- Definition
- Pathophysiology
- Systems Affected
- Genetics
- Incidence/Prevalence
- Geographic Distribution
- Signalment
- Signs
- Causes
- Risk Factors
BASICS
IH is a variably pruritic summer seasonal dermatitis characterized by self-inflicted trauma and sometimes urticaria, resulting from a hypersensitivity response to biting insects such as midges (Culicoides spp.) and black flies (Simulium spp.).
Susceptible animals become sensitized to arthropod antigens, principally salivary proteins, by producing allergen-specific IgE, which binds to receptor sites on mast cells; further allergen exposure leads to a type I hypersensitivity reaction with mast cell degranulation and the release of histamine and other inflammatory mediators with recruitment of eosinophils and basophils. Delayed type IV hypersensitivity is also commonly a component.
A familial tendency and heritability associated with certain equine leukocyte antigen haplotypes demonstrated. A genetic basis has been demonstrated in imported Icelandic horses.
- IH is the commonest allergic skin disease
- Reported incidence rates range from 2.8% in the UK to 32% in Australia
- In cooler climates, IH presents as recurrent and seasonal
- In warmer climates the incidence may present as a nonseasonal disease
- Worldwide. Limited to areas conducive to arthropod breeding.
- Culicoides spp. require still water or marshy areas that also support mosquito development
- Simulium spp. require moving water for larval development
- Stomoxys calcitrans breeds in decaying materials such as compost piles
Any breed of horse or pony may be affected, but some breeds may be at increased risk in certain locations (Icelandic Ponies, Shire Horses in Germany, Friesians, Shetlands, Welsh Ponies, Arabs, Connemaras, Swiss Warmbloods, Quarter Horses).
Typically seen in horses from 2 years of age onwards as previous sensitization is required. Can start later in life.
Lesions occur at the site of insect feeding, resulting in clinical signs in specific distribution patterns. Primary acute lesions are papules or crusted papules and rarely papular urticaria. Pruritus results in typical lesions of self-trauma such as excoriation, represented as erosions and ulcers, serous effusions, scale, crusts, exfoliation, lichenification, pigmentary disturbances, and various degrees of patchy alopecia represented by mild hypotrichosis to severe hair loss. The mane and tail are reduced to sparse, broken, and distorted hairs that give the appearance of a “roached mane” and “rat tail.”
- Note the age of onset of disease, seasonality, duration, locations of the initial disease, and how it has progressed
- Inquire if a response to strict insect control and/or administration of anti-inflammatory therapy has had an effect
- IH can have various clinical presentations based on the offending insect(s) and their preferred feeding sites
- In general, the appearance of 3 lesional distribution patterns can be recognized:
- Dorsal—involves the face, pinnae, poll, mane, withers, and tail-head; insects implicated are various Culicoides spp. and Simulium spp.
- Ventral—involves the intermandibular space, ventral thorax and abdomen, axillae, ventral midline, and groin; insects implicated are some Culicoides spp., Simulium spp., and Haematobia irritans
- A combination of 1 and 2—the caudal lateral aspects of both the front and hindlimbs are preferred feeding areas of Aedes and S. calcitrans, typically resulting in papulourticarial lesions
- Secondary moderate to severe bacterial dermatitis is common
Insect salivary protein, venom, excrement, or other proteinaceous body parts acting as allergens.
DIAGNOSIS
TREATMENT
MEDICATIONS
FOLLOW-UP
MISCELLANEOUS