Serious severity

Histoplasmosis

A fungal infection caught by inhaling spores from soil enriched with bird or bat droppings. In cats it rarely stays in the lungs — by the time it is diagnosed it has usually spread through the body.

⚠️ Common symptoms

Gradual weight loss and poor appetite over weeksPersistent fever that does not respond to antibioticsLaboured or rapid breathing, sometimes with coughingLethargy and marked weaknessPale gums from anaemiaLameness or reluctance to walk, from bone involvementEye changes, including cloudiness or sudden blindnessEnlarged lymph nodes, or skin nodules that ulcerate

A Diagnosis That Gets Missed

Histoplasmosis is a great imitator. A cat that is quietly losing weight, running a fever and eating poorly looks like a cat with lymphoma, or FIP, or a chronic bacterial infection, and it is frequently treated for several of those before anyone thinks of a fungus. The tell is a fever that does not answer to antibiotics in a cat with a plausible soil exposure, and the diagnosis is often made by finding the organism rather than by suspecting it first.

How It Is Confirmed

The quickest route is cytology. The yeast is small, round and found inside macrophages, and it can be identified in samples from lung washes, lymph nodes, bone marrow, skin lesions or the buffy coat of a blood sample — bone marrow is a particularly high-yield site in cats with anaemia. Urine antigen testing has become widely used and is both sensitive and convenient, with the added benefit that falling antigen levels are a good way to track response to treatment. Chest radiographs typically show a diffuse pattern through the lungs, and an eye examination is standard, since inflammation at the back of the eye is common and is sometimes the finding that unlocks the case.

What Recovery Looks Like

Cats treated early, with disease confined largely to the lungs, do reasonably well. The prognosis worsens considerably with widespread dissemination, bone marrow involvement or nervous system disease, and a proportion of severely affected cats do not survive the first weeks regardless of treatment. Owners should be prepared for months of daily medication, repeat blood tests, and a bill that accumulates — this is one of the conditions where an honest conversation about cost and commitment at the outset is kinder than discovering it halfway through.

Relapse after apparently successful treatment is well recognised, usually within the first year. That is the reason for treating well past the point at which the cat looks entirely better, and for repeat antigen testing rather than judging by appearance alone.

🔍 Causes

Histoplasma capsulatum is a dimorphic fungus that lives in the soil as a mould and converts to a yeast form at body temperature. It thrives in damp soil enriched with nitrogen from bird or bat droppings, which is why river valleys, chicken coops, old barns, caves and roosting sites are the classic hazards. Cats acquire it by inhaling spores disturbed from contaminated ground; the fungus is then taken up by immune cells and carried through the bloodstream, which is precisely why feline cases so often present as disseminated disease affecting lungs, bone marrow, eyes, bone, gut and skin at once. It is not contagious between animals or from cat to human — shared infection in a household reflects shared exposure to the same soil, not transmission.

🛡 Prevention

Prevention is largely a matter of limiting exposure in endemic regions, which for the classic form means the Ohio and Mississippi river valleys and comparable river systems elsewhere. Keeping cats indoors removes most of the risk. Avoid letting cats into old barns, poultry houses, derelict outbuildings and any area where bat or bird droppings have accumulated, and be aware that disturbing such soil — demolition, digging, clearing a loft — aerosolises spores in large numbers. There is no vaccine. Cats that are immunosuppressed, whether from FeLV, FIV or medication, are less able to contain an early infection and deserve more caution.

💊 Treatment

Antifungal therapy is the mainstay, and it is a long commitment rather than a course. Itraconazole is the usual first choice in cats, given daily for a minimum of four to six months and frequently longer — treatment continues for at least a month past the resolution of all clinical signs, and stopping early is a common cause of relapse. Fluconazole is used where the eyes or nervous system are involved, since it penetrates those sites better; amphotericin B is reserved for severely ill cats needing rapid control. Liver values are monitored throughout, because azole antifungals can be hepatotoxic and cats sometimes go off their food on them. Supportive care carries many patients through the first weeks: oxygen for those struggling to breathe, appetite stimulants, assisted feeding, and occasionally transfusion for severe anaemia.

When to see a vet: See a vet for any cat losing weight steadily over weeks, running an unexplained fever, or breathing faster than usual at rest. Urgent attention is needed for laboured breathing, open-mouth breathing, collapse, sudden blindness, or very pale gums. Tell the vet if the cat has had access to barns, chicken runs, caves or areas with bird or bat droppings, and mention any travel — a geographic history frequently changes which tests are run and how quickly.

⚕️ Medical disclaimer

  • This information is educational only and is not a substitute for veterinary advice.
  • Symptoms overlap between conditions — only a vet can diagnose accurately.
  • Never withhold treatment or self-medicate based on information found online.
  • When in doubt, call your vet. Early intervention almost always leads to better outcomes.

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