History
A 1.5-year-old 2.72-kg (6-lb) spayed female domestic shorthair cat was taken to a veterinarian because of abnormal breathing and lethargy of several days’ duration. At this visit, radiography revealed a diffuse miliary pattern in all lung fields. The cat was initially treated with cefovecin sodium,a but there was no response to treatment after 1 week. At the 1-week recheck examination, cevofecin treatment was discontinued, and systemic administration of ketoconazole was initiated for 2 weeks. Additional diagnostic testing was declined by the owner.
Clinical and Gross Findings
Following 14 days of treatment, the cat had become anorectic and its respiratory tract disease had progressively worsened with the development of severe dyspnea. Given the lack of response to treatment and poor prognosis, the cat was euthanized by IV injection of pentobarbital solution and submitted to the Tifton Veterinary Diagnostic and Investigational Laboratory for necropsy. Primary pulmonary disease was suspected.
At necropsy, the cat's oral and conjunctival mucosae were pale gray. When the thoracic cavity was opened, all lung lobes appeared mottled dark red to beige. The lung parenchyma was expanded by poorly defined, coalescing to diffuse, beige-gray, firm, slightly raised foci that elevated the pleural surface (Figure 1). Lung sections sank in formalin solution. No other notable findings were observed in the carcass. Samples from all major organ systems were fixed in neutral-buffered 10% formalin and routinely processed for histologic examination. Fresh tissue samples were submitted for bacterial culture, fungal culture, and virologic and parasitological assessments.
Photograph of the lungs of a 1.5-year-old domestic shorthair cat that was initially evaluated because of abnormal breathing and lethargy of several days’ duration. Treatment with cefovecin was ineffective. The cat received ketoconazole for 2 weeks, after which it became anorectic and developed severe dyspnea. Given the lack of response to treatment and poor prognosis, the cat was euthanized and underwent necropsy. Notice that the lung lobes are mottled dark red to beige with coalescing, slightly raised, firm beige-gray foci.
Citation: Journal of the American Veterinary Medical Association 255, 1; 10.2460/javma.255.1.63
Formulate differential diagnoses from the history, clinical findings, and Figure 1—then turn the page→
Histopathologic Findings
Formalin-fixed sections of paraffin-embedded tissue from all major organ systems were routinely processed and stained with H&E stain. Following histologic examination of the tissue sections, the most prominent lesions were found in the lungs. The pulmonary parenchyma was diffusely disrupted by severe inflammation. The alveoli were distended with large numbers of foamy macrophages, lesser numbers of neutrophils, and occasional multinucleated giant cells (Figure 2). Most macrophages and multinucleated giant cells contained numerous, intracytoplasmic, round to ovoid (maximum dimension, 2 to 4 μm), basophilic organisms bordered by a thin halo of clear space (Figure 3). Interalveolar septa were thickened by macrophages and mild smooth muscle hyperplasia. The bronchi and bronchioles had a hyperplastic to attenuated epithelium and the lumina contained similar aggregates of macrophages and neutrophils. Occasional, lesser affected alveoli were filled with an eosinophilic homogeneous material and low numbers of macrophages (pulmonary edema). The intrahistiocytic basophilic organisms were argyrophilic when stained with Grocott methenamine-silver stain.
Photomicrograph of a section of lung tissue from the cat in Figure 1. Diffusely, alveoli contain high numbers of macrophages with fewer neutrophils and multinucleated giant cells. Centrally, a bronchiole is filled with similar aggregates of inflammatory cells. H&E stain; bar = 100 μm.
Citation: Journal of the American Veterinary Medical Association 255, 1; 10.2460/javma.255.1.63
High-magnification photomicrograph of inflamed alveoli in a lung tissue section from the cat in Figure 1. Macrophages are filled with numerous, intracellular, round to ovoid (maximum dimension, 2 to 4 μm in diameter), basophilic yeast. H&E stain; bar = 50 μm. Inset—Photomicrograph of the lung tissue highlighting numerous intralesional and intracellular, argyrophilic yeast. Grocott methenamine-silver stain; bar = 50 μm.
Citation: Journal of the American Veterinary Medical Association 255, 1; 10.2460/javma.255.1.63
Liver tissue was congested and disrupted by randomly distributed foci of mild to moderate mixed inflammation. In these areas, hepatic cords were obscured by moderate-density aggregates of foamy macrophages and fewer neutrophils intermixed with mild amounts of karyorrhectic debris. Portal triads contained low numbers of lymphocytes, plasma cells, and macrophages. Among other tissues examined, histologic changes included mild, chronic lymphocytic-plasmacytic interstitial nephritis and rare, small crypt abscesses in the jejunum.
Microbiological and Laboratory Findings
Lung tissue samples were inoculated on a Sabouraud dextrose plate and a mycobiotic agar plate and incubated at 28°C. After 14 days, both plates yielded moderate growth of a fungal isolate; Histoplasma capsulatum was suspected. A PCR assay was performed on samples of the fungal culture isolate with fungus-specific primers to amplify the internal transcribed spacer regions 1 and 2 and the 5.8S ribosomal DNA.1 Following the PCR assay, an amplified 600-bp segment of DNA was purified and sequenced. A bioinformatic searchb of the sequence revealed 100% identity for H capsulatum.
Lung tissue samples were inoculated for aerobic culture on both tryptic soy agar with 5% sheep blood and MacConkey agar plates and incubated at 37°C. No bacterial growth was identified after 7 days. Fresh tissue samples of lungs, spleen, intestinal tract, brain, and heart underwent fluorescent antibody testing for feline parvovirus, FeLV, Toxoplasma gondii, and Neospora caninum, and results were negative.
Morphologic Diagnosis and Case Summary
Morphologic diagnosis and case summary: severe, diffuse, pyogranulomatous pneumonia with numerous intrahistiocytic yeasts (consistent with H capsulatum) in a cat.
Comments
For the cat of the present report, the clinical suspicion was primary fungal pneumonia with differential diagnoses of heartworm disease or neoplasia (pulmonary metastasis). The histologic features of granulomatous pneumonia were consistent with severe H capsulatum infection, and the causative agent was confirmed via fungal culture and PCR assay. The random foci of histiocytic inflammation in the liver were indicative of developing embolic hepatitis, suggesting early dissemination of the infection; however, no intralesional yeasts were identified in the examined liver tissue sections.
Other common agents involved in systemic mycotic infections in cats are Cryptococcus neoformans, Blastomyces dermatitidis, and Coccidioides immitis. However, the geographic distribution of these fungi, the morphology of the yeast in examined tissue sections, and fungal culture results made systemic mycoses other than histoplasmosis unlikely in the case described in the present report. Also, certain systemic protozoal infections of felids, namely T gondii or N caninum infection, can be associated with similar histopathologic changes and were initially considered less likely differential diagnoses for this cat; fluorescent antibody testing results and microbiological findings excluded such protozoal infections.
Histoplasma capsulatum is a dimorphic soil-borne fungus that exists in yeast form in the host and in mycelial form in the environment.2,3 The yeasts primarily infect macrophages in the host allowing for dissemination of the organism via the mononuclear phagocyte system.3 The organism is endemic to the central United States, especially along the Missouri, Mississippi, and Ohio River Valleys.2–4 Interestingly, the cat of the present report came from an area along the southeastern-most border of the typical geographic range of H capsulatum. Nevertheless, occasional infections outside this endemic area and cases of histoplasmosis in cats outside the Americas (ie, in Japan and Europe) have also been reported.3,5,6 Furthermore, soil containing high levels of bird or bat feces (eg, on poultry farms or near caves) can favor growth of the fungus in the environment.7
In general, affected immunocompetent animals do not develop clinical signs and clear the infection.2,3 Clinical disease most often develops in dogs and cats. The primary site of infection is typically the lungs with subsequent potential spread to the lymph nodes, spleen, liver, kidneys, adrenal glands, gastrointestinal tract, eyes, and bone marrow.2,3 Dissemination to the gastrointestinal tract is fairly common in dogs, yet rare in cats.2 The possibility of underlying immunosuppression was considered in the case described in the present report. However, this cat had no history of previous illness or immunocompromise (eg, previous or ongoing treatment with corticosteroid). Moreover, fluorescent antibody testing of tissue samples yielded negative results for FeLV and feline parvovirus.
In cats with histoplasmosis, common clinical signs include lethargy, weight loss, anorexia, respiratory tract signs (dyspnea, tachypnea, nasal discharge, and coughing), splenomegaly, and lymph-adenopathy.3,4,7,8 Less common signs are vomiting, diarrhea, and blindness.8 Affected cats will often have mild, nonresponsive, normochromic, normocytic anemia.3 Yeast can be identified within the cytoplasm of macrophages in cytologic preparations of affected organ samples (eg, fine-needle aspirate or bronchoalveolar lavage specimens).3
At necropsy, affected organs of cats and dogs with histoplasmosis are typically expanded by multifocal to coalescing, gray to light beige, nodular lesions of variable diameter.3 Histologically, such lesions correspond to areas of severe histiocytic to granulomatous inflammation that often distort the tissue parenchyma.2,3 Yeast can be identified microscopically within the cytoplasm of affected macrophages. The organisms appear as round to ovoid (maximum dimension, 2 to 4 μm) basophilic bodies that are surrounded by a thin clear halo.2–4 The yeast can be highlighted in histologic sections with periodic acid-Schiff stain or Grocott methenamine-silver stain.2,3 Additional diagnostic testing (eg, immunohistochemical analysis, fungal culture, and PCR assay) of affected tissue samples can be beneficial in instances where intralesional yeasts are not apparent on histologic examination of tissue sections.3
The morphological features of fungal organisms in tissue sections and the geographic location of affected animals can help differentiate H capsulatum infection from other common systemic mycoses. Cryptococcus neoformans has a worldwide distribution and these yeasts are 4 to 8 μm in diameter with a 1- to 30-μm-thick mucinous capsule; in addition, the organism often induces a minimal inflammatory response.9 Blastomyces dermatitidis is found mainly in North America, especially in the Mississippi, Missouri, and Ohio River Valleys; mid-Atlantic states; Ontario; and Quebec. The organism is comparatively larger (20 to 35 μm in diameter) and undergoes broad-based budding.10 Coccidioides immitis is endemic to desert areas of the southwest United States and Mexico. Compared with H capsulatum, C immitis organisms are much larger (10 to 80 μm in diameter), are double walled, and contain endospores.11
Treatment for histoplasmosis in cats relies on systemic administration of antifungal drugs, typically oral treatment with azoles, such as itraconazole, ketoconazole, or fluconazole.4,7 Amphotericin B can also be used in combination with azoles to treat H capsulatum infections.4,6-8 The prognosis for cats with histoplasmosis is variable; mortality rates range from 33% to 45%, even with treatment.4,8
Humans who reside in regions endemic for H capsulatum and laboratory personnel who culture the organism are also at risk for infection. However, direct transmission of H capsulatum from dogs and cats to humans has yet to be reported, and consequently, histoplasmosis is not considered zoonotic.6,7
Footnotes
Convenia, Zoetis Inc, Kalamazoo, Mich.
BLAST, National Center for Biotechnology Information, National Institutes of Health, Bethesda, Md. Available at: blast. ncbi.nlm.nih.gov/. Accessed Mar 23,2016.
References
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