Causes of Sweet Itch

Sweet Itch is an allergic reaction and therefore an immune system problem. Unfortunately these are notoriously complicated and difficult to deal with.

The disease is a delayed hypersensitivity to insect bites and results from an over-vigorous response by the animal's immune system. In the process of repelling invading insect saliva (which actually contains harmless protein) the horse attacks some of its own skin cells 'by mistake' and the resulting cell damage causes the symptoms described as Sweet Itch.

In the UK several species (of the 1,000 or so that exist) of the Culicoides midge and, to a lesser extent, the larger, hump-backed Simulium Equinum, a member of the blackfly family, are responsible. Each has a preferred feeding site; Culicoides tend to be body feeders and the Simulium ear feeders.

Culicoides adults mainly rest among herbage and are most active in twilight, calm conditions. Breeding sites are commonly in wet soil or moist, decaying vegetation. They are tiny, with a wing length less than 2 mm and able to fly only a short distance (100 metres or so).

Male Culicoides are nectar feeders, but soon after hatching the females mate and require a blood meal to mature their eggs. They do not fly in strong wind, heavy rain or bright, clear sunshine. They dislike hot, dry conditions. The grey light at dusk and dawn suits them well, and they are at their most active at these times. However, as they are poor fliers, if there is too strong a wind, or rain during early morning they will simply wait until later to feed. Likewise they may feed at any time during humid days with cloud cover.

Culicoides are on the wing and breeding from as early as late March until the end of October, depending on geographical location. There is only a short breeding season each year in the north of Scotland, while in the south of England larvae will hatch throughout the spring, summer and autumn, depending on weather conditions. Seasonal variations in the weather can have an impact - recent winters have been milder and damper allowing breeding to start earlier. Summers that are alternately sunny and rainy cause an increase in midge breeding habitats and therefore an increase in the numbers of midges that are around to bite. Under these conditions most horses will show symptoms of Sweet Itch to some degree. Culicoides numbers are the critical factor.

Culicoides larvae are able to survive severe frosts but they do not survive prolonged drought conditions.

Research into how Culicoides locate their prey indicates that they primarily do so by sight.


Symptoms of Sweet Itch

Sweet Itch, or Summer Seasonal Recurrent Dermatitis (SSRD), is a problem that affects thousands of horses, ponies and donkeys in many countries of the world to a greater or lesser degree. Virtually all breeds and types of ponies and breeds can be affected, from tiny Shetland ponies to heavyweight draught horses, although the condition is rare in English Thoroughbreds. In South Australia reports say that as many as 60% of horses and ponies are affected. About 5% of the UK horse population are thought to suffer. Although known by different names (e.g. Sommer Ekzem in Germany, Kasen in Japan, Queensland Itch in Australia), the symptoms are the same.

Symptoms include severe pruritus [itching], hair loss, skin thickening and flaky dandruff. Exudative dermatitis [weeping sores, sometimes with a yellow crust of dried serum] may occur. Without attention sores can suffer secondary infection.

The top of the tail and the mane are most commonly affected. The neck, withers, hips, ears and forehead, and in more severe cases, the mid-line of the belly, the saddle area, the sides of the head, the sheath or udder and the legs may also suffer.

The animal may swish its tail vigorously, roll frequently and attempt to scratch on anything within reach. It may pace endlessly and seek excessive mutual grooming from field companions. When kept behind electric fencing with nothing on which to rub, sufferers may scratch out their mane with their hind feet and bite vigorously at their own tail, flanks and heels. They may drag themselves along the ground to scratch their belly or sit like a dog and propel themselves round to scratch the top of their tail on the ground.

There can be a marked change in temperament - lethargy with frequent yawning and general lack of 'sparkle' may occur, or the horse may become agitated, impatient and, when ridden, lack concentration. When flying insects are around he may become agitated, with repeated head shaking.

Diagnosis is not usually difficult - the symptoms and its seasonal nature (spring, summer and autumn) are strong indicators. However symptoms can persist well into the winter months, with severely affected cases barely having cleared up before the onslaught starts again the following spring.

Horses that go on to develop Sweet Itch usually show signs of the disease between the ages of one and five and it is common for the symptoms to appear first in the autumn.

There is anecdotal evidence that stress (e.g moving to a new home, sickness, or severe injury) can be a factor when mature animals develop Sweet Itch.

Hereditary predisposition may be a factor in Sweet Itch and work to identify the gene(s) responsible is at an early stage. However environmental factors play a major part - where the horse is born and where it lives as an adult are at least as significant as the bloodlines of its sire and dam.

Sweet Itch is not contagious, although if conditions are particularly favourable to a high Culicoides midge population, more than one horse in the field may show symptoms.

In the UK Sweet Itch is classed by Vets as a reportable condition, which must be disclosed by an owner to a prospective purchaser before the sale. For the purpose of a vetting the allergy may be regarded as seriously as an unsoundness.

 

 

 

Cavalesse is a feed additive which promotes skin heath and can greatly reduce sweet itch symptoms. 

 

Allergy Awareness and Cavalesse

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CONTROL

At present there is no cure for Sweet Itch. Once an animal develops the allergy it generally faces a 'life-sentence' and every spring, summer and autumn are a distressing period for horse and owner alike. The animal's comfort and well being are down to its owner's management.

Some methods to reduce the syptoms -

MINIMISE MIDGE ATTACK 

Avoid marshy, boggy fields. If possible move the horse to a more exposed, windy site, e.g. a bare hillside or a coastal site with strong onshore breezes. Chalk-based grassland will have fewer midges than heavy clay pasture. 
 

Ensure pasture is well drained and away from rotting vegetation (e.g. muck heaps, old hay-feeding areas, rotting leaves). 

Stable at dusk and dawn, when midge feeding is at its peak, and close stable doors and windows (midges can enter stables). The installation of a large ceiling-mounted fan can help to create less favourable conditions for the midge. 

For slight to moderate cases of Sweet Itch this can help. However a seriously itchy, stabled horse has hours of boredom during which to think up new ways of relieving his itch - manes and tails can be demolished in a few hours of scratching against a stable wall. If stabling can be avoided it is best to do so.

Use an insect repellent.

Some are effective against flies but their effectiveness against Culicoides is unproven.

DEET (the acronym for N,N-Diethyl-m-toluamide), has a track record stretching back over 40 years and has proven to be highly effective. It is the active ingredient in many midge and mosquito repellents for use by people.

Use an insecticide. 

Some owners achieve good results with insecticides whilst others find they have shown little benefit in controlling Sweet Itch. 

Benzyl benzoate was originally used to treat itch-mites (scabies) in humans and has been used for many years to combat Sweet Itch. In its neat form it is a transparent liquid with an aromatic smell, but it is more commonly obtained from Vets or pharmacies as a diluted milky-white suspension. It is listed as an ingredient in several proprietary formulations, including Carr, Day & Martins' 'Kill Itch' and Pettifer's 'Sweet Itch Plus'.

Benzyl benzoate should be thoroughly worked into the skin in the susceptible areas every day. However it is a skin irritant and should not be used on the horse if hair loss and broken skin have occurred - application should therefore start before symptoms develop in the spring. If used later its irritant properties can cause areas of skin to slough-off, in the form of large flakes of dandruff. 

Other insecticides, including permethrin and related compounds, tend to be longer lasting but should also be used with care. Permethrin is available by veterinary prescription (e.g. Day, Son & Hewitt 'Switch' pour-on liquid). Application instructions should be followed. 

Note: Gloves should be worn when applying insecticides, including benzyl benzoate. Particular care should be taken if they are used on ponies handled by children - they can cause eye irritation, for example if fingers transfer the chemical from the pony's mane to the eyes.

Coat the susceptible areas of the horse with an oil 

Midges dislike contact with a film of oil and they will tend to avoid it. Commonly used preparations include Medicinal Liquid Paraffin, and 'Avon Skin-so-Soft' bath oil (diluted with water). There are several oil-based proprietary formulations, for example Day Son & Hewitt's 'Sweet Itch Lotion'. 

Oils and other repellents that are effective usually work for a limited time: In summer a horse's short coat-hair does not retain the active ingredient for long and it can be easily lost through sweating or rain. Re-application two or three times every day may be necessary.

Greases (usually based on mineral oils) stay on the coat longer, but they are messy and therefore not ideal if the horse is to be ridden. They can be effective if only a small area of the horse is to be covered. However it is impractical and often expensive to cover larger areas. 

Some preparations contain substances (e.g. eucalyptus oil, citronella oil, tea tree oil, mineral oil or chemical repellents) that can cause an allergic skin reaction. Always patch test first on the neck or flank of the horse - apply to an area about 3 cm across and look for any sign of swelling or heat over a 24 hour period before using more extensively.
 

Use a Boett® veterinary blanket. This is by far the most effective Sweet Itch protection to date and avoids the need need to use insecticides, oils or greases.

ALLOW MIDGE ATTACK, BUT TRY TO MINIMIZE THE RESULTANT ALLERGIC REACTION BY:

Depressing the immune system with corticosteroids (e.g. by injection of 'Depo-Medrone' or 'Kenalog', or in tablet form as 'Prednisolone') may bring temporary relief but there can be side effects, including laminitis, in some animals. With time, corticosteroids may become less effective, requiring ever larger and more frequent doses.

The use of anti-histamines may bring some relief but high dose rates are required and they can make the horse drowsy. 

Applying soothing lotions to the irritated areas. Soothing creams such as Calamine Cream or 'Sudocrem' can bring relief and reduce inflammation, but they will not deter further midge attack. Steroid creams can reduce inflammation. 

It is often difficult to assess the effectiveness of a particular treatment. The incidence and severity of Sweet Itch is so highly dependent on midge numbers, apparent success may simply reflect a temporary fall in numbers due to a change in the weather, for symptoms only to return again later when weather conditions are more midge-favourable.