Sweet itch
Summer seasonal recurrent dermatitis, commonly referred to as sweet itch, is a common and unpleasant skin disease. It is caused by an allergic reaction or hypersensitivity to the bite of a Culicoides midge, a tiny fly, which incidentally is the same type of midge that carries African Horse Sickness. All horses are bitten by the midge, but just like with African Horse Sickness, only some develop the condition.
Pathology
The Culicoides midge is so tiny that it can get through conventional mosquito nets to bite the horse. A local allergic reaction occurs to a protein in the saliva of the midge, causing intense irritation in the region of the bite. Basically, the horse’s immune system over-reacts to the protein, which is essentially a harmless substance, by attacking its own skin cells, and it is the damage caused by this attack on their own skin that leads to the symptoms of sweet itch. Sweet itch is therefore, fundamentally, a problem of the immune system and as such is very difficult to deal with.
Culicoides midges mostly feed at dusk and dawn in warm weather and they breed in wet conditions, rotting vegetation and water troughs. The most affected areas are under the mane and at the root of the tail, along with the neck, ears, forehead, withers and across the hindquarter, but in severe cases the condition also appears along the midline of the belly, the saddle area, the sides of the head, the sheath or udder and the legs. In fact the condition can really affect just about any part of the horse’s body!
Animals who are predisposed to sweet itch will initially show signs at between one and four years of age, with the condition becoming worse in the summer months. Older animals who show the disease for the first time are thought to do so because of some outside stressor, such as moving to a new home, disease or serious injury. However, in both younger and older horses, the major contributing factor in developing sweet itch is obviously the prevalence of the Culicoides midge in the area, so managing this environmental factor is the focus in the treatment of the condition.
Sweet itch, while immensely distressing for the affected horse, is not contagious. One or more animals in the same stable yard may have the condition, but this is simply due to the environmental factors (the high load of the Culicoides midge), not the disease being passed from horse to horse. It is by far the most common skin allergy in horses and ponies and occurs throughout the world.
Diagnosis
If you suspect that your horse or pony has sweet itch, it is important to get a proper clinical diagnosis from your vet. Diagnosis should not be difficult, especially with the disease being seasonal, but proper diagnosis does eliminate other causes of similar symptoms, which are much more easily treatable, like lice and ringworm, so is an essential part of management.
Symptoms of the disease start to appear at the end of spring and continue into autumn. However, in a few rare cases, the condition can be present almost all year round with last year’s symptoms having barely cleared up before the condition flares again the following spring.
Sweet itch is characterised by intense itching (pruritis), causing the horse or pony to vigorously rub or scratch the affected parts of the body on anything they can get their body up against. If there is nothing convenient to rub on, they may even pull themselves along the ground to scratch their belly or sit like a dog and wriggle around on the floor to scratch their tail. Sufferers frequently pester their field companions for extra mutual grooming or pace up and down continuously. Vigorous tail swishing or excessive rolling is also seen. Animals enclosed with electric fencing may resort to biting their flanks and the base of the tail and scratching the mane with a hind hoof.
Constant scratching and rubbing causes the skin to thicken and the hair to become broken and fall out. It is not uncommon for the condition to worsen each year until eventually the skin is so damaged that the hair no longer grows back. Constant damage to the hair may also result in the hair growing back white, so if you are buying a pony or horse over the winter months, check for white hairs in the mane and tail as this may suggest a case of chronic sweet itch that is just less obvious in the cooler months.
Other symptoms of sweet itch include flaky skin and weeping sores, sometimes with a yellow crust. Secondary infection with bacteria, viruses or fungi can affect these sores if they are not kept clean. If you suspect secondary infection you must contact your vet immediately to get the area treated.
Psychological and performance impact
Apart from the physical signs of the condition, the psychological impact on the horse can be dramatic. Suffering from sweet itch is genuinely a miserable predicament. Horses with sweet itch need help to minimise their symptoms, and a lot of sympathy and TLC.
Symptoms of sweet itch commonly cause horses to lose their ability to concentrate. These horses typically become lethargic and yawn a lot or become fidgety and impatient. They generally lose their interest in work, and their performance deteriorates. Tack may become unbearable for them. Insects buzzing around can produce general agitation and can set off a bout of head-shaking. With all of this extra anxiety it is not unusual for these horses to drop weight and lose condition.
Treatment
The key to managing sweet itch is prevention and persistence. Preventative environmental measures to reduce exposure to the midge are certainly the most effective, but if the midges are still biting, you need to be diligent in applying treatments and fly sprays regularly to get the condition under control.
Environmental management
There are a number of measures you can take to reduce the midge’s access to the horse, which is fundamentally the most important arm of treatment. Firstly, horses must be removed from areas near to woods, ponds, lakes, rivers, standing water, wet ground or rotting vegetation, as these are the main breeding grounds of the fly. An ideal location for the horse is in fact an exposed windy site as the midges cannot cope with the windy conditions. It is easy to see therefore why a fan in the stable would help to keep the midges at bay. The fan should be directed towards the door or window to prevent the midges flying in and getting access to the horse. However, fans must be securely fastened and well out of the way of your horse, as they can cause nasty injuries if not handled properly. Similarly, when riding your horse, avoid areas like those mentioned above that may be home to plenty of midges.
Rugging the horse with a lightweight rug that has a hood and covers the belly is also helpful. Many tack shops stock specific blankets for this purpose. Fly nets unfortunately won’t help hugely as the fly is small enough to get through the holes of the standard nets.
Culicoides midges do tend to feed on specific parts of the horse, but fly repellents need to be applied all over the body, possibly twice a day, if they are to have any effect at driving away the midges. They must also be applied before every riding session. One other point with fly sprays before you liberally pour them all over your horse is to do a patch test. This involves testing the spray on a small patch of skin and observing the patch for 24 hours before applying to the rest of the horse. This checks for any adverse reactions to the chemicals in the spray, by testing only a small area initially. This is especially important in sweet itch horses as they are already hypersensitive, so exposure to additional irritants must be avoided.
Pharmacological treatment
If the allergic reaction is severe, vets can prescribe corticosteroids to minimise a bad reaction, but these drugs will only bring temporary relief and carry the risk of various side effects, including an increased risk of developing laminitis. Similarly, some vets will prescribe antihistamines (specifically hydroxyzine), which have been shown to be of benefit in some cases of sweet itch. Antibiotics may be required in cases where the skin has been damaged and has become infected. Steroid and antihistamine creams can also be used, and some people have experienced good results with these.
It is thought that there is a hereditary component to sweet itch, so many people would advise not to breed horses with sweet itch as the condition is so debilitating.
Useful additional treatments
Certain creams and oils can reduce the itch but will need regular application. Some horses experience relief from cold hosing and ice packs on the affected areas. Finally, many alternative therapies are recommended for the treatment of sweet itch, and it is worth discussing these with a specialist.
Remember
Don’t expect your treatments to be effective immediately. Horses who have suffered from sweet itch have often become habitual scratchers, and therefore it will take a while to break the itch-scratch cycle.
With sweet itch there are no cures and no quick fixes, and even a short exposure to a few biting Culicoides midges can result in three weeks of itchiness. Throughout the main ‘biting’ season preventative measures must not be relaxed if the programme is to successfully control the condition. This is time-consuming but must be done if you are to get the best for your horse.
As sweet itch can be such a debilitating condition, it is essential that future owners are told of the issue of sweet itch at the time of a sale.