By Melanie Huggett

Most of us know what the itch of an insect bite feels like. But what if it wouldn’t go away? And what if it covered your whole stomach or head instead of just one small spot?

For some horses, certain flies cause an incessant, unbearable itch that won’t go away no matter how hard or long they scratch. This condition, called Recurrent Seasonal Pruritis or “Sweet Itch,” is a hypersensitivity to the bite of the tiny Culicoides fly, commonly known as midges, “punkies,” or “no-see-ums.” With many names throughout the world — “Queensland Itch” in Australia, “Kasen” in Japan, “Sommer Ekzem” in Germany, as well as summer eczema and summer seasonal recurrent dermatitis in other English speaking countries — sweet itch is the most common skin allergy in horses.

The irritation occurs seasonally, from April to November, upon the appearance of the midge fly. Cases in young horses and foals are rare, though once the hypersensitivity begins, it tends to worsen with each successive year. There may also be a genetic factor, as certain breeds seem to be particularly susceptible to the condition; for example, sweet itch is estimated to occur in as much as 54 percent of all Icelandic Horses. It is also common in some draft and pony breeds.

Horses and ponies with sweet itch will scratch themselves frequently, often causing hair loss. Photo: Bentley Wood

Signs and Symptoms

Sweet itch symptoms include continuous, severe pruritis (itching), hair loss, blisters, scabs, and flaky dandruff. Weeping sores may also appear and can lead to secondary infection if not treated. Over a long period, skin thickening, ridges, and hair pigmentation loss may occur.

Horses will typically bite themselves and scratch on anything in reach. They may pace, seek excessive mutual grooming, roll frequently, or drag themselves along the ground in an attempt to relieve the itch. In fact, hair loss, lesions, and ridges are due to the horse scratching himself incessantly rather than the midge bites themselves.

Affected areas are typically along the mane and at the dock, or underneath the abdomen. In more severe cases the irritation can move down the back, onto the head and ears, into the sheath area, and onto the legs.

The species of midge in your area will cause either the mane/tail form or dorsal abdomen form to be more common. The severity of attacks will depend on midge numbers; mild winters and rainy summers will tend to increase midge numbers.  

Prevention

Sweet itch is notoriously difficult to handle. Preventing midges from contacting sensitive horses is the best approach to reduce problems, rather than trying to treat the sores after bites have occurred.

Keep sensitive horses stalled when midge flies are most active, during the twilight hours and on calm, humid days. Photo: Robin Duncan Photography

Good stable hygiene is a must. Culicoides breed in wet soil or moist, decaying vegetation, so ensure you remove compost, plant debris (e.g. leaves, cut grass, etc.), dirty stall bedding, old feed, and other potential breeding material far away from the stable and turnout areas. Midges, with a wing span of only two millimetres, can fly only short distances, approximately 100 metres.

Avoid turning sensitive horses out near marshes, bogs, or swamps, as these are havens for midges. Turnout in well-drained paddocks or pastures is best, especially in windy areas as midges are poor fliers unable to cope with air speeds over ten kilometres per hour.

Stable the horse during the twilight hours, morning and evening, when midges are most active. Calm, humid days may have more midge activity as conditions are ideal for Culicoides, so stabling time may need to be longer. Fitting screens to the doors and windows of the barn can help, but they must be made of a very fine mesh as the tiny midge can fit through typical screens. Installing a large interior fan can also help.

If stabling for long periods, provide your horse with some stimulation as he may take to scratching himself in order to relieve boredom as well as itching, and damage himself in the process. A neighbour horse, something to munch on, or toys can all help reduce the time your horse has to think up new ways to relieve the irritation.

Horses prone to sweet itch should wear full coverage fly sheets and masks when turned out, to protect them from midges.

Fly sheets, hoods, and masks should be worn when the horse is turned out. Choose products with as much coverage as possible to reduce the areas that midges may attack. For example, a sheet that covers the neck and belly is best, as well as masks that cover the ears and further down the face.

Fly repellents and insecticides can help, but often need to be reapplied two to three times a day to be effective. Oiling or greasing affected areas will deter midges, who dislike landing on an oily film, but can be messy, especially if the horse is still to be ridden.

Treatment

Once the horse has been attacked by midges, there are few options.

Lotions and creams applied to irritated areas can provide some relief, but will not reduce further attack. 
Daily antihistamines can be given, but often have disappointing results. They may also cause drowsiness. Corticosteroids can also be given to suppress the immune system and provide relief, but side-effects can include laminitis.

A study done at the University of Guelph, however, showed that feeding crushed flaxseed can reduce the response and inflammation in horses affected with sweet itch. In the study, one group of horses was given one pound of flaxseed supplement per 1000 pounds of body weight per day, and another group was given a placebo. After 42 days, the horses were given no supplements for 14 days and then the groups were crossed over and the trial repeated. Skin tests were done with a Culicoides extract at zero, 21, and 42 days. At both 21 and 42 days, the group that was fed flaxseed showed a significantly reduced reaction to the extract, with the highest reduction at 42 days. No negative side-effects were observed.

A Future Cure?

In the UK, a group of researchers at the University of Bristol School of Veterinary Science is working towards a cure for sweet itch. They believe sweet itch is caused by proteins present in the saliva of the fly. Because it is an allergic reaction, administering low doses of the proteins to desensitize the horse’s immune system — a technique known as immunotherapy — could effectively “cure” the horse of the condition.

So far, the research team has isolated over 20 potential protein allergens in midge saliva and identified some of these proteins’ genes. The proteins have also been successfully manufactured through recombinant DNA techniques. The next step is to find out which of these proteins causes an immune response, and then give horses regular doses of those proteins to reduce their reaction. They hope the desensitization can occur through feeding the proteins to the horses.

While sweet itch immunotherapy is not yet proven and available to the public, perhaps one day horses will no longer be plagued by this itchy and troublesome condition.

More by Melanie Huggett

Main photo: Ponds and other marshy areas are havens for midge flies, who breed in wet soil and vegetation. Keep sensitive horses at least 100 metres away from these areas if possible.