Issues

Dermatophytosis

• A cutaneous fungal infection affecting the cornified regions of hair, nails, and occasionally the superficial layers of the skin.
• Microsporum and Trichophyton dermatophytes are most commonly isolated: the majority of cases are caused by Microsporum canis; infection with Trichophyton mentagrophytes, M. gypseum, or M. persicolor also occurs.
• PATHOPHYSIOLOGY
• Dermatophytes—grow in the keratinized layers of hair, nail, and skin; do not thrive in living tissue or persist in the presence of severe inflammation.
• Exposure to or contact with a dermatophyte does not necessarily result in an infection.
• Infection may not result in clinical signs.
• Incubation period from exposure to clinical lesions: approximately 2–4 weeks.
• Infective spores must contact the skin surface and defeat host protective mechanisms (innate immunity, normal flora, sebum, grooming), in order for infection to occur.
• Factors that favor the development of disease: stress, trauma, ectoparasite infestations, and immunosuppression.
• An infected animal may remain as an asymptomatic (inapparent) carrier for a prolonged period of time; some animals never become symptomatic.

 

SYSTEMS AFFECTED

Skin/Exocrine

 

INCIDENCE/PREVALENCE

• Lesions may mimic many dermatologic conditions; over-diagnosis is likely common.
• Infection rates (inapparent and clinical) vary widely, depending on the population studied.
• GEOGRAPHIC DISTRIBUTION
• More common in hot, humid climates.
• Incidence of dermatophyte species may vary seasonally and geographically (e.g., northern vs. southern hemisphere, rural vs. urban environment, indoor vs. outdoor housing).

SIGNALMENT

Species

Dog and cat

 

Breed Predilections

• Cat—more common in longhaired breeds (i.e., Persian and Himalayan)
• Dog: Yorkshire terrier and Manchester terrier

 

Mean Age and Range

• M. canis is more common in younger animals; other species (associated with rodents or wildlife) seen more often in adults.
• Generalized dermatophytosis in older dogs associated with immunosuppression.

 

SIGNS

Historical Findings

Previously confirmed infection or exposure to an infected animal or environment (e.g., a cattery) is a useful but not consistent finding.

 

Physical Examination Findings

• Inapparent carrier state–cats.
• Only consistent finding is extreme variability of clinical signs.
• Classical lesion: slowly expanding circular patch of alopecia with scale.
• Seborrheic or greasy hair coat.
• Papular or pustular eruptions.

 

CAUSES

Multiple species identified; majority of cases caused by Microsporum canis, Microsporum gypseum, Tricophyton mentagrophytes, and Microsporum persicolor (non-follicular).

 

RISK FACTORS

• Immunocompromised caused by disease (FeLV, FIV) or by medications (glucocorticoids).
• High population density.
• Poor management practices.

DIAGNOSIS

DIFFERENTIAL DIAGNOSIS

• Staphylococcal folliculitis
• Demodicosis
• Allergic dermatitis
• Pemphigus (especially foliaceus)
• Keratinization defects

DIAGNOSTIC PROCEDURES

Wood’s Lamp Examination

• Can be misleading: only 50% of M. canis isolate fluoresce; most other pathogenic dermatophytes do not fluoresce.
• True positive reaction consists of apple-green fluorescence of the hair shaft.

 

Microscopic Examination of Hair

• Choose hairs that fluoresce under Wood’s lamp illumination to increase success.
• Hyphae and arthrospores seen invading hair shafts.

 

Fungal Culture with Identification

• “Gold standard” for diagnosis.
• Choose hairs that fluoresce under Wood’s lamp if possible.
• Sampling methods: Pluck hairs from the periphery of an alopecic area. Brush haircoat with a sterile toothbrush or carpet square (especially inapparent or treated patient.
• Dermatophyte test media—dermatophytes change media color to red during the early growing phase of the culture; saprophytes cause color change after significant colony growth; examine inoculated media daily.
• Fungal colonies are non-pigmented.
• Microscopic examination of the growth for microconidia and macroconidia—necessary to confirm pathogenic dermatophyte and to identify genus and species; helps identify source of infection.
• Positive culture—indicates presence of a dermatophyte; however, organisms may be transient (i.e., geophilic dermatophytes on the feet).

Skin Biopsy

• Not usually required for diagnosis.
• Can be helpful in confirming true invasion and infection, or to diagnose suspicious cases with negative fungal culture.

 

PATHOLOGIC FINDINGS

• Folliculitis, perifolliculitis, or furunculosis.
• Hyperkeratosis, intraepidermal pustules, and pyogranulomatous reaction patterns may occur.
• Fungal hyphae seen in H&E-stained sections; special stains allow easier visualization of the organism.

 

TREATMENT

APPROPRIATE HEALTH CARE

• Most animals are treated as outpatients.
• Consider quarantine owing to the infective and zoonotic nature of the disease.

 

CLIENT EDUCATION

• Many shorthaired cats in a single-cat environment and many dogs will undergo spontaneous remission within 3 months.
• Longhaired animals should be clipped to reduce environmental contamination.
• Decontamination of the environment reduces the risk of false positive fungal cultures which can lead to prolonged treatment and confinement.
• Infective spores are shed into the environment, but do not multiply in the environment; transmission of the disease strictly from a contaminated environment (i.e., no direct contact with an infected animal) is extremely rare.
• Effective disinfectants
• Sodium hypochlorite (0.5%): potential to react with other chemicals to create toxic gases; can be irritating and result in “bleaching” of colors.
• Enilconazole: available as a concentrated spray or fogger; 10-minute contact time is recommended.
• Accelerated hydrogen peroxide: should not be mixed with concentrated sodium hypochlorite products; 10-minute contact time recommended.
• Potassium peroxymonosulfate (2% solution): recent studies report antifungal properties against M. canis and Trichophyton spp.
• Advise that treatment can be both frustrating and expensive, especially in multi-animal households or with recurrent cases; consider referral to a veterinarian with expertise in treatment of dermatophytosis.

 

MEDICATIONS

 

DRUG(S) OF CHOICE

• Topical therapy and clipping—recommended concurrently with systemic therapy; may help prevent environmental contamination; may be associated with an initial exacerbation of signs
• Rinses: lime sulfur (1:16 dilution or 8 oz. per gallon of water), miconazole/chlorhexidine (0.2%), or enilconazole (0.2%) applied once to twice weekly; lime sulfur is odoriferous and can stain; enilconazole is not currently approved for use in companion animals in the US. Shampoos containing 1–2% ketoconazole, miconazole, or 0.5% climbazole; a minimum of a 3-minute contact time is recommended; have little to no residual effect.
• Use of an Elizabethan collar, particularly in cats, is recommended to prevent ingestion of these products.
• Griseofulvin—effective but use is declining due to relatively high costs and side effects.
• Ketoconazole—true efficacy unknown; some studies have shown in vitro resistance of M. canis: dogs, 10 mg/kg PO q24h or divided q12h for 4–8 weeks; anorexia and vomiting are the most common side effects; not recommended in cats.
• Itraconazole—similar to ketoconazole, but more effective; fewer side effects, expensive: dogs, 5–10 mg/kg PO q24h for 4–8 weeks; cats, 10 mg/kg PO q24h for 4–8 weeks or until cured. Alternate dosing—20 mg/kg q48h cats and dogs. In some cats, dosage regimen is altered after 4 weeks of therapy to every other week schedule for a total of 8–10 weeks of therapy; alternative schedule—one-week-on, one-week-off with apparent efficacy to reduce drug cost; manufactured drug preferred over compounded formulations due to absorption/concentration variability.
• Terbinafine—may be helpful in cases resistant to azole drugs; dogs, 20–30 mg/kg q12–24h for 4–8 weeks; cats, 20–40 mg/kg q24–48h for 4–8 weeks; dermatophyte carriers, 8.25 mg/kg q24h for 4–8 weeks; side effects may include gastrointestinal upset, hepatotoxicity, neutropenia, and pancytopenia.

 

CONTRAINDICATIONS

• Corticosteroids: can modulate inflammation and prolong the infection.
• Griseofulvin: cats with FeLV or FIV; teratogen.

 

PRECAUTIONS

• Ketoconazole
• Hepatopathy has been reported and can be severe in cats.
• Inhibits endogenous production of steroid hormones in dogs.
• Itraconazole
• Rare vasculitis and ulcerative skin lesions at doses of 5 mg/kg q12h; not noted in patients receiving 5 mg/kg q24h.
• Hepatotoxicity reported infrequently in dogs.
• Terbinafine
• Gastrointestinal upset, hepatotoxicity, and bone marrow suppression (pancytopenia, neutropenia). Decrease dosage with renal and/or hepatic insufficiency. Cimetidine increases blood concentration; rifampin decreases blood concentration.

 

• Lime-Sulfur Solution – Ingestion of lime sulfur may lead to oral erosions.

 

ALTERNATIVE DRUG(S)

• Lufenuron—a chitin synthesis inhibitor used in flea control; not effective in controlled studies.
• Fluconazole—effectiveness not well documented in studies; less expensive than itraconazole.

 

FOLLOW-UP

PATIENT MONITORING

• Dermatophyte culture is the only appropriate method for monitoring response to therapy; many animals clinically improve but remain culture-positive.
• Repeat fungal cultures toward the end of the treatment regimen and continue treatment until at least two subsequent cultures are negative.
• In resistant cases, cultures should be repeated weekly using the toothbrush technique.
• Weekly or biweekly CBC if treated with griseofulvin; periodic evaluation of liver enzymes if treated with ketoconazole, itraconazole, or terbinafine.

 

PREVENTION/AVOIDANCE

• Initiate a quarantine period and obtain dermatophyte cultures of all animals entering the household to prevent reinfection from inapparent carriers.
• Consider the possibility of rodents aiding in the spread of the disease.
• Decontaminate the environment.
• Avoid infective soil if a geophilic dermatophyte is cultured.
• Consider prophylactic treatment of exposed animals.

 

POSSIBLE COMPLICATIONS

False-negative dermatophyte culture

 

EXPECTED COURSE AND PROGNOSIS

Many animals will “self-clear” infection over a period of a few months.

Treatment hastens clinical cure and helps reduce environmental contamination.

Some infections, particularly in longhaired cats or multi animal situations, can be persistent.

 

MISCELLANEOUS

ZOONOTIC POTENTIAL

• Dermatophytosis is a significant zoonosis.
• Considered a low level pathogen; disease is not life-threatening, can be easily treated, but may cause scarring.

 

PREGNANCY/FERTILITY/BREEDING

• Griseofulvin is teratogenic.
• Ketoconazole can affect steroidal hormone synthesis, especially testosterone.

 

SYNONYMS

Ringworm

 

ABBREVIATIONS

FeLV = feline leukemia virus

FIV = feline immunodeficiency virus

H&E = hematoxylin and eosin 

 

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

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