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- Histoplasmosis in cats
Susan E Fielder 1, James H Meinkoth 1, Theresa E Rizzi 1, Andrew S Hanzlicek 2, Ruth Mackenzie Hallman 2
Background
Histoplasmosis, caused by the soil-borne fungus Histoplasma capsulatum, is the second most common fungal infection in domestic cats. While typically presenting with non-specific systemic signs and respiratory involvement, bone and joint manifestations are considered rare. However, clinical experience in endemic areas like Oklahoma suggests musculoskeletal involvement may be underreported. This study aimed to describe the clinical and diagnostic findings associated with bone and joint disease due to histoplasmosis in cats.
Methods
This retrospective study reviewed medical records of cats diagnosed with histoplasmosis between 2011 and 2017 at Oklahoma State University facilities and collaborating clinics. Inclusion criteria were confirmation of histoplasmosis via cytology, histopathology, antigen testing, or culture, and clinical evidence of lameness or joint effusion. Data collected included signalment, clinical presentation, diagnostic imaging, laboratory tests, and follow-up outcomes. Remission was defined by resolution of clinical signs and supporting diagnostic criteria.
Results
Twenty-five cats met the inclusion criteria. Lameness was reported in 81% of cases and was the sole complaint in 43%. Synovial fluid cytology identified H. capsulatum in 13 of 15 cats sampled and showed inflammation in all cases, with pyogranulomatous inflammation being most common. Diagnosis was confirmed by cytology in 22 cats; others were diagnosed by culture, antigen testing, or necropsy. Osseous lesions, particularly in the tarsus and carpus, were frequent. Most cats showed radiographic evidence of bone involvement, and hematological abnormalities included non-regenerative anemia and neutrophilia. Urine antigen testing had limited sensitivity in localized disease. Resolution of clinical signs occurred in eight cats with a mean recovery time of 16 weeks following antifungal therapy.
Limitations
As a retrospective study, data completeness and standardization were variable. Four cases had limited medical records, and follow-up data were inconsistent, hindering comprehensive assessment of long-term outcomes. Diagnostic imaging and testing protocols were not standardized across cases.
Conclusions
Histoplasmosis should be considered in cats presenting with musculoskeletal signs in endemic areas. Synovial fluid cytology is frequently diagnostic, although additional testing may be necessary. This study highlights inflammatory arthritis as a significant and potentially underrecognized manifestation of feline histoplasmosis.
Radiographic abnormalities in feline pulmonary histoplasmosis. A, Lateral thoracic radiograph from an 8-month-old intact male domestic shorthair cat with disseminated histoplasmosis. There is a diffuse bronchointerstitial pulmonary pattern and evidence of sternal lymphadenopathy (arrow). In the viewable abdomen, marked splenomegaly can be appreciated. B, Lateral thoracic radiograph from a 7-month-old male neutered domestic shorthair cat with histoplasmosis that was evaluated for tachypnea and dyspnea of 10 days duration. There is a diffuse, severe, interstitial-to-coalescing and patchy pulmonary pattern that obscures visualization of the pulmonary vasculature and cardiac contours. (Courtesy of Lorrie Gaschen, Louisiana State University.)
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