Sweet Itch in a Horse

What is sweet itch?

This is an intense skin irritation affecting principally the mane and tail regions of the horse. It is caused by a hypersensitivity reaction to the bite of the Culicoides midge. These midges are usually active in the summer months but may be active year-round in tropical regions. The midges are most active in the early morning and late afternoon.

Firstly to avoid confusion, other names for this condition are as follows,

  • Queensland Itch
  • Summer Itch
  • Summer Eczema
  • Allergic dermatitis

What are the symptoms?

  • Sweet itch is very itchy and the horse rubs and bites itself often to the extent that it may cause bleeding.
  • The bites cause a local allergic reaction with the development of papules and pustules. The sores can weep and may even bleed.
  • With the continual rubbing, the skin can become thickened and often there is substantial hair loss in the most affected areas. In fact, often a horse may lose most of its mane or tail if left untreated.
  • Most affected areas are the tail and mane but places that are also affected include ears, poll, face, chest and ventral abdomen.

Seldom the condition affects horses younger than two years as its basis is in allergic disease so the horse must first become sensitized to the midge bite.

How does the midge bite have such a profound effect?

The saliva of the midge is the principle, allergic agent. Some horses may have a genetic predisposition to the disease which partly explains the individual nature of the condition. When the horse has become exposed to the saliva it will develop antibodies which precipitate a hypersensitivity reaction when next exposed to the bites. It is very hard to control in that it only takes so few bites to trigger the allergic response. The inflammatory response involves the release of various chemical mediators including histamine which is responsible for the intense itchiness observed in sweet itch cases.

Apart from the Culicoides midge other fly species have been associated with similar hypersensitivity reactions including the stable fly, horsefly and the blackfly.

How can we be sure of the diagnosis?

The history of seasonal recurrence of the condition in warmer months with it possibly getting worse with each subsequent year would be suggestive.

The appearance of the clinical signs as described above usually follow a classical distribution pattern on the horse’s body.

Your vet may decide to take a skin biopsy or carry out some intradermal skin allergy tests to make a definitive diagnosis of sweet itch.

It is important to eliminate the possible differential conditions that may be confused with sweet itch. Some of these would include lice infestation, chorioptic mange, rain scald, ringworm, food hypersensitivity and onchocercal dermatitis.

How do we treat or control this condition ?

By knowing the habits of the midges we can try to reduce the horse’s exposure. Midges breed in areas such as ponds or marshy ground where there is still water. Keep your horses out of these areas and if possible at least a half mile away.

The midges are most active around sunset but also feed at night and at sunrise. Knowing this, affected horses should be stabled at least an hour before sunset until a similar period after sunrise. The stable should have an insect-proof screen so as to prevent the midges from gaining entry.

If stabling is not an option rugging with a sheet and hood should be considered.

Insect repellents can be applied to the mane and tail region. Benzyl benzoate is one that is commonly used. Preparations with citronella are also effective as a deterrent.

Certain drugs are also used in the systemic treatment of the symptoms of sweet itch. Anti-histamines are used but results are variable. Corticosteroids are very effective at reducing the signs of itch. Oral prednisolone can be effective when given as alternate day therapy. However, with all usage of corticosteroids they can inrease the risk of inducing laminitis in the horse.

Can Herbal preparations make any difference?

In Practice, I have certainly helped many cases of sweet itch using herbs. As the bite in itself is very irritant , a certain amount of irritation can still be found in some cases but the intense itchiness that characterizes the complaint is significantly reduced. The herbal formula I use is Aniscratch Equine which myself and our herbalist developed over the past five years. The herbal ingredients can be examined by visiting the Aniscratch equine product page.

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