Bacterial folliculitis (superficial pyoderma) is usually caused by a coagulase positive Staphylococcus species. Both S. aureus and S. intermedius have been isolated.1,2 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.3 Interestingly, in another study, lysozymes from equine neutrophils were only slightly bactericidal for S. aureus.4 Many isolates are resistant to penicillin G3. Occurrence of pyoderma has been linked to poor nutrition and husbandry in some cases.5 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.6 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.7 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 plaques and crusts.
Deep pyoderma followed by ulceration may develop over large areas of the body, especially on the neck, sides of the thorax, inner surface of the thighs, or the prepuce. A pastern bacterial infection (pastern folliculitis) is often seen. Again, the causative agent is usually a coagulase positive Staphylococcus species. As with most “primary pyodermas,” the mechanism(s) whereby the organism gains its foothold is unknown (not contagion and not poor sanitary conditions). 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 (Fig. 4). If left untreated, the 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. A relatively uncommon nodular disease termed “botryomycosis” mimics actinomycosis or a deep fungal infection, but it is most often caused by Staphylococcus species in the horse. These may require surgical excision as well as long-term antibiotics.