Dermatologic fungal diseases in animals include both superficial infections of 1 of 18 different Malassezia species and superficial or subcutaneous infections of 1 of more than 30 species of dermatophytes.1,2 Here, we review some important background information on these 2 classes of fungal infections, known risk factors, and evidence-based steps for prevention and treatment.
Malassezia species
Malassezia dermatitis is a very common superficial fungal disease in dogs and cats, according to Daniel O. Morris, DVM, MPH, DACVD, professor of dermatology and allergy at the University of Pennsylvania School of Veterinary Medicine in Philadelphia. Malassezia species, especially Malassezia pachydermatis, are considered to be normal superficial skin flora in dogs.1
The World Association for Veterinary Dermatology (WAVD) reports that the factors that cause this organism to transition from a normal, innocuous inhabitant of the surface of the skin to a pathogen are still under investigation. They likely include the health of the host’s immune response and interactions with other commensal organisms, such as Staphylococcus spp. Other predisposing factors can include conditions that cause increased moisture, changes in the surface lipids of skin cells, and disruption of the stratum corneum barrier layer. It is especially common in dogs with atopic dermatitis and, in many cases, results in Malassezia hypersensitivity.1
Diagnosing yeast infections is key for therapy
One of the biggest challenges in managing Malassezia yeast infections in dogs is a failure to appropriately diagnose them, explained Daniel O. Morris, DVM, MPH, DACVD, professor of dermatology and allergy at the University of Pennsylvania School of Veterinary Medicine in Philadelphia. Diagnosis requires performing skin surface cytology in every cat and dog with pruritus. This should be done before using an antipruritic drug such as oclacitinib (Apoquel) or lokivetmab (Cytopoint), according to Morris, because these drugs will generally not do a good job of controlling the itch related to Malassezia infections.
There are multiple possible diagnostic tests for dermatophytosis, with none being shown to be a true gold standard, according to the World Association for Veterinary Dermatology Clinical Consensus Guidelines. Morris added that the most common mistake he sees in the diagnosis of dermatophytosis is the reliance on in-house fungal cultures. He reported that he has seen multiple cats who were needlessly prescribed antifungal therapy based solely on a color change on the dermatophyte test medium slant. Not knowing how to properly identify the macroconidia under the microscope can contribute to both false positive and false negative results.
Prevention
Malassezia infections require identification, and prevention of the underlying cause of the infection may require chronic or pulse antifungal therapy to prevent or control recurrence. Because of concerns about toxicity and/or resistance to oral antifungal medications, topical therapy is often preferred for long-term use. Findings from some studies have demonstrated that bathing dogs with a mixture of 2% chlorhexidine 2% miconazole shampoo every 3 days can significantly reduce the amount of Malassezia found on the skin and hair.1
Morris reported that most dermatologists will now include Malassezia yeast extract in their canine allergen vaccines. “Malassezia allergen immunotherapy seems to work for many dogs to reduce the frequency/recurrence of their Malassezia skin infections, which helps reduce our reliance on oral [and topical] antifungal drugs,” Morris said.
Treatment
Because the organism is located in the stratum corneum, topical formulations such as shampoos, gels, and lotions can be very effective at managing the infection. Medicated shampoos can be particularly helpful, because many of them contain ingredients that can reduce scaling and greasy exudate. The WAVD consensus guidelines suggest 2% chlorhexidine 2% miconazole shampoo used twice a week as the topical treatment of choice.1 Morris added that there is no one-size-fits-all recommendation on frequency of topical therapy and that twice a week is just a starting point that may need to be adjusted through a process of trial and error. He reported that he will often incorporate systemic therapy in dogs, especially when the paws are involved. He found it difficult to ensure that topical therapy alone will adequately reach all the areas between the toes and in the claw beds.
The WAVD consensus guidelines rank commonly used treatments for Malassezia infections based on how strong the evidence is for their efficacy in previous studies. Only 2% chlorhexidine 2% miconazole shampoo used twice weekly received a rating of “strong evidence” for use in dogs. Oral ketoconazole and itraconazole received a rating of “moderate evidence” for use in dogs. Fluconazole and terbinafine received a rating of “weak evidence” for use in dogs because of limited reports of successful use and “questionable” levels of terbinafine measured in the stratum corneum in dogs. Compounded formulations are not recommended because of inconsistent bioavailability.1
Dermatophytosis
In small animal medicine, dermatophytosis (most commonly Microsporum canis, Microsporum gypseum, and Trichophyton mentagrophytes) is more common in cats than in dogs and is much less common than Malassezia infection, atopic dermatitis, and superficial pyoderma. Although it is normally a self-limiting condition in animals with a competent immune system, it is contagious, zoonotic, and can cause chronic infections in animals who are not immunocompetent.2
In a review of prevalence studies, risk factors for dermatophytosis include young age (kittens and puppies), overcrowding, being allowed to roam freely outside, and living in a warm climate. No clear association could be made between feline leukemia virus or feline immunodeficiency virus status and risk for dermatophytosis in cats.2,3 Studies looking for a correlation between immunosuppressive medication use and increased risk for dermatophytosis yielded mixed results.2
Persian cats have been shown to be overrepresented in many studies of feline dermatophytosis. Suggested reasons for this include the propensity for ineffective grooming of the haircoat, alterations in the cutaneous mycobiota of Persian cats, and a genetic immunodeficiency that prevents clearance of fungal organisms.4 In dogs, at-risk breeds can include Yorkshire terriers (who are suspected to have a genetic predisposition for subcutaneous nodular dermatophytosis) and various hunting and working breeds. The increased risk for working and hunting breeds is thought to be due to their increased exposure to environmental sources of the infectious organisms.2
Prevention
Prevention of dermatophytosis involves reducing the known risk factors for
the disease where possible. Limiting exposure to known infected animals, preventing free roaming, and preventing overcrowding and other conditions that cause physiologic stress can reduce the risk of infection. Live and killed fungal vaccines have been studied as possible preventive options against experimental dermatophyte infection. A protective benefit was seen in dogs who were administered a live M canis vaccine before challenge with the infective organism but not in cats.3
Treatment
WAVD consensus guidelines recommend twice-weekly topical application of lime sulfur, enilconazole, or miconazole/chlorhexidine shampoo for treatment of cats and dogs.2 In cats, management of dermatophytosis must always include systemic therapy, according to Morris, with no exceptions. This is because topical therapy can reduce environmental contamination and transmission of the organism to others, but it is not sufficient to clear the organism from the infected patient.
In a review of the available systemic therapies for management of dermatophytosis in cats and dogs, WAVD has identified itraconazole (noncompounded) and terbinafine as the safest and most effective treatments. Griseofulvin was also determined to be effective but with a higher risk of adverse effects. Ketoconazole and fluconazole rank lower in efficacy, and ketoconazole has a higher risk of adverse effects when compared with itraconazole and terbinafine, especially in cats. Although lufenuron has been identified as a possible treatment option, findings from in vitro studies have demonstrated no effectiveness in the management of dermatophytosis.2
Environmental decontamination is also important to help shorten the duration of the infection. This involves using appropriate disinfecting agents in the environment, clipping of hair over affected areas (if the patient will tolerate this), confining patients to an area that is easy to clean (without over restricting socialization of kittens and puppies), and daily removal of hair from the environment by dusting, mopping, and/or sweeping.2
Emily Singler, VMD, is a 2001 graduate of Pennsylvania State University in State College and a 2005 graduate of University of Pennsylvania School of Veterinary Medicine in Philadelphia. Her career in veterinary medicine has included experience in shelter medicine, private practice, and as a relief veterinarian. She currently works as a veterinary writer, consultant, and mentor and enjoys writing for both pet owners and veterinary professionals. Her writing interests include public health, preventive medicine, the human-animal bond, and life as a working mom. She is the author of Pregnancy and Postpartum Considerations for the Veterinary Team, which is being published by CRC Press in November 2023 and is available for preorder now at www.emilysinglervmd.com.
References
- Bond R, Morris DO, Guillot J, et al. Biology, diagnosis and treatment
of Malassezia dermatitis in dogs and cats Clinical Consensus Guidelines of the World Association for Veterinary Dermatology. Vet Dermatol. 2020;31(1):28-74. doi:10.1111/vde.12809 - 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):266-e68. doi:10.1111/vde.12440
- Moriello KA. Dermatophytosis in dogs and cats. Merck Veterinary Manual. October 2022. Accessed August 23, 2023. https://www.merckvetmanual.com/integumentary-system/dermatophytosis/dermatophytosis-in-dogs-and-cats
- 4. Myers AN, Older CE, Diesel AB, Lawhon SD, Rodrigues Hoffmann A. Characterization of the cutaneous mycobiota in Persian cats with severe dermatophytosis. Vet Dermatol. 2021;32(4):319-e88. doi:10.1111/vde.12969
