What is Sweet Itch?
Sweet itch, also known as insect bite hypersensitivity (IBH) or summer eczema is a type of skin inflammation caused by allergies (allergic dermatitis). Most commonly the allergy is to the saliva of Culicoides (midges). These midges emerge when the daytime temperature remains above 10⁰C and are most prevalent at dawn and dusk. The mane and tail are the most common areas to be bitten, and therefore become itchy, but irritation of the belly, sheath, armpits, and face is common too.
Another type of allergic dermatitis is atopy. This is caused by environmental allergens, like pollen. Atopy can closely resemble sweet itch, but may affect the horses skin rather than the mane and tail. Depending on the allergen, atopy may or may not be seasonal.
Both sweet itch and atopy have a similar underlying cause which is an abnormal immune response. For both problems the initial immune response is over-active, followed by an under-active resolution of the inflammatory response.
Treatment of Sweet Itch
Treatment options available for horses with allergic skin diseases are relatively limited. Steroids have their place in managing flare ups, but are not favoured for long term use. Vaccines which influence the immune response have been tested, and are beginning to be used in equine practice for sweet itch. However feed back tends to be mixed, with fantastic results seen in some horses, and little difference seen in others. The same can be said for allergen-specific immune therapy which is used to treat atopy.
For both problems the mainstay of long-term management is allergen avoidance- sometimes easier said than done! For the majority of cases allergen avoidance is going to involve specifically designed rugs, management changes, topical ointments and dietary support.
Rugs
Where itching is caused by or made worse by midges and flies, the use of a specifically designed rug to prevent the flies from being able to bite the horse will make a significant difference to the severity of the itching. It is very important that the fabric thread density is high, as midges can work their way through normal fly rug mesh. Many affected horses will need to wear these rugs 24/7 during the spring and summer, or as a minimum, during dawn and dusk. It is useful to have a spare, so regular rug washing can be facilitated. A top tip is to start using the rug before the midges emerge.
Management
Avoidance of allergens is the most important step towards successful management. Where the cause is known (midges) this is some what easier, as the horse can be stabled at dawn and dusk, rugs can be used, and fans can be installed in stables (these stop midges being able to land on the horse). The horse should be kept away from wooded areas and standing water, as this is where midges like to live.
Sometimes allergy testing is useful to help owners of horses with atopy identify allergens. However, it is not uncommon for 5-10 allergens to be identified and avoidance of all of them in impossible! It is sensible to implement changes for atopic horses which avoid flies, midges, dust and high pollen counts.
Topical Ointments
Topical treatments are aimed at repelling biting flies and midges, and improving the skin barrier. Affected skin can often become dry, inelastic, thickened and irritated. This will only make the problem worse as skin will become more itchy, and more attractive to biting flies. Itchy skin is normally very sensitive, so owners should be very careful with topical product choice. If owners are unsure of what to use on irritated skin veterinary advice should be saught.
Dietary Support
Before specific supplements are discussed, it is imperative that the itchy horse or pony is being fed a suitably balanced diet (fibre, fat, protein, vitamins, minerals) for their type and workload. Research has shown that obesity increases the risk of developing a skin condition by five times (1), highlighting the importance of a correct diet in the management of allergic skin disease.
Fat, or oil is essential for an effective skin barrier. Not only are fatty acids responsible for maintaining the strength of the connections between skin cells, they are also needed to maintain skin hydration, and prevent it from becoming dry.
Researchers (2) have looked more closely at the role of linseed on reactivity of the skin of horses with sweet itch. The study focused on whether feeding micronized linseed would influence how a horse with sweet itch reacted to midge saliva. The horses were fed 500g of micronized linseed for 42 days, and found it reduced the skin reaction of supplemented horses. It is important to note, in a previous study that used linseed oil rather than micronized linseed, no difference was seen. This suggests that perhaps the contents of the seed (phytochemicals) also contributes to the improvements seen.
The same study examined the oil content of the skin and hair of supplemented horses. No changes were noted in the oil composition of the skin, but there were differences in the oil content of the hair. The oil on the hair is produced by sebaceous glands within the skin. The sebum produced by the sebaceous glands is responsible for protecting the skin surface and hairs. More research is needed to fully understand if there is a connection between the change in the composition of the sebum and reduced skin reactivity to midge saliva.
While it appears that supplementation with micronized linseed could be of benefit to itchy horses, it is worthwhile remembering that 500g of micronized linseed contributes a similar number of calories to the diet as a stubbs scoop of horse and pony nuts.
Research into other nutritional aspects of skin health are lacking in horses, but we can draw some sensible conclusions by looking at deficiencies which cause skin problems. Deficiencies of copper and zinc commonly first manifest as changes to coat and skin colour and quality. While researchers found no difference in blood levels of copper and zinc in horses with sweet-itch verses those without (3), it would still be sensible to maintain a well balanced trace mineral profile in the diet.
Vitamin E is a fat soluble anti-oxidant found in sebum, and the walls of cells. Vitamin E helps to protect cells from damage and in human research it has been found to support the resolution of inflammation. The natural source of Vitamin E for horses is lush green grass, as it is poorly preserved in hay. If your horse or pony has restricted or no access to grazing, or the grazing is poor quality, then supplementing vitamin E is recommended.
Science Supplements Skin, Coat & Hoof contains chelated trace minerals Copper and Zinc alongside Vitamin E and other essential nutrients and is suitable for horses and ponies who suffer from itchy skin.
References
1. Salonen, Hollands, T., (2009). Epidemiology of equine obesity in south east England: preliminary findings. Proceedings of 48th BEVA Congress p247.
2. O'Neill, W., McKee, S., (2002). Flaxseed (Linum usitatissimum) supplementation associated with reduced skin test lesional area in horses with Culicoides hypersensitivity. Canadian journal of veterinary research = Revue canadienne de recherche veterinaire, 66(4), 272–277.
3. Stark, G., Schneider, B., (2001). Zinc and copper plasma levels in Icelandic horses with Culicoides hypersensitivity. Equine veterinary journal, 33(5), 506–509.