Folliculitis is a common skin disease in the horse. It can be caused by a number of etiologies — including infections with bacteria (Staphylococcus, Streptococcus or Dermatophilus), fungi (dermatophytes), and parasites (Demodex), or by autoimmune diseases (pemphigus foliaceus). Folliculitis is an inflammation of the hair follicle with accumulation of inflammatory cells within the lumen of the follicle. If the inflammation results in degeneration of the follicular wall with associated inflammation beyond the confines of the follicle and into the surrounding der-mis and subcutis, the lesion is referred to as furunculosis.
Clinical signs of folliculitis include papules, serum exudation, crusts, alopecia, and easily epilated hairs. Nodular lesions and draining tracts are lesions that suggest furunculosis. Instances of infectious folliculitides due to staphylococcal pyoderma, dermatophilosis, and dermatophytosis are the most common in the horse.
Staphylococcal Bacterial Folliculitis
Bacterial folliculitis due to gram-positive staphylococcal species is seen worldwide. Other less specific synonyms for this skin disease include acne, summer rash, summer scab, sweating eczema, and saddle scab. No known age, sex, or breed predisposition exists, but horses that are poorly groomed appear to be at increased risk to develop this condition. Incidence usually increases in the spring and summer months, when humidity is higher and horses are shedding their hair coats. Localized disturbance to the normal skin barrier predisposes to infection. Maceration of skin by moisture (water or sweat) and frictional trauma contribute disruption of the normal skin barrier. The areas most at risk are under the saddle and tack, the cervical region, and the distal limbs. The most common bacterial isolates are Staphylococcus aureus and Staphylococcus intermedius. Staphylococcus hyicus is less commonly isolated and may be cultured from lesions of pastern folliculitis. The antimicrobial susceptibility patterns between these three staphylococcal species in the horse are not reported to be substantially different.
Pastern dermatitis — also called scratches, cracked heal, grease heal, mud fever, or mud rash — is a dermatitis that occurs after trauma to the palmar or plantar aspects of the pastern (see “Pastern Dermatitis”). Chronically moist environmental conditions, long hair, and ectoparasites (mites, Pelodera, or Strongyloides larvae) also can contribute to the development of this localized form of folliculitis. Infection with bacteria other than Staphylococcus spp. and with fungal organisms can occur in pastern folliculitis. Pastern folliculitis also often has associated furunculosis, and scar tissue can form because the infection is often deep.
Staphylococcal Bacterial Folliculitis: Clinical Signs
Early clinical lesions are often more easily palpated than visualized and are characterized by small 2- to 3-mm in diameter foci of erect hairs that project from papules. Lesions can increase in diameter up to 1 cm. Papules often develop central ulceration with a purulent or serosangineous exudate that results in crust formation. As lesions heal, they become flattened and develop alopecia. Expanding areas of circular alopecia and scale may develop as the infection progresses. Hairs in affected areas may epi-late more easily. These lesions are often painful, and horses may resent palpation and saddling. Pruritus is uncommon but, if present or severe, possible concurrent hypersensitivity reactions should also be considered. Nodules with draining tracts and the lesions that indicate furunculosis are much less common but can result in scarring with leukoderma and leukotrichia.
Diagnosis of Staphylococcal Bacterial Folliculitis
Differential diagnoses include dermatophytosis, dermatophilosis, other bacterial pyodermas, and folliculitis (Streptococcus spp., Corynebacterium spp.), demodicosis, and pemphigus foliaceus. Diagnosis is made based on compatible history (time of year, management concerns such as poor grooming), physical examination findings, and cytologic evaluation of impression smears of any exudate. More challenging cases that have failed to respond to appropriate therapy may require skin biopsies for bacterial and/or fungal culture and for histologic examination.
Treatment of Staphylococcal Bacterial Folliculitis
Treatment should include removal or control of any underlying precipitating factors such as poor-fitting or unclean tack or inadequate grooming. Some cases of staphylococcal folliculitis are self-limiting and resolve within a few weeks without any therapy. Mild cases may respond to topical therapy with povidoneiodine or chlorhexidine-based shampoos. More severe or chronic cases may require systemic antibiotics. In general, most Staphylococcus spp. often resist penicillins and tetracyclines. Potentiated sulfonamides are the antibiotic of choice. Trimethoprim/sulfonamide at a dose of 30 mg/kg every twelve hours for 3 to 4 weeks usually resolves staphylococcal folliculitis. Treatment should be continued for 7 to 10 days beyond clinical cure.
Prognosis of Staphylococcal Bacterial Folliculitis
The prognosis for staphylococcal folliculitis is good, but when it is localized to the pastern, staphylococcal folliculitis can be more difficult to resolve.