Sweet itch is caused by an allergic reaction to the protein in the saliva of biting insects. This causes the immune system to attack its own cells and leads to an extreme reaction. In the UK, insects that cause sweet itch are the midge (Culicoides) and potentially the black fly (Simulium).
Horses are mostly affected between March – November when the biting insects are most active.
Sweet itch can affect all breeds and types of horses, ponies and donkeys and can be genetic, meaning some horses are at an increased risk of developing the condition. Breeds such as the Belgian warmblood1 and Icelandic horses2 as well as native ponies such as the Exmoor3 have been found to be at an increased risk.
Signs
Horses predominantly show signs of sweet itch at the base of the mane and tail, and on the face. The withers, back, belly and rump can also be affected. These are the areas where insects will normally bite and feed from the horse.
Signs include:
- Intense itchiness - itching can become so severe that the horse scratches itself on anything available including posts, stable doors and trees.
- Vigorous tail swishing in an effort to keep the insects away.
- Excessive mutual grooming from field companions.
- Excessive rolling and scratching at their mane with their hind hooves.
- Hair loss can occur through scratching and the skin may become inflamed, crusty and sore, with broken skin potentially leading to infection.
- As the condition progresses, skin thickens, folds develop and the hair becomes sparse and coarse with flaky dandruff.
- Changes in the horse’s behaviour such as seeming lethargic or agitated/restless and impatient, with a lack of concentration when ridden.
- A horse may also shake its head or become restless if flying insects are close by.
Management and control
There is currently no known cure for sweet itch and the condition recurs every year at the first contact with midges.
Reduce exposure to the biting insects
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Management and control measures aim to reduce exposure to the biting insects. It is important to be as proactive as possible by starting these measures before signs develop:
- Midges thrive in warm and moist conditions such as on droppings. Poo picking regularly and keeping your muck heap away from grazing and stabling may help to reduce the number of midges near your horse.
- Midges are poor fliers so a more exposed windy site that is well drained may have fewer midges; avoid turning out in marshy, boggy fields.
- The midges’ egg, larval and pupa developmental life stages require the presence of water to create a warm, moist environment. Therefore, try and avoid turning out near water sources such as ponds. Clean water troughs regularly and fix any leaks quickly to prevent stagnant water accumulating.
- Special sweet itch rugs which cover all the areas of the horse susceptible to bites can provide protection.
- Stabling horses from dusk to dawn, especially in hot and humid conditions when midges are at their most active, can help prevent horses being bitten. You could also consider installing fans to discourage midges if safe to do so.
Methods to ease the itching
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- Soothing lotions may help relieve itching and reduce inflammation.
- Steroid medication may be considered in severe cases. Speak to your vet about whether this is a suitable option for your horse.
Insect insecticides and repellents
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There are many insect repellents and insecticides available but some are more effective than others. Products containing pyrethroid or permethrin are generally more effective and may help to deter midges from biting. The products should be applied with care following the manufacturers’ instructions and should not be applied to inflamed or broken skin.
Vaccination
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Speak to your vet for guidance on the current vaccination options.
References
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1) Peeters, L.M., et al 2015. Genetic parameters and estimated breeding values of insect bite hypersensitivity in Belgian Warmblood horses. The Veterinary Journal, 206(3), pp.420-422.
2) Eriksson, S., et al 2008. Genetic analysis of insect bite hypersensitivity (summer eczema) in Icelandic horses. Animal, 2(3), pp.360-365.
3) Andersson, L.S.,et al 2012. The same ELA class II risk factors confer equine insect bite hypersensitivity in two distinct populations. Immunogenetics, 64(3), pp.201-208