Fungal skin infections in animals put under the spotlight

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Alternaria sp. in a horse’s skin under 400x magnification: a) & b) proliferating spores, c) chains of spores in the horse epithelium.

Researchers have literally scraped the surface in a fresh study examining the various fungi that can infect the skin of animals.

The research team from the Warsaw University of Life Sciences in Poland noted that fungal infections in animals or humans are common all over the world.

Some micro-organisms such as fungi exist on the skin and can be transmitted to other individuals, species or even humans, triggering skin infections.

They can also be behind severe generalized infections, especially in those with a weakened immune system.

Bożena Dworecka-Kaszak, Małgorzata Biegańska and Iwona Dąbrowska set out to evaluate the micro-organisms behind fungal skin diseases, analyzing the results of cultures performed on skin samples from a range of animals, including horses.

The research began with a pool of 5335 specimens, such as hair, skin scrapings, skin and ear swabs investigated during a 10-year period for fungal infection at the Microbiology Lab, which is part of the Department of Preclinical Sciences at the university.

The clinical specimens were obtained from 4150 dogs, 689 cats, 88 rodents, 274 riding horses, 11 birds and 123 other pet animals. They focused their research on the samples from 2399 from animals diagnosed with dermatitis and 2936 from animals with otitis externa — inflammation of the external ear canal.

The fungi most frequently isolated from animals with skin lesions were Malassezia pachydermatis (29.14%), Candida yeasts (27.07%), and dermatophytes (23.5%), with Microsporum canis (59.25%) and Trichophyton genus (40.7%) making up most of the latter.

Mycological investigations of samples collected from animals with dermatitis. Colour bars represent indicated fungi and the number of isolates

Malassezia pachydermatis represented (80%) of isolates in animals with inflammation of the external ear canal. Nearly all of the rest of the positive fungal cultures obtained from external ear canals involved Candida yeasts, mainly Candida albicans.

Alternaria moulds were identified in 127 samples taken from skin lesions, sometimes as the only isolated fungi.

“All of these isolates were counted separately from other moulds because of the presence of hyphae and typical spores in skin scrapings (especially in samples from horses),” the study team reported in the journal BMC Veterinary Research.

The study team noted that Alternaria moulds are widely distributed in the environment and colonize various plants, causing various diseases.

Alternaria spores are present in the air, soil and water, or on the surface of human and animal skin. However, in veterinary medicine, infections due to moulds such as Alternaria alternata, especially in horses, as well as hypersensitivities noted frequently in pet animals, have become an emerging clinical problem.

“Considering the increasing number of cases in which this fungus was found as the only agent in skin lesions, it should be noticed that the growing mycelium (the vegetative part of a fungus) may mechanically damage the neighbouring tissues.”

Many authors have noted cases of equine dermatitis involving Alternaria fungi, identified through biopsies of skin nodules, the study team said.

“In some clinical samples of this study, Alternaria was isolated as the only infectious agent.

Alternaria alternata has been recorded as a common indoor allergen, causing different hypersensitivity reactions in humans, sometimes leading to asthma. Although serious infections in the immunocompetent hosts are rare, this fungus may cause opportunistic infections in immunocompromised patients, especially in those undergoing solid organ or bone marrow transplantation.”

Ringworm in a cattlebeast. Photo: I. Kaszak

They concluded that fungal skin infections (dermatomycoses) in companion animals are caused by both mycelial fungi and yeasts.

Among the animals with dermatitic lesions, yeasts and yeast-like fungi from Malassezia and Candida genera were the most frequent fungal cause.

“In more than 20% of positive cultures, dermatophytes were identified, mostly belonging to the Microsporum genus, while Trichophyton were less common.

They noted the increasing frequency in which Alternaria moulds are found in clinical cases of dermatitis. These fungi should be considered as the causative agents of skin fungal infections, based on the presence of their proliferating spores in the skin samples as the sole organism.

Malassezia pachydermatis was isolated in over 80% of otitis externa cases in dogs and remains the most frequent etiological agent of this disease. In this group of animals Candida yeasts were revealed in less than 20% of cases, which makes them the second causative agent.”

Dworecka-Kaszak, B., Biegańska, M.J. & Dąbrowska, I. Occurrence of various pathogenic and opportunistic fungi in skin diseases of domestic animals: a retrospective study. BMC Vet Res 16, 248 (2020). https://doi.org/10.1186/s12917-020-02460-x

The study, published under a Creative Commons License, can be read here

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