Equine Dermatology: I. Diagnosis and Treatment of the Pruritic Horse

Pyoderma (Bacterial Skin Infections)

1. Introduction

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.

Figure 1. Staphylococcal folliculitis: crusts in a circular pattern. (Courtesy of Elsevier Publishing.)

Figure 2. Staphylococca lfolliculitis: widespread, coalescing areas of alopecia and scaling. (Courtesy of Elsevier Publishing.)

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.

Figure 3. Corynebacterium pseudotuberculosis folliculitis: circular areas of crust and alopecia. (Courtesy of Elsevier Publishing.)

2. Public Health Considerations-Staphylococcus spp.

In a 2000 study, methicillin-resistant coagulase-negative staphyloccal species were cultured from healthy horses in Japan; Yusada et al [8]. concluded that "these organisms must be considered a potential threat to horses and veterinarians who care for them." In a 2006 study from the Netherlands, methicillin-resistant coagulase-negative staphylococci were found frequently [9]. The organism was usually S. sciuri, not S. epidermidis, which was found in the humans in close contact with these horses. No me-thicillin-resistant S. aureus (MRSA) was found in healthy horses.

In contrast, a single strain of MRSA was isolated from both humans (13%) and horses (4.7%) on horse farms in Canada and New York state [10]. In looking at horses admitted to a university teaching hospital (Ontario Veterinary College, University of Guelph, Guelph, Ontario, Canada), MRSA was isolated from 120 (5.3%) of 2,283 horses. Of these 120 horses, 50.8% were positive at the time of admission, and clinical infections attributable to MRSA were present or developed in 14 horses. Horses colonized at admission were more likely to develop clinical MRSA infection. Administration of ceftiofur or aminoglycosides during hospitalization was the only risk factor associated with nosocomial MRSA colonization. Another strain of MRSA was isolated from a small number of horses at the Veterinary University in Vienna, Austria [11].

Of most concern is the finding of humans reporting skin lesions after contact with a community MRSA-positive affected foal, despite short-term contact with standard protective barriers. The isolates from the foal were indistinguishable from the ones from the humans [12].

Figure 4. Pastern folliculitis. (Courtesy of Elsevier Publishing.)

3. Treatment of Equine Pyoderma

The antibiotic usually used for many bacterial skin infections in the horse is trimethoprim sulfa orally (30 mg/kg, q 12 h for 2 - 6 wk, longer for deep infections) [6]. Interestingly, dosing intervals for IV administration of trimethoprim-sulfamethoxazole in horses may not be appropriate for use in donkeys or mules. Donkeys eliminate the drugs rapidly compared with horses [13]. In cases of Staphylococcus sp. resistance to trimethoprim-sulfa drugs, enrofloxacin may be used. Use of enrofloxacin in young horses (<2 yr old) should be avoided because of concerns of damage to the articular cartilage [14]. A recent report [15] on the use 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 diarrhea, and 10% had oral lesions. Epstein et al [15]. felt that this latter side effect could be overcome with administration of tap water rinse of the oral cavity. Interestingly, enrofloxacin binds to melanin in equine hair, although the clinical implication is unknown [16]. In one report of 15 horses, vancomycin was used, alone or in combination with an aminoglycoside, to treat MRSA and enterococcal infections. The average vancomycin dosage was 7.5 mg/kg,q8h,IV over 30 min. The antibiotic, alone or in combination with an aminoglycoside, was safe and effective. Because of the problems with emerging resistance, Orsini et al [17]. recommended that the use of vancomycin in horses be limited to cases in which culture and susceptibility indicate effectiveness and no reasonable alternative treatment is available.

For localized lesions, generic mupirocin ointment 2% or silver sulfadiazine cream [a] may be effective. Shampoos such as ethyl lactate [b] or chlorhexidine (2% - 4%) are helpful.

Dermatophilosis is caused by an actinomycete bacteria Dermatophilus congolensis. Three conditions must be present for Dermatophilus to manifest itself: a carrier animal, moisture, and skin abrasions. Chronically affected animals are the primary source of infection. However, they only become a serious source of infection when their lesions are moistened. This results in the release of zoospores, the infective stage of the organism. Mechanical transmission of the disease occurs by both biting and non-biting flies and possibly, fomites. Because normal healthy skin is quite impervious to infection with D. congolensis, some pre-disposing factor that results in decreased resistance of the skin is necessary for infection to occur; prolonged wetting of the skin by rain is one of the most prevalent causes.

The disease is usually seen during the fall and winter months with the dorsal surface of the animal most commonly affected. Occasionally, the lesions involve the lower extremities when animals are kept in "wet pastures" ("dew poisoning") or if horses are left in the stall while the stall is cleaned with high-pressure water hoses. In the early stages of the disease, the lesions can be felt better than they can be seen. Thick crusts can be palpated under hair coat (Fig. 5). Removing the crusts and attached hair exposes a pink, moist skin surface with both the removed hair and the exposed skin assuming the shape of a "paintbrush." The under surface of the crusts are usually concave with the roots of the hairs protruding.

Figure 5. Dermatophilosis: severe scaling and alopecia. (Courtesy of Dr. V. Fadok and Elsevier Publishing.)

Diagnosis is made by the "railroad track" cocci on impression smears: a portion of one of the crusts should be minced and mixed with a few drops of sterile water on a glass slide, gram stained, and examined microscopically (Fig. 6). Alternatively, bacterial culture or histopathology may be used for diagnosis. A thick crust composed of alternating layers of parakeratotic stratum corneum, dried serum, and degenerating neutrophils is the most characteristic change. A superficial folliculitis may be a prominent feature of the disease [1]. In sections stained with gram stain, the branching, filamentous organisms can be observed in the crusts and in the follicles. Treatment is removal from the wet environment, removal of crusts (with care because these may be painful), washing with iodophors or lime sulfur, and use of antibiotics (penicillin at 22,000 mg/kg procaine pen G, q 12 h, IM or trimethoprim sulfa orally with the same dosage used for staphylococcal pyoderma) for 7 days [18]. As the crusts are important in contagion, these should be disposed of rather than brushed on to the ground.

Figure 6. Dermatophilosis: branching chains of cocci ("railroad tracks") modified Wright’s stain times 100. (Courtesy of Dr. V. Fadok and Elsevier Publishing)

Figure 7. Dermatophytosis: circular alopecia and scaling caused by Trichophyton Mentagrophytes Infection. (Courtesy of Elsevier Publishing.)

4. Dermatophytes and Malassezia

Dermatophyte infections, like pyoderma, can be variably pruritic. The most common equine dermatophyte species isolated from horses are Trichophyton equinum, M. equinum, T. mentagrophytes, and T. verrucosum [1,3,19]. Tack (bridles, halters, and saddle blankets) often act as fomites. The lesions usually appear first on the axillary/girth area and may spread over the trunk, rump, neck, head, and limbs (Fig. 7). Initial lesions may be urticarial in nature and can progress to multi-focal, sharply demarcated scaling and crusting areas (Figs. 8 and 9). Lesions may be superficial or deep. Superficial infections are more common and are manifested by the development of thick crusts or more generally, a diffuse moth-eaten appearance with desquamation and alopecia. Less commonly, deeper structures are infected through the hair follicles, which causes small foci of inflammation and suppuration. A small crust forms over the follicle, and the hair is lost. However, extensive alopecia and crust formation do not occur; some irritation and itching may be caused by this type. Rarely, dermatophytosis may be limited to the coronary band (Fig. 10).

Figure 8. Dermatophytosis: urticarial lesions caused by Tricho-phytonmentagrophytesinfection. (Courtesy of Elsevier Publishing.)

Diagnosis is by fungal culture; biopsy is less reliable (Trichophyton species may cause acantholysis, which mimics pemphigus on histopathology) [20]. Hair is the specimen most commonly collected for the isolation of dermatophytes. Using forceps, hairs should be selected that appear broken, espe-cially at the advancing periphery of an active, non-medicated lesion. In addition, surface keratin may be gathered by forceps or skin scrapings from similar areas and inoculated onto the culture medium.

Figure 9. Dermatophytosis: urticarial lesion caused by Tricho-phyton Mentagrophytes Infection that transitions into a circular area of alopecia. (Courtesy of Elsevier Publishing.)

Figure 10. Dermatophytosis: scaling of the coronary band caused by Microsporum Gypseum Infection. (Courtesy of Dr. V. Fadok and Elsevier Publishing.)

The hair and surface keratin of large animals have large numbers of saprophytic fungi and bacteria. Therefore, it is recommended by some clinicians to cleanse the skin before taking samples for culture. This may be done by gently cleansing the area to be sampled with water and allowing it to air dry, although the authors do not routinely do this.

Sabouraud’s dextrose agar has been used traditionally in veterinary mycology for the isolation of fungi; however, other media are available with bacterial and fungal inhibitors, such as dermatophyte test medium (DTM). DTM is essentially Sabouraud’s dextrose agar containing cycloheximide, gentamicin, and chlortetracycline as antifungal and antibacterial agents and to which the pH indicator phenol red has been added. Dermatophytes use protein in the medium first, and alkaline metabolites turn a medium red. Most other fungi use carbohydrates first and give off acid metabolites, which do not produce a red color change. These saprophytic fungi will later use the protein in the medium, resulting in a red color change. However, this usually occurs only after a prolonged incubation (10 - 14 days or more). Consequently, DTM cultures should be examined daily for the first 10 days. Some Aspergillus species and others cause a red color change in DTM, and therefore, microscopic examination is essential to avoid an erroneous presumptive diagnosis. It has been recommended that 1 - 2 drops of a sterile injectable B complex vitamin preparation be added to culture plates when culturing horses, because one strain of T. equinum (T. equinum var. equinum) has a unique niacin requirement.

However, the authors do not routinely do this. Skin scrapings and hair should be inoculated onto Sabouraud’s dextrose agar and/or DTM and incubated at 30°C with 30% humidity. A pan of water in the incubator will usually provide enough humidity. Cultures should be checked every day for growth. DTM may be incubated for 21 days, but cultures on Sabouraud’s agar should be allowed 30 days to develop. The authors usually use DermDuet [c], which has DTM on one side, rapid sporulating media (RSM) on the other side, and a well of water in the center. It is routinely incubated at room temperature. T. verrucosum has been reported not to grow on DTM [21].

Topical treatment alone is often curative. Although 50% captan (2 tablespoons of the powder in 1 gallon of water) has been touted in the past, and while certainly safe for tack, its effectiveness has been questioned. Lime Sulfur [d] (1 cup to 1 gallon of water) or bleach (1:10 with water) are both effective but messy and odiferous. Miconazole or ketoconazole veterinary shampoos are becoming more widely used and may be as effective. In Europe and Canada, an enilconazole rinse [e] is highly effective.

Systemic treatment is occasionally needed. Griseofulvin’s efficacy in horses (as well as an effective dose) has not been thoroughly researched. However, a dosage of 100 mg/kg daily for 7 - 10 days has been advocated and has been used with good success on a small number of horses by the authors. Griseofulvin is a teratogen and should not be used in pregnant mares. Additionally, it is no longer available. Alternatively, 20% NaI may be given IV (250 ml/500 kg horse every 7 days, 1 - 2 times). This also is contraindicated in pregnant mares, because it may cause abortion. Although medications such as itraconazole and fluconazole have been used to treat horses with systemic mycotic infections such as coccidioidomycosis and aspergillosis, there have not been any studies on their effectiveness in dermatophytosis. However, the safety record in horses in the face of the doses used (2 - 5 mg/kg, q 12 h) are encouraging [22-24]. Vaccination to T. equinum may reduce the incidence of new infections and protect a high percentage (>80%) of vaccinates from infection. This data is based on results with an inactivated vaccine containing both conidia and mycelial elements [25].

The exact species of Malassezia growing on horses’ skin is just beginning to be investigated [26]. In one study, the Malassezia sp. isolated were identified as M. furfur, M. slooffiae, M. obtusa, M. globosa, and M. restricta [27]. The authors have examined several mares with Malassezia infections between their mammary glands that were intensely pruritic. The mares rubbed their tails and ventral abdomens. Physical examinations showed dry, greasy-to-thetouch exudate. Cytology of the exudate showed numerous yeast organisms, which were identified on culture as Malassezia species (Fig. 11). Treatment with a topical 2% miconazole/chlorhexidine shampoo was curative. The authors are aware of other similar cases. However, healthy non-pruritic mares may also have large numbers of yeasts in the intramammary area [28].

Figure 11. Cytology of Malasseziasp. from intermammary debris from a healthy mare. (Courtesy of Elsevier Publishing.)

Footnotes

[a] Silvadene, Monarch Pharmaceuticals, Inc., Bristol, TN 37620.
[b] Etiderm, VIRBAC, Ft. Worth, TX 76137.
[c] DermDuet, Bacti-Labs, Mountain View, CA 94042.
[d] LymDyp, Miami, FL 33169.
[e] Imaveral, Janssen-Cilag Animal Health 1 rue Camille, Desmoulins, France.