Rothacker, Jaffey et al. (2020) — Medical Mycology
Novel Penicillium Species Causing Disseminated Disease in a Labrador Retriever Dog
Published: July 31, 2026
In the history of veterinary medicine, the discovery of a pathogen novel enough to represent an entirely new species is rare. A case report published in Medical Mycology documents exactly that — a 3-year-old Labrador Retriever who developed severe disseminated fungal disease caused by a Penicillium species that had never previously been described in the scientific literature. The species has since been formally named Penicillium labradorum after the breed in which it was first identified. 🐾
Researchers Tatiana Rothacker, J. Jaffey, A. Royal, and their team described the clinical presentation, diagnostic workup, treatment course, and mycological characterisation of this case — a case that began as a challenging diagnostic puzzle and ended with the formal description of a new species of fungus, added to the scientific record approximately 587 days after the dog first presented with symptoms.
How the Case Unfolded
The Labrador Retriever presented with a cluster of signs that individually might suggest several different conditions but together pointed toward systemic disease: lethargy, enlarged lymph nodes, rapid breathing, moderate pitting edema, and non-weight-bearing lameness of the right hind limb. Fine-needle aspirate biopsies from lymph nodes produced the first diagnostic clarity — cytological examination revealed pyogranulomatous inflammation with abundant fungal fragments and hyphae both outside cells and within phagocytic immune cells attempting to contain them.
The morphology of the organisms was consistent with a category of fungal infection called hyalohyphomycosis — infections caused by hyaline, or non-pigmented, moulds. Within that category, the differential included Fusarium, Penicillium, and Paecilomyces species. Fungal culture from the lymph nodes eventually yielded an isolate that molecular identification testing placed within Penicillium section Exilicaulis — but as an undescribed species. It did not match any known species in the databases available.
Subsequent phylogenetic analysis conducted approximately 21 months after initial isolation confirmed the isolate’s position within the Penicillium parvum clade but at phylogenetic distance sufficient from all other known species in the section to warrant formal description as a new species. Penicillium labradorum was named and entered into the scientific record as the first formally described member of this lineage.
Antifungal susceptibility testing produced clinically actionable data alongside the taxonomic finding. The isolate showed low minimum inhibitory concentrations — indicating potentially useful activity — against terbinafine, voriconazole, and posaconazole. It showed in vitro resistance to fluconazole. This was significant context given that the dog had already been treated with fluconazole — the resistance profile was consistent with the treatment failure observed clinically. Subsequent treatment attempts included itraconazole, amphotericin B lipid complex, voriconazole, and terbinafine across the 587-day course of the disease. The dog was ultimately euthanised as clinical signs worsened and quality of life could no longer be maintained.
What This Case Tells Us About Emerging Fungal Disease in Dogs ⚠️
Disseminated fungal infections in immunocompetent dogs are uncommon, and when they occur they are frequently caused by geographically associated pathogens — Blastomyces, Histoplasma, Aspergillus, Cryptococcus — whose epidemiology and clinical behaviour are reasonably well characterised. A Penicillium species causing disseminated systemic disease severe enough to involve multiple lymph node regions, cause respiratory signs, edema, and lameness, and resist multiple antifungal agents over nearly two years represents an unusual and poorly understood clinical scenario.
The discovery that the causative organism was a previously undescribed species adds a further dimension. When a pathogen has not been described before, there is no established clinical literature to guide treatment decisions, no known natural history of the infection to set prognostic expectations, and no empirical treatment data beyond in vitro susceptibility testing that may or may not predict clinical response. The treating team was navigating genuinely novel diagnostic and therapeutic territory — and the 587-day treatment course reflects both the severity of the disease and the absence of an established protocol to follow.
The practical implication for owners is not heightened alarm about a specific new pathogen — Penicillium labradorum appears to be exceedingly rare, and this case represents its only documented occurrence in a dog. The more relevant takeaway is the broader principle: unusual or treatment-resistant infections in dogs sometimes reflect pathogens that have not yet been adequately characterised, and in those cases, specialist referral to veterinary internal medicine and access to mycology laboratory services with molecular identification capability can be the difference between a diagnosis and a prolonged diagnostic uncertainty.
What a 587-Day Fight Reveals About the Bond 🐕
The clinical timeline in this case — 587 days from initial diagnosis to euthanasia — represents nearly two years of intensive veterinary management, owner commitment, and the kind of sustained care that a seriously ill animal requires. Multiple treatment protocols. Repeated reassessments. Ongoing monitoring. The decision, eventually, that quality of life could no longer be maintained.
At Zoeta Dogsoul, cases like this one sit at the intersection of physical reality and relational depth that NeuroBond points toward in its most serious dimension. Caring for a dog through prolonged severe illness is one of the most demanding expressions of what the human-dog bond can require of an owner. It asks for presence, for informed decision-making, for the capacity to hold the dog’s welfare as the primary measure when the cost of continued treatment no longer serves them. That is not a training question or a behaviour question. It is the deepest layer of what it means to be genuinely responsible for another living being. 🐾
Source: Rothacker, T. R., Jaffey, J., Royal, A., et al. (2020). Novel Penicillium species causing disseminated disease in a Labrador Retriever dog. Medical Mycology. Published April 3, 2020.
