Dermatophilosis

“Mud rash” or “mud fever” is a rare crusting dermatitis of dogs and cats.
A rare nodular subcutaneous and oral disease in cats.
Dermatophilus congolensis—causative agent; Gram-positive, branching filamentous bacterium classified as an actinomycete; very common cause of crusting dermatoses in hoofed animals; persists in the environment within crusts.
Dogs, cats, humans, and other animals can rarely be secondarily infected.

SYSTEMS AFFECTED

Skin/Exocrine

SIGNALMENT

SIGNS

Historical Findings

Association with cattle, sheep, or horses.
Occasionally free-roaming dogs.
Cats with subcutaneous disease—episodes of trauma; existence of a foreign body; lesions generally chronic; no systemic clinical signs, except when internal organs or large oral lesions develop.
Physical Examination Findings
Dogs—circular to coalescing, papular, crusted lesions on the head and/or trunk; lesions resemble superficial bacterial pyoderma caused by Staphylococcus pseudintermedius; lesions may resemble dermatophilosis in horses (adherent thick, gray-yellow crusts that incorporate hair and leave a circular, glistening, shallow erosion when removed); variable pruritus.
Cats—subcutaneous, oral, or internal ulcerated and fistulated nodules or abscesses similar to lesions caused by other actinomycetes; superficial pyogenic crusting disease of the face has been reported.

CAUSES & RISK FACTORS

Dogs, cats, and humans can be exposed directly from lesions on large animals or from environmental exposure.
Infectious stage—requires wetting for activation; cannot penetrate intact epithelium; minor trauma or mechanical transmission by biting ectoparasites (Amblyomma variegatum) may help in establishing infection.
Deeper infection—requires traumatic inoculation of infectious materials.

DIAGNOSIS

DIFFERENTIAL DIAGNOSIS

Dogs

Cats

CBC/BIOCHEMISTRY/URINALYSIS

Usually normal; possible neutrophilic leukocytosis in cats.

DIAGNOSTIC PROCEDURES

Dogs

Cytologic examination of crusts—most important procedure; differentiates from more typical bacterial pyoderma.
Organism—distinctive morphology in cytologic and histopathologic preparations; resembles “railroad tracks” as the bacterium forms branching chains of small diplococci.
Cytologic diagnosis—impression smears of exudate from under crusts or by preparation of macerated crusts; mince crusts finely in a drop of water and allow to macerate several minutes; dry the preparation and stain with Wright-Giemsa.
Histopathologic specimens—crusts contain organisms; submit with tissue samples.

Cats

Histopathologic examination—biopsy of nodules; procedure of choice.
Cytologic examination—exudate obtained from aspiration of nodule or swabbing of a draining tract.
Culture of biopsy specimens—may yield the organism; facilitated if the laboratory is alerted to the differential diagnosis of Dermatophilus (aerobic, relatively slow growing, and easily obscured by contamination).
Culture from crusts—requires the use of a special selective medium; isolation is possible but usually very difficult.

PATHOLOGIC FINDINGS

Dogs—crusting and superficial pustular dermatitis; palisading of the crusts with orthokeratotic and parakeratotic hyperkeratosis; organism visualized within the crusts.
Cats—pyogranulomatous inflammation; central necrosis; fistulous tract formation; organism visualized near the necrotic center of granulomas, especially with Gram stain.

TREATMENT

Dogs—antibacterial shampoo and gentle removal (and disposal) of crusts; shampoo containing benzoyl peroxide, ethyl lactate, chlorhexidine, or selenium disulfide; one or two applications suffice in most cases. Iodine or lime-sulfur may also be used.
Cats—for pyogranulomas and abscesses: surgical debridement; exploration for foreign bodies; establishment of drainage for exudate.

MEDICATIONS

DRUG(S)

Penicillin V 10 mg/kg PO q12h for 10–20 days; drug of choice.
Tetracycline 22–30 mg/kg, q8h, PO; doxycycline 5–10mg/kg, q12h, PO; or minocycline 5–12 mg/kg, q12h, PO.
Ampicillin 10–20 mg/kg PO q12h for 10–20 days; some isolates resistant in vitro.
Amoxicillin 10–20 mg/kg PO q12h for 10–20 days; some isolates resistant in vitro.

CONTRAINDICATIONS/POSSIBLE INTERACTIONS

Penicillin and ampicillin hypersensitivity

FOLLOW-UP

PATIENT MONITORING

Dogs—reexamine after 2 weeks of treatment to ensure complete resolution; give an additional 7 days of systemic therapy if indicated.
Cats—monitor biweekly for 1 month after apparent resolution of lesions, depending on location.

EXPECTED COURSE AND PROGNOSIS

Dogs—excellent.
Cats—varies with the location of lesions; recurrence possible due to persistence of organism in crusts and extent of surgical debridement; complete resolution can be achieved with early diagnosis and medical/surgical therapy.

MISCELLANEOUS

ZOONOTIC POTENTIAL

Veterinarians and animal care workers—seldom infected, even after traumatic exposure when working with infected farm animals.

Dogs and cats—unlikely but possible to serve as a source for human infection; caution is warranted for exposure of immunocompromised individuals.

Visit your veterinarian as early recognition, diagnosis, and treatment are essential.