humidity is reduced to <60%.

Clinically normal, as well as hypersensitive, animals may test positive to house dust mites, therefore serological results must be evaluated in conjunction with clinical signs.

STORAGE MITES

Commonly encountered storage mites in the UK include Acarus siro, Tyrophagus putrescentiae and Lepidoglyphus destructor. Mites and their faeces are considered highly allergenic and increasing numbers of dogs have raised levels of IgE to storage mites. Mite-sensitive individuals frequently show co-sensitisation to house dust and storage mites. In some cases this may reflect parallel sensitisation however, ELISA cross-inhibition studies have demonstrated extensive cross-reactivity between the house dust mites, between D. farinae and the storage mites T.putrescens and Acarus siro and between the latter two mites (Saridomichelakis et al 2008).

Cross-reactivity may explain positive reactions in dust mite sensitised patients where exposure to storage mites is thought to be unlikely.

Whether or not there is in-vivo cross-reactivity to the mites is currently unclear; nevertheless the results of both serology and intradermal skin testing support the possibility of the development of clinical signs following exposure to a novel but cross-reacting antigen (Saridomichelakis et al 2008).

Storage mites are found in cereal based foods; their presence will eventually result in significant spoilage. Populations expand rapidly in warm humid conditions in contaminated food.

Control measures include allergen avoidance; changing the diet to tinned/wet food should be considered if hypersensitivity to storage mites is suspected. If this is not practical, dried food should be purchased in small quantities and stored in dry, airtight containers that are thoroughly cleaned once emptied. Crumbs and residue in the bottom of the container should be discarded.

After feeding it may be helpful to wipe the animal's face or muzzle clean.

Cockroach allergy is common in man; although less of a problem in domestic animals positive serological reactions may occur also as a result of cross-reactivity with dust mites. The structural protein tropomyosin, common to some species of cockroach, mites including the Dermatophagoides spp. and shell fish, may be responsible for this cross-reactivity (Eggleston, Arruda 2001).

MOULDS

Fungal spores are present indoors and are widely distributed in the outdoor environment. They may be present in bedding, soft furnishings, clothing, and damp walls or airborne. Fungal spores are well documented as a cause of atopic dermatitis in man, their involvement in companion animal hypersensitivity is less clear; approximately 3% of canine and feline sera assayed have elevated levels of IgE against fungal spores (allervet® unpublished data).

POLLENS

Outdoor allergens are mainly pollens (grasses, weeds and trees). Pollens show seasonal and geographic variation in their prevalence in the environment. Patients suffering from pollen allergies may initially have seasonal pruritus, usually over the spring and summer months with clinical signs coinciding with high levels of the particular pollen(s) in the environment; with chronic disease they may become symptomatic all year round.

The Airborne Pollen Calendar illustrates the seasonal incidence of major pollen allergens in the UK.

FLEA ALLERGIC DERMATITIS (FAD)

This is the most common form of allergic skin disease seen in the dog and cat in flea-endemic regions. Type 1 and type IV hypersensitivity reactions to allergens in flea saliva are involved in the pathogenesis in dogs, mechanisms of FAD in the cat are unclear.

This pruritic skin condition shows a classical distribution affecting the back, dorsal lumbosacral region, perineum, tail, caudal/medial thighs and abdomen. Unlike flea infestation severity is independent of the numbers of fleas on the animal.

SWEET ITCH

Biting flies such as midges, black flies, mosquitos, horse and stable flies can be highly problematic for some horses. In most cases this is due to a hypersensitivity reaction to insect saliva which is inoculated during feeding.

STAPHYLOCOCCI AND MALASSEZIA

These are commensals at various sites on canine skin including the nares, axilla, groin and perineum. If the epidermal barrier is breached (trauma, epidermal barrier dysfunction, endocrine disease) colonisation and infection may occur. Some patients develop hypersensitivity to these organisms which then becomes a major factor contributing to the level of pruritus. Serological testing may identify such cases and direct therapy.

THE SIGNIFICANCE OF AIRBORNE PARTICLE SIZE

The size of airborne particles determines both the time taken for them to "settle out" in still air, and their destination within the respiratory tract following inhalation.

Small particles, up to 2 microns, are airborne for up to 6 hours, following inhalation they may reach the alveoli.

Particles between 2-10 microns may settle in as little as 15 minutes, following inhalation some reach the bronchi/bronchioles. Examples include fungal spores, dander, smoke and diesel particles.

Particles between 10-20 microns settle in 4-15 minutes in still air. They are filtered out in the nasal cavity and do not reach the bronchi. Examples include small pollen grains, mite droppings, certain cockroach allergens and some fungal spores.

Particles larger than 40 microns settle rapidly and are rarely inhaled.

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