By Stephen Champion, BSc (Hons) BVM&S MRCVS,
Veterinary Surgeon, CHAMPIONVET.com
Written for Equi-Ads Magazine, July 2003

Well, its that time of year again. In fact, we’re well into it by now. As you go out to feed your horse this evening, the feeling of impending doom will fall upon you once again as you approach him in the field. His rump against the fence, he may be rubbing as if demented. The moth eaten appearance of his coat, the cropped mane and bald poll tell their story. He might be irritable, might look like a “welfare” case and will almost certainly not feel like going for a hack. Yes, its “Sweet Itch” season again and no doubt it’s not just your horse who is pulling his hair out – its likely to be you too.

Sweet itch, otherwise known as Queensland Itch or Allergic Dermatitis, is caused by a small, mosquito-like insect called Culicoides¸ which breeds in ponds and marshes near to your field. There are many different species of Culicoides, and the different species feed on different areas of the horse. Hence some horses are affected along the poll, neck back and tail-head, whilst others are affected underneath. It depends to some degree on which species of Culicoides are breeding locally and you can often see them landing on the affected areas.

A real problem occurs when the horse becomes allergic to the insects, or rather to their saliva. (Not all horses become allergic and some just remain “irritated” by the landing of the Culicoides.) The worst affected animals, however, have almost certainly developed an allergy and most likely have developed it from previous seasons. Thus the mere site of a Culicoides in the field is enough to send the horse to the nearest fence post, and you for the citronella spray and the gin bottle. The allergy (or predisposition to react to the saliva) will rarely leave the horse and often augments with progressive seasons. Worst of all, there may be a hereditary predisposition – so when buying a youngster, ask about his parents, or see them in the field during the Summer!

For those who are lucky enough to have not experienced this condition first hand – a potted history. These wee beasts feed most voraciously an hour each side of sunset, but also around sunrise and throughout the night. They are most, if not exclusively, active, in wind speeds less than 7km/h. Very quickly in sensitive horses, they cause severe itching of the face, ears, chin, chest, poll, withers, rump, underbelly and groin (not all in the same horse – fortunately!). This in turn leads to hair-loss, red inflamed spots, loss of the mane and in many cases, the skin over the neck and withers becomes thickened and wrinkled, not unlike that of a rhino. The hair becomes coarser and more brittle. The horse may even lose weight due to the constant irritation – not a bad thing perhaps? Often in the Autumn, as the air becomes cooler and the stable more draughty, the spots heal and the hair grows back. But…only to reappear the following Spring – aaagghh!

So, what can we do about it, short of pulling your hair out (sorry…insensitive). As in most things veterinary, one must kick off with a good and reliable diagnosis. It may seem obvious (or you may not have a clue) but the clinical signs of this disease can mimic a number of other common culprits of skin disease. Circumstantial evidence may stack up in favour of Sweet Itch, I don’t deny, but keep an eye out for lice, mange, ringworm, mud-fever, Oxyuris equi worms, ticks, food allergy, contact allergy and then some of the really rare things! If in any doubt, get your vet out to have a look, as he or she should be able to rule out many of the offenders using simple tests by taking samples from the horse’s skin. And don’t forget that some horses can become sensitive to the “normal” flies that you see buzzing around them – the stable fly, the horsefly, the blackfly and the buffalofly. So good fly control is essential.

Which brings us on to control of the Culicoides. When your horse is stabled, perhaps erect fine-mesh screens across doors and windows. (Pre-plan before next fly season, which is generally April to October. Ceiling or wall-mounted fans can create a breeze and discourage the beasts. When grazing, rugging with sheets and hoods can be useful, although in my experience, they are not as effective as they look! Consider the natural history of the beast. It breeds in ponds and marshes (and water troughs) and cannot fly more than half a mile Therefore, consider finding alternative grazing away from water (come on – this is Scotland we’re talking about!). Anyway, keep water troughs clean, improve pasture drainage and drain ponds if possible. Stable one hour before and one hour after sunset and sunrise. Stabling at night may also help. It is also realised that individual horses may be sensitive to individual species of Culicoides – so moving grazing, even if not away from water, may improve the symptoms in some horses.

Insecticides and repellents are mandatory in all but the mildest cases. Try several different preparations to find the most effective and your vet may be able to help suggest a more reliable treatment. I find a group of compounds called synthetic pyrethroids are very useful. They can be applied once weekly and are great a keeping the larger flies away too. Citronella seems to work quite well and repellents such as the various lotions, and benzyl benzoate preparations have their place in the armoury.

In the most persistent cases of Sweet Itch, when your horse’s welfare is affected, your vet may choose to prescribe corticosteroids to alleviate the condition. Low doses of prednisolone, given by mouth every second or third day, will often make the horse considerably happier. However, there is a risk of adverse effects and these drugs should only be used under strict guidance from your vet. Antihistamines may also be tried, although I have found a variable response to this group of drugs.

In summary, good luck – there is no easy answer. Your horse should show a reasonable improvement if adequate control measures are implemented. Consider the natural history of the beast you are trying to conquer and remember – most successful treatments start with an accurate diagnosis. Roll on the season of mellow fruitfulness!

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