Equine Summer Seasonal Recurrent Dermatitis (SSRD)

Other Names

SSRD; sweet itch; Culicoides hypersensitivity; summer itch; Queensland itch (Australia); equine insect bite hypersensitivity (EIBH); pruritic dermatitis.

What is Equine Summer Seasonal Recurrent Dermatitis?

Equine Summer Seasonal Recurrent Dermatitis (SSRD), more commonly known as sweet itch, is the most prevalent skin condition affecting horses and ponies in the UK. It is caused by an allergic hypersensitivity reaction — specifically a Type I and Type IV immunological response — to the saliva of Culicoides midges (biting midges), with Culicoides obsoletus and Culicoides pulicaris being the species most responsible in the UK.

The condition is seasonal, typically flaring from spring through to late autumn when Culicoides midges are active. It is recurrent, meaning it returns every year with the midge season and frequently worsens progressively as the horse ages and repeated sensitisation drives a stronger allergic response. Between midge seasons, affected horses typically show no skin signs at all — the abrupt return of symptoms in spring is characteristic.

Sweet itch is an important welfare issue because of the intense, persistent pruritus it causes. Affected horses often mutilate their own skin through relentless rubbing, causing hair loss, open wounds, thickened skin, and permanent scarring if the condition is poorly managed over many years. Ponies, cobs, and draught breeds — including Icelandic horses, Welsh ponies, and Connemaras — are disproportionately affected, and there is a clear heritable component.

Causes

The underlying cause is an IgE-mediated allergic reaction to proteins in Culicoides midge saliva. Genetically predisposed horses mount an exaggerated immune response to these salivary proteins upon midge feeding, triggering mast cell degranulation and histamine release — causing intense pruritus. Repeated exposure and sensitisation amplifies the response year on year.

Culicoides midges are most active at dawn and dusk, in warm, still, humid conditions, near water and woodland. Horses in exposed upland, coastal, and well-ventilated areas have lower midge exposure than those grazing in sheltered, low-lying, boggy paddocks near standing water.

While Culicoides is the primary cause of SSRD, other biting insects including Simulium (black flies), Stomoxys (stable flies), and Haematobia (horn flies) can contribute to or exacerbate the condition in some horses.

Symptoms and Signs

Signs are primarily driven by intense, localised pruritus. Common presentation includes:

Intense rubbing of the mane, tail base, withers, poll, and along the dorsal midline
Hair loss — initially mild, becoming extensive in chronic or severe cases
Scaling, crusting, and excoriation over rubbed areas
Broken, sparse mane and tail hairs from persistent rubbing
Thickened, wrinkled, lichenified skin at chronically affected sites
Open weeping wounds where rubbing has been severe
Restlessness and irritability — constant fence or post rubbing behaviour
Ventral midline involvement (belly, sheath) in some horses, depending on which midge species predominates locally

Secondary bacterial infection of open lesions may develop, adding a moist, malodorous component to the clinical picture.

Diagnosis

Diagnosis is primarily clinical, based on the characteristic seasonal pattern, the distribution of lesions, and the breed predisposition. Intradermal allergy testing or serum allergy testing can confirm hypersensitivity to Culicoides allergens and are particularly useful before undertaking allergen-specific immunotherapy (desensitisation).

Differential diagnoses include other causes of pruritic skin disease — mite infestations (Chorioptes, harvest mites), ringworm, contact dermatitis, and onchocerciasis — and these should be excluded before confirming SSRD as the sole diagnosis.

Treatment

Management of sweet itch requires a combined approach of insect avoidance, barrier protection, and medical management of active inflammation:

Full-body sweet itch rugs — covering the neck, body, belly, and tail to prevent midge access; these are the single most effective protective measure
Stabling during peak midge activity — dusk to dawn stabling significantly reduces midge exposure
Application of insect repellents to exposed areas — products containing permethrin or DEET
Fine mesh fly screens over stable windows and doors
Corticosteroids (prednisolone or dexamethasone) for managing acute flares and breaking the itch-scratch cycle — not for long-term daily use
Antihistamines — limited efficacy in horses compared to small animals, but occasionally useful as adjunctive therapy
Omega-3 fatty acid supplementation to support skin barrier function
Topical anti-pruritic shampoos and leave-on preparations to soothe affected skin
Allergen-specific immunotherapy (desensitisation injections) — requires allergy testing first and takes several months to show effect; evidence in horses is developing

Prevention

Complete prevention is not possible in sensitised horses, but excellent management substantially reduces suffering:

Avoid grazing affected horses near water, woodland edges, and boggy ground during midge season
Use sweet itch rugs from early spring before the first midge activity of the season
Maintain stable hygiene to reduce stable-associated insects
Paddock fans near shelters can disrupt midge flight and reduce exposure
Consider breed and family history before purchasing ponies prone to the condition — particularly Icelandic, Welsh, and Connemara breeds

Prognosis

Sweet itch is a lifelong condition in affected horses. It cannot be cured but can be managed effectively with committed, consistent management. The welfare impact varies enormously depending on the quality of management applied. Well-managed horses can maintain excellent skin condition and a comfortable quality of life. Poorly managed horses may suffer significant skin damage and psychological distress from chronic pruritus.

When to Call the Vet

Consult your vet at the start of each season if your horse has a history of sweet itch to establish a management plan before lesions develop. Call promptly if lesions become infected, if the horse’s behaviour and welfare are significantly affected, or if over-the-counter management approaches are not controlling the condition adequately. Steroid treatment may be necessary to break a severe flare cycle.

Frequently Asked Questions

Is sweet itch hereditary?

Yes. There is strong evidence for a heritable component in sweet itch susceptibility. Icelandic horses are notably predisposed — the prevalence increases dramatically when they are exported from Iceland (where Culicoides midges are absent) and exposed to them for the first time. Breeding from severely affected animals is inadvisable as it is likely to produce offspring with similar susceptibility.

Can sweet itch be cured?

No. Once a horse is sensitised, it remains sensitised. However, excellent management substantially reduces suffering and some horses on allergen-specific immunotherapy protocols show long-term improvement in their reactivity. The condition must be actively managed every midge season for the rest of the horse’s life.

Does moving a horse to a different location help?

Yes, potentially. Horses moved to coastal, windy, or upland areas with fewer Culicoides midges often show marked improvement. This is particularly well-documented in Icelandic horses. However, midge species composition varies by region, and some horses react to multiple species, limiting the benefit of relocation.

Are there any new treatments on the horizon?

Allergen-specific immunotherapy is an area of active research and shows promise for some horses. Biologic therapies targeting IgE or specific cytokines involved in allergic responses are under investigation in equine dermatology. Your vet or a veterinary dermatology specialist can advise on what is currently available and appropriate for your horse.

The British Equine Veterinary Association (BEVA) publishes guidance on managing equine skin conditions including insect bite hypersensitivity.