History
A 10-year-old 30-kg (66-lb) castrated male Belgian Malinois military working dog was evaluated because of a sudden onset of bloody diarrhea and vomiting. This dog had been treated for dirofilariasis in 2010.
Clinical, Clinicopathologic, and Gross Findings
On physical examination, the dog was weak and lethargic with dull mentation, tachycardia, tachypnea, and weak pulses. Thoracic and abdominal radiography and ultrasonography revealed free abdominal fluid and bilateral pleural effusion with lung compression. No thoracic or abdominal masses were evident. Approximately 1.3 L of serosanguineous pleural fluid (that did not clot) was obtained via thoracocentesis; the fluid's total protein concentration was 1.8 g/dL, and the PCV was 4.5%. Abdominocentesis yielded similar serosanguineous fluid that had a total protein concentration of 2.8 g/dL and a PCV of < 1%. The pleural and peritoneal fluids were classified as transudates, and a sample of the pleural fluid obtained via thoracocentesis was examined cytologically. A thoracic tube was placed, and supportive care was provided. However, the dog's condition continued to deteriorate; euthanasia by IV injection of pentobarbital was elected, and a necropsy was performed.
Cytologic Findings
Cytologic evaluation of the pleural fluid specimen revealed numerous individualized and clustered, round to polygonal cells admixed with moderate numbers of foamy macrophages and a few neutrophils and small lymphocytes (Figure 1). The round to polygonal cells were each characterized by a moderate amount of dark blue cytoplasm and an oval nucleus with coarse chromatin and an occasionally prominent nucleolus. Cells were often encircled by an eosinophilic to basophilic coronal region. Anisocytosis and anisokaryosis were moderate to marked, and there were frequent binucleated and multinucleated cells.
Photomicrographs of an aspirate specimen of pleural fluid obtained from a Belgian Malinois military working dog that was evaluated because of a sudden onset of bloody diarrhea and vomiting. Admixed with moderate numbers of foamy macrophages and a few neutrophils and small lymphocytes, there are individualized and clustered round to polygonal cells, each with a moderate amount of dark blue cytoplasm and an oval nucleus that has coarse chromatin and an occasionally prominent nucleolus. Anisocytosis and anisokaryosis are moderate to marked, and there are frequent binucleated and multinucleated cells. Romanowsky stain; bar = 50 μm. Inset—Photomicrograph of one of the round to polygonal cells. This cell is encircled by an eosinophilic to basophilic coronal region. Romanowsky stain; bar = 25 μm.
Citation: Journal of the American Veterinary Medical Association 256, 9; 10.2460/javma.256.9.999
Photomicrographs of an aspirate specimen of pleural fluid obtained from a Belgian Malinois military working dog that was evaluated because of a sudden onset of bloody diarrhea and vomiting. Admixed with moderate numbers of foamy macrophages and a few neutrophils and small lymphocytes, there are individualized and clustered round to polygonal cells, each with a moderate amount of dark blue cytoplasm and an oval nucleus that has coarse chromatin and an occasionally prominent nucleolus. Anisocytosis and anisokaryosis are moderate to marked, and there are frequent binucleated and multinucleated cells. Romanowsky stain; bar = 50 μm. Inset—Photomicrograph of one of the round to polygonal cells. This cell is encircled by an eosinophilic to basophilic coronal region. Romanowsky stain; bar = 25 μm.
Citation: Journal of the American Veterinary Medical Association 256, 9; 10.2460/javma.256.9.999
Photomicrographs of an aspirate specimen of pleural fluid obtained from a Belgian Malinois military working dog that was evaluated because of a sudden onset of bloody diarrhea and vomiting. Admixed with moderate numbers of foamy macrophages and a few neutrophils and small lymphocytes, there are individualized and clustered round to polygonal cells, each with a moderate amount of dark blue cytoplasm and an oval nucleus that has coarse chromatin and an occasionally prominent nucleolus. Anisocytosis and anisokaryosis are moderate to marked, and there are frequent binucleated and multinucleated cells. Romanowsky stain; bar = 50 μm. Inset—Photomicrograph of one of the round to polygonal cells. This cell is encircled by an eosinophilic to basophilic coronal region. Romanowsky stain; bar = 25 μm.
Citation: Journal of the American Veterinary Medical Association 256, 9; 10.2460/javma.256.9.999
Formulate differential diagnoses from the history, clinical findings, and Figure 1—then turn the page →
Necropsy Findings
At necropsy, severe thickening of the pericardium and epicardium were noted. There was bicavitary effusion. The perihilar and tracheobronchial lymph nodes were enlarged and dark red. Numerous hard, gritty, sand-like, irregularly shaped (maximum dimension, 1 to 2 mm) intraparenchymal foci were randomly scattered in all lung lobes. These foci were easily detected by palpation of the lungs, but many were situated near the surfaces of the lungs and could be seen on cut sections. There was also hepatomegaly. Careful examination and sectioning of the abdominal organs failed to reveal a solid neoplasm. All tissues were fixed in neutral-buffered 10% formalin prior to sectioning. No microbial culture of tissue or fluid specimens was performed because such testing was cost prohibitive for the owner.
Histopathologic Findings
On histologic examination, multiple sections of pericardium, epicardium, aortic adventitia, and pleura had moderate to severe mesothelial hyperplasia with multifocal formation of papillary projections (Figure 2). The subcapsular and medullary sinuses of a tracheobronchial lymph node were markedly expanded by individual and clustered round to polygonal cells that each had a moderate amount of eosinophilic cytoplasm and an oval nucleus with dense chromatin and an indistinct nucleolus (Figure 3). Anisocytosis and anisokaryosis were moderate, and there were frequent binucleated and multinucleated cells. These cells (which were consistent with mesothelial cells) often contained a discrete vacuole that peripheralized the nucleus. There was also evidence of local hemorrhage draining to the tracheobronchial lymph node. The hemorrhage filled the sinuses and included free erythrocytes and hemosiderin-laden macrophages; some macrophages had undergone erythrophagocytosis. Immunohistochemical analysis of sections of the tracheobronchial lymph node was performed. The round to polygonal cells within the lymph node sinuses had diffuse, strong cytoplasmic immunoreactivity to pancytokeratin but were negative for vimentin, with the exception of scattered, individual cells. Immunohistochemical staining with antibodies against calretinin and Wilms tumor-1 protein (markers used for diagnosis of mesothelioma in humans) were equivocal.
Photomicrograph of a section of parietal pleura with moderate to severe mesothelial hyperplasia, which is characterized by tight clusters of large mesothelial cells that multifocally form papillary projections. Mesothelial cells often contain a discrete vacuole that peripheralizes the nucleus. H&E stain; bar = 100 μm.
Citation: Journal of the American Veterinary Medical Association 256, 9; 10.2460/javma.256.9.999
Photomicrograph of a section of parietal pleura with moderate to severe mesothelial hyperplasia, which is characterized by tight clusters of large mesothelial cells that multifocally form papillary projections. Mesothelial cells often contain a discrete vacuole that peripheralizes the nucleus. H&E stain; bar = 100 μm.
Citation: Journal of the American Veterinary Medical Association 256, 9; 10.2460/javma.256.9.999
Photomicrograph of a section of parietal pleura with moderate to severe mesothelial hyperplasia, which is characterized by tight clusters of large mesothelial cells that multifocally form papillary projections. Mesothelial cells often contain a discrete vacuole that peripheralizes the nucleus. H&E stain; bar = 100 μm.
Citation: Journal of the American Veterinary Medical Association 256, 9; 10.2460/javma.256.9.999
Photomicrograph of a section of a tracheobronchial lymph node. The subcapsular and medullary sinuses are markedly expanded by individual and clustered round to polygonal cells; however, the nodal architecture appears intact and there is no invasion through the capsule or into adjacent tissue. There is also hemorrhage and hemosiderosis. H&E stain; bar = 800 μm. Inset—Higher-magnification view of a medullary sinus. Clusters of round to polygonal cells, each with a moderate amount of eosinophilic cytoplasm and an oval nucleus with dense chromatin and an indistinct nucleolus (consistent with mesothelial cells) are visible. Anisocytosis and anisokaryosis are moderate. There are frequent binucleated and multinucleated cells, which often contain a discrete vacuole that peripheralizes the nucleus. H&E stain; bar = 100 μm.
Citation: Journal of the American Veterinary Medical Association 256, 9; 10.2460/javma.256.9.999
Photomicrograph of a section of a tracheobronchial lymph node. The subcapsular and medullary sinuses are markedly expanded by individual and clustered round to polygonal cells; however, the nodal architecture appears intact and there is no invasion through the capsule or into adjacent tissue. There is also hemorrhage and hemosiderosis. H&E stain; bar = 800 μm. Inset—Higher-magnification view of a medullary sinus. Clusters of round to polygonal cells, each with a moderate amount of eosinophilic cytoplasm and an oval nucleus with dense chromatin and an indistinct nucleolus (consistent with mesothelial cells) are visible. Anisocytosis and anisokaryosis are moderate. There are frequent binucleated and multinucleated cells, which often contain a discrete vacuole that peripheralizes the nucleus. H&E stain; bar = 100 μm.
Citation: Journal of the American Veterinary Medical Association 256, 9; 10.2460/javma.256.9.999
Photomicrograph of a section of a tracheobronchial lymph node. The subcapsular and medullary sinuses are markedly expanded by individual and clustered round to polygonal cells; however, the nodal architecture appears intact and there is no invasion through the capsule or into adjacent tissue. There is also hemorrhage and hemosiderosis. H&E stain; bar = 800 μm. Inset—Higher-magnification view of a medullary sinus. Clusters of round to polygonal cells, each with a moderate amount of eosinophilic cytoplasm and an oval nucleus with dense chromatin and an indistinct nucleolus (consistent with mesothelial cells) are visible. Anisocytosis and anisokaryosis are moderate. There are frequent binucleated and multinucleated cells, which often contain a discrete vacuole that peripheralizes the nucleus. H&E stain; bar = 100 μm.
Citation: Journal of the American Veterinary Medical Association 256, 9; 10.2460/javma.256.9.999
The pericardium was thickened (up to 6 mm) by dense fibrous connective tissue with abundant hemorrhage and scattered hemosiderin-laden macrophages (Figure 4). Multifocally, pericardial lymphatic vessels and blood vessels contained clusters of the previously described polygonal cells, suggestive of mesothelial cells. Among the polygonal cells, mitoses were not evident. The liver had evidence of chronic passive congestion with dilated or congested sinuses, moderate to severe edema, centrilobular fibrosis, and scattered foci of hemosiderin-laden macrophages. Within the lungs, multiple pulmonary arteries were variably occluded by marked granulomatous and proliferative arteritis with mineralized thrombi and adult nematodes consistent with Dirofilaria immitis. There was multifocal mesothelial hyperplasia affecting the pleurae, as observed in the heart and aorta.
Photomicrograph of a section of the pericardium, which is thickened (≤ 6 mm) by dense fibrous connective tissue with abundant hemorrhage and scattered hemosiderin-laden macrophages. The box denotes an area where a pericardial lymphatic vessel contains clusters of round to polygonal cells. H&E stain; bar =1,000 μm. Inset—Higher-magnification view of a pericardial lymphatic vessel with clusters of round to polygonal cells consistent with mesothelial cells. Mitoses are not seen. H&E stain; bar = 100 μm.
Citation: Journal of the American Veterinary Medical Association 256, 9; 10.2460/javma.256.9.999
Photomicrograph of a section of the pericardium, which is thickened (≤ 6 mm) by dense fibrous connective tissue with abundant hemorrhage and scattered hemosiderin-laden macrophages. The box denotes an area where a pericardial lymphatic vessel contains clusters of round to polygonal cells. H&E stain; bar =1,000 μm. Inset—Higher-magnification view of a pericardial lymphatic vessel with clusters of round to polygonal cells consistent with mesothelial cells. Mitoses are not seen. H&E stain; bar = 100 μm.
Citation: Journal of the American Veterinary Medical Association 256, 9; 10.2460/javma.256.9.999
Photomicrograph of a section of the pericardium, which is thickened (≤ 6 mm) by dense fibrous connective tissue with abundant hemorrhage and scattered hemosiderin-laden macrophages. The box denotes an area where a pericardial lymphatic vessel contains clusters of round to polygonal cells. H&E stain; bar =1,000 μm. Inset—Higher-magnification view of a pericardial lymphatic vessel with clusters of round to polygonal cells consistent with mesothelial cells. Mitoses are not seen. H&E stain; bar = 100 μm.
Citation: Journal of the American Veterinary Medical Association 256, 9; 10.2460/javma.256.9.999
Morphologic Diagnosis and Case Summary
Morphologic diagnosis: bicavitary effusion; embolized polygonal cells in the tracheobronchial lymph nodes and pericardial lymphatic vessels; severe, diffuse pericardial fibrosis with multifocal hemorrhage, hemosiderosis, and mesothelial hyperplasia; severe chronic, diffuse hepatic congestion with centrilobular fibrosis, edema, and hemosiderosis; and severe, multifocal, granulomatous, and proliferative arteritis with mineralized thrombi and adult D immitis in the lungs.
Case summary: suspected benign mesothelial emboli in a dog with constrictive pericarditis and bicavitary effusion.
Comments
For the case described in the present report, the histologic features in combination with the reported physical examination and gross necropsy findings were consistent with constrictive pericarditis with bicavitary effusion and presumed lymphatic mesothelial emboli. Severe pericardial fibrosis likely resulted in chronic cardiac compression, impaired diastolic filling, venous engorgement with diminished cardiac output, and right-sided (possibly with left-sided) heart failure along with subsequent pleural effusion, ascites, and chronic passive hepatic congestion.1–3
We speculated that the dog's pleural effusion resulted in mesothelial hyperplasia with embolization of mesothelial cells to regional lymph nodes. Mesothelial cells are typically cytokeratin and vimentin positive.4 For the dog of the present report, the embolic cells were pancytokeratin positive but largely vimentin negative, so we were unable to definitively establish a mesothelial origin or to rule out metastatic carcinoma. Interestingly, although remaining foci of normal pleural and pericardial mesothelium were immunoreactive for both cytokeratin and vimentin, some areas of mesothelial hyperplasia were negative for vimentin. The reason for this is unclear; however, there are reported cases of vimentin-negative benign and malignant mesothelial cells.4–6 Additionally, our veterinary pathology diagnostic service shares a histology laboratory with human physician pathologists, so the available immunohistochemical stains are typically not optimized for domestic animal species. In this case, despite equivocal immunohistochemical staining characteristics, a diagnosis of reactive mesothelial hyperplasia with mesothelial emboli seemed reasonable given the morphologic features identified, the lack of a solid tumor, and the presence of pleural effusion and mesothelial hyperplasia.
Reactive mesothelial hyperplasia is a well-documented lesion associated with pleural effusion,7 whereas embolization of benign mesothelial cells to regional lymph nodes in humans and animals is rarely reported. All reported cases8–11 have been associated with pleural or peritoneal effusions or pericarditis, as for the case described in the present report. The proposed pathogenesis of multicentric embolic reactive mesothelium involves desquamation of hyperplastic mesothelial cells into an effusion with subsequent clearance through intercellular mesothelial stomata that connect with submesothelial lymphatic vessels and accumulation in draining lymph nodes.5 It can be very difficult to distinguish multicentric embolic reactive mesothelium from metastatic mesothelioma or carcinoma because reactive mesothelium often has features of malignancy.12 Lack of a solid tumor and a histologic lack of papillary formations, stromal reaction, and invasiveness within the lymph nodes are all suggestive of multicentric embolic reactive mesothelium rather than neoplasia.9
The most common causes of constrictive pericarditis in humans (and likely in domestic animals as well) are infectious (bacterial, viral, or fungal) or idiopathic, although neoplasia, trauma, and various other causes have been reported.1,2 Typical clinical findings associated with constrictive pericarditis include tachycardia, dyspnea, ascites, pleural effusion, pericardial effusion, and hepatosplenomegaly.1,13 The underlying cause of the pericardial fibrosis in the dog of the present report was unclear. Microbial culture of tissue or fluid specimens was not performed; thus, an infectious cause could not be ruled out.
Although there was no histologic evidence of active infection in the dog of the present report, it is possible that the lesions represented the late stage or reparative phase of a chronic infection. Canine idiopathic hemorrhagic pericardial effusion, a disease characterized by accumulation of serosanguineous pericardial fluid without evidence of any underlying disorder, was also considered as a possible cause of the dog's pericardial fibrosis. Idiopathic hemorrhagic pericardial effusion has been associated with pericardial thickening and fibrosis; however, it tends to be slowly progressive in nature.14 Furthermore, the dog had no history of clinical signs attributable to pericardial effusion, and the presence of gross pericardial effusion was not noted during the necropsy.
In light of the presence of pulmonary proliferative arteritis with mineralized thrombi and adult nematodes, consideration was given to whether the previous heartworm disease could have had a role in the dog's clinical condition. Although pulmonary arteritis and subsequent chronic pulmonary hypertension, right-sided heart failure, ascites, chronic passive hepatic congestion, and pleural effusion are well-documented sequelae to dirofilariasis in dogs,2 we are not aware of any reported cases of fulminant constrictive pericarditis secondary to heartworm disease in dogs. Overall, we believe that the case described in the present report was one of embolization of benign, reactive mesothelial cells to regional lymph nodes in a dog with constrictive pericarditis and pleural effusion, a lesion that is rarely identified in domestic animals.
Acknowledgments
The views expressed in this case report are those of the author and do not reflect the official policy of the Department of the Army, Department of Defense, or US Government.
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