Sweet itch – an itchy business

Sweet itch is the most common cause of itching in horses leading to hair loss, especially from the mane and tail, with crusting and scab formation. Preventing your horse from developing sweet itch can be challenging, but regular checks and the use of preventative measures can ensure you are one step ahead. Prevention is definitely the name of the game!

Sweet itch is best documented as an allergic reaction to bites from midges/”no-see-ums” (Culicoides spp), but it also occurs with bites from black flies (Simulium spp), horn flies (Haematobia irritans), stable flies (Stomoxys calcitrans), and occasionally hypersensitivity to mosquito bites, horse flies and Onchocerca cervicalis causes similar symptoms. The allergic reaction to these insect bites leads to an itching sensation in the skin, causing the horse to bite, rub and scratch the affected areas. The disorder is familial suggesting a genetic predilection. Sweet itch also seems to get worse with age.

You will notice your horse suffering from intense itching, which will almost definitively lead to excoriation, hair loss, pigment disturbances, “rat tail” and “buzzed off” mane. You may also notice the development of diffuse midline dermatitis with crusts and scabs, and in severe cases your horse may self-mutilate and suffer from weight loss and nervousness. The condition is usually seasonal, occurring over the summer period, with the possibility of it worsening each year.

When your vet examines your horse they will need to know the history of the development of the problem and will carry out a physical examination to rule out any other causes.
Your vet may carry out an intradermal test to identify the cause of the sweet itch – this involves the application or intradermal injection of a substance to the skin to test the body’s reaction to it. The test detects the horse’s sensitivity to micro-organisms believed to be the cause of the sweet itch. Your vet will shave an area on your horse’s neck before starting the test; depending on your horse’s temperament, your vet may or may not need to sedate your horse.
An ELISA test may also be carried out – this test is used to detect the causative antigen (bacteria) of the sweet itch. Your vet will take a blood sample that will be tested in the laboratory – this test is not as reliable as intradermal testing.
If your vet is finding it difficult to make a diagnosis, and other non-allergic causes need to be ruled out first, then it may be necessary for your vet to take a skin biopsy which will be taken for histopathological evaluation.

Prevention is the most important factor when dealing with sweet itch. Fly control should begin before the start of the fly season and exposure to insects should be limited. Application of residual insecticides (insect repellent) and stabling during peak feeding times of the insect (dusk and dawn) will help limit the contact your horse has with the biting flies. Fly rugs and masks are now available for use when your horse is not stabled.
You should try and keep your horse away from areas where the flies breed, such as the muck heap or stagnant water; ensure that all possible areas where the files may breed are kept scrupulously clean, so washing out troughs on a regular basis and improving pasture drainage and draining ponds, etc should be considered. If this is not possible, then you should consider moving your horse to a new area, at least half a mile from any possible breeding habitats, and away from any resident cattle. Individual horses may be particularly sensitive to individual species, so may improve in a new area. Other possible prevention methods can include the use of fine mesh screens for stable openings, and the use of box fans in stable opening (“no-see-ums” are weak fliers!).
When avoidance is not possible short-term use of steroids or, if itching is less intense, antihistamines may be used to alleviate the itching. Allergen-specific-immunotherapy (allergy shots/hyposensitisation) is another option, with 50-75% reported success rate, but a response can take several months, so year round treatment is recommended.

If adequate fly control methods are implemented before the start of the fly season, then prognosis is good, however, it is likely that your horse will suffer from the problem every year, so you must be ready to repeat the control methods every year.
If none of the prevention methods are implemented then prognosis is poor with seasonal recurrence and clinical signs worsening with subsequent seasons a definite possibility.

Sweet itch is familial, suggesting a genetic predilection, so chances are higher that offspring of affected horses will have allergic skin disease as well.