EQUINE VETERINARY EDUCATION / AE / MARCH 2015


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effective include: ivermectin 0.2 mg/kg bwt orally q. 14 days for 6 weeks; doramectin (Dectomax)7 0.3 mg/kg bwt subcut. q. 14 days × 2; topical lime sulfur (LimePlus Dip)8, 1 cup to 1 gallon of water q. 48 h for 6 weeks, or 50 ml in one litre q. 7 days × 4; or topical fipronil (Frontline)9 spray (Littlewood et al. 1995; Littlewood 2000; Rendle et al. 2007; Paterson and Coumbe 2009). A 500 ml bottle of fipronil is sufficient to treat the legs of one horse; the clinician must spray to dampen haircoat and skin from elbow and stifles down, and treat affected cases twice at a 3–4 week interval (this can be expensive). Regardless of the treatment used, there is a high risk of asymptomatic carriers, so all in-contact horses should be treated at least once. The environment should be thoroughly cleaned as well, as the mites have been reported to survive off the horse. A study showed efficacy using a single dose of moxidectin 2% oral gel (Quest)10 at the manufacturer’s recommended therapeutic dose of 0.4 mg/kg bwt (Osman et al. 2006).


Lice (biting lice: Damalinia equi, Werneckiella equi equi; sucking louse: Haematopinus asini) may cause pruritus. These wingless insects (3–6 mm) typically cause a winter problem due to the close confining of a number of horses in less than ideal sanitary conditions. As the lice numbers increase, the pruritus increases as well. Diagnosis is usually by finding the lice or the lice eggs (nits; 1–2mm long) on the horse; these can often be identified on visual inspection, although a magnifying glass may be helpful. Sucking lice tend to be stationary, whereas biting lice tend to be mobile. D. equi has been reported as preferring the dorsolateral trunk, whereas H. asini favours the mane, fetlocks and tail. Topical lime sulfur, permethrin or fipronil, or oral ivermectin, should be effective; some clinicians feel that ivermectin is not as successful in the treatment of the biting lice. All in-contact horses should be treated.


Trombiculidiasis (‘chiggers’) is an occasionally encountered parasitic skin disease of animals and man. There is a definite geographic distribution of the parasite. The aetiological agents are the larval forms of Eutrombicula alfreddugesi (North America) or Neotrombicula autumnalis (Europe). The adult stage is a parasite of vegetation. The 6-legged larval form (0.2–0.4 mm) requires a blood meal to complete the life cycle. The larvae attach to the horse’s skin, take a blood meal and then fall off after 48–72 h. The disease occurs in the late summer, autumn and early winter, and is primarily associated with ‘undeveloped’ pastures and fields of natural vegetation. The lesions occur on the face, sides of the neck and thorax,


and the extremities. The lesions, 1–2mm crusts, can be palpated better than they can be seen. If these crusts are removed there is frequently a small amount of underlying suppuration. The disease in the horse is variably pruritic, with self-rubbing, stamping and, rarely, head shaking reported (Mair 1994). With diligent searching, the larvae, which are bright orange or red in colour and measure approximately 0.2mmin diameter can usually be seen with the naked eye in the centre of some of the lesions. Frequently most of the larvae have dropped off by the time the animal is presented for examination. Finding the minute brightly coloured larvae is diagnostic. Treatments are similar to those for lice (with some of the same controversy of efficacy attached to the use of ivermectin).


Fig 3: Atopic dermatitis causing urticaria and facial pruritus in an 8-year-old mix-breed mare.


Oxyuriasis refers to the infection of horses with the nematode parasite Oxyuris equi (‘pinworms’). The primary pathological effects of this nematode are cutaneous rather than internal. The adults are free-living in the caecum and colon, apparently feeding only on faecal material. Following fertilisation, the females migrate to the rectum and then through the anal sphincter to deposit ova in the perineal area. The eggs are ‘cemented’ together in yellowish-white gelatinous masses. The ova develop rapidly and contain infective larvae within 3–5 days. Infection of horses occurs when ova containing infective larvae are ingested. Further development takes place in the large intestine and the parasites reach sexual maturity after 5 months. The adult parasites are not usually a problem. However, the


gelatinous material holding the clusters of eggs together is highly irritating and results in intense pruritus in the anal region. Affected animals are constantly rubbing at the base of their tail in an effort to relieve the anal pruritus. The hairs become broken leading to alopecia in the region (‘rat tail’ appearance). In severe cases erosions and ulcerations of the tail head develop. Oxyuriasis should be a consideration in the differential diagnosis of a ‘tail rubber’. Horses with oxyuriasis rub only at the tail region; thus this diagnosis should not be considered in horses rubbing at both the tail and mane regions (which is more likely Culicoides hypersensitivity). A clear acetate (‘scotch’) tape impression smear of the perineal region is used to demonstrate the ova; the sticky side is placed down on a glass slide that has been coated with mineral oil. The ova are elongated, slightly flattened on one side, provided with a plug at one pole and measure about 42 × 90 μm. Pyrantel pamoate is usually effective as an antihelmentic, although both ivermectin and moxidectin have also been used.


Allergies


Atopic dermatitis is probably more common than reported. It may be seasonal or in California, often year round. Urticaria, urticaria with pruritus or pruritus alone are the 3 common presentations, in that order (Fig 3). Pruritus most commonly affects the face and trunk. Seasonal signs would be most consistent with allergy to pollens; year-round signs would be more consistent with an allergy to moulds or barn dust, or a food allergy (but food allergies affecting the skin are very rare in horses).


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