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Sweet Itch (or Summer Itch, as it is sometimes referred to), is the common name for Summer Seasonal Recurrent Dermatitis (SSRD), or more technically ‘Insect Bite Hypersensitivity’, which, as the name suggests, is an allergic reaction to the saliva of biting midges. There are numerous types of flying insects that can cause skin irritation in horses, however, in the case of Sweet Itch, allergies are most likely to develop due to one of the species of Culicoides midges, or in some cases, a member of the black fly family called Simulium Equinum.
As an allergic reaction disease, it is also an immune system problem that is notoriously complicated and difficult to handle. The delayed hypersensitivity to the insect bites results from an over-vigorous response by the horse’s immune system. In the process of repelling invading insect saliva the horse’s system also mistakenly attacks some of its own skin cells, with the resulting cell damage causing the external symptoms we see.
This is an incredibly common problem that affects thousands of horses, ponies, and donkeys throughout the world and is not discerning of breed or size, although more typically it is a condition that affects adults over about four years of age, and can worsen with age. There is strong evidence to show that susceptibility to the disease may be hereditary, meaning the offspring of an affected mare or stallion is more likely to be affected than the offspring of unaffected horses. This also means that the disease is more prevalent in certain breeds, with some native breeds seemingly more often affected. There is also some belief that evidence of stress (for example, moving home, sickness, or severe injury) can be a factor when mature animals develop Sweet Itch.
Sweet Itch is a seasonal complaint and is at its worst when midges are most active, between the months of April and early November. The midges are especially active on calm, humid days, biting mostly in the early morning and late afternoon/early evening. Different species of midges tend to bite in different areas on the body, but the most commonly affected areas are around the mane, along the back (close to the spine), and around the dock (base of the tail). This can then progress to covering areas such as the face, ears, neck, belly, and shoulders.
There is no cure for Sweet Itch, only prevention and management, so once a horse has the disease the reaction will likely continue to worsen, year after year. The skin of an affected horse can recover almost completely during the winter months, but will flare up again on first contact with biting midges in the spring. For this reason, it is not advisable to buy a horse in winter, as there may not be any visual indicators to alert you to the fact that the horse has the disease.
Symptoms of Sweet Itch include severe pruritus (intense itching), hair loss, a thickening of the affected skin, flaky dandruff, and in severe cases, weight loss. Weeping sores may appear with a yellow crust of dried serum which, if left untreated, can trigger secondary infection. The lesions are commonly found along the spine, mostly at the junction with the mane, forelock, or tail. In more severe cases the sores become more extensive, moving down the body to the mid-line of the abdomen, the saddle area, sides of the head, sheath, and eventually onto the legs. The horse will scratch itself incessantly causing the hair to break and the skin to become bald, hard and rigid.
To relieve the intense irritation a horse may swish its tail vigorously, roll frequently and attempt to scratch on anything in reach. Pacing endlessly and seeking excessive mutual grooming from field companions can be observed, with some horses even resorting to dragging themselves along the ground in order to scratch their abdomen. The horse’s temperament may also change, becoming visibly lethargic with frequent yawning, or conversely agitated, impatient and showing a lack of concentration when ridden. When flying insects are around they may become restless with repeated head shaking.
It should be noted that despite being regarded as a mainly seasonal disease, symptoms can persist right into the winter months, with severely affected cases barely having healed before the onslaught starts again the following spring. Sweet Itch is not contagious, although if conditions are favourable to midge populations, more than one horse in the field may show symptoms.
With such obvious symptoms and appearance as an indicator, along with the seasonal nature of the disease, diagnosis is not usually difficult. However, there are other skin allergies that can have a very similar appearance to the symptoms displayed in cases of Sweet Itch, so if you suspect your horse has Sweet Itch we recommend having them checked out by a qualified veterinarian, just to confirm this is the case. They may want to perform intradermal allergy testing, which involves injecting midge extracts into the skin (among other substances) to check for a reaction. This will usually be done amongst other forms of testing, to rule out any other allergies or underlying causes.
If your horse appears to be suffering from skin allergy symptoms that differ slightly from the listed symptoms for Sweet Itch, then it could be one of the following conditions instead; Mud Fever, Mites, Lice, or Rain Rot (to name just a few).
More information on Diagnosing & Treating Horse Lice >
More information on Diagnosing & Treating Horse Mites (Mange) >
Although there is no complete cure for Sweet Itch, it is important to take action as soon as you see any of the telltale symptoms, as the affected horse can do a lot of further damage to itself in a short period of time, in their battle to relieve the itch.
As mentioned above, the first step is to get an official diagnosis of Sweet Itch from a vet. Once you have had confirmation that this is the cause of their irritation, there are a number of things you can do to ease their discomfort and in turn, reduce their stress levels. Each method has varying results, with no one method providing effective relief in all cases.
All of the above methods are simply suggestions for ways to reduce the symptoms once already present in an affected horse, and will require a fair bit of trial and error to find what works for each individual.
As with most things, prevention is much better than the cure (especially when there isn’t one!), and there are numerous things you can do to prevent and control the midge attacks.
In the United Kingdom the Culicoides midge and, to a lesser extent, a member of the black fly family called Simulium Equinum are responsible. Each of these insects has a preferred feeding site on the horse, the midge preferring the body and black fly the ears. The breeding sites of the Culicoides are commonly in wet soil or moist, decaying vegetation. Whereas Simulin prefer oxygenated water sources such as slow moving streams or rivers. Most of these midges are nectar feeders but soon after hatching the females mate and require a blood meal to mature their eggs. Hence the biting of nearby blood carriers occurs.
As poor fliers, weather conditions can delay their normal mealtimes of dusk and dawn, but all they do is simply wait until the wind calms or the rain stops and then feed later. Their larvae are able to survive severe frosts but they do not survive prolonged drought conditions. Marshy, boggy fields are a haven for the biting midges.
Minimisation of midge attack is a primary tactic in the avoidance of Sweet Itch. With all this in mind, here are some of the things you can do to reduce the chance of midge attacks;
Prevention of midge attacks is challenging but, without a doubt, worth the effort. However, with so many possible tactics and remedies available for prevention and relief, it is very difficult to assess the effectiveness of any particular course of management of this disease. The incidence and severity of Sweet Itch are so highly dependent on midge numbers, that apparent success may simply reflect a temporary fall in the numbers due to a change in the weather, only for symptoms to return again later when weather conditions are more favourable to the biting midge. The best course of action, therefore, is to trial a series of methods in the longer term, until you find something that works for your specific case.