Ben Sturgeon, BSc, BVM&S, Cert EP, MRCVS

It's May, hooray, lambs are skitting about the fields, you can even have a beer outside, everyone's out riding past eight o'clock and everyone's getting bitten by those blood sucking monsters the midge. Actually as a point of fact the midge season officially started on the 19th of March, we know this because DEFFRA monitors the situation predicting when and where new outbreaks of condition such as Bluetongue (spread by the midge) will arise. And this year March 19th was the date of invasion and who knows what with global warming the date will probably get pushed back earlier but then sweet itch will be the least of our worries.

Midges are members of the Culicoides genus that attack mammals in general. Importantly individual species tend to attack different areas of the horse with the neck, tail and ventral abdomen being the most common areas to be bitten. This can be very difficult to explain to owners who having read the text books expect the bites and reaction to be only on the neck and tail, however the midges not having read the text books will bite anywhere causing diagnostic confusion. More importantly still, not every horse bitten by a midge will develop sweet itch, most in fact simply show signs of irritation, restlessness and itchiness. Under favourable climatic conditions enormous populations of the flies are present and such horses will show severe irritation during these wave attacks. Self inflicted trauma can cause moderate to severe skin abrasions and excoriation with serum oozing and secondary bacterial infections. Compare this with true sweet itch and the picture is quite different - here despite the possible cessation of the wave attacks or a single biting fly the reaction continues and we have a true allergy or more correctly a type 1 (immediate) hypersensitivity. There is no sex or hair/skin colour predilection. Cases in foals and young horses are rare with most horses becoming affected after 4-5 years of age with the condition worsening with advancing age. Certain breeds are particularly sensitive (Icelandic and Welsh ponies) suggesting a genetic basis to the condition. The true hypersensitivity appears to be commoner to the head/neck/tail biting insects although it should be stressed that hypersensitivity can develop to any group of biting insects, not just midges.

Culicoides are blood feeders and as their life cycle demands specific warmth, moisture and vegetation conditions they tend to be very seasonal. Clinical signs then tend also to be seasonal (beware buying horses in the dead of winter) but worsen over succeeding years.

General signs: itching is always present and worse in the early evening (dusk) and very early morning (still in bed). Tail switching, rubbing, restlessness and eventual weight loss due to constant irritation.

Acute form: itching with very aggressive rubbing of the tail base, neck, head and back. Papules occur along the back of the horse from ears to tail. Self inflicted trauma causes exfoliation, serum exudation and hair loss. Crusting and darkening of the hair are common effects.

Chronic form: Variously thickened skin on the withers, neck and tail head. Chronic hair loss with coarsening of the hair coat quality in affected areas. The tail may develop a rat tail appearance.

Diagnosis is reasonably straight forward with characteristic clinical signs and seasonality often with a history of slow deterioration. Other parasites will need to be eliminated from the investigation (Lice, mange mites, oxyuris equi worms, onchocerca cervicalis filaroid worm) generally by treatment of some kind (Ivermectin wormers or washes) and the actual biting fly may need to be identified by use of night lights and meshes, since other fly bites can equally cause the hypersensitivity - Tabanus (Horse flies) Stomoxys (Stable flies), Simulium (Black fly) and Haematobia (Buffalo fly) again to identify appropriate treatment and control measures. Biopsy (to identify allergic eosinophil cells) has been used on occasion where signs are atypical but is generally not necessary. Finally, allergy testing is now being used more commonly, this can take the form of a blood sample or skin tests with the former being more usual. The aim with these tests is to identify the various allergen or allergens causing the reaction. Whilst the testing is reported to give reliable positive results their main drawback is that they tend to identify a lot more than one causal allergen making the follow up desensitisation protocols complicated.

Individual horses can be treated daily with antihistamines but the results are disappointing. For longer lasting effects in seriously affected horses long acting steroids can be administered intramuscularly at 3-4 week intervals. Prolonged usage is not advisable since there small but significant risks. Daily treatment with oral steroid is during the worst of the season is more preferable and can reduce the amount of itching to a minimal but again secondary effects from prolonged use must be considered. Treatment of unrugged, grazing horses is not just extremely difficult but virtually impossible, applications of a light-oil (water and veg oil) to the back of the horse may provide some relief. Success with various insect repellants has been claimed but the applications do need to be frequent, minimum weekly and more so if rain or climatic/vegetation conditions necessitate. Immunotherapy and hyposensitisation with specific allergens have shown some success and although most testing will throw up many more allergens than is likely the horse is suffering from you have no choice but to try and eliminate them all. This can be by feed or bedding withdrawal although the usual treatment therapy is a series of small (0.1ml), gradually reducing concentration injections, given over a 8-26 week course with then a maintenance dose given if clinical signs require it. Little testing has been done on the results but the “selling companies” claim a minimum of 60% improvement (not cure). How this is measured is difficult since the effects cannot be assessed until the following season although many owners do claim good improvements in both skin, respiratory and in the general well-being of the horse. A final consideration to point out is blood testing (rather than skin testing) ideally needs to be done during the time the horse is actually reacting to the stimuli so that the allergen is at its highest measurable level, and production of the treating hyposensitising agents takes up to 3 months.

The most important control measure is the protection of the horse against further contact with Culicoides. Unless this can be accomplished, all other measures are likely to be unsuccessful. Stabling in a protective environment between dawn and mid-morning and mid-afternoon and dusk combined with rugging with sheets and hoods may prevent serious skin damage. Making an stable insect proof is the ideal but rarely possible. Culicoides seem unable to cope with air speeds over 5-6km/hr so moving the affected horse to a windy area away from the water courses and woodlands is sometimes the best measure. Equally individual horses may be sensitive to a particular species of Culicoides so simply moving the horse away from the area may be a last resort.

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