Rhodotorulae are environmentally ubiquitous yeasts that were originally considered non-pathogenic. However, over the last fifty years, Rhodotorula mucilaginosa (R. mucilaginosa) has established itself as an emerging opportunistic pathogen incriminated in several systemic and localized infections in humans and animals. Most of the rhodotorulosis cases were linked directly or indirectly to an immunocompromising event in the affected cases. Nevertheless, recently, a few non-immunocompromised cases were reported. In the current study, performed throughout the period from April/2023 to April/2024, 450 samples were collected from pet animals and investigated for the existence and coexistence of Rhodotorula spp. in different clinically diagnosed infectious cases. 173 (38.5%) samples showed positive direct microscopic slides of different sizes of Gram-positive budding yeast cells, 21 (4.7%). Rhodotorula isolates mixed with other yeasts and/or bacterial pathogens were recovered from nasal passages and ear canal swabs collected from dogs and cats suffering from nasal affection and otitis externa. Laboratory investigations were based on sample collection, microscopic examination, primary isolation and identification, biochemical and post-culturing characterization, antifungal susceptibility testing, VITEK 2 Compact Identification System, DNA extraction, PCR amplification, sequencing, and phylogenetic analysis. Moreover, antifungal susceptibility testing based on the standard broth microdilution test was applied to the recovered Rhodotorula isolates. In conclusion, the present findings spotlighted a prospective insight into the role of the emergence of R. mucilaginosa among pet animals and its possible public health concerns.

Hippocrates was the first to describe a yeast infection caused by Candida albicans in the fifth century and named it “thrush”. Since then, the primary and the most frequently isolated yeast associated with human and animal infections is Candida albicans. Non-albicans Candida and other new and emerging yeast agents have been demonstrated in various forms and types of disease, some of which are systemic life-threatening and others are local superficial conditions. Despite the recognition of different yeasts as agents of disease, little medical or scientific concern was given to them, in contrast to the many serious and highly prevalent bacterial infections recognized in the late 1800s. However, the advent in the 1960s of new modalities to treat cancer, increasing use of central venous catheters, an explosion in new antibacterial agents’ development, increases in average life expectancy, and other medical developments soon paved the way for innocuous yeasts to cause serious infections.

With further developments in medical intervention, increasing the population of immunosuppressed and immune-deficient patients, and raising animal-human interaction habitual occasions, the list of yeasts that can cause disease continues to grow. Disturbances of the microbiome niche environment and conditions that adversely affect the host immune system predispose and facilitate the pathogenicity transition of opportunistic microbes and emerging new infectious diseases. Factors predisposing the human or animal host to fungal infection establishment are long-term or repeated prescription of broad-spectrum antibiotics, impairments of epithelial barriers especially that reflect on the skin and the gastrointestinal tract e.g., by chemotherapy, surgical interventions, or catheter-based medical processes, and treatment with immunosuppressive drugs such as corticosteroids. Rhodotorula spp. are saprophytic yeasts that are usually isolated from dairy products and dumpy or moist environmental sources including bathroom surfaces, swimming pools, and planted places as commensal inhabitants of the gastrointestinal tract and skin of mammals.

Reported infections with Rhodotorula spp. in pet animals are rare and usually confirmed by laboratory isolation, identification, antifungal susceptibility testing, and the degree of treatment response during the case follow-up. Opportunistic Rhodotorula infection of humans, which is known as rhodotorulosis, is increasing in predisposed immunocompromised cases and is associated with high mortality rates despite interceptive antifungal treatments. Therefore, rhodotorulosis showed an increasing concern as a zoonotic threat that can be transmitted directly to susceptible humans from infected or apparently healthy companion animals, or to a patient from a medical occupation member who raises an infected pet animal, or indirectly through medical equipment manipulation such as catheters and other medical prosthetic transplants.

Nowadays, fungal infections, whether zoonotic or sapronotic, are increasingly important to public health worldwide. A number of these infections are due to established potentially pathogenic fungal agents such as dermatophytosis, histoplasmosis, and candidiasis. On the other hand, it is notable that some emerging opportunistic pathogens with zoonotic potential have inadequate attention by international public health efforts, leading to insufficient attention to their precautionary measures. Therefore, the current study highlighted the incrimination of one of those neglected causes; R. mucilaginosa in several mixed infection cases among pet animals.

The current study represents the first elucidation of the predicted evolution of the emerging opportunistic fungal pathogen, R. mucilaginosa, focusing on the cross-kingdom-talks conceptualization as well as hitting the warning bell to be ready for any uprising microbial-based danger that may face humanity in the upcoming few years. Also, it is worth mentioning that controlling human contact with animal reservoirs helps to safeguard vulnerable groups and is an essential part of any prospective prevention strategy. To more accurately describe the burden, distribution, mortality, and socioeconomic effects of any possible neglected zoonotic or sapronotic potentials and to provide an integrated platform of prevention and control techniques, greater awareness-raising initiatives are required. Additional research investigations on the pathogenesis, expected risks, molecular and epidemiological tracing and follow-up, diagnostic techniques, as well as efficient and early therapeutic protocols are currently of importance. The current findings leave us with a very urgent question; Does R. mucilaginosa pose a public health risk or is it an exaggerated concern? Therefore, and due to the scarce contribution of the scientific community in this area of inquiry, further studies should be performed to ascertain the etiologic significance of different Rhodotorula spp. in other and more clinical diseases of humans and animals that may be incriminated in, currently and prospectively.