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Fungal dermatological diseases in animals include both superficial infections and 1 in 18 different Malassezia species and superficial or subcutaneous infections of one species among more than 30 species of dermatophytes.1.2 Here we review some important general information about these 2 classes of fungal infections, known risk factors, and evidence-based prevention and treatment steps.
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, in particular Malassezia pachydermatisare considered normal superficial skin flora in dogs.1
The World Association of Veterinary Dermatology (WAVD) reports that the factors that change this organism from a normal, harmless inhabitant of the skin’s surface to a pathogen are still being studied. They likely include the health of the host immune response and interactions with other commensal organisms, such as Staphylococcus spp. Other predisposing factors may include conditions leading to increased humidity, changes in skin cell surface lipids, and disruption of the barrier layer of the stratum corneum. It is particularly common in dogs with atopic dermatitis and in many cases leads to Malassezia hypersensitivity.1
Diagnosing Yeast Infections Is Key to Treatment
One of the biggest challenges in management Malassezia Yeast infections in dogs are a failure to properly 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 each cat and dog presenting with pruritus. This should be done before using an antipruritic medication such as oclacitinib (Apoquel) or lokivetmab (Cytopoint), according to Morris, because these medications are usually not enough to control itching related to Malassezia infections.
There are several possible diagnostic tests for dermatophytosis, but none is a true gold standard, according to the World Association of Veterinary Dermatophytology clinical consensus guidelines. Morris added that the most common mistake he sees in diagnosing dermatophytosis is the reliance on internal fungal cultures. He reported seeing several cats who were unnecessarily prescribed antifungal treatment based solely on a color change on the tilt of the dermatophyte test medium. Not knowing how to correctly identify macroconidia under the microscope can contribute to false positive and false negative results.
Prevention
Malassezia Infections require identification and prevention of the underlying cause of infection may require chronic or pulsed antifungal therapy to prevent or control recurrence. Due to concerns regarding toxicity and/or resistance to oral antifungal medications, topical treatment is often preferred for long-term use. The results of some studies have demonstrated that bathing dogs with a mixture of 2% chlorhexidine and 2% miconazole shampoo every 3 days can significantly reduce the amount of Malassezia found on skin and hair.1
Morris said most dermatologists will now include Malassezia yeast extract in their canine allergen vaccines. “Malassezia Allergen immunotherapy appears to work for many dogs to reduce the frequency/recurrence of their symptoms. Malassezia skin infections, which helps reduce our reliance on oral (and topical) antifungal medications, Morris said.
Treatment
Since the organism is located in the stratum corneum, topical formulations such as shampoos, gels, and lotions can be very effective in managing the infection. Medicated shampoos can be particularly helpful, as many of them contain ingredients that can reduce flaking and oily exudate. The WAVD consensus guidelines suggest 2% chlorhexidine and 2% miconazole shampoo used twice weekly as the topical treatment of choice.1 Morris added that there is no single recommendation on the frequency of topical treatment and that twice a week is only a starting point that may need to be adjusted through a process of testing and evaluation. errors. He said he will often incorporate systemic therapy in dogs, particularly when the paws are involved. He has had difficulty ensuring that the topical treatment alone will adequately reach all areas between the toes and in the claw beds.
The WAVD consensus guidelines classify commonly used treatments for Malassezia infections based on the strength of evidence for their effectiveness in previous studies. Only 2% chlorhexidine and 2% miconazole shampoo used twice weekly received a “strong evidence” rating for use in dogs. Oral ketoconazole and itraconazole were given a rating of “moderate evidence” for their use in dogs. Fluconazole and terbinafine were given a rating of “weak evidence” for use in dogs due to limited reports of successful use and “questionable” levels of terbinafine measured in the dog’s stratum corneum. Compounded formulations are not recommended due to inconsistent bioavailability.1
Dermatophytosis
In small animal medicine, dermatophytosis (most often 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 disease in animals with competent immune systems, it is contagious, zoonotic, and can cause chronic infections in animals that are not immunocompetent.2
In a review of prevalence studies, risk factors for dermatophytosis include young age (kittens and puppies), crowding, being able to move freely outdoors, and living in a warm climate. No clear association could be established between feline leukemia virus or feline immunodeficiency virus status and the risk of dermatophytosis in cats.2.3 Studies looking for a correlation between the use of immunosuppressive medications and an increased risk of dermatophytosis have 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 coat grooming, alterations in the skin mycobiota of Persian cats, and a genetic immunodeficiency that prevents the elimination of fungal organisms.4 In dogs, breeds at risk may include Yorkshire terriers (which are believed to have a genetic predisposition to subcutaneous lumpy 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 infectious organisms.2
Prevention
Prevention of dermatophytosis involves reducing known risk factors for
disease when possible. Limiting exposure to known infected animals, preventing free movement, and preventing crowding and other conditions causing physiological stress can reduce the risk of infection. Live and killed fungal vaccines have been investigated as possible preventive options against experimental dermatophyte infections. A protective benefit was observed in dogs given a live dose M canis vaccine before challenge with the infectious organism, but not in cats.3
Treatment
The WAVD consensus guidelines recommend twice-weekly topical application of lime sulfur, enilconazole, or miconazole/chlorhexidine shampoo for the treatment of cats and dogs.2 In cats, according to Morris, management of dermatophytosis should always include systemic treatment, without exception. Indeed, topical treatment can reduce environmental contamination and transmission of the organism to others, but it is not enough to eliminate the infected patient’s organism.
In a review of systemic therapies available for the management of dermatophytosis in cats and dogs, WAVD identified itraconazole (uncompounded) and terbinafine as the safest and most effective treatments. Griseofulvin has also been shown to be effective, but with a higher risk of adverse effects. Ketoconazole and fluconazole are less effective and ketoconazole has a higher risk of adverse effects than itraconazole and terbinafine, especially in cats. Although lufenuron has been identified as a possible therapeutic option, results from in vitro studies have demonstrated no efficacy in the management of dermatophytosis.2
Environmental decontamination is also important to help shorten the duration of infection. This involves using appropriate disinfectant agents in the environment, trimming hair on affected areas (if tolerated by the patient), confining patients to an easy-to-clean area (without unduly restricting kitten socialization and puppies) and daily removal of hair from the environment by dusting, wiping and/or sweeping.2
Emily Singler, VMD, is a 2001 graduate of Pennsylvania State University in State College and a 2005 graduate of the University of Pennsylvania School of Veterinary Medicine in Philadelphia. Her career in veterinary medicine includes 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 pet owners and veterinary professionals. Her writing interests include public health, preventive medicine, the human-animal bond, and life as a working mother. She is the author of Pregnancy and Postpartum Considerations for the Veterinary Team, which will be published by CRC Press in November 2023 and is available for pre-order now at www.emilysinglervmd.com.
The references
- Bond R, Morris DO, Guillot J et al. Biology, diagnosis and treatment
of Malassezia dermatitis in dogs and cats World Association of Veterinary Dermatology Clinical Consensus Guidelines. Veterinarian 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 from the World Association of Veterinary Dermatology. Veterinarian 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 skin mycobiota in Persian cats with severe dermatophytosis. Veterinarian Dermatol. 2021;32(4):319-e88. doi:10.1111/vde.12969