History
A 12-year-old 3-kg (6.6-lb) castrated male American domestic shorthair cat was evaluated at the University of Missouri's veterinary teaching hospital because of a 3-week history of inappetence, weight loss, and lethargy. This cat was housed indoors but had access to an outdoor enclosure where birds often perched.
Clinical and Gross Findings
Radiography revealed a diffuse bronchial nodular to miliary pattern in the lungs and decreased abdominal serosal detail. Hematologic findings were indicative of severe pancytopenia and included moderate to severe leukopenia, moderate to severe neutropenia, moderate lymphopenia, severe eosinopenia, mild monocytopenia, and nonregenerative anemia (Table 1). There was marked thrombocytopenia (the automated platelet count was not validated by blood smear examination). Serum biochemical abnormalities included very mild hypercalcemia and hyperglobulinemia. The cat's clinical signs did not improve with supportive treatment, leading to a suspicion of advanced disease. The owner elected euthanasia and complete necropsy of the cat.
Hematologic and serum biochemical data obtained from a 12-year-old cat that was evaluated because of a 3-week history of inappetence, weight loss, and lethargy.
| Variable | Result | Reference interval |
|---|---|---|
| RBC count (× 106 cells/μL) | 3.54 | 6.54–12.20 |
| Hemoglobin (g/dL) | 5.6 | 9.8–16.2 |
| Hct (%) | 17.4 | 30.3–52.3 |
| Mean corpuscular volume (fL) | 49.2 | 35.9–53.1 |
| Mean corpuscular hemoglobin concentration (g/dL) | 32.2 | 28.1–35.8 |
| Absolute reticulocyte count (× 103 reticulocytes/μL) | 1.4 | 3.0–50.0 |
| Platelet count (× 103 platelets/μL)* | 11 | 151–600 |
| WBC count (× 103 cells/μL) | I.43 | 2.87–17.02 |
| Neutrophil count (× 103 cells/μUL) | 0.96 | 1.48–10.29 |
| Lymphocyte count (× 103 cells/μL) | 0.42 | 0.92–6.88 |
| Monocyte count (× 103 cells/μL) | 0.04 | 0.05–0.67 |
| Eosinophil count (× 103 cells/μL) | 0.01 | 0.17–1.57 |
| Basophil count (× 103 cells/μL) | 0.00 | 0.01–0.26 |
| Total protein (g/dL) | 8.0 | 5.7–58.9 |
| Globulin (g/dL) | 5.5 | 2.8–5.1 |
| Albumin (g/dL) | 2.5 | 2.3–3.9 |
| Creatinine (mg/dL) | 0.6 | 0.8–2.4 |
| Phosphorus (mg/dL) | 5.8 | 3.1–7.5 |
| Calcium (mg/dL) | 12.2 | 7.8–11.3 |
The automated platelet count was not validated by blood smear examination.
Formulate differential diagnoses from the history, clinical findings, and Figure 1—then turn the page→
Photographs of the surface of the lungs of a 12-year-old cat that was evaluated because of a 3-week history of inappetence, weight loss, and lethargy. A—On the pleural surface, there are multifocal to coalescing, 2- to 10-mm-diameter, tan to white nodules (arrows). B—Higher-magnification image of the pulmonary lesions taken from the area identified by a box in panel A. Multifocal tan to white nodules are randomly scattered throughout the lung parenchyma (arrows).
Citation: Journal of the American Veterinary Medical Association 256, 4; 10.2460/javma.256.4.431
Necropsy Findings
At necropsy, the lung failed to collapse. There were tan to white and irregular to round (maximum dimension, 2 to 10 mm) nodules randomly distributed on the pleural and cut surfaces of the lung lobes (Figure 1) Similar dark red nodules (maximum dimension, ≤ 1 cm) were randomly distributed throughout the hepatic parenchyma.
Histopathologic Findings
Representative samples of the lung, liver, and spleen lesions as well as samples of kidney tissue and bone marrow were collected and placed in neutral-buffered 10% formalin. All tissue samples were routinely processed and stained with H&E stain. Multifocal granulomas had effaced the normal parenchymatous architecture and had expanded the sinusoids of the liver. The alveolar septae were effaced by multifocal granulomas. Alveolar spaces contained large numbers of epithelioid macrophages admixed with areas of hemorrhage, few multinucleated giant cells, lymphocytes, and plasma cells and rare neutrophils (Figure 2) Large numbers of epithelioid macrophages had infiltrated the bone marrow and expanded the red pulp of the spleen. Sections of kidney tissue were not affected by granulomatous inflammation. Within the cytoplasm of macrophages and multinucleated giant cells, there were variable numbers of round, 2- to 4-μm-diameter yeasts with a thin outer capsule and a central 1- to 2-μm-diameter basophilic area surrounded by a clear halo. The yeasts reacted positively with periodic acid-Schiff and Gomori methenamine silver stains. To confirm the identity of the organisms, immunohistochemical analysis was performed with immunoperoxidase-labeled rabbit polyclonal antibodya that reacts with the H and M antigens of Histoplasma capsulatum. Numerous intracellular yeasts in the bone marrow (Figure 3) and lung tissue sections were immunopositive for H capsulatum antigens (data not shown).
Photomicrographs of sections of affected lung tissue (A, E, F), liver tissue (B), and bone marrow (C, D) from the cat in Figure 1. A—The multifocal nodules seen during gross examination of the lungs are composed of large numbers of macrophages that expand the alveolar spaces and efface the alveolar septa (arrows). H&E stain; bar = 100 μm. B—In the liver, sinusoids are expanded and normal hepatic parenchyma is effaced by multiple similar granulomatous foci with occasional hemorrhage (arrows). H&E stain; bar = 100 μm. C—Granulomatous inflammation has multifocally infiltrated the bone marrow as evidenced by the focally extensive pale area delimited by the arrows. H&E stain; bar = 100 μm. D—Macrophages are multifocally distributed throughout the bone marrow and contain intracytoplasmic yeasts (arrows) similar to those observed in pulmonary and hepatic macrophages. H&E stain; bar = 10 μm. E—The yeasts in lung tissue react with periodic acid-Schiff stain (arrows). Periodic acid–Schiff stain; bar = 10 μm. F—Gomori methenamine silver staining highlights the intracytoplasmic yeasts, which appear as empty black rings (arrows), in a lung tissue section. Gomori methenamine silver stain; bar = 10 μm.
Citation: Journal of the American Veterinary Medical Association 256, 4; 10.2460/javma.256.4.431
Photomicrograph of a section of the bone marrow tissue following immunohistochemical staining for Histoplasma capsulatum. Notice that the yeasts have reacted with rabbit polyclonal antibody against the H and M glycoproteins of H capsulatum (arrows). Histoplasma capsulatum–specific immunohistochemical stain; bar = 10 μm.
Citation: Journal of the American Veterinary Medical Association 256, 4; 10.2460/javma.256.4.431
Morphologic Diagnosis and Case Summary
Morphologic diagnosis: severe, multifocal, granulomatous pneumonia with intrahistiocytic yeasts consistent with H capsulatum var capsulatum; and mild to moderate multifocal, hepatitis, splenitis, and myelitis with intrahistiocytic H capsulatum.
Case summary: pulmonary and disseminated histoplasmosis in an American domestic shorthair cat.
Comments
The case described in the present report involved a domestic cat from an endemic area for histoplasmosis (Missouri) that developed severe pulmonary granulomatous inflammation as a result of H capsulatum infection. The differential diagnoses for cats with weight loss, lethargy, and multifocal pulmonary lesions include histoplasmosis, toxoplasmosis, cytauxzoonosis, blastomycosis, and neoplasia, such as lymphoma or metastatic neoplasia.1–8 The hematologic findings (severe pancytopenia) and pulmonary radiographic interpretation (bronchial to military pattern) for the cat of the present report were highly suggestive of a neoplastic process such as lymphoma or metastatic neoplasia. This case has highlighted the fact that granulomatous inflammation attributable to histoplasmosis should be considered as a differential diagnosis for a cat with pancytopenia or a bronchial miliary pattern on thoracic radiographs (or both). Detection of very mild hypercalcemia may help to refine the list of differential diagnoses toward granulomatous disease. Hypercalcemia may develop in any mammal with granulomatous disease as a result of increased production of 1,25-dihydroxyvitamin D by macrophages and increased intestinal absorption of calcium.
In the cat of the present report, the granulomatous inflammation was disseminated and affected multiple organs including the liver, spleen, and bone marrow. The cat had a nonregenerative anemia, leukopenia, and thrombocytopenia, which are highly indicative of bone marrow involvement. For cats with histoplasmosis, normocytic, normochromic, nonregenerative anemia has been commonly described. In addition, concurrent leukopenia and thrombocytopenia secondary to granulomatous disease-associated effacement and severe depletion of bone marrow developed in the cat of the present report. However, the cat was not tested to determine the presence of FIV or FeLV infection, which can cause bone marrow suppression.
Histoplasma capsulatum, the causative agent of histoplasmosis in multiple species, is a soil-borne, dimorphic fungus that exists as a filamentous mycelial form (mold form) in the environment and a yeast form, which is considered the virulent form affecting animals.1–3 Histoplasma capsulatum is widely distributed in North America, South America, Africa, eastern Asia, Australia, and Europe.9–11 In the United States, the prevalence of the disease is high in the Mississippi, Ohio, and Missouri River valley regions.9–11
The filamentous mycelial forms of H capsulatum grow in nitrogen-enriched soil. In the environment at 25°C (77°F), the mycelial stage produces macroconidia (5 to 18 μm in diameter) and microconidia (2 to 5 μm in diameter); the latter are considered to be the infective form for mammals.12 Histoplasma capsulatum infection is often associated with exposure to bird or bat excrement.13 With desiccation over time, dust from bird or bat droppings has resulted in outbreaks of respiratory histoplasmosis. For the cat of the present report, contamination of the outdoor enclosure by H capsulatum-laden bird feces may have been the source of infection.
The principal routes of infection are inhalation (pulmonary form) or ingestion (gastrointestinal form) of microconidia.12 After infection and approximately 12 to 16 days of incubation at body temperature (37°C [98.6°F]), the microconidia transform into the yeast phase and start the reproductive stage by budding.1
The yeasts can spread to other tissues in mononuclear phagocytes via lymphatic and hematogenous routes.14 Ingestion of H capsulatum microconidia from contaminated soil can lead to a primary gastrointestinal tract infection with similar systemic dissemination.15
Severe clinical signs of histoplasmosis develop after a host is exposed to large numbers of microconidia (spores) or when the immune system of the infected host is compromised. The immune system can clear minor infections predominantly via cytokine-mediated macrophage killing. However, large numbers of spores overwhelm the cell-mediated immune response of affected animals. In immunosuppressed patients, severe H capsulatum fungemia may develop and death may occur.1,16 Infected cats are occasionally retrovirus positive (ie, infected with FIV or FeLV [or both]), but the FeLV status of cats with histoplasmosis is negative in most reports. Therefore, for cats with histoplasmosis, a diagnostic test to confirm concurrent retrovirus infection is warranted because retrovirus-associated immunosuppression may influence the prognosis.17,18
Disseminated disease involving the lungs, lymph nodes, liver, spleen, kidneys, adrenal glands, and bone marrow, rather than confinement of disease to 1 primary organ (portal of entry), most commonly develops in H capsulatum-infected cats. In most infected cats with disseminated disease, a wide range of nonspecific clinical signs including lethargy, weight loss, anorexia, and pyrexia are evident rather than respiratory tract signs (dyspnea or coughing). Other frequent clinical findings with disseminated infection are peripheral or visceral lymphadenomegaly, splenomegaly, and hepatomegaly.
In all forms of Histoplasma infection (respiratory, gastrointestinal, and disseminated), the presumptive diagnosis requires the detection of granulomatous to pyogranulomatous inflammation with multiple intracytoplasmic yeasts in a cytologic preparation or tissue section.19 Identification of 2- to 5-μm-diameter yeasts with a basophilic oval to spherical center surrounded by a clear halo (caused by shrinkage of the yeasts during sample processing) after application of routine stains to cytologic preparations or tissue sections enables a presumptive diagnosis of histoplasmosis. Definitive diagnosis of histoplasmosis requires collection of appropriate specimens and application of molecular diagnostic techniques (PCR assay and partial ribosomal gene sequencing) or immunohistochemical analysis.
The prognosis for cats with histoplasmosis ranges from good to guarded depending on the extent of the disease. Although all azole drugs and amphotericin B can be used successfully in treatment of systemic fungal infections in cats and dogs, itraconazole is most commonly used. The pulmonary form of histoplasmosis may be self-limiting, but the disseminated form can be difficult to treat; therefore, treatments of long duration or with combinations of drugs may be neccessary.3,19
Histoplasma capsulatum is an important human pathogen, particularly in immunocompromised individuals. Regarding the public health aspects, although humans and animals both develop pulmonary histoplasmosis after environmental exposure to H capsulatum, transmission from animal to animal or animal to human has not been reported.20 Prevention should not be neglected and should include restriction of animal or human access to areas rich in wild bird droppings in endemic regions.
Acknowledgments
The authors thank Drs. Tamas Nagy and Michael L. Divine for technical assistance.
Footnotes
Gibson Bioscience, Lexington, Ky.
References
1. Greene CE. Histoplasmosis. In: Greene CE, ed. Infectious diseases of the dog and cat. 4th ed. St Louis: Saunders Elsevier, 2012;614–620.
2. Uzal FA, Plattner BL, Hostetter JM. Alimentary system: fungal diseases of skin. In: Maxie MG, ed. Jubb, Kennedy, & Palmer's pathology of domestic animals. 6th ed. St Louis: Elsevier, 2016;202–203.
3. Lin Blache J, Ryan K, Arceneaux K. Histoplasmosis. In: Compend Contin Educ Pract Vet 2011;33:E1–E11.
4. Evans NA, Walker JM, Manchester AC, et al. Acute respiratory distress syndrome and septic shock in a cat with disseminated toxoplasmosis. J Vet Emerg Crit Care (San Antonio) 2017;27:472–478.
5. Houseright RA, Webb JL, Claus KN. Pathology in Practice. Blastomycosis in an indoor-only cat. J Am Vet Med Assoc 2015;247:357–359.
6. Klopfleisch R. Respiratory system tumors. In: Klopfleisch R, ed. Veterinary oncology. Cham, Switzerland: Springer International Publishing Switzerland, 2016;255–265.
7. Treggiari E, Pedro B, Blackwood L, et al. A descriptive review of cardiac tumours in dogs and cats. Vet Comp Oncol 2017;15:273–288.
8. Wang JL, Li TT, Liu GH, et al. Two tales of Cytauxzoon felis infections in domestic cats. Clin Microbiol Rev 2017;30:861–885.
9. O'Sullivan MV, Whitby M, Chahoud C, et al. Histoplasmosis in Australia: a report of a case with a review of the literature. Aust Dent J 2004;49:94–97.
10. Wheat LJ. Histoplasmosis: a review for clinicians from non-endemic areas. Mycoses 2006;49:274–282.
11. Kobayashi R, Tanaka F, Asai A, et al. First case report of histoplasmosis in a cat in Japan. J Vet Med Sci 2009;71:1669–1672.
12. Knox KS, Hage CA. Histoplasmosis. Proc Am Thorac Soc 2010;7:169–172.
13. Jülg B, Elias J, Zahn A, et al. Bat-associated histoplasmosis can be transmitted at entrances of bat caves and not only inside the caves. J Travel Med 2008;15:133–136.
14. Atiee G, Kvitko-White H, Spaulding K, et al. Ultrasonographic appearance of histoplasmosis identified in the spleen in 15 cats. Vet Radiol Ultrasound 2014;55:310–314.
15. Schumacher LL, Love BC, Ferrell M, et al. Canine intestinal histoplasmosis containing hyphal forms. J Vet Diagn Invest 2013;25:304–307.
16. de Souza EW, Gremião IDF, Pereira SA, et al. Feline histoplasmosis: a case of supraorbital involvement. R Bras Ci Vet 2015;22:77–80.
17. Johnson LR, Fry MM, Anez KL, et al. Histoplasmosis infection in two cats from California. J Am Anim Hosp Assoc 2004;40:165–169.
18. Clinkenbeard KD, Cowell RL, Tyler RD. Disseminated histoplasmosis in cats: 12 cases (1981–1986). J Am Vet Med Assoc 1987;190:1445–1448.
19. Taylor AR, Barr JW, Hokamp JA, et al. Cytologic diagnosis of disseminated histoplasmosis in the wall of the urinary bladder of a cat. J Am Anim Hosp Assoc 2012;48:203–208.
20. Brömel C, Sykes JE. Histoplasmosis in dogs and cats. Clin Tech Small Anim Pract 2005;20:227–232.
