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 or anaphylactic 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 of horses with Sweet Itch.

Researchers found that active vaccination against the IL-5 cytokine decreased lesion scores in horses. This vaccination can be given each year before the active Sweet Itch season begins. [1]

IL-31 is a second therapeutic vaccine that works similarly to the IL-5 vaccine. However, the IL-31 vaccine targets a different cytokine responsible for itching in lesion areas. Clinical trials showed that this vaccine also reduced symptoms in affected horses. [5]

Although these vaccines will not prevent the allergic reaction itself, they might help stop the vicious cycle of itching to alleviate the clinical symptoms of Sweet Itch. [11]

Managing & Preventing Sweet Itch

There are a number of strategies that horse owners can use to help manage Sweet Itch symptoms in their horse. Most aim to prevent midges from coming in contact with horses. These strategies include:

  • Stable horses from dusk until dawn – especially in hot, humid conditions when Culicoides are most active;
  • Avoid turnout in marshy areas, overly wet fields or near water sources such as ponds
  • Use mesh blankets to keep insects from biting
  • Use fly masks and mesh leg coverings to protect the entire body
  • Apply insect repellents
  • Keep manure regularly cleaned from grazing areas and stables to reduce the number of midges, which thrive in warm and moist conditions on or around droppings
  • Clean water troughs regularly
  • Use industrial fans in barns
  • Apply a fly screen to the stable door

Insect Repellants & Topical Treatments

Several products have been developed to provide relief for horses suffering from Sweet Itch. Essential oil extracts from plants, including camphor, lemongrass, May Chang, peppermint, and patchouli are purported to have immune-supportive, antihistamine, anti-itching, anti-inflammatory, and insect repellent effects.

One study examined the effect of an herbal spray made from the above essential oil extracts on Sweet Itch symptoms. The researchers found a significant association between treatment and resolution of itching symptoms, as well as improvement of disease severity.

Though this product did not cure the condition, it may present another effective management strategy for horses with Sweet Itch. [6]

Other products that help to reduce itching include shampoos, topical therapies, emollients, and anti-itch substances, but these products need to be applied frequently in order to be effective.

Nutrition

Nutrition can also play an important role in helping to manage the symptoms of Sweet Itch and supporting skin health.

It is recommended to reduce grains and sweet feeds in the diets of horses with Sweet Itch. These feeds are high in non-structural carbohydrates, which can promote inflammation.

Instead, to support your horse’s immune health and reduce inflammation, feed more low-starch forage and provide a diet with balanced vitamin and mineral levels.

Studies show that supplementing horses with omega-3 fatty acids from flaxseed oil can help reduce skin lesions and inflammation in horses with Culicoides hypersensitivity. [12] The researchers concluded that omega-3s could significantly reduce the allergic response associated with Sweet Itch.

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Another supplement that can help with skin allergies in horses is Spirulina – a rich source of the anti-inflammatory omega-6 fatty acid, gamma linolenic acid (GLA).

Spirulina has been shown to inhibit histamine release from mast cells and decrease the production of pro-inflammatory antibodies. This may reduce allergic over-responses and help to decrease itchiness. [18]

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Sweet Itch is not easy to deal with, but with vigilant care and patience, you may be able to help manage symptoms and make your horse more comfortable.

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References

  1. Fettelschoss-Gabriel, A. et al. Treating insect-bite hypersensitivity in horses with active vaccination against IL-5. J Allergy Clin Immunology. 2018.
  2. Fettelschoss-Gabriel, A. et al. Molecular mechanisms and treatment modalities in equine Culicoides hypersensitivity. Vet Journal. 2021.
  3. Meulenbroeks, C. et al. Allergen-Specific Cytokine Polarization Protects Shetland Ponies against Culicoides obsoletus-Induced Insect Bite Hypersensitivity. PLoS One. 2015.
  4. van den Boom, R. et al. The effect of a supplement containing sunflower oil, vitamins, amino acids, and peptides on the severity of symptoms in horses suffering insect bite hypersensitivity. Tijdschr Diergeneeskd. 2010.
  5. Jonsdottir, S. et al. New Strategies for Prevention and Treatment of Insect Bite Hypersensitivity in Horses. Veterinary Dermatology. 2019.
  6. Cox, A. et al. Essential oil spray reduces clinical signs of insect bite hypersensitivity in horses . Aust Vet Journal. 2020.
  7. Lanz, S. et al.  Insect Bite Hypersensitivity in Horses is Associated with Airway Hyperreactivity. J of Intern Vet Medicine. 2017.
  8. Jonsdottir, S. et al. Comparison of recombinant Culicoides allergens produced in different expression systems for IgE serology of insect bite hypersensitivity in horses of different origins. Vet Immunol Immunopathol. 2021.
  9. Torsteinsdotti, S. et al. A prospective study on insect bite hypersensitivity in horses exported from Iceland into Switzerland. Acta Vet Scand. 2018.
  10. Hallamaa, R.E.  Characteristics of equine summer eczema with emphasis on differences between Finnhorses and Icelandic horses in a 11-year study. Acta Vet Scand. 2009.
  11. Olomski, F. et al. Interleukin 31 in insect bite hypersensitivity—Alleviating clinical symptoms by active vaccination against itch. European J of Allergy and Clin Immunology. 2019.
  12. O’Neill, W. et al. Flaxseed (Linum usitatissimum) supplementation associated with reduced skin test lesional area in horses with Culicoides hypersensitivity. Can J Vet Res. 2002.
  13. Cvitas, I. et al. Investigating the epithelial barrier and immune signatures in the pathogenesis of equine insect bite hypersensitivity. PLoS One. 2020.
  14. Birras, J. et al. First clinical expression of equine insect bite hypersensitivity is associated with co-sensitization to multiple Culicoides allergens. PLoS One. 2021.
  15. Björnsdóttir, S. et al. Summer eczema in exported Icelandic horses: influence of environmental and genetic factors. Acta Vet Scand. 2006.
  16. Kolm, G. et al. Variations in the concentration of zinc in the blood of Icelandic horses. Vet Rec. 2005.
  17. Huhmann, R. and Mueller, R.S. A cream containing omega-3-fatty acids, humectants and emollients as an aid in the treatment of equine Culicoides hypersensitivity. Vet Dermatol. 2019.
  18. Nemoto-Kawamura et al. Phycocyanin enhances secretary IgA antibody response and suppresses allergic IgE antibody response in mice immunized with antigen-entrapped biodegradable microparticles. J Nutri Sci Vitaminol. 2004.
  19. Schurink, A. et al. Genetic parameters of insect bite hypersensitivity in Dutch Friesian broodmares. J Anim Sci. 2011.