History and Clinical Findings

A 1.5-year-old 1.22-kg (2.68-lb) neutered male ferret that had a 6-week history of diarrhea and intermittent melena, bruxism of increasing frequency and severity, increasing lethargy, and 2 recent episodes of vomiting was evaluated. Prior treatment with antimicrobials, sucralfate, buprenorphine, and predniso-lone was ineffective; therefore, referral for further diagnostic investigation was pursued. In the ferret's household, there was 1 other ferret and a cat; the ill ferret had close contact with the other ferret and no contact with the cat.

The ferret underwent abdominal ultrasonography, and a well-marginated, irregularly shaped, soft tissue mass was identified in the right cranial portion of the abdomen. The mesenteric lymph nodes were also markedly enlarged. Fine-needle aspirate specimens of the abdominal mass and mesenteric lymph nodes were collected for cytologic examination(Figure 1).

View Full Size

View Full Size

Photomicrographs of fine-needle aspirate specimens of a mesenteric mass (A) and an enlarged mesenteric lymph node (B) obtained from a 1.5-year-old neutered male ferret that was evaluated because of diarrhea with intermittent melena and progressive lethargy of 6 weeks' duration. A—Large, round to oval cells with indistinct cellular borders are present. They have moderate basophilic cytoplasm and an oval to lobulated nucleus with finely to coarsely stippled chromatin. Anisocytosis and anisokaryosis are mild to moderate. Modified Romanowsky stain; bar = 20 μm. B—Cells in the lymph node specimen are similar to those in the mesenteric mass specimen. Modified Romanowsky stain; bar = 20 μm. Inset—Notice the multinucleated cell. Modified Romanowsky stain; bar = 20 μm.

Citation: Journal of the American Veterinary Medical Association 259, 3; 10.2460/javma.259.3.257

• Download Figure
• Download Figure
• Download figure as PowerPoint slide

Cytologic Findings

The fine-needle aspirate specimen obtained from the mesenteric mass was moderately cellular and revealed a monomorphic cell population; the cells were present individually and in loose aggregates (Figure 1). The cells were large (20 to 50 µm in width) and round to spindle shaped with indistinct cell borders. The nucleus-to-cytoplasm ratio was variable but approximately 1:1 in general. Cells each had moderate to abundant, basophilic cytoplasm that contained a low number of clear vacuoles and an eccentrically located, oval to lobulated nucleus with finely to coarsely stippled chromatin. There was mild to moderate anisocytosis and anisokaryosis. Occasionally, these large cells had evidence of erythrophagocytosis, and rare mitotic figures were present. Cytologically, the fine-needle aspirate specimens of the mesenteric lymph nodes contained cells similar to those in the mesenteric mass specimen, which were scattered among a heterogeneous population of lymphocytes; occasional multinucleated cells were noted.

Additional Gross Findings

Cytologically, it was unclear whether the large cells were reactive macrophages or neoplastic cells; therefore, the ferret underwent an exploratory laparotomy. The abdominal mass was 4 × 5 cm and associated with the pyloric region of the stomach and an adjacent portion of the duodenum. The mass was removed (Figure 2) and submitted for histologic examination. Shortly thereafter, the owners elected to euthanize the ferret because of its poor prognosis and worsening condition.

View Full Size

Photograph of the abdominal mass (star) after surgical removal from the ferret. The mass was associated with the pyloric region of the stomach (arrow) and with an adjacent portion of the duodenum. Bar = 1 cm.

Citation: Journal of the American Veterinary Medical Association 259, 3; 10.2460/javma.259.3.257

• Download Figure
• Download figure as PowerPoint slide

Histopathologic Findings

Histologic examination of sections of the abdominal mass revealed several pyogranulomas circumscribed by fibrous tissue, which together markedly expanded the tunica muscularis and serosa of the stomach and duodenum and extended to the pancreas. The centers of the granulomas were composed of macrophages, variable numbers of degenerated and nondegenerate neutrophils, and rare lymphocytes, with occasional necrotic debris. The pyogranulomas were surrounded by a band of lymphocytes and plasma cells and thick bands of connective tissue. The granulomas multifocally extended into and expanded the submucosa (Figure 3).

View Full Size

Photomicrograph of a section of the mesenteric mass. Notice the severe pyogranulomatous inflammation, with macrophages and neutrophils in the center surrounded by lymphocytes and plasma cells. H&E stain; bar = 200 µm.

Citation: Journal of the American Veterinary Medical Association 259, 3; 10.2460/javma.259.3.257

• Download Figure
• Download figure as PowerPoint slide

Laboratory Findings

The mass underwent immunohistochemical analysis for coronavirus. A monoclonal antibody against feline infectious peritonitis (FIP) virus^a was applied to 1 section of the mass, and low numbers of macrophages in multiple pyogranulomas had intense cytoplasmic staining (Figure 4). Results of staining sections of the mass with Gram, modified acid-fast, or Gomori-Grocott methenamine silver stain were all negative.

View Full Size

Photomicrograph of a section of the mesenteric mass after immunohistochemical staining for coronavirus. In a granuloma, a low number of macrophages have intense cytoplasmic staining for coronavirus. Anti–feline infectious peritonitis virus antibody; bar = 200 µm.

Citation: Journal of the American Veterinary Medical Association 259, 3; 10.2460/javma.259.3.257

• Download Figure
• Download figure as PowerPoint slide

Morphologic Diagnosis and Case Summary

Morphologic diagnosis: severe, chronic, multi-focal to coalescing, pyogranulomatous gastritis and duodenitis with intralesional coronavirus.

Case summary: FIP-like syndrome in a ferret.

Comments

For the ferret of the present report, examination of the fine-needle aspirate specimens obtained from the mesenteric mass and mesenteric lymph nodes did not yield a definitive diagnosis but did suggest 2 primary differential diagnoses: severe granulomatous inflammation or mesenchymal neoplasia (histiocytic sarcoma, poorly differentiated sarcoma, or amelanotic melanoma) with lymph node metastasis. Histologic examination and immunohistochemical analysis findings supported a diagnosis of FIP-like syndrome.

Ferret systemic coronavirus (FRSCV) infection, also known as FIP-like syndrome, is an emerging disease that was first identified in Spain in 2006 and has since been detected in pet ferrets in the United States and Europe.^1,2 Ferret systemic coronavirus is closely related to ferret enteric coronavirus (FRECV). Infection with FRSCV causes clinicopathologic abnormalities similar to those associated with FIP virus infection.³ Much like the dry form of FIP, infection with FRSCV primarily affects juvenile and young adult ferrets with a mean age of onset of 11 months.^2,4 Both FRSCV and FIP virus infections are associated with a high mortality rate and short duration of illness; with regard to FRSCV infection, the mean duration of disease in ferrets is 69 days.² Common clinical signs of infection with FRSCV include weight loss, lethargy, anorexia, vomiting, loss of body condition, ataxia, paraparesis, seizures, and head tilt.^2,4 Less commonly, affected ferrets may develop sneezing, coughing, nasal discharge, dehydration, bruxism, a systolic heart murmur, jaundice, and green urine.^2,4 The variable clinical signs are attributable to immune complex–related damage of multiple organs.⁵ Other concurrent findings of FRSCV infection include anemia, hyperproteinemia as a result of hyperglobulinemia, and leukocytosis with marked neutrophilia, as well as nonspecific biochemical findings; these findings are also related to the immune-mediated nature of the disease.² Gross pathological changes associated with FRSCV infection include granulomatous lesions throughout the abdominal cavity (including masses and nodules on the spleen, liver, kidneys, mesentery, and lymph nodes) with concurrent organomegaly and mesenteric lymphadenopathy.² Histologic examination of the masses reveals severe pyogranulomatous inflammation centering on or near vessels, similar to features of the dry form of FIP in cats.^2,3

The pathogenesis of FRSCV-induced disease is still largely unknown. Phylogenetic analysis of isolated strains has shown that the virus is more similar to FRECV (the causative agent of epizootic catarrhal enteritis) than other group 1 coronaviruses, including FIP virus.⁶ However, results of genetic analysis of the coronavirus spike protein indicate that FRSCV is markedly different from FRECV and is in fact a different pathotype.^4,6 On the basis of the striking similarities FRSCV and FRECV share with FIP virus, it has been hypothesized that FRSCV and FRECV may be associated in a manner similar to the interactions of FIP virus and feline coronavirus.⁶ Whether FRSCV evolved from in vivo mutations of FRECV or whether both FRSCV and FRECV circulate independently of one another cannot be determined from the available data.⁶ Given the many similarities between FIP virus and FRSCV, the viral mutations required for macrophage tropism may also be similar. The ability of FIP virus strains to infect macrophages has potentially arisen from mutations in the spike protein gene.⁶ Notably, analyzed strains of FRSCV also have mutations in the spike protein genes, compared with the FRECV sequences, which may mediate viral tropism to macrophages.⁶ Further analysis of FRSCV and FRECV strains are required to elucidate the pathogenesis and patho-physiology of FRSCV infections.

Specific diagnostic tests for FRSCV include immunohistochemical analysis for detection of group 1c coronavirus, PCR assay for detection of the coronavirus spike protein, and electron microscopy for detection of cytoplasmic viral inclusion bodies within macrophages.^2,6 Immunostaining cannot discriminate among the various strains of coronavirus⁷; however, the intracellular presence of virus within a granulomatous lesion is indicative of a pathogenic strain. As yet poorly understood host and viral factors combine to result in progression from subclinical to pathogenic disease in an infected individual.⁸ However, sequencing can be used to further characterize the phylogenic characteristics of the viral strain present.^2,6 Differential diagnoses for intestinal masses in ferrets include lymphoma, metastatic adrenal neoplasia, mycobacteriosis, nocardiosis, and other causes of chronic inflammation.²

In the case described in the present report, the pleomorphism of the cell population identified in the mesenteric mass and lymph node fine-needle aspirate specimens made it difficult to determine whether the cells were reactive macrophages or neoplastic. A definitive diagnosis was achieved on the basis of histopathologic and immunohistochemical findings that confirmed the presence of coronavirus in the cytoplasm of macrophages in the abdominal cavity granulomas. The findings of this case highlighted the importance of including FRSCV infection as a differential diagnosis for ferrets with an abdominal mass.

Ferrets with FRSCV infection have a poor prognosis and will die or have to be euthanized. However, affected individuals can survive for extended periods with appropriate supportive medical care.^4,5 Because FRSCV-associated disease is an immune-mediated disease, treatment of affected ferrets requires immunosuppressant therapy (often administration of prednisolone) and interventions to ameliorate clinical signs and improve the quality of life.^4,5,8