Bit & Bridle

Equine Insect Hypersensitivity

Other Names

  • Sweet itch
  • Queensland itch
  • Summer eczema
  • Kasen disease
  • Insect bite hypersensitivity (IBH)
  • Summer seasonal recurrent dermatitis

What is Equine Insect Hypersensitivity?

Equine insect hypersensitivity, commonly known as sweet itch, is an allergic skin condition caused by a hypersensitive immune response to the bites of certain insects — most notably midges of the genus Culicoides. It is one of the most prevalent skin diseases in horses worldwide and can cause severe discomfort, chronic skin damage, and a significant reduction in quality of life.

The condition is seasonal in temperate climates, typically worsening in spring and summer when midge populations peak, and improving or resolving during the colder months when midges are inactive. In warmer regions where Culicoides are active year-round, horses may suffer persistently without seasonal relief.

Any breed can be affected, but some — including Icelandic horses, Shire horses, and certain warmblood lines — show a higher genetic predisposition. Horses exported from Iceland to countries where Culicoides are prevalent are particularly susceptible, having had no prior exposure to trigger species.

Causes

The primary trigger is an IgE-mediated (Type I) hypersensitivity reaction to proteins in the saliva of Culicoides midges. When an allergic horse is bitten, the immune system mounts an exaggerated inflammatory response, releasing histamine and other mediators that cause intense itching and skin inflammation.

Other biting insects can trigger similar reactions, including:

  • Black flies (Simulium spp.)
  • Stable flies (Stomoxys calcitrans)
  • Horse flies (Tabanidae)
  • Mosquitoes
  • Biting gnats and no-see-ums

Genetic predisposition plays a significant role — not all horses exposed to the same midge population will develop the allergy. Once sensitised, the reaction typically worsens with each subsequent season.

Symptoms and Signs

Clinical signs are driven by intense, often unbearable itching. Affected horses will rub themselves against any available surface — fence posts, trees, stable walls — causing progressive skin damage. Signs include:

  • Intense rubbing and scratching, particularly of the mane, tail, withers, rump, and belly
  • Hair loss and broken hairs in affected areas
  • Thickened, lichenified (elephant-like) skin with repeated trauma
  • Raw, excoriated patches that may bleed or weep serum
  • Scabs and crusting over affected regions
  • Secondary bacterial skin infections (pyoderma) from repeated self-trauma
  • Restlessness, poor concentration under saddle, and general irritability
  • In severe cases, weight loss due to distraction from grazing

The distribution pattern can help differentiate the trigger insect: Culicoides tend to bite the dorsal midline (mane, back, tail), while black flies often affect the ventral midline and ears. For more on equine skin conditions, see our entry on Equine Sporotrichosis.

Diagnosis

Diagnosis is based on the characteristic clinical history and seasonal pattern, combined with physical examination findings. There is no single definitive test, but the following may support the diagnosis:

  • Clinical history — seasonal onset, known insect exposure, improvement in winter, breed predisposition
  • Skin scrapings and cytology — to rule out other causes such as mange mites (Chorioptes) or fungal infections
  • Intradermal allergy testing — injection of Culicoides extract into the skin to identify reactivity. Useful in confirming IBH and guiding immunotherapy, though not universally available
  • Serum IgE testing — measures allergen-specific antibodies, increasingly available through specialist equine laboratories

It is important to rule out other pruritic conditions including lice, mange, ringworm, and contact dermatitis before confirming IBH.

Treatment

There is currently no cure for equine insect hypersensitivity, but the condition can be managed effectively with a combination of insect avoidance and medical therapy:

  • Corticosteroids — systemic prednisolone or dexamethasone can provide significant relief from itching during acute flare-ups. Long-term use carries risks including laminitis, so they should be used at the lowest effective dose for the shortest period necessary.
  • Antihistamines — have limited efficacy in horses compared to small animals, but hydroxyzine may provide modest relief in some cases.
  • Omega-3 fatty acid supplementation — dietary supplementation with fish oil or linseed has shown anti-inflammatory benefits in some horses, reducing skin reactivity over time.
  • Topical treatments — corticosteroid creams, soothing aloe-based products, and antimicrobial shampoos can help manage localised lesions and secondary infections.
  • Allergen-specific immunotherapy (ASIT) — desensitisation using gradually increasing doses of Culicoides extract. Results have historically been modest, but newer formulations and protocols continue to be investigated.
  • Cyclosporine — an immunosuppressive agent showing promising results in refractory cases, though cost and monitoring requirements limit its routine use.

Prevention

Prevention focuses on reducing the horse’s exposure to biting insects. The following measures are most effective:

  • Stable the horse from one hour before sunset to one hour after sunrise — the peak feeding times for Culicoides
  • Use fine-mesh fly rugs covering the entire neck, body, and belly, including a fly hood and tail flap
  • Install fans in stables and field shelters — Culicoides are weak fliers and cannot tolerate airflows above 5 mph
  • Apply insect repellents containing DEET or permethrin, particularly to high-risk areas such as the mane base and tail
  • Remove standing water and manage manure promptly — these are prime midge breeding sites
  • Avoid fields near water bodies such as ponds, streams, and marshes, which harbour large midge populations
  • Consider housing affected horses well away from the field margins at dusk and dawn

For horses with severe disease, a combination of physical barriers and medical management provides the best outcomes. The BEVA has published guidance on managing allergic skin diseases in horses, available via their website at beva.org.uk.

Prognosis

Sweet itch is a lifelong condition — once a horse becomes sensitised, the allergy does not resolve. However, with diligent management, most affected horses can live comfortably and continue in work. Without appropriate insect control, the condition will worsen with each season, leading to progressive skin damage and reduced welfare. Early intervention and consistent management are key to a good quality of life.

When to Call the Vet

  • Your horse is showing signs of intense itching for the first time, especially in spring or summer
  • Skin lesions are raw, bleeding, or showing signs of infection
  • The condition is not improving despite insect control measures
  • You wish to explore immunotherapy or discuss longer-term medical management options
  • The horse is losing weight or showing marked changes in behaviour and temperament

Frequently Asked Questions

Can sweet itch be cured permanently?

No — equine insect hypersensitivity is a lifelong allergy. Once sensitised, the horse will always react to Culicoides bites. The goal of management is to minimise exposure to trigger insects and control the inflammatory response when it occurs. Some horses are managed very successfully and lead full working lives with appropriate care.

What breeds are most susceptible to sweet itch?

Icelandic horses are particularly susceptible, especially when moved to countries where Culicoides are present. Shire horses, Fell ponies, Welsh ponies, and some warmblood lines also show higher-than-average prevalence. However, any breed can be affected. Explore our horse breed guides for breed-specific health considerations.

Is sweet itch worse in certain parts of the UK?

Yes. Areas near water — marshes, rivers, ponds, and coastal wetlands — tend to have much higher Culicoides populations. Similarly, warm, humid summers increase midge activity. Horses in these locations may need more intensive management during peak season.

Can a horse with sweet itch still be ridden?

In most cases, yes. Horses with well-managed sweet itch can remain in full work. However, during severe flare-ups, the discomfort may make it difficult for the horse to concentrate, and saddle fit can become an issue if lesions develop on the back. Work closely with your vet to keep symptoms controlled during the riding season.