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Sweet Itch is a common skin condition in horses that is caused by an allergic reaction to insect bites. It is also known by the names Queensland itch, summer dermatitis, insect-bite hypersensitivity, summer eczema, recurrent seasonal pruritus, and equine Culicoides sensitivity.
This debilitating and chronic seasonal condition tends to cause severe itching, inflammation, hair loss, and skin lesions in horses. It can be very frustrating for horse owners and result in a lot of discomfort for the horse.
Not all horses develop an allergic reaction after being bitten by flies or midges, but horses with Sweet Itch are hypersensitive to the saliva of biting insects. They may engage in intense rubbing or scratching behaviour to relieve itchiness, resulting in damage to the skin.
First described in 1840, Sweet Itch is now a well-recognized allergic disease affecting approximately 10% of all horses worldwide. [1] It is one of the most common allergic conditions that veterinarians see today.
Unfortunately, there is currently no cure for Sweet Itch, but it can be managed to reduce irritation in horses.
What Causes Sweet Itch?
Allergies are immune disorders caused by complex interactions of genetic as well as environmental factors. Researchers do not fully understand why some individual horses are affected by specific allergens while others are not, but genetics are believed to play a role. [3]
Researchers have classified four different allergen hypersensitivity reactions: Type I through IV. Sweet Itch is considered to be a Type I allergy, also called immediate hypersensitivity reaction. [2]
Sweet Itch is typically caused by an allergy to biting insects in the Culicoides genus. Other insect species including Simulium, Tabanus, and Stomoxys spp. may also cause the condition. [4]
These types of insects are commonly referred to as midges. After the midge bites a horse and starts feeding, various salivary proteins are transferred to the horse’s skin.
Skin cells release cytokines (small proteins) that alert nearby immune cells which become activated to produce allergen-specific antibodies, primarily immunoglobulin E (IgE). This allergen-specific IgE is then found on mast cells in the skin. When IgE binds the allergen (salivary proteins) it initiates an allergic reaction that causes the mast cells to release inflammatory mediators such as histamine and prostaglandins. The histamine release is what causes itchiness in the skin. Other immune cells also are involved, including eosinophils and basophils. [5]
The insects that cause Sweet Itch are found in many areas around the world, and species vary between countries. The condition is quite common in Australia, with a reported incidence rate of 60% (hence the name “Queensland Itch”). Incidence rates in the UK are estimated at 3% and in Germany, they are estimated at 37%. [6]
Studies also show that horses affected by Sweet Itch are at higher risk for also having recurrent airway obstruction (RAO or heaves) and vice versa. [7] [6]
Researchers believe that the current increase in allergic diseases may be linked to the biodiversity hypothesis in which improved hygiene, altered nutrition, and changes in the gut microbiome all play a part. [2]
Genetics and Early Exposure
Sweet Itch affects horses of all breeds, but an individual horse’s risk is determined by genetics and environmental factors. Horses born to affected dams or that have affected maternal granddams are more likely to experience Sweet Itch than those descending from unaffected dams and granddams. The genetic component of Sweet Itch seems to be passed from dam to foal, although shared environmental exposure can also influence this. [19]
The maternal effect of Sweet Itch can be related to: [19]
- Maternal IgE antibodies against Culcoides allergens passed to the foal in colostrum
- Similar level of exposure in the shared environment
- Similar management factors such as feeding and housing protocols
- Genetic susceptibility
Evidence that early exposure can reduce future Sweet Itch comes from Icelandic horses. When they are exported to other areas of the world as adults they appear to be more at risk for developing the condition.
Icelandic horses are more prone to Sweet Itch likely due to the fact that these horses are only exposed to Culicoides and similar insect species after export at an adult age. The insects are not found in Iceland. One study found that 50% of Icelandic horses brought from Iceland to Switzerland developed Sweet Itch within three summers after export. [9]
Early life exposure appears to be important for the development of tolerance to allergens. For example, Icelandic horses born in mainland Europe have less of an immune response to allergens than Icelandic horses born in Iceland and exported to mainland Europe. [8]
Symptoms of Sweet Itch
Sweet Itch leads to skin lesions which are often hairless, weeping, and in some cases, ulcerative (non-healing).
Horses tend to have severe itching at the site of these lesions along the horse’s back, especially at the base of the mane and tail. Less commonly, lesions can form under the belly, on the horse’s sides, as well as on the head and legs.
Sweet Itch lesions may bleed, swell, or appear scaly or crusty. Horses often attempt to rub lesions on trees, fence posts, stall walls, or the ground, leading to further hair loss and inflammation.
Other signs that your horse may be experiencing Sweet Itch include:
- Vigorous tail swishing (in attempt to keep insects away)
- Excessive mutual grooming with pasture mates
- Excessive rolling
- Scratching at mane with hind hooves
- Changes in behavior including lethargy, agitation, and restlessness
- Head shaking when insects are nearby
- Skin folds that develop as the condition progresses, leading to sparse hair coat with flaky dandruff
Signs of Sweet Itch are most commonly seen from May to October, though the time of onset can vary depending on location. Some horses show signs in late spring while others may not show signs until late summer. [10]
It is also common for secondary infections to occur. Lesions in the skin allow bacteria, mites, and fungi to proliferate leading to further irritation and more lesions. [1]
Sweet Itch Treatments
Allergen-specific immunotherapy (ASIT) is the only treatment that has proven to be successful for Type I hypersensitivities. This treatment has been used for over 100 years with human allergies. However, ASIT has limited efficacy for the treatment of Sweet Itch in horses. [5]
Treatment strategies for Sweet Itch are typically aimed at providing relief from itching symptoms and making the horse as comfortable as possible. Hyposensitization therapy and steroids have been used in some instances, but these treatments have less than a 50% success rate. [6]
Steroids also carry a risk of adverse side effects including immunosuppression, adrenocortical dysfunction, pituitary dysfunction, muscle wasting, altered bone metabolism, increased susceptibility to infection, and laminitis, among others. [2]
Research is still ongoing and recently two vaccines were produced for use as Sweet Itch treatments. The IL-5 vaccine targets eosinophils, the major inflammatory cells found in lesions o