Sweet Itch

Also Known As

Queensland Itch, Summer seasonal recurrent dermatitis (SSRD)

Description

Sweet Itch is a problem that affects thousands of horses, ponies, and donkeys in many countries of the world to a greater or lesser degree. Sweet itch is the result of a hypersensitive reaction to allergens in the saliva of Culicoides midges, also known as "sand flies" or "no see'ms."

This hypersensitive response to the bites of Culicoides midges leads to severe itching, causing horses to become distressed and rub and bite themselves constantly once the midges begin biting them.

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.

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. 

Horses that are susceptible to developing 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.

Prolonged rubbing and biting results in hair loss and damage to the skin to such an extent that open sores develop which often become infected by bacteria leading to further distress.

Symptoms

  • Intense itching
  • Allergic reaction causing blisters at the site where the insects bite
  • Open sores with crusting, scabbing, and scaling
  • Lesions around ears and head
  • Loss of patches of hair caused by rubbing and biting
  • Skin thickening and loss of hair pigmentation
  • Secondary infections

Cause

Sweet Itch 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 tend to feed around the ears.

Culicoides are on the wing and breeding from as early as late March until the end of October, depending on geographical location. Seasonal variations in the weather can have an impact. Mild, damp winters allow 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.

Hereditary predisposition may also be a factor in Sweet Itch and work is being done to identify the gene(s) responsible. 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 favorable to a high Culicoides midge population, more than one horse in the field may show symptoms.

In the UK, Sweet Itch is classified as an unsoundness and, as such, should be declared when a horse is sold.

Prevention

The best way to prevent sweet itch is to minimize midge attack as much as possible. The following suggestions have been used successfully:

Ensure that pastures are well-drained and avoid putting horses in marshy, boggy fields.

Keep horses in the stable at dusk and dawn when midge feeding is at its peak, and close stable doors and windows. The installation of a large ceiling-mounted fan can help to create less favorable conditions for the midge.

Use an insect repellent. Some are effective against flies, but their effectiveness against Culicoides is generally unproven. DEET (the acronym for N,N-Diethyl-m-toluamide), has a track record stretching back over 40 years. It is the active ingredient in many midge and mosquito repellents for use by people. Research has shown that the higher the concentration of DEET in a repellent, the more effective and long-lasting it is likely to be.

Use an insecticide. Some owners achieve good results with insecticides, while others find they have shown little benefit in controlling sweet itch. In any event, they should be used with care. Permethrin is available by veterinary prescription.

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, such as Day, Son & Hewitt's 'Sweet Itch Lotion'.

Oils and other repellents that are effective usually work for a limited time because 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, such as 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 product provides effective sweet itch protection and avoids the need to use insecticides, oils, or greases. The Boett (pronounced Bo-ett, as in Go-get!) Blanket was invented in Sweden 16 years ago to offer protection to horses and ponies suffering from insect-bite allergy. It has been continually developed since then and is now used around the World to manage sweet itch, while avoiding undesirable side effects.

Ideally, the horse should start wearing the blanket before symptoms appear, but even later in the season, once the blanket is fitted, sores will quickly heal and mane and tail growth restart. Typically, it will take from one to three weeks after the blanket is fitted for damaged skin cells to recover and itchiness to decline. Horses wearing the blanket all summer keep their full manes and tails and have glossy, clean coats and those susceptible to sun sensitivity and contact nettle rash are also helped.

Animals that require stabling should continue to wear their blanket inside, unless the building is completely midge-free.

Two out of three horses with Sweet Itch suffer damage to the head area. The ears, forehead, and area around the eyes are commonly affected. For these animals, the Boett Hood offers protection. It has ample adjustment and is secured to the blanket by a loop behind the ears, a long elastic strap, which is fastened to a point inside the blanket by the wither area, and by two snap-clips below the cheeks. No head collar is required.

Use of a Midge Mask prevents damage around the eye and protects the face from forehead to muzzle. The mask also stops fly-borne 'runny-eye' infections and is suitable for all horses, ponies, and donkeys. The mask can be worn alone or with the Boett Hood.

Treatment

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 becomes a distressing period for horse and owner alike. The animal's comfort and well-being depend on its owner's management of the affliction.

In general, treatment for sweet ttch focuses on minimizing the resultant allergic reaction following midge attack. Consultation with a veterinarian will be helpful.

Depressing the immune system with corticosteroids, such as 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 antihistamines may bring some relief, but high dose rates are required and they can make the horse drowsy.

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

Benzyl benzoate was originally used to treat itch-mites (scabies) in humans and has been used for many years to combat sweet itch. 

Regular shampooing can benefit the condition of allergic horses and ponies.

Many horses with sweet itch have very sensitive skin. Some reports indicate that some shampoos containing additives such as Tea Tree Oil, Citronella, Oil of Eucalyptus, and others may cause dramatic allergic reactions. A non-allergenic shampoo contains no added perfume or color and no cheapening additives, such as salt. and should be pH neutral. Low-lather formulations make thorough rinsing after shampooing easier and leaves the coat clean and shiny.

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, with symptoms only to return again later.

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