Dermatophytosis (ringworm) is a superficial fungal skin infection of animals. In dogs and cats, lesions can include hair loss, scaling, crusting erythema, papules, hyperpigmentation, and variable pruritus. The infection is diagnosed most commonly via direct examination of hair and scales or via fungal culture. A Wood's lamp tool can be used to find suspect hairs for direct examination. Topical therapy is used to treat (disinfect) infective material on the hair coat, and systemic treatment is used to eradicate the infection from within hair follicles. Resolution of infection takes 6–12 weeks. Dermatophytosis is a zoonotic disease.
Dermatophytosis is a superficial fungal infection of the skin and hair and less commonly of the claw or hoof. The following are pathogens of veterinary importance:
Microsporum canis (affects cats, dogs, and, to a lesser extent, large animals)
Trichophyton mentagrophytes (small and large animals), T verrucosum (primarily large animals), and T erinacei (primarily hedgehogs)
Nannizzia gypsea (formerly M gypseum; soil organism that causes inflammatory lesions)
The genera Microsporum and Trichophyton have been reclassified into the genus Arthroderma.
Dermatophytosis is a self-limiting disease that resolves without treatment in otherwise healthy animals. The disease is considered zoonotic and in humans causes skin lesions that are easily treated.
Transmission is by direct contact with an infected animal, but mere exposure does not always result in disease. Transmission from spores in the environment is inefficient unless there is microtrauma (eg, clipping of hair coat with contaminated clippers, tack) to the skin.
The prevalence of dermatophytosis is difficult to determine because it is not a reportable disease in pets, and many studies reporting prevalence do not distinguish between fomite carriage and true infection. Overall, this disease is uncommon, with true disease prevalence reported at < 4% of all skin disorders. It is not the most common skin disease of cats, contrary to what is reported in lay literature.
For dogs, young age, free-roaming or hunting activities, and warm environments are risk factors. For cats, positive status for feline immunodeficiency virus or feline leukemia virus does not predispose to infection. Disease is more common in animals under physiological stress or in overcrowded environments, such as animal hoarding situations. Infections in animals that are ill, under physiological stress, or have some other underlying disease or factor are more challenging to treat. It can be difficult to administer oral or topical medication to ill animals, and they may not mount a good cell-mediated immune response, which is needed for recovery.
Lesion development requires exposure to a sufficient number of infective spores, microtrauma to the skin, and moisture on the skin. Dermatophyte arthrospores can germinate and start invading skin and hair shafts within 6–8 hours under ideal conditions. The severity of infection reflects the host's overall health; severe infections are not caused by more virulent strains but rather by a failure of host cell-mediated immunity to respond to the infection.
Clinical Findings of Dermatophytosis in Dogs and Cats
Clinical signs of dermatophytosis can include any combination of hair loss, scaling, crusting, erythema, papules, hyperpigmentation, and variable pruritus (see hair loss in a cat and a puppy).
Nodular lesion (kerion) reactions can develop in dogs (see dermatophytosis, nodular lesions image). Persian cats can develop nodular lesions (pseudomycetoma). Cats can also develop exudative paronychia. Pustular dermatophytosis can mimic clinical signs of dermatophytosis.
Courtesy of Dr. Sheila Torres.
Courtesy of Dr. Sheila Torres.
Courtesy of Dr. Sheila Torres.
Diagnosis
Direct examination
Fungal culture
PCR assay
No single test is a gold standard for diagnosis of dermatophytosis; typically, multiple tests are used to confirm infection. The most helpful diagnostic tests include direct microscopic examination of hair and scales for spores (see photomicrograph direct examination of infected hairs); superficial fungal culture of potentially infected hair, most commonly using dermatophyte test medium (DTM); and PCR assay of hair and crust.
Courtesy of Dr. Karen A. Moriello.
Skin biopsy, though not indicated in routine cases of dermatophytosis, is indicated when there are nodular lesions or an unusual presentation of skin disease. If a biopsy is ordered, the laboratory should be informed that dermatophytosis is a possible differential diagnosis so that special stains can be performed. Submitting at least several 8-mm skin biopsy punch samples or a nodule is important to ensure adequate material for ruling in or ruling out infection.
Direct microscopic examination of hair and scales scraped and plucked from lesions and mounted in mineral oil can confirm infections in > 85% of cases. Clearing agents are not needed. Microscopically, fungal spores may be seen, and infected hairs typically appear thicker than normal hairs (see photomicrograph comparison between normal and infected hairs).
Courtesy of Dr. Karen A. Moriello.
A Wood's lamp (320–400 nm) is a tool used to find M canis–infected hairs. These hairs can be used for direct microscopic examination or for fungal culture. Using a plug-in Wood's lamp with built-in magnification is important. It does not need to warm up, but the user's eyes must adapt to the darkened room.
The user must hold the lamp close to the skin (within 2–4 cm), start at the head, and move slowly. Hairs infected with M canis glow/fluoresce apple green. Only hair shafts, not crusts, glow; crusts should be lifted to look for glowing hairs. The hairs can then be collected via plucking or scraping of material onto a drop of mineral oil for direct microscopic examination.
An evidence-based review found that 91–100% of untreated, infected animals will have glowing hairs (1). Fluorescence will be less common as animals recover. Sometimes only the tips of hair will glow, and these may or may not yield a positive test result.
A dermatoscope, a handheld tool used to examine the skin, can locate abnormal hairs for direct examination or culture.
Fungal culture can determine whether spores are present on the hair coat and must be used in conjunction with clinical examination findings.
Only lesions should be sampled; the entire hair coat should not be sampled for fungal testing. For small animals, a new, unopened toothbrush is brushed over the lesions and then inoculated onto fungal culture medium. Hairs should not be plucked from the bristles because this increases contamination. If hair plucked from lesions is used, it should not be wiped with alcohol because doing so may result in a negative culture result.
Fungal culture can be done as a point-of-care test only if the user follows directions for storage and performs gross and microscopic examinations of colonies daily. If this is not possible, a diagnostic laboratory should be used.
M canis and N gypsea are not difficult to isolate and identify; however, Trichophyton species are hard to identify to genus level. (See photographs comparing fungal cultures and gross colony morphology; see also photomicrograph of M canis and photomicrograph of N gypsea.)
Courtesy of Dr. Karen A. Moriello.
Courtesy of Dr. Karen A. Moriello.
Courtesy of Dr. Karen A. Moriello.
Courtesy of Dr. Karen A. Moriello.
The following are fungal culture key points:
Use flat plates that are easy to inoculate. Do not use jars or small flat plates intended for samples from humans.
Stab bristles of toothbrush onto the surface 5 to 6 times; do not overinoculate the plate, or sporulation will not be apparent.
Incubate at room temperature; seal the plate in a plastic bag to prevent desiccation.
Examine daily for 14 days.
Suspect pathogens are pale or light in color and never heavily pigmented.
If DTM is used, there will be a color change (red) around the colony.
Posttreatment cultures can have abnormal gross and microscopic appearances.
Pathogen type can be confirmed by direct microscopic examination of the colony (with an adhesive tape prep) using lactophenol cotton blue stain.
PCR assay confirms the presence or absence of fungal DNA on the hair coat. It cannot distinguish between viable and nonviable spores. Availability of the validated testing protocols limits use. Adequate samples (> 20 hairs and crusts) should be submitted to the laboratory for PCR assay.
Treatment
Topical therapy for disinfection
Systemic antifungal drugs for active infection
Patients can be treated to shorten the course of dermatophytosis and to minimize contagion to other susceptible animals or humans. Infected small animals should remain isolated from other pets until there is clear evidence of clinical cure. However, infected young animals should not be confined for long periods or undersocialized, or lifelong behavioral problems may result.
Cats can be treated with itraconazole (5 mg/kg, PO, every 24 hours on a week on/week off schedule). Most infections are resolved after 3 or 4 cycles. An evidence-based review found that itraconazole is well tolerated in cats and was not associated with liver toxicity or vasculitis (1). Compounded itraconazole has poor bioavailability and should not be used; instead, a commercial veterinary liquid formulation should be used.
Small dogs can be cost-effectively treated with itraconazole (5 mg/kg, PO, every 24 hours); pulse therapy also is likely to be effective, but this has not been documented. Larger dogs can be treated with ketoconazole (5 mg/kg, every 24 hours) or terbinafine (30–40 mg/kg, every 24 hours), which may be more cost-effective. Dogs should be treated until mycological cure (eg, a negative result of fungal culture or PCR assay), which can take ≥ 4–6 weeks (1).
Other points to keep in mind regarding drug therapy for dermatophytosis include the following:
Ketoconazole should not be used in cats because it causes anorexia.
Fluconazole should not be used because this is the least effective drug for dermatophytes.
Griseofulvin is no longer recommended because itraconazole and terbinafine are superior drugs with lower risk of adverse effects.
Lufenuron is ineffective.
In addition to systemic treatment, topical therapy is required because it disinfects the hair coat. This is important because infective spores are the source of contagion and transmission, and disinfection minimizes environmental contamination. A whole-body rinse (lime sulfur 1:16 or enilconazole 1:100) should be used twice a week; these products have residual activity. Shampoo containing 2% chlorhexidine/2% miconazole is effective and may be the only option in countries where lime sulfur or enilconazole is not available; shampoo therapy has no residual activity and should be done 2–3 times a week. Even if systemic therapy is stopped, topical therapy should be continued until a mycological cure is demonstrated.
Adjunct focal topical therapy can be used for lesions in hard-to-treat locations such as the ears and face. A 1–2% over-the-counter vaginal miconazole cream can safely be used on the face. For the ears, otic products that contain clotrimazole or miconazole/chlorhexidine or ketoconazole/chlorhexidine combinations are available.
Environmental cleaning removes infective material from the environment. Spores do not multiply in the environment and do not invade the environment like mildew. Spores are a normal dormant life stage of dermatophytes and are easily removed from the environment. Mechanical removal of organic material and hair with a vacuum or wipes, followed by washing surfaces with detergent until visibly clear, is the most important step for environmental cleaning/disinfection.
After cleaning, disinfectant should be used. Any bathroom disinfectant labeled as effective against Trichophyton will kill spores not removed by mechanical cleaning. Disinfectant use alone will not remove environmental contamination. Thorough cleaning once or twice a week is adequate. Between cleanings, remove organic material with wipes or other means. Continue cleaning until mycological cure.
Soft surfaces such as bedding or towels can be disinfected via thorough washing in a washing machine; bleach is not needed. Carpets can be disinfected via shampooing or steam cleaning.
The end of treatment includes both clinical and mycological cure. Clinical cure is the resolution of all lesions and the lack of any new lesions. A Wood's lamp examination can be used to look for residually infected hairs in animals with M canis infections.
Once lesions have resolved and there is a clear clinical cure, a fungal culture or PCR assay can test for mycological cure. One negative PCR assay result supports a mycological cure. Unless the animal has systemic illness, one negative fungal culture result also supports mycological cure.
Key Points
Dermatophytosis is caused by several Microsporum and Trichophyton spp and can affect a wide range of species.
Infections in otherwise healthy animals are self-limiting and do not require treatment.
Animals can be treated both systemically and topically to shorten the disease course and minimize contagion to other susceptible animals or people.
Systemic antifungal drugs eliminate active infection in hair follicles.
Topical therapy is required in addition to systemic treatment to disinfect the hair coat.
For More Information
Moriello K. Dermatophytosis in cats and dogs: a practical guide to diagnosis and treatment. In Pract, 2019;41(4):138-147.
Summary of recommendations for general practitioners based on the clinical consensus guidelines: dermatophytosis. World Association for Veterinary Dermatology.
Moriello K. Clinical aspects of small animal dermatophytosis. World Association for Veterinary Dermatology webinar.
Also see pet owner content regarding ringworm in dogs and ringworm in cats.
References
Moriello KA, Coyner K, Paterson S, Mignon B. Diagnosis and treatment of dermatophytosis in dogs and cats: clinical consensus guidelines of the World Association for Veterinary Dermatology. Vet Dermatol. 2017;28(3):259-e68, 304-e69.
