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


  • Chronic, seasonally recurring, IgE-mediated pruritus and superficial dermatitis.
  • Cause: hypersensitivity to bite of Culicoides spp or mosquitoes.
  • Signs: pruritus, papules, alopecia, crusting, excoriation of ears, base of mane, withers, tail head, ventral abdomen (pattern depends on feeding areas of midge), flanks in the case of mosquitoes.
  • Diagnosis: seasonality, clinical signs, elimination of differentials, intradermal and/or ELISA testing, response to treatment.
  • Treatment: fly control +/- antihistamines or corticosteroids, hyposensitization.
  • Prognosis: dependent on prevention and management.
  • See also:
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  • Type I and IV hypersensitivity reaction to salivary antigens of Culicoides spp Culicoides spp (over 100 species) or mosquito spp (100s of species).
  • Hypersensitivity to other flies also reported, eg Stomoxys spp (stable fly) Stomoxys calcitrans, Tabanus spp (horse fly), Simulium spp (black fly) Simulium spp and Haematobia spp (buffalo fly, horn fly) Biting and nuisance flies.

Predisposing factors


  • 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 cause a primary irritation in all horses, but signs improve relatively quickly.
  • Bites extremely irritating even on non-allergic animals - not all animals develop hypersensitivity.
  • Antigens in Culicoides spp Culicoides spp saliva and mosquito saliva induce type I and/or type IV or late phase IgE hypersensitivity reactions in a sensitized host → clinical signs.
  • Preferred landing sites of Culicoides spp varies between species of Culicoides 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.
  • Culicoides are vectors for bluetongue in sheep and African horse sickness African horse sickness.
  • Black flies transmit Onchocerca cervicalis Onchocerca cervicalis, vesicular stomatitis virus Vesicular stomatitis and possibly papillomaviruses causing aural plaques Ear: pinnal acanthosis.
  • Tabanidae transmit a variety of infections to hosts: equine infectious anemia Equine infectious anemia (EIA), anthrax Anthrax, hog cholera, surra, filariasis, tularemia, anaplasmosis Anaplasmosis, Lyme disease Borreliosis, Bovine leukemia virus, papillomaviruses Papilloma virus and dermatophytes Dermatophytosis.
  • Mosquitoes are vectors for viral diseases such as Eastern EEE, Western WEE and Venezuelan equine encephalomyelitis VEE, West Nile virus West Nile fever virus and vesicular stomatitis Vesicular stomatitis.
  • The decreased risk of developing hypersensitivity when foals are first exposed to Culicoides spp at a very young age may be explained by the differences in the immune system responses between neonatal or young foals and adult horses.


  • First clinical signs at 1-4 years of age - 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.
  • Fettelschoss-Gabriel A, Fettelschoss V, Olomski F et al (2019) Active vaccination against interleukin-5 as long-term treatment for insect-bite hypersensitivity in horses. Allergy 74 (3), 572-582 PubMed.
  • Radwanski N E, Morris D O, Boston R C, Cerundolo R & Lee J W (2019) Longitudinal evaluation of immunological responses to allergen-specific immunotherapy in horses with IgE associated dermatological disease, a pilot study. Vet Derm 30 (3), 255-e78 PubMed.
  • Fettelschoss-Gabriel A, Fettelschoss V, Thoms F et al (2018) Treating insect-bite hypersensitivity in horses with active vaccination against IL-5. J Allergy Clin Immunol 142 (4), 1194-1205 PubMed.
  • Ginel P J, Hernandez E, Lucena R et al (2014) Allergen-specific immunotherapy in horses with insect bite hypersensitivity: a double-blind, randomized, placebo-controlled study. Vet Derm 25, 29-e10 PubMed.
  • 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.
  • Schaffartzik A, Hamza E, Janda J, Crameri R, Marti E & Rhyner C (2012) Equine insect bite hypersensitivity: What do we know? Vet Immunol Immunopathol 147 (3-4), 113-126 PubMed.
  • Sommer-Locher B, Endriss V & Fromm E (2012) Various circumstances regarding initial allergen exposure and their influence on development of insect bite hypersensitivity in horses. J Equine Vet Sci 32 (3), 158-163 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.
  • 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.
  • 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.
  • 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 its relevance to summer seasonal recurrent dermatitis (sweet itch). Equine Vet J 20 (6), 426-429 (Demonstrates landing and biting sites of Culicoides spp, also effect of windspeed on numbers Culicoides spp collected) 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.
  • Brostrom H, Larsson A and Troedsson M (1987) Allergic dermatitis (sweet itch) of Icelandic horses in Sweden: An epidemiologic 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.

Other sources of information

  • Noli C, Foster A & Rosenkrantz W (2014) Culicoides Hypersensitivity and Other Insect Allergies. In: Veterinary Allergy. 1st edn. Wiley Blackwell, UK. pp 273-325.
  • Scott D W & Miller W H Jr (2011) Skin Immune System and Allergic Skin Diseases. In: Equine Dermatology. 2nd edn. Saunders, USA. pp 263-313.
  • Knottenbelt D C (2009) Immune-Mediated/Allergic Diseases. In: Pascoe’s Principles and Practice of Equine Dermatology. 2nd edn. Saunders, USA. pp 253-297.

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