Insect bite hypersensitivity

Veterinary advice should be sought before applying any treatment or vaccine.

Insect Bite Hypersensitivity

Queensland Itch, Sweet Itch, Allergic Dermatitis, Recurrent Seasonal Pruritus

Insect bite hypersensitivity (IBH) is a common seasonal allergic skin condition in horses. It occurs in horses worldwide and is the result of an immediate, type I hypersensitivity reaction to the saliva of biting flies (Simulium and Stomoxys species) and midges (Culicoides spp).

IBH affects between 3 to 60% of horses worldwide, of all age groups. Although the condition has been found to worsen with age. IBH is a multi-factorial disorder influenced by both genetic and environmental factors. Certain breeds of horses have a higher likelihood of developing IBH than others; Icelandic, Shetland ponies, Thoroughbreds, Quarter horses, and German Shires have a genetic predisposition to suffering from IBH.

Horses with IBH develop severe itchiness and discomfort; they are often seen scratching and biting themselves, sometimes using objects they find in the pasture such as shrubs and small trees. The constant scratching often causes skin irritation which leads to hair loss, crusts, papules, and lichenification of the skin. The parts of the horse's body most affected include the mane and tail, ears, dorsal midline and rarely ventral thorax/abdomen.

IBH tends to occur seasonally in horses, with clinical signs coinciding with months where flying insects are most abundant---between April to October. Horses tend to demonstrate increased signs of discomfort during the times of the day that insects are most likely to feed, which is usually the morning and evening hours.

Symptoms

Intensive itchiness, apparent by excessive rubbing
Crusts and papules
Hair loss
Lichenification

Diagnosis

  • History
  • Clinical signs
  • Physical exam
  • Laboratory tests

Treatment


TreatmentDetails
Corticosteroids
Management/environmental modificationskeep horse stabled at desk and dawn; Modify horse's turnout schedule so that they are not out in the pasture during dusk or daybreak, as this is when sand flies undergo peak activity. Utilize insect repellents, fly traps, better manure management, remove stagnant water, use of fans in stalls
Provide protective barrierfly sheets, fly masks, fly boots
FlaxseedAs a supplement in diet
Fatty acidsadded as a dietary supplement

Prevention

  • Apply long lasting insect repellents, especially in the evening and early morning
  • Use a fly sheet, fly mask with ear protection, and fly leg guards when horse is turned out
  • Utilize cooling fans when the horse is inside the stall since biting insects are not great flyers
  • Routinely clean or treat water troughs
  • Spread fly predators
  • Apply mineral oil to horse prior to turnout along ventral midline

Scientific Research References

Good Overviews

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Seasonality

WinterSpringSummerAutumn