Sweet itch

by Peter Fenton BVM&S, MRCVS

Sweet itch is a delayed hypersensitivity reaction to insect bites, mainly midges (culicoides) and results from an over vigorous response by the animal’s immune system. In the process of repelling invading insect saliva the horse attacks some of its own skin cells ‘by mistake’ and the resulting cell damage causes the symptoms described as Sweet Itch.

For most horse owners the most distressing aspect of sweet itch is seeing the severe reaction their horse can have to the midge saliva, mostly the intense itching exhibited often until they have open bleeding wounds. The other symptoms of sweet itch can include hair loss, thickening of the skin and flaky dandruff. Weeping sores, sometimes with a yellow crust of dried serum may occur and often open sores can become infected and may require antibiotics to resolve the infection.

The most commonly affected areas are the mane and tail but other areas affected can also include the neck, abdomen, withers, hips, ears and forehead. Horses with severe sweet itch may have a change in temperament and can become lethargic, agitated or even aggressive when handled or ridden.
Culicoides midges mainly live in wet, marshy and wooded areas and are most active in dusk and dawn in calm conditions and they do not fly in strong wind or heavy rain. They are active from as early as March to October. Breeding sites are commonly in wet soil or moist, decaying vegetation therefore making horse stables and grazing fields an ideal place to live and breed.

Diagnosing sweet itch is not usually difficult and the time of year and symptoms seen are strong enough to suggest that a horse has sweet itch. There is currently no cure for sweet itch and so it is important to try to minimise contact with midges for horses with sweet itch.

There are many commercial products available for horses including feed supplements, herbs, creams and sprays that claim to treat or prevent sweet itch, most of them have limited efficacy and do not work. Prevention of sweet itch is much more important than trying to treat it.

To minimise the risk of sweet itch;
• Try to avoid marshy, boggy fields and avoid grazing horses next to streams, ponds or areas of stagnant water as these attract the midges. This includes ensuring water troughs are cleaned regularly.
• Ensure pasture and all turn out is well drained and away from rotting vegetation such as muck heaps or compost heaps.
• If possible stable the horse at dusk and dawn, when midge feeding is at its peak, and close stable doors and windows or use mesh screens to stop midges entering the stable.
• The installation of a large ceiling-mounted fan can help to create less favourable conditions for the midge.
• Use a good quality fly repellent and apply frequently. Such as a permethrin based one such as coopers fly repellent or switch.

The best way to protect your horse from midges is to use a good quality fly sheet with face mask and belly flap. The best flysheets are made from material that the midges can’t bite through, so a heavy mesh with larger holes is less suited to this job. The Boett blanket has been specifically designed for the use in horses with sweet itch and had very good results and there are several of this type available now. Always make sure the rug is clean and dry and fits the horse correctly, ideally have a spare rug available so if one rug gets wet or rips you have another to use. The horse will, ideally, wear the rug for 24 hours a day even when stabled at very high risk periods of the year. Some owners will say that their horse will not wear a rug or rips them, persistence will hugely benefit the horse. Turning the horse out in a paddock with electric fencing and soft plastic water buckets instead of wooden fencing and metal troughs will limit the damage a horse will do to the rug and will also limit the trauma a horse with sweet itch can inflict on itself. Some owners find that keeping a horse turned out in an electric fence paddock stops the horse rubbing out its mane and tail and also stops the horse developing sores but it is important to remember that although the horse is not itching, it is still itchy!

Cavalesse is a product that is being increasingly used to treat allergic skin diseases in horses. It contains nicotinamide that has been shown to reduce levels of histamine produced and also boost levels of natural fats in the skin therefore improving the skins natural barrier. There is a topical gel in combination with an in feed supplement that can be used on affected lesions. Many people have found that their horses have responded very well to treatment with cavalesse often showing dramatic improvement within days. The product is best started prior to the nice weather and turnout usually end of February beginning of March.
Exposure to midges during early years of life appears to be important in helping young horses become tolerant to the proteins present in midge saliva. It is these proteins that, if not recognised by the horse’s immune system as being harmless, trigger the hypersensitivity reaction which we recognise as sweet itch.
These findings helped researchers to develop ways of tricking the immune system of sweet itch affected horses into believing that midges pose no threat. This re-programming of the immune system, known as immunotherapy has been used in asthma and other human allergies. It involves repeated exposure to small amounts of the proteins that would otherwise trigger the hypersensitivity reaction. Scientists at Bristol University have identified the offending proteins present in midge saliva and work has started to find the most effective way of administering these in a course of immunotherapy. This work should not be confused with the trials of sweet itch ‘vaccine’. This has been available for several years and is actually based on bacterial cell extracts that are designed to modify the immune response. This product is available for oral administration as BioEos Sweet Itch Capsules. It has been reported that some 70% of horses treated with this product show a 50% improvement in clinical signs, with 10% showing substantial reduction in signs and 10% minor improvement. The capsules are given monthly and cost approximately £280 for a year’s treatment. Further information can be obtained from the National Sweet Itch Centre
In conclusion, no horse should ever just be dismissed as having a little bit of sweet itch. Severely affected horses become so irritated and restless that they often lose weight and show negative changes in behaviour and demeanour. This can become a welfare issue that should be addressed by the owner as soon as possible. Most horses show a dramatic improvement in general well being when a treatment is successful, which is telling of just how detrimental the condition would be if left untreated.

http://www.equinevetclinic.co.uk/Documents/Insectbitehypersensivity.pdf

Images supplied by www.brinicombe-equine.co.uk

Author: The Editor

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