Pruritus is the most common manifestation of skin disease in the horse. Pruritus is exhibited in a number of ways including the obvious scratching, rubbing, chewing and biting, but also in more subtle fashion such as head shaking, foot stamping or "irritability". When a clinician begins the work up for a pruritic horse, the history should include questions such as length of time of the skin disease; does the condition appear to be contagious? Seasonal? Recurring? Are multiple horses affected? The most common causes of pruritus in horses include hypersensitivity reactions, bacterial and fungal infections, infestations with mites or lice, and irritation or hypersensitivity reactions caused by biting insects. This lecture will cover only some of the more common causes of pruritus in horses seen by this author.

Cutaneous infections (folliculitis)

Two common infections of the skin include bacterial folliculitis and dermatophytosis which may also involve hair follicles. Many different bacteria are known to infect or colonize the skin, and most infections will stimulate some degree of pruritus, however Staphylococcus species accounts for much of the true skin infections seen in horses. Clinically lesions can be localized or widespread and start as a papular to pustular dermatosis which can progress into serous our hemorrhagic crusting lesions. Alopecia is also common and may produce a "moth eaten" appearance to the coat. A Staphylococcal infection may be secondary to other underlying dermatopathies, particularly hypersensitivity reactions. Cytology of lesions (crust or pustules) revealing intracellular coccoid bacteria will confirm the diagnosis. Culture and sensitivity testing is normally only performed if the patient fails to respond to appropriate systemic antibiotics. Unfortunately methicillin resistant Staphylococcal infections are becoming increasingly common around the world in both veterinary and human medicine. At the Veterinary Microbiological Diagnostic Center, the Netherlands, the percentage of methicillin-resistant Staphylococcus aureus (MRSA) isolates found in equine clinical samples increased from 0% in 2002 to 37% in 2008. Their study found that nosocomial transmission occurs in equine clinics and that personnel played a role in the transmission. An increased awareness of this epidemic should motivate all veterinary personnel to utilize more complete sanitation practices between handling patients, especially hand sanitation with frequent washing and antiseptic rinses or gels. If a patient with folliculitis is failing to respond to appropriate, empirically chosen antibiotics, then a resistant strain should be suspected and cultures of an intact pustule, or fresh exudate underneath a crust should be obtained. Topical therapy is also useful when treating superficial skin infections. Chlorhexidene shampoos, mupirocin ointment and 0.4% stannous fluoride gel all have efficacy, especially for localized infections, or as adjunctive therapy with systemic antibiotics. The author commonly examines horses with bacterial folliculitis which have been treated with antibiotics for seven to ten days, instead of the necessary 21 days.

Microsporum and Trichophyton are the two most common genus of ringworm in the horse. Clinically lesions are similar to bacterial induces lesions, although the sites affected are most commonly at points of friction or under saddle or tackle. Cytology (trichogram) of infected hairs is difficult to perform, and most patients are diagnosed with Dermatophyte cultures. Sabouraud's agar is required to recover some of dermatophyte species which infect horses, such as Trichophyton equinum. Sab-Duet™ (Hardy diagnostics) are ideal culture plates since they contain both Sabouraud's on one side, and a DTM on the other. Others advocate placing 1-2 drops of an injectable multi-B vitamin on the media. Due to the large number of saprophytic mold spores on the coat of horses, cleaning the hair coat prior to sampling is recommended. Wiping the coat clean with alcohol, or even washing the site with a gentle shampoo or detergent is recommended to minimize the numbers of contaminants on the culture. Because many cases spontaneously resolve, therapy may be conservative and limited to topical antifungal products, many of which are available in shampoo, spay, ointment and rinse formulations.