Malassezia pachydermatis carriage in dog owners

Emerging Infectious Diseases, Jan, 2005 by Daniel O. Morris, Kathleen O'Shea, Frances S. Shofer, Shelley Rankin

Yeasts of the genus Malassezia serve as both commensal microorganisms and pathogens on the skin of humans and domestic animals. Although rare, cases of life-threatening fungemia in people have been attributed to Malassezia pachydermatis, for which dogs are a natural host. Zoonotic transfer has been documented from dogs to immunocompromised patients by healthcare workers who own dogs. We investigated the role of pet dogs as risk factors for mechanical carriage of M. pachydermatis on human hands. Dogs and their owners were sampled as pairs, by fungal culture and nested polymerase chain reaction (PCR). Although fungal culture was not a reliable means by which to detect carriage of the yeast on human hands, PCR identified M. pachydermatis on most ([approximately equal to] 93%) human participants. Human carriage of ubiquitous opportunistic pathogens such as M. pachydermatis underscores the importance of good hand hygiene healthcare professionals.

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Yeasts of the genus Malassezia, part of the normal cutaneous microflora of mammals, can cause life-threatening fungemia and other nosocomial infections in immunocompromised humans, especially in preterm neonates (1-3). While disease in humans is most commonly caused by Malassezia furfur, a commensal of human skin (4), it has also resulted from M. pachydermatis, for which dogs are a natural host (5-8). In some cases, the sources of human infections have been traced to pet dogs owned by healthcare workers (9).

In normal dogs with healthy skin, M. pachydermatis colonizes the stratum corneum in very low numbers (10). In dogs with allergic skin disease, however, the numbers of M. pachydermatis may increase dramatically on the skin and within the ear canals (11-13). The potential for human exposure to the organism is therefore quite great. While no evidence has shown that dogs represent an overt health concern to immunocompromised humans, the increasing incidence of immune suppression in humans worldwide suggests that a survey of the zoonotic potential of this organism is relevant to modern hospital hygiene practices.

We hypothesized that mechanical transfer of M. pachydermatis from the inflamed skin of dogs with M. pachydermatis infection to the healthy skin of humans occurs commonly. We also hypothesized that atopic dermatitis of dogs, which is a widely documented risk factor for M. pachydermatis infection, would be a risk factor for human carriage. The purpose of this study was to evaluate the prevalence of M. pachydermatis in dogs and their owners as determined by microbiologic culture and polymerase chain reaction (PCR). The ultimate goal was to assess whether pet owners could be reservoirs for mechanical transfer of the organism.

Materials and Methods

Study Population

Approvals for privately owned animal use and sampling of human participants were obtained from the University of Pennsylvania's Institutional Animal Care and Use Committee and the biomedical institutional review board, respectively, and informed consent was obtained from participants.

Dogs referred to the Dermatology and Allergy Service of the Matthew J. Ryan Veterinary Hospital of the University of Pennsylvania (VHUP) for evaluation of allergic skin or ear canal disease were screened for secondary M. pachydermatis overgrowth (i.e., infection, commonly referred to as malassezia dermatitis or malassezia otitis) by using the tape strip and ear swab methods described below. Dogs with positive cytologic results and their human companions were recruited for the disease group.

A control group of healthy dogs and their human companions were recruited from the faculty, staff, and students at the VHUP. Samples were taken from dogs with normal skin and ear canals (defined as no episodes of skin disease in the preceding calendar year and no evidence of inflammation at the time of sampling) and their human companions by using the same techniques as for the disease group.

Cytology

The cytologic collection technique used for skin was the tape strip method (14). A piece of clear cellophane tape (5 cm x 2 cm) was applied to the surface of the skin 2 times in succession, removed, stained with a modified Wright's stain (Diff Quik, Dade Behring, Deerfield, IL), and applied to a glass slide for microscopic analysis. From dogs with atopic dermatitis, inflamed skin, which was typically alopecic from self-trauma due to pruritus, was sampled. One or more of the following regions were sampled from each dog: axilla, groin, chin, ventral neck fold, paronychium, and interdigital spaces (dorsal or plantar) according to clinical signs. From dogs with ear canal infections, cotton-tipped swabs of ear canal exudates were collected and streaked onto glass slides, which were then heat fixed and stained.

All slides were examined at 1,000x (high power under oil immersion) magnification. This technique allows for microscopic visualization of any microorganisms that reside on the surface of the skin or within the ear canal cerumen. When a minimum number of yeast cells per oil immersion field (oif) was exceeded (>1 yeast/oif on the skin, >5 yeast/oif in ear canal exudates) (10,15,16), excessive colonization by the organism (i.e., infection) was diagnosed, and these dogs were assigned to the disease group.