Overland Park Veterinary Emergency and Specialty
Histoplasmosis is a fungal infection caused by Histoplasma capsulatum, a soil-borne fungus found worldwide, particularly in the Ohio, Missouri, and Mississippi River valleys. It exists in a hyphal form (structures that branch out to form the body of a fungus) in the environment and converts to yeast in the tissues of infected animals. Bats are the primary reservoir for Histoplasmosis, and this fungal organism is found in bat guano and decaying bird feces, especially from blackbird or starling roosts and chicken coops. Risk factors include disruption of soil, such as from construction.
How It Spreads and Affects Pets
The primary mode of infection is inhalation of fungal spores, which settle in the lungs and transition into yeast. These yeast cells replicate inside immune cells, spreading through the bloodstream and lymphatic system to multiple organs, including the lungs, gastrointestinal tract, liver, spleen, eyes, skin, and even the nervous system. Pets may also be infected by ingesting spores. The incubation period (period between infection and development of illness) is 2-3 weeks but can be as long as several years.
Clinical Signs and Diagnosis
Histoplasmosis can present with various clinical signs that often mimic other diseases. For cats, Histoplasmosis tends to occur from ages 4-9 but has been seen in cats as young as two months and as old as fifteen and can occur in indoor cats. The mean age is 4.3 years for dogs, but it has been seen in dogs as young as 5 months and as old as 10. Sporting and working dogs may be at greater risk. Specific breeds, including Pointers, Weimaraners, and Brittany Spaniels, are at an increased risk of contracting Histoplasmosis.
Symptoms in dogs and cats include:
Weight loss (dogs and cats)
Chronic diarrhea (more common in dogs)
Respiratory signs such as coughing (dogs and cats)
Fever and lethargy (dogs and cats)
Enlarged lymph nodes (dogs and cats)
Ocular abnormalities (more common in cats)
Common diagnostic changes include anemia (low red blood cells), low blood proteins (hypoalbuminemia, reported in >75% of dogs and cats.
Diagnosis is confirmed through:
Cytology and histopathology (identifying yeast in samples from affected tissues)
Antigen testing (urine antigen tests more sensitive than serum), through MiraVista Diagnostic Labs
Antibody testing (useful in localized infections or if antigen testing is negative but you are still suspicious), through MiraVista Diagnostic Labs
Avoid Fungal antibody panels – they use antibody testing that is not sensitive or specific
Imaging (X-rays and ultrasounds can reveal internal involvement)
Treatment and Management
The primary treatment for histoplasmosis is antifungal medication, typically itraconazole. Compounded itraconazole is not recommended because it is poorly absorbed and is not equivalent to commercially available itraconazole. Fluconazole may be used in cases involving the central nervous system or eyes and other azole antifungals can be used in refractory cases or when pets are not tolerating the side effects of itraconazole or fluconazole. Severe cases may require amphotericin B for rapid fungal reduction.
Note: Anecdotally, dogs with gastrointestinal histoplasmosis can get worse before they get better and they can benefit from a short course of anti-inflammatory steroids.
Key points in treatment:
Duration: At least six months, continuing one month beyond negative antigen tests
Monitoring: Regular antigen testing one month into therapy, then every three months during treatment, and post-recovery checkups and antigen testing at 3, 6, and 12 months post-finishing treatment
Supportive Care: Anti-inflammatory steroids, nutritional support, and symptom management
Prognosis and Prevention
The prognosis depends on disease severity and organ involvement. With early detection and appropriate treatment, many pets recover well. However, disseminated cases with severe lung may have a guarded prognosis.
In dogs, a JAVMA 2018 paper reviewing 79 canine cases of histoplasmosis found that clinical remission was achieved in 67% of cases. Older studies reported that pulmonary involvement had a fair to excellent prognosis, but more recent findings suggest that dyspnea and the need for oxygen supplementation are poor prognostic indicators. Prognosis in dogs with disseminated or gastrointestinal disease varies from guarded to good, depending on organ involvement and severity. Other factors associated with lower survival or remission rates include palpable abdominal organomegaly, icterus, anemia, thrombocytopenia, hypercalcemia, and elevated ALP at initial evaluation. Interestingly, dogs that presented with diarrhea had a higher survival rate at six months compared to those without gastrointestinal symptoms.
For cats, a JFMS 2018 paper stated that more severe respiratory diseases (such as dyspnea or increased lung sounds) were significantly more common in cats that didn’t survive hospital discharges and one month after diagnosis. Additionally, a longer hospital stay, neurologic signs, circulating fungal organisms, severe anemia, pancytopenia, and hyperbilirubinemia were associated with a worse prognosis. Prednisolone use was also associated with a worse prognosis, though it was more commonly used in severe cases, particularly those involving respiratory distress. Unlike in humans, pre-treatment Histoplasma urine antigen levels do not correlate with disease outcomes in cats.
While histoplasmosis cannot be directly transmitted between pets and humans, pet infections may indicate environmental exposure risks for owners. Avoiding soil disturbances in endemic areas and reducing outdoor exposure for immunocompromised pets may help reduce the risk of infection.
Missy, a 7-year-old Chihuahua mix, presented to our Internal Medicine team with a nine-month history of chronic diarrhea. Previous treatments, including diet changes and a steroid trial, provided temporary relief but no resolution. Diagnostics revealed mild hypoalbuminemia, hyperglobulinemia, and intestinal abnormalities on ultrasound.
A Histoplasma urine antigen test returned positive, leading to a diagnosis of gastrointestinal histoplasmosis. Treatment with itraconazole was initiated, but Missy’s condition temporarily worsened, which can happen with GI histoplasmosis, before improving with the addition of a short course of anti-inflammatory steroids. After several weeks of therapy, her symptoms resolved, and follow-up testing confirmed a successful response.
Conclusion
Histoplasmosis is a critical but treatable fungal disease in pets. Awareness of clinical signs, early diagnostics, and appropriate antifungal therapy are crucial for successful outcomes. Please contact your veterinarian for more information regarding this disease.
