Feline Sporotrichosis Is Becoming Harder to Treat. The Data From Thailand Should Be on Every Clinician's Radar.

Sporotrichosis does not get the attention in North American veterinary practice that it probably deserves. It is a fungal disease that is frequently underrecognized, genuinely difficult to treat, and critically zoonotic. Veterinary professionals who handle infected cats are at real risk of infection through scratches, bites, or contact with wound exudates. The fungus does not care whose skin it enters.

A study just published in Emerging Infectious Diseases analyzed 1,178 confirmed feline Sporothrix schenckii complex isolates collected in Thailand between 2023 and 2025 and evaluated their susceptibility to eight antifungal drugs. The findings are a clinical warning worth understanding regardless of where you practice, because the patterns documented in this study reflect what happens when a zoonotic fungal pathogen meets prolonged antifungal treatment pressure at scale.

The Disease Is Spreading and Case Numbers Are Rising

Sporothrix schenckii complex is a group of thermally dimorphic fungi that live as molds in the environment and convert to yeast form inside host tissue. Infection occurs when fungal spores from contaminated material enter subcutaneous tissue through cuts or scratches. Cats are the most susceptible species and serve as a highly efficient reservoir and transmitter. Infected cats can shed infectious yeast organisms through saliva, nasal secretions, wound exudates, and scratches or bites to humans, other cats, and other species.

In Thailand, confirmed feline sporotrichosis cases at the study's laboratory increased from 225 in 2023 to 383 in 2024 to 570 in 2025. That is a 153% increase over three years at a single referral laboratory. The authors note that these represent laboratory-confirmed cases and that the true number of infections is almost certainly higher, since not every infected cat undergoes confirmatory testing.

The demographic picture of affected cats was consistent with previous reports from the region. Most were young adult males, likely because roaming behavior increases exposure to environmental pathogens. Domestic shorthairs represented 91.6% of cases, reflecting their prevalence in Thailand's cat population rather than any breed-specific susceptibility. Retroviral infection status, FeLV or FIV positive in 28.1% of the cats tested, did not show a significant association with antifungal resistance patterns, which suggests that resistance in these isolates is more closely tied to treatment pressure and strain virulence than to the immune status of the host.

The First-Line Drug Is Failing

Itraconazole is the antifungal drug of choice and first-line treatment for sporotrichosis in both humans and animals. It has been the backbone of feline sporotrichosis management for years. The data from this study should prompt serious reconsideration of empirical itraconazole use in clinically challenging cases.

Of the 1,178 isolates tested, 687 showed reduced susceptibility to itraconazole. That is 58.3% of all isolates. The MIC50 for itraconazole, the concentration required to inhibit 50% of isolates, increased 16-fold compared to data from an earlier outbreak period in Thailand. The MIC90 increased more than 8-fold. A greater than 4-fold increase in these values is the established threshold for indicating acquired reduced susceptibility in non-wild-type strains. These isolates cleared that threshold by a wide margin.

Among the itraconazole non-wild-type isolates, cross-reduced susceptibility was common. A significant proportion showed co-reduced susceptibility to itraconazole and posaconazole, which shares overlapping molecular targets in the ergosterol biosynthesis pathway. Multidrug reduced susceptibility involving itraconazole, posaconazole, and amphotericin B was also documented. Only 31.2% of all isolates in the study showed susceptibility to all tested antifungal drugs.

The authors identify several contributing factors to this resistance profile. Sporotrichosis treatment requires antifungal drugs for several months. In Thailand, treatment compliance is affected by drug cost, the difficulty of medicating cats consistently, owner concern about zoonotic transmission, and the absence of structured control programs for stray cats. Incomplete or interrupted therapy creates precisely the selection pressure that drives resistance. This is not a story unique to Thailand. Any setting where prolonged antifungal therapy faces compliance and access challenges is a setting where this pattern can develop.

What Still Works

The picture on resistance is not uniformly bleak. Three drugs maintained strong susceptibility profiles across the dataset.

Terbinafine was the standout. Wild-type susceptibility was observed in 98.2% of isolates. MIC values for terbinafine remained stable compared to previous data from Thailand, with only a 1.3-fold increase in MIC50 and no change in MIC90. Cross-reduced susceptibility involving terbinafine was extremely rare. The authors propose a study-specific cutoff of 4 micrograms per milliliter for terbinafine as a surveillance and treatment guidance tool, and their findings support the use of terbinafine as an alternative for itraconazole non-wild-type isolates, either as monotherapy or in combination with posaconazole, based on in vitro susceptibility testing.

Voriconazole showed wild-type susceptibility in 97.3% of isolates. Amphotericin B maintained wild-type susceptibility in 79% of isolates, though the authors note a concerning 8-fold increase in MIC50 compared to earlier data, and the nephrotoxicity risk of amphotericin B at the concentrations required to treat borderline-susceptible isolates requires careful consideration.

Fluconazole and flucytosine showed consistently high MIC values across the dataset and are not useful options for this pathogen.

The Zoonotic Risk for Veterinary Professionals

This is the part of the sporotrichosis story that veterinary professionals sometimes underestimate. Cats with sporotrichosis are efficient transmitters. The infectious yeast organisms are present in wound exudates, saliva, and nasal secretions. A scratch or bite during examination or treatment is a meaningful exposure event. Veterinary staff who handle cats with cutaneous ulcerative lesions without appropriate protective measures are at real risk.

Human sporotrichosis from cat-to-person transmission is well documented in outbreak regions. In Brazil, where Sporothrix brasiliensis drives a large ongoing epidemic, veterinary professionals have been disproportionately represented among human cases. The Thailand outbreak involves a specific clade of Sporothrix schenckii s.s. with documented cat-to-cat and cat-to-human transmission.

For practices that see cats with chronic, non-healing cutaneous lesions, sporotrichosis should be on the differential. For those cases, appropriate protective equipment during handling is not optional. Gloves, eye protection, and care around scratches and bites are the practical prevention measures. And when a diagnosis is confirmed, the conversation with the owner about their own exposure risk is a clinical and ethical obligation.

What This Means for Clinical Practice

The core message from this study is that empirical itraconazole therapy in suspected or confirmed feline sporotrichosis cases, particularly those that have already failed treatment or that present as clinically complex, is increasingly unreliable. Susceptibility testing is not a research tool in this context. It is a clinical decision-support tool that the data from this study supports using in refractory cases.

For practices outside of Asia where sporotrichosis remains relatively uncommon, the direct clinical application may seem distant. But the dynamics documented in this study, prolonged treatment pressure on a fungal pathogen with zoonotic transmission potential, producing escalating resistance patterns that compromise first-line therapy, describe a process that can unfold anywhere the disease is present and treatment is challenging. Understanding the susceptibility landscape now, and knowing that terbinafine currently represents a more reliable alternative than itraconazole in resistant cases, is the kind of clinical knowledge that prepares practitioners to make better decisions when they encounter cases that do not respond as expected.

The full study is published in Emerging Infectious Diseases, a peer-reviewed journal of the Centers for Disease Control and Prevention.

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