Pyoderma, Otitis Externa, and Otitis Media

Chapter 84

Pyoderma, Otitis Externa, and Otitis Media


Etiology and Epidemiology

Pyoderma is a cutaneous infection with pyogenic (pus-forming) bacteria. Although the term pyoderma literally means “pus in the skin,” the pus may not always be visible to the naked eye. Pyoderma is the one of the most common disorders of canine skin. It results from impaired local defense mechanisms, which permit secondary bacterial invasion of the skin. The most common underlying skin diseases are allergic dermatitis and endocrine diseases, but a variety of other skin diseases can also predispose to pyoderma. In cats, pyoderma is uncommon to rare, although feline acne can be classified as a pyoderma. Three broad classifications of pyoderma exist, based on the depth of skin and follicle involvement: surface pyoderma, superficial pyoderma, and deep pyoderma.

Surface pyoderma occurs when bacteria proliferate on the surface of the skin and incite an inflammatory response, without invading the skin. Surface pyodermas include the “fold” pyodermas (also known as intertrigo), “hot spots” (also known as pyotraumatic dermatitis), and mucocutaneous pyoderma, which commonly affects German shepherd dogs.1 The last probably has an immunologic as well as a bacterial etiology. Mucocutaneous pyoderma is histologically identical to intertrigo, and differentiation between the two entities must be made clinically.2 Microbial overgrowth has also been categorized as a surface pyoderma when it involves bacteria (as opposed to just Malassezia spp.).

In superficial pyoderma, bacteria infect the superficial epidermal layers that lie immediately under the stratum corneum (the outermost layer of the skin) and the portion of the hair follicle above the sebaceous duct (the infundibulum) (Figure 84-1). This is the most common type of pyoderma. It includes superficial bacterial folliculitis, superficial spreading pyoderma, and “puppy pyoderma” (also known as impetigo or juvenile pustular dermatitis).

Deep pyoderma is defined by infection deep within the hair follicle, with or without follicular rupture (furunculosis) (Figure 84-2) German shepherd dogs seem prone to a more severe and extensive form of deep pyoderma.3 Other examples include pedal folliculitis and furunculosis, pressure-point pyoderma, pyotraumatic folliculitis and furunculosis, and muzzle folliculitis and furunculosis (“canine acne”).

The most common cause of pyoderma in dogs is the coagulase-positive Staphylococcus pseudintermedius (previously misidentified as Staphylococcus intermedius) (see Chapter 35 for more information on staphylococci).4 S. pseudintermedius is a normal resident of mucosal sites such as the anus and nares and is thought to colonize the skin transiently following grooming and excessive licking in dogs with pruritus.5 In contrast, the resident flora of the canine skin includes coagulase-negative staphylococci, Micrococcus spp., α-hemolytic streptococci, aerobic coryneforms, Acinetobacter spp., and some anaerobes. Other organisms implicated in canine pyoderma are Staphylococcus schleiferi and, uncommonly, Staphylococcus aureus.68 Gram-negative bacteria such as Pseudomonas aeruginosa, Proteus spp., and Escherichia coli may be isolated from dogs with deep pyoderma.9,10 These organisms are thought to invade secondary to alteration of local environmental conditions by S. pseudintermedius. Complete genome sequencing of S. pseudintermedius isolated from a dog with pyoderma has shown that S. pseudintermedius possesses a number of virulence factors that resemble those produced by S. aureus.11

Over the past 5 years, clonal spread of methicillin-resistant S. pseudintermedius has occurred across Europe and North America.12 Methicillin resistance has also been described among S. schleiferi and S. aureus isolates from dogs with pyoderma.8 These organisms encode an altered penicillin binding protein that incurs resistance to all β-lactam antimicrobials, and many also demonstrate resistance to fluoroquinolone antimicrobials.

Clinical Features

Signs and Their Pathogenesis

Surface Pyoderma

Surface pyoderma is usually caused by superficial skin trauma, such as the rubbing together of lip, facial, vulvar, and tail folds in dogs with skinfold pyoderma. “Hot spots” occur secondary to self-trauma, such as that secondary to flea allergy dermatitis, and are commonly found in the lumbosacral area. Clinical signs include erythema, variable pruritus, alopecia, moist exudation, and a foul odor. Chronic lesions may be characterized by lichenification and hyperpigmentation. The exact causes of mucocutaneous pyoderma and surface microbial overgrowth have not yet been elucidated. Mucocutaneous pyoderma is characterized by erosions and ulcerations with or without crusting, which involve the lips, nasal planum, nares, perioral skin, and sometimes the anus, vulva, prepuce, and eyelids. It has also been reported in the axillary and inguinal region.1 Microbial overgrowth syndrome usually involves the ventral aspect of the body and is characterized by erythema, pruritus, a foul odor, erythema, alopecia, hyperpigmentation, lichenification, and excoriations, but no evidence of papules, pustules, epidermal collarettes or crusts.13

Superficial Pyoderma

Early lesions of superficial pyoderma are erythematous follicular papules (papules from which a hair shaft protrudes). As pus accumulates within the epidermis and hair follicles, these lesions become pustules. Papules and pustules are primary skin lesions. When pustules rupture, crusted papules result. Epidermal collarettes are circles of epidermal scale with a free edge facing toward the center of the circle (see Figure 84-1), and may be remnants of ruptured papules. In thick-coated or shaggy dogs, the scale may become trapped throughout the haircoat as epidermal collarettes exfoliate. Hair fragments are shed from damaged follicles, which results in alopecia. Crusted papules, epidermal collarettes, and alopecia are secondary lesions.

Impetigo generally occurs in dogs less than a year of age. In impetigo, pustules are confined to the epidermis, without follicular involvement, and are most commonly found in the poorly haired (glabrous) areas on the ventral abdomen and inguinal region and around the chin. Superficial bacterial folliculitis usually occurs secondary to underlying allergic or atopic dermatitis or endocrinopathies such as hyperadrenocorticism or hypothyroidism. Pruritus is less common in dogs with underlying endocrinopathies. Other conditions that predispose to pyoderma include cornification disorders, genetic skin disorders, and ectoparasitism. In dogs with atopic dermatitis, invasion of the skin by staphylococci can in turn lead to a hypersensitivity response to staphylococcal antigens, which exacerbates the pyoderma.14

Physical Examination Findings

Because the lesions of bacterial pyoderma are directly visible, in many cases, diagnosis can be made based on a thorough physical examination. Use of a hand lens may be helpful. In some cases, clipping of small areas of the haircoat may be required. This can help to identify epidermal collarettes in dogs with dry scale. Careful attention should be paid to the presence of follicular papules, pustules, crusted papules, epidermal collarettes, alopecia, nodules, fistulous tracts, and the presence or absence of pruritus (Box 84-1). The distribution of lesions should be noted because it can be a clue to underlying disease, such as demodicosis. In dogs with deep pyoderma, the skin may feel warm to the touch, and peripheral lymphadenomegaly may be detected.


The diagnostic approach to pyoderma should involve confirmation of the presence of pyoderma and a thorough search for possible underlying causes. The differential diagnosis includes many diseases that predispose to pyoderma, such as demodicosis, dermatophytosis, and pemphigus foliaceus (see Overview). When crusting is present, cornification disorders should be considered, although crusting may also result from self-trauma. The presence or absence of Malassezia spp. must also be determined (Chapter 59).

In addition to a thorough physical examination, diagnostic procedures that may be necessary include skin scrapings, cytologic examination, aerobic bacterial culture and susceptibility, and skin biopsy (Table 84-1). Dermatophyte culture is indicated when underlying dermatophytosis is suspected (see Chapter 58). Identification and subsequent treatment of underlying allergic skin disease may require additional testing such as intradermal skin testing, for which referral to a veterinary dermatologist should be considered.

Microbiologic Testing

Cytologic Examination

Cytology can be performed on tape preparations of the skin or smears made after swabbing pustules or draining tracts. Cytologic examination is essential in order to identify concurrent infection with Malassezia pachydermatis. Slides should be air-dried and stained with a modified Wright’s stain such as Diff Quik. The presence of cocci suggests S. pseudintermedius infection (Figure 84-3). Infection is supported by the presence of degenerate neutrophils and intracellular cocci. Inflammatory cells may be absent in dogs with underlying immunosuppressive disorders or those receiving glucocorticoid treatment.

Culture and Susceptibility Testing

Given the rapidly increasing prevalence of methicillin-resistant staphylococci worldwide, aerobic bacterial culture and susceptibility testing is playing a more important role in diagnosis of canine pyoderma. Although probably not necessary for dogs with a first-time diagnosis of pyoderma, culture and susceptibility testing is never contraindicated and should always be offered in dogs with recurrent or refractory pyoderma, or in dogs that have a history of recent treatment with systemic antimicrobial drugs. Refractory pyoderma is pyoderma that fails to respond to treatment within a 3- to 4-week treatment period. Culture also can definitively identify the staphylococcal species involved, which may have public health implications (see Public Health Aspects, later). The results of culture and susceptibility testing must always be interpreted in light of the clinical signs present and any history of antimicrobial drug treatment.

The best lesions for culture in dogs with superficial pyoderma are pustules. A thorough search for pustules is recommended. If pustules cannot be detected, culture can be performed on swab specimens collected from the skin that lies beneath crusts or from epidermal collarettes. No surface antisepsis should be performed. Any hair should be clipped from the lesion using sterile scissors, and crusts should be lifted with sterile forceps. Pustules can be ruptured using a sterile needle, and a swab used to obtain purulent material. Papules can be biopsied using local anesthesia and submitted for culture. In this case, surface antisepsis with a single 70% alcohol wipe is indicated before collection of the biopsy. Culture of biopsy specimens from nodules and furuncles is best for dogs with deep pyoderma. After collection of the biopsy, the epidermis can be removed using a sterile blade and the deeper tissues submitted for macerated tissue culture.

Pathologic Findings

Histopathologic Findings

Findings on histopathology in dogs with uncomplicated pyoderma include a neutrophilic infiltrate and abundant bacterial organisms. Histopathologic evaluation of skin biopsy specimens allows classification of the extent and type of pyoderma and identification of other conditions such as allergic dermatitis, demodicosis, dermatophytosis, pemphigus foliaceus, neoplasia, deep mycoses, and vasculitides. Biopsy should be considered in recurrent or refractory cases, in dogs with deep pyoderma, or when the underlying cause is not apparent. Biopsies can be performed under local anesthesia using a 6-mm punch biopsy. An attempt should be made to include both affected and adjacent healthy tissue when identifying sites for biopsy collection. The biopsy site should be sutured after the specimen has been collected.

Treatment and Prognosis

Factors to consider when treating dogs for pyoderma include the underlying disease present, the severity and extent of lesions, and the local prevalence of staphylococcal resistance. Veterinarians’ reliance on systemic antimicrobial drug treatment has been challenged by the increasing prevalence of multidrug-resistant staphylococci. Topical treatments that help to restore skin structure and function, as well as topical antimicrobial therapy, should be considered as alternatives.15 In dogs with mild disease, treatment aimed at the underlying disorder may be sufficient to resolve infection (see Prevention).

Topical Treatments

Antibacterial shampoos may be effective alone in some dogs with surface or mild superficial pyoderma and should be used as adjunctive therapy in dogs with deep pyoderma. They aid in debridement, reduce surface bacterial numbers, and decrease pain and pruritus. Options include shampoos that contain benzoyl peroxide, ethyl lactate, chlorhexidine, or triclosan. Initially, they should be used at least twice weekly with a 10-minute contact time. Benzoyl peroxide shampoos can be drying and so are best reserved for dogs with greasy dermatitides. Daily shampooing may be required for dogs with deep pyoderma. If available, treatment with daily whirlpool baths containing chlorhexidine may also be helpful for these dogs.

Topical antimicrobial drugs and antiseptics result in high concentrations of drug at the skin surface that can overwhelm bacterial drug resistance mechanisms, and can be useful when pyoderma is limited to small areas of skin. Aside from having the potential to be messy, these drugs have minimal side effects and result in minimal exposure of bystander organisms (such as gut flora) to antimicrobial drugs. Examples of topical drugs with excellent activity against staphylococci include neomycin, gentamicin, polymyxin B, bacitracin, hydroxyl acids (such as acetic acid), novobiocin, and silver sulfadiazine. Mupirocin and fusidic acid are additional topical antimicrobial drug preparations that may also be useful for treatment of pyoderma that is restricted to small areas. Unfortunately, resistance to mupirocin and fusidic acid has been documented in methicillin-resistant S. aureus isolates from humans.16,17

Systemic Antimicrobial Drug Therapy

When pyoderma is severe, treatment with systemic antimicrobial drugs may be required. The drug selected for empiric treatment should be based on the local prevalence of resistance, and narrow-spectrum drugs that target staphylococci are preferable. Clindamycin, first-generation cephalosporins (such as cephalexin), and amoxicillin-clavulanic acid are reasonable first choices (Table 84-2). Other acceptable alternatives when the local regional susceptibility of S. pseudintermedius is known include doxycycline, trimethoprim- or ormetoprim-potentiated sulfonamides, lincomycin, and erythromycin. The use of third-generation cephalosporins for empiric therapy is controversial because their use in particular has been associated with selection for methicillin-resistant staphylococci in humans and they have the potential to select for resistant populations of gram-negative bacteria in the gastrointestinal tract.18 More research is required to determine whether this is also true in dogs and for all third-generation cephalosporins. When pyoderma is severe, and only when culture and susceptibility indicate the presence of multidrug-resistant staphylococci, the use of chloramphenicol, minocycline, doxycycline, third-generation cephalosporins, rifampin, clarithromycin, azithromycin, amikacin, or fluoroquinolones (such as enrofloxacin, marbofloxacin, orbifloxacin, pradofloxacin, or ciprofloxacin) could be considered as indicated based on culture and susceptibility results. The use of fluoroquinolones in humans and dogs is a risk factor for selection of methicillin-resistant S. pseudintermedius and also has the potential for selection of resistant populations of gram-negative bacteria in the gastrointestinal tract.

Treatment should be for a minimum of 4 weeks for superficial pyoderma and 8 weeks for deep pyoderma. A small percentage of dogs with superficial pyoderma require 6 weeks of treatment. For superficial pyodermas, treatment should be continued for at least 7 days beyond clinical resolution of lesions, because inflammation subsides before infection is completely resolved. For deep or recurrent superficial pyoderma, treatment should be continued for 14 days beyond clinical resolution.

Although intermittent administration of antimicrobials on a regular basis (“pulse therapy”) has been used to treat some dogs with recurrent pyodermas, there is concern for induction of resistance using these protocols. If this appears to be necessary, referral to a veterinary dermatologist is recommended.


Prevention of pyoderma relies on addressing the underlying cause or causes whenever possible. This could include treatment with antihistamines and essential fatty acids, intradermal skin testing and hyposensitization injections, frequent bathing to restore skin condition, dietary management for dogs with food allergies, or breeding practices that aim to select against atopy and dogs with abundant skin folds. Specific prevention of methicillin-resistant staphylococcal infections relies on hand-washing and disinfection practices and judicious use of antimicrobial drugs, as outlined earlier, although more data are required in regard to risk factors for development of resistant pyodermas in dogs.

When treatment of the underlying cause or use of topical therapy is unsuccessful in preventing recurrence of pyoderma, subcutaneous administration of autogenous bacterins or commercial bacterial antigens (such as Staphage Lysate, Delmont Laboratories, USA) may be beneficial in some dogs.19,20 These are generally given once or twice weekly (see Chapter 7).

Public Health Aspects

Although S. pseudintermedius prefers not to colonize humans, human infection has been reported occasionally, including among veterinarians.2123 A methicillin-susceptible S. pseudintermedius was detected as a cause of endocarditis in a human patient,23 and methicillin-resistant S. pseudintermedius has been isolated from human patients with sinusitis.21 In all cases, pet dog contact was reported and suggested to be the source of infection. In one case, an isolate from the dog matched that isolated from the human as determined using pulsed field gel electrophoresis.

In contrast, contact with human hospitals and children may be risk factors for acquisition of methicillin-resistant S. aureus infections by dogs, and S. aureus infections appear to be a reverse zoonosis. Colonization is generally transient in dogs and cats, so specific treatment to decolonize dogs and cats is not necessary.24

Two subspecies of S. schleiferi have been described: S. schleiferi subsp. coagulans, which is coagulase positive, and S. schleiferi subsp. schleiferi, which is coagulase negative. Although it may be carried elsewhere on the body, S. schleiferi subsp. coagulans has been isolated primarily from dogs with otitis externa and was reported as a cause of endocarditis in an immunocompromised human.25 Coagulase-negative S. schleiferi are thought to be a commensal of the skin of both humans and dogs, and although they are common contaminants, they can cause disease in both species.8,26

Frequent and proper hand washing using soap and water and disinfectant foams or gels is the number one practice that limits spread of these infections in human hospitals. Hands should be washed before and after handling each patient. Clean clothing should be worn and laboratory coats changed regularly. Attention must be paid to regular disinfection of other fomites such as stethoscopes, cell phones, rectal thermometers, computer keyboards and mice, calculators, and bandage scissors. Disinfectant wipes that contain alcohol or accelerated hydrogen peroxide can be used to clean these surfaces.

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Jul 10, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on Pyoderma, Otitis Externa, and Otitis Media

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