Equis ISSN 2398-2977

Insect hypersensitivity

Synonym(s): Sweet itch, Culicoides hypersensitivity, allergic dermatitis, Summer itch/dermatitis/eczema/sores, Muck itch, Queensland itch, Kasen disease, Dhobie itch, Sommerekzem

Contributor(s): Sheelagh Lloyd, Dawn Logas, Graham Munroe, Sue Paterson, Vetstream Ltd


  • Chronic pruritus, seasonally recurring, superficial dermatitis.
  • Cause: hypersensitivity to bite ofCulicoidesspp or mosquitoes.
  • Signs: pruritus, papules, alopecia, excoriation of base of mane, withers, tail head, ventral abdomen (pattern depends on feeding areas of midge)   Skin: trauma 01 - self mutilation  , flanks in the case of mosquitoes.
  • Diagnosis: seasonality, clinical signs, elimination of differentials, response to treatment.
  • Treatment: fly control +/- antihistamines or corticosteroids, hyposensitization.
  • See also:
Print off the Owner factsheet Sweet itch - an itchy business to give to your clients.



  • Type I and IV hypersensitivity to salivary antigens ofCulicoidesspp   Culicoides spp   (over 100 species) or mosquito spp (100s of species).
  • Hypersensitivity to other flies also reported, egStomoxysspp (stable fly)   Stomoxys calcitrans  ,Tabanusspp (horse fly),Simuliidaespp (black fly)   Simulium spp   andHaematobiaspp (buffalo fly, horn fly)   Biting and nuisance flies  .


  • Spring through summer.
  • Wind speeds <5 km/h.
  • Dawn and dusk.
  • Night.
  • All day if cool.
  • Hereditary predisposition (genes of the major histocompatibility complex [MHC] and genes outside the MHC), further data is required to substantiate this.


  • Swarms create apprehension.
  • Bites extremely irritating even on non-allergic animals - not all animals develop hypersensitivity.
  • Antigens inCulicoidesspp saliva and mosquito saliva induce type I and/or type IV or late phase IgE hypersensitivity reactions in a sensitized host   →   clinical signs.
  • Release of IgG is stimulated in all cases of fly/mide/mosquito bite, but release of IgG is only stimulated in hypersensitive animals.
  • Preferred landing sites ofCulicoidesspp varies between species ofCulicoides  Flies: landing and biting sites  .
  • Biting sites correspond to landing sites.
  • Dorsal feeders   →   pruritus of ears, poll, mane, withers, rump and tail head.
  • Ventral feeders   →   pruritus of face, ears, inter-mandibular space, chest, ventral abdomen, groin.
  • Mosquitoes pruritis on lateral areas.
  • Feeding reduced at wind speeds >5 km/h.
  • Maximum feeding time: 1 h prior to and 0.5 h after sunset.
  • Feeding also occurs around sunrise and throughout the night.
  • Midges transmitOnchocerca cervicalis  Onchocerca cervicalis  .
  • Mosquitoes transmit a variety of viruses to horses.
  • Exposure to a particular antigen in the presence of maternally derived antibody may lead to long-term tolerance of that antigen. This may explain the decreased risk of developing hypersensitivity when foals are first exposed toCulicoidesspp at a very young age.


  • 1-4 years - once sensitized likely to recur each year, damage may become permanent.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Boerma S, Back W & Sloet van Oldruitenborgh-Oosterbaan M M (2012) The Friesian horse breed: A clinical challenge to the equine veterinarian? Equine Vet Educ 24 (2), 66-71 VetMedResource.
  • Craig M (2011) Culicoides hypersensitivity in horses. UK Vet 16, 5-9 Wiley Online Library.
  • Hallamaa R E (2010) Autoserum preparation in the treatment of equine summer eczema: Findings over 12 years. Equine Vet Educ 22 (12), 610-615 VetMedResource.
  • Loewenstein C & Mueller R S (2009) A review of allergen-specific immunotherapy in human and veterinary medicine. Vet Derm 20 (2), 84-98 PubMed.
  • van Grevenhof E M, Ducro B, Heuven H C M & Bijma P (2007) Identification of environmental factors affecting the prevalence of insect bite hypersensitivity in Shetland and Friesian horses in the Netherlands. Equine Vet J 39 (1), 69-73 PubMed.
  • Baselgia S et al (2006) Evaluation of an in vitro sulphidoleukotriene release test for diagnosis of insect bite hypersensitivity in horses. Equine Vet J 38 (1), 40-46 PubMed.
  • Pilsworth R C & Knottenbelt D C (2004) Equine insect hypersensitivity. Equine Vet Educ 16 (6), 324-325 VetMedResource.
  • Steinman A, Peer G & Klement E (2003) Epidemiological study of Culicoides hypersensitivity in horses in Israel. Vet Rec 152 (24), 748-751 PubMed.
  • Swiderski C E (2000) Hypersensitivity disorders in horses. Vet Clin North Am Equine Pract 16 (1), 131-151 PubMed.
  • Marti E, Gerber H and Lazary S (1992) On the genetic basis of equine allergic diseases: II. Insect bite dermal hypersensitivity. Equine Vet J 24 (2), 113-117 (Demonstrates that certain animals can transfer hereditary susceptibility to Culicoides hypersensitivity) PubMed.
  • Anderson G S, Belton P, Jahren E, Lange H & Kleider N (1996) Immunotherapy trial for horses in British Columbia with Culicoides (Diptera: Ceratopogonidae) hypersensitivity. J Med Entomol 33, 458-466 (efficacy in 9 of 10 horses) PubMed.
  • Barbet J L, Bevier D and Greiner E C (1990) Specific immunotherapy in the treatment of Culicoides hypersensitive horses: a double-blind study. Equine Vet J 22 (4), 232-235 (No statistical difference between horses on Culicoides antigen and control horses) PubMed.
  • Braverman Y (1988) Preferred landing sites of Culicoides species on a horse in Israel and it's relevance to summer seasonal recurrent dermatitis (sweet itch). Equine Vet J 20 (6), 426-429 (Demonstrates landing and biting sites ofCulicoidesspp, also effect of windspeed on numbers Culicoides spp collected) PubMed.
  • Brostrom H, Larsson A and Troedsson M (1987) Allergic dermatitis (sweet itch) of Icelandic horses in Sweden: An epidemiolologic study. Equine Vet J 19 (3), 229-236 (Demonstrates a higher incidence of Culicoides hypersensitivity in horses imported from Iceland to Sweden relative to that of horses born in Sweden) PubMed.
  • Littlewood J D (1988) Incidence of recurrent seasonal pruritus ('sweet itch') in British and German shire horses. Vet Rec 142 (3), 66-67 PubMed.