Reactive mesothelial hyperplasia associated with chronic peritonitis in a 20-year-old Quarter horse

Abstract

A 20-year-old gelding was diagnosed with peritonitis and severe reactive mesothelial hyperplasia. Exploratory laparotomy findings were suggestive of a neoplastic etiology; however, additional diagnostics ruled this out and the horse made a full recovery. This report demonstrates the difficulty and value of differentiating between reactive and neoplastic mesothelial processes.

Figure 1

Intraoperative examination of the serosal surface of the large colon. There are numerous, multifocal, bright red, highly vascularized, soft, irregularly marginated, villiform structures ranging in size from 2.0 cm (length) × 1.0 cm (width) × 0.5 cm (height) up to 6.0 cm (length) × 2.0 cm (width) × 1.0 cm (height) that are firmly attached to the serosa (arrows).

Figure 2

Cytologic and histologic evidence of reactive mesothelial hyperplasia. A — Impression smears have predominantly neutrophilic inflammation (asterisk) and clusters of reactive mesothelial cells (arrow) each with a round nucleus, coarsely stippled chromatin, small to moderately sized, variably distinct nucleoli and a small to moderate amount of deeply basophilic cytoplasm rarely containing fine pink granules. Wright-Giemsa stain; 1000× magnification; Bar = 50 μm. B — Serosal tags are composed of papillary projections with highly vascularized collagenous stroma and mixed inflammation. Hematoxylin and eosin (H&E); 100× magnification; Bar = 500 μm. C — Projections are lined by a single layer of pleomorphic mesothelial cells (arrows). Centrally, there is vascularized collagenous stroma and marked numbers of plasma cells and lymphocytes with fewer neutrophils and macrophages. H&E; 500×magnification; Bar = 100 μm. D — Immunohistochemical labeling highlights mesothelial cells aligning along the surface of projections without invasion into deeper tissue. Mesothelial cells are pleomorphic and vary from plump and reactive (arrow), to flat and bland (arrowhead). Pancytokeratin immunohistochemistry; 500× magnification; Bar = 100 μm.