Equine dermatology.

Bacterial folliculitis (superficial pyoderma) is usually caused by a coagulase positive Staphylococcus species. Both S. aureus and S. intermedius have been isolated. In one study, S. aureus accounted for twice as many isolates as S intermedius; the same study isolated some strains of S. hyicus as well. Interestingly, in another study, lysozymes from equine neutrophils were only slightly bactericidal for S. aureus. Many isolates are resistant to penicillin G. Occurrence of pyoderma has been linked to poor nutrition and husbandry in some cases. Clinical signs of staphylococcal pyoderma are most often crusts, usually in a circular pattern suggestive of dermatophytosis (this may be the reason that equine pyoderma is underdiagnosed), epidermal collarettes (circular skin lesions with an exfoliative border as seen in dogs with superficial pyoderma; Figs. 1 and 2), or encrusted papules similar to the miliary dermatitis reaction pattern in cats. 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. A truncal form of bacterial folliculitis (contagious acne, contagious pustular dermatitis, or Canadian horsepox) is often associated with poor grooming, 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. This organism is more commonly a cause of deep pyoderma, as discussed below (Fig. 3). In horses, 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