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6. Allergen specific immunotherapy (hyposensitisation) is controversial; success may vary with the presence of an adjuvant, or the actual antigen used.
7. Prednisolone (1 mg/kg bwt orally to effect, then taper) to manage the pruritus
8. Occasionally, topical medications may be helpful. Those reported as being effective in reducing the pruritus are oatmeal-based shampoos or washes, and 2 corticosteroid sprays: Genesis Spray (0.015% triamcinolone acetonide)3 and Cortavance Spray (0.0584% hydrocortisone aceponate)4.
The author has not found antihistamines to be of
substantial benefit in treating pruritus due to insect bites. In the UK an oral product containing niacinamide, accompanied by a aloe-containing gel (Cavalesse, fidavet)5 is available for the treatment of ‘summer itch’. The author has no experience with this product. Other flies such as stable fly (Stomoxys calcitrans) 6–8 mm;
horn fly (Haematobia sp.) 3–4 mm; horse fly (Tabanus sp.) and deer fly (Chrysops sp.) as long as 25 mm; and black fly (Simulium sp.) 1–5 mm, often cause a pruritic dermatitis in horses. Stomoxys calcitrans has been reported to cause an exudative dermatitis on the legs of mature horses (Yeruham and Braverman 1995). Fly control and good sanitation (stable flies lay eggs in rotting vegetation, and horn flies lay eggs in faeces) are important control measures. Stable flies ‘like’ sunlight, whereas horn flies do not. Horn flies may produce a focal pattern of ventral midline dermatitis. The use of parasitic wasps (Fly Predators)6 has been recommended as a way to control stable, horn, house and other flies. Simulium spp. (black flies) occur in practically all parts of
the world especially in regions with warm climates. The flies are dark coloured, vary in length from 1–5 mm, and have a rather characteristic appearance with the thorax humped over the head. They are thus often referred to as buffalo gnats. The flies are most active in the morning and late afternoon causing considerable annoyance and irritation. The females suck blood and produce small skin wounds that are quickly covered with droplets of dried blood. The usual sites include the udder, scrotum, prepuce, inner surface of the thigh and upper forelimbs, throat, ears, ventral abdomen, chest, and natural body orifices. It is likely that various species of black fly have a favoured site on which to feed. Black flies may produce a generalised pattern of ventral midline dermatitis. Urticaria and angioedema may be a prominent sign when larger numbers of flies attack an animal. There may be considerable pruritus leading to excoriation. It seems likely that a hypersensitivity reaction plays a role in the pathogenesis of the skin lesions. Generalised signs only occur when large numbers of flies attack an animal and are evident either immediately or within a few hours of the attack. Affected animals are weak and listless with a rapid pulse and respiration rate. The body temperature is normal or subnormal; elevation appearing only in cases that are advanced or terminal. Pregnant animals may abort. In severe cases death may occur within a few hours to several days.
Horse and deer fly bites are usually swellings with a centre
of blood or haemorrhagic crust, whereas the swellings caused by mosquito bites lack the centre of blood or crust. Treatment of the skin lesions is entirely symptomatic. The affected areas should be gently cleansed to remove the
© 2015 EVJ Ltd
Fig 2: Crust, lichenification and erythema caused by Chorioptes infestation in a 13-year-old Shire gelding.
crusts. If the skin lesions are confined to a small area, a topical corticosteroid preparation may be applied twice daily until the pruritus is controlled, at which time the frequency of application may be reduced. If large areas of skin are involved or the pruritus is intense, a short course of oral prednisolone (1 mg/kg bwt/day × 1 week, then 1 mg/kg bwt q. 48 h × 2 weeks) is indicated. A major objective should be the prevention of further fly bites. As simulids are primarily daytime biters, horses should be stabled during the day preferably in screened stalls. Insect repellents should be applied especially to sites of predilection. It is important to remember the relationship of these various
insects to the environments they favour/require for breeding and when they feed:
Horse flies, deer flies
Horn flies
Stable flies Black flies
Culicoides Mosquitoes vegetation and water daytime feeders cattle
rotting vegetation running water
standing water, manure/vegetation water
dusk to 2 h past sunset
Chorioptes mites are most frequently found on the lower limbs (pasterns, fetlocks) of draught type breeds. This predisposition seems to be inherent in these breeds, not merely because of the long ‘feathers’ present on the legs. Pruritus is the hallmark of this disease, with secondary crusts and alopecia (Fig 2). The mites live on the skin, and are not always as easy to find with skin scrapings as the literature would suggest. Numerous scrapings may need to be performed, and the clinician should not hesitate to treat the horse on a presumptive basis if the clinical signs are suspicious. These are relatively large mites (0.3–0.5 mm) and, if present, can often be found using the 4×, and certainly the 10× power objective. Treatments reported as
daytime feeders daytime feeders
morning and evening
twilight to dawn
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