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lesions coalesce and may produce large areas of ulceration and suppuration, which may be quite painful. The disease is usually not associated with systemic signs and the general health of the horse is not affected. Important differential diagnoses are vasculitis, dermatophytosis, photosensitisation (in white-coloured legs), chronic progressive lymphoedema (in Belgians and other draught breeds), pemphigus foliaceus and chorioptic mange. Both


methicillin-resistant, coagulase-negative


staphylococci and coagulase-positive staphylococci (usually Staphylococcus aureus) have been cultured from horses worldwide (Yasuda et al. 2000; Busscher et al. 2006; Vengust et al. 2006). On occasion the same strain of methicillin-resistant S. aureus has been cultured from both horses and veterinary personnel (Weese et al. 2005a,b, 2006). A complete discussion of the zoonotic potential of staphylococci is beyond the scope of this article; the reader is referred to the cited references. The antibiotic usually used for many bacterial skin


Fig 4: Circular alopecia with crusting due to Staphylococcus aureus infection in a 16-year-old Quarter Horse gelding.


species S. pseudintermedius and S. delphini were cultured from a skin lesion of a horse (Devriese et al. 2005). Recent work would tend to suggest that what was previously termed S. intermedius is more correctly identified as S. pseudintermedius, and that S. delphini may be the most common staphylococcus species isolated from horses. Clinical signs of staphylococcal pyoderma are most often


crusts, usually in a circular pattern suggestive of dermatophytosis (this may be the reason that equine pyodermais under-diagnosed), epidermal collarettes (circular skin lesions with an exfoliative border as seen in dogs with superficial pyoderma) or encrusted papules similar to the miliary dermatitis reaction pattern in cats (White 2005; Fig 4). These infections tend to be variable in their intensity of pruritus. Histology usually shows folliculitis and/or furunculosis, but bacterial colonies are not always seen (Scott and Manning 1980). A truncal form of bacterial folliculitis (contagious acne, contagious pustular dermatitis, Canadian horsepox) is often associated with poor grooming and trauma from tack and saddle, warm wet weather and heavy work. It is painful and interferes with working and riding. It is usually caused by a coagulase positive Staphylococcus species but may also be caused by Corynebacterium pseudotuberculosis (Scott and Manning 1980; Heffner et al. 1988) although this organism is more commonly a cause of deep pyoderma. Folliculitis often develops in the saddle and lumbar region, particularly in the summer. The affected area initially may be swollen and very sensitive; this is followed by formation of follicular papules and pustules. These may become confluent or rupture, forming plaques and crusts. Staphylococcal pyoderma of the tail has been linked to pruritus due to an underlying hypersensitivity, ectoparasites, or abnormal behaviour (Scott and Miller 2011). Another presentation is pastern bacterial infection (pastern


folliculitis). Again, the causative agent is usually a coagulase-positive Staphylococcus species. The lesions are usually limited to the posterior aspect of the pastern and fetlock regions; one or more limbs may be involved. The initial lesions consist of papules and pustules. If left untreated, the


infections in the horse is trimethoprim-sulfa per os (30 mg/kg bwt q. 12 h for 2–6 weeks, longer for deep infections) (White 2005). Interestingly, dosing intervals for i.v. administration of trimethoprim-sulfamethoxazole in horses may not be appropriate for use in donkeys or mules. Donkeys eliminate the drugs rapidly, compared with horses (Peck et al. 2002). In cases of Staphylococcus sp. resistance to TMS, enrofloxacin (Baytril)10 or doxycycline may be used. Doxycycline is less expensive, but associated with a higher incidence of colic. Dosage is usually 10 mg/kg bwt q. 12 h although a more recent report suggests that 20 mg/kg bwt q. 24 h may also be used (Davis et al. 2006). Off-label usage of the oral enrofloxacin formulations for


poultry, ruminants or swine has been suggested; a dose for the poultry formulation has been suggested as 7.5 mg/kg bwt per os once daily. These formulations are not available in all countries. Use of enrofloxacin in young horses (age <2 years) should be avoided, due to concerns of damage to the articular cartilage (Egerbacher et al. 2001). A report of the usage of an oral gel formulation of enrofloxacin (100 mg/ml of gel) showed good clinical efficacy for infections in several organs; however, almost one-third of the horses had some diarrhoea, and 10% developed oral lesions. The article stated that this latter side effect could be overcome with following administration with a tap water rinse of the oral cavity (Epstein et al. 2004). However, the author has seen oral lesions and a swollen tongue, suggestive of a contact reaction, develop in a horse that was receiving the oral water rinse. Ceftiofur sodium (Naxcel)7 2.2 mg/kg bwt, q. 12–24 h, i.m.


or i.v., may also be used, although its usefulness over a long period is limited by its parenteral route. For localised lesions, mupirocin ointment (Muricin)8 or silver


sulfadiazine cream (Silvadene)13 may be effective. As shampoos, ethyl lactate (Etiderm)3 or chlorhexidine-tris-EDTA (4%; TrizChlor4)8 are helpful. Recently, a gel with 0.4% stannous fluoride (MedEquine Gel)14 was reported as resolving bacterial skin infections in horses (Marsella and Akucewich 2007). Topical fusidic acid, not available in the USA, has also been used.


Infections: fungal


Dermatophyte infections, like pyoderma, can be variably pruritic. The most common equine dermatophyte


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