What exactly is blastomycosis and how rare is it in cats?

Blastomycosis is caused by Blastomyces dermatitidis, a dimorphic fungus that lives as a mold in soil and converts into a yeast form once it reaches body temperature inside a mammal. The organism favors moist, acidic soil enriched with decaying leaves or animal droppings, conditions common along riverbanks and in hardwood forests, and it is documented as endemic to the Mississippi, Ohio, and St. Lawrence-Great Lakes river valleys in North America (Centers for Disease Control and Prevention, 2024). Cats are infected far less frequently than dogs — most published feline case series describe only a handful to a few dozen confirmed cats compared to hundreds of canine cases from the same endemic regions (Merck Veterinary Manual, 2023).

Cats typically acquire the infection by inhaling airborne spores that have been disturbed from soil, so outdoor or barn cats digging, hunting, or rolling in disturbed dirt near wetlands carry higher exposure risk than strictly indoor cats. Direct skin inoculation through an existing wound is also possible but considered a secondary route (Merck Veterinary Manual, 2023). The organism cannot cross intact skin, which is one reason indoor-only cats in endemic counties are rarely diagnosed.

How long after exposure do cats start acting sick?

Once spores reach the lungs, the incubation period before visible illness commonly runs from several weeks up to roughly three months, reflecting the time it takes the fungus to establish itself in lung tissue and begin disseminating through the bloodstream (Merck Veterinary Manual, 2023). Because of this lag, owners often can't connect a cat's current lethargy or weight loss to a hike or barn visit that happened a month or two earlier, which delays the decision to test for a fungal cause rather than a bacterial one.

This incubation window is also why veterinarians ask detailed questions about a cat's outdoor access and recent travel to river-valley states even when the presenting complaint looks like ordinary upper respiratory illness. A cat that seemed fine after a weekend at a lake house can present five to twelve weeks later with progressive breathing difficulty that no longer responds to standard antibiotics.

What symptoms show up first, and which organs get hit hardest?

Because Blastomyces enters through the respiratory tract, the lungs are almost always affected first, producing labored or rapid breathing, coughing, exercise intolerance, fever, and weight loss as the fungus triggers granulomatous inflammation in lung tissue (Merck Veterinary Manual, 2023). From there the yeast form can disseminate through the bloodstream to the eyes, skin, lymph nodes, bones, and central nervous system, meaning a cat may arrive at the clinic with draining skin nodules or sudden vision changes rather than any obvious respiratory complaint.

Ocular involvement is particularly common and serious in disseminated cases — uveitis, retinal detachment, and glaucoma have all been reported, sometimes leading to permanent blindness in the affected eye if the infection isn't controlled quickly (Merck Veterinary Manual, 2023). Skin lesions typically appear as firm nodules that ulcerate and drain a serosanguineous discharge, and lymph nodes near the site can become visibly swollen.

How does a veterinarian actually diagnose blasto in a cat?

Diagnosis usually starts with chest radiographs, which in blastomycosis often show a diffuse, miliary-to-nodular lung pattern that can mimic other fungal or even neoplastic disease, so imaging alone is not confirmatory (Merck Veterinary Manual, 2023). Definitive diagnosis comes from identifying the characteristic broad-based budding yeast organisms under a microscope, obtained from cytology of lymph node aspirates, skin lesion exudate, or fluid from the lungs via bronchoalveolar lavage or fine-needle aspiration.

When cytology is inconclusive, veterinarians may send blood or urine for a Blastomyces urine antigen test, an assay more commonly validated in dogs but increasingly used in cats when biopsy samples are hard to obtain, alongside fungal culture to confirm species identification (Merck Veterinary Manual, 2023). Because blastomycosis mimics bacterial pneumonia and even some cancers on imaging, a specific fungal diagnosis before starting months of antifungal treatment is important for both the cat's welfare and the owner's expectations.

What does treatment for feline blastomycosis actually involve?

The mainstay of treatment is itraconazole, an oral antifungal drug that must typically be continued for a minimum of two to three months and often six months or longer, with treatment length guided by resolution of clinical signs and follow-up chest radiographs rather than a fixed calendar date (Merck Veterinary Manual, 2023). Fluconazole is sometimes substituted, particularly when central nervous system or ocular disease is present, because it crosses the blood-brain barrier and penetrates ocular tissue more reliably than itraconazole.

Cats with severe respiratory compromise at diagnosis may need supplemental oxygen and hospitalization during the first days of treatment, since a rapid die-off of fungal organisms can transiently worsen lung inflammation right after antifungal therapy starts. Routine bloodwork to monitor liver values is standard during treatment, since itraconazole is metabolized by the liver and can cause elevated liver enzymes or appetite loss in some cats (Merck Veterinary Manual, 2023).

What does home care and monitoring look like during months of treatment?

Owners managing a cat through itraconazole therapy should expect regular rechecks, generally every four to eight weeks, that include a physical exam, weight check, and often repeat chest radiographs to confirm the lung lesions are shrinking rather than progressing (Merck Veterinary Manual, 2023). Giving the antifungal with a meal improves absorption for most formulations, and missed or inconsistent dosing is one of the more common reasons treatment stretches longer than expected.

Keeping the cat indoors and away from the soil or water source where exposure likely occurred is a practical step during recovery, both to reduce reinfection risk and because a cat with reduced lung capacity tolerates outdoor exertion poorly. Watching appetite and energy day to day matters more than watching for any single symptom, since a cat quietly eating less or hiding more can be an early sign that the current drug or dose isn't controlling the infection.

What's the realistic long-term outlook for a cat diagnosed with blastomycosis?

Prognosis depends heavily on how disseminated the disease is at diagnosis: cats with infection confined mainly to the lungs generally have a better chance of full recovery with prolonged antifungal therapy than cats with central nervous system or severe multi-organ involvement, which carries a guarded to poor prognosis (Merck Veterinary Manual, 2023). Blastomycosis is described as potentially fatal if untreated, underscoring why early, aggressive diagnosis matters more than waiting to see if symptoms resolve on their own.

Cats that respond well to treatment and complete the full antifungal course can return to normal activity, though relapse is possible if treatment is stopped too early or if imaging hasn't confirmed full resolution of lung lesions. There is no vaccine against Blastomyces, so reducing a recovered cat's future exposure to high-risk soil and water areas remains the only practical way to lower reinfection risk (Centers for Disease Control and Prevention, 2024).