A biphasic malignant mesothelioma of the peritoneum and pleura in a horse.

The research describes a rare case of severe biphasic malignant mesothelioma found in the peritoneal and pleural cavity of a 13-year-old Icelandic pony mare, detailing its macroscopic, histologic, immunohistologic, and ultrastructural characteristics along with the tests and evaluations performed.

Case Description and Macroscopic Examination

• The study revolves around a 13-year-old Icelandic pony mare exhibiting recurrent ascites clinically.
• Upon post-mortem examination, the researchers observed severe ascites and hydrothorax – a condition where fluids build up in the cavity where the lungs are located.
• The serosal surfaces of the peritoneum and pleura showcased poorly-demarcated, multifocal to coalescing laminar masses and small nodules – indicating an abnormal growth of tissue.

Microscopic Analysis and Histology

• Upon histological examination, a bimorphic mass consisting of spindle-shaped cells and microcystic epithelioid areas was revealed. This suggests the occurrence of biphasic malignant mesothelioma, presenting both epithelioid and sarcomatoid (spindle-shaped) components.
• A transcoelomic and local invasive growth pattern along with lymph node metastases were also observed, suggesting the spread of the disease.

Immunohistology and Ultrastructural Examination

• Through immunohistology, a strong expression of cytokeratins (CK) was detected. Cytokeratins are proteins of keratin-containing intermediate filaments found in the intracytoplasmic cytoskeleton of epithelial tissue. They are generally used as markers to identify the origin of various types of cancerous cells.
• Though a low and moderate expression of CK5/6 and CK20 respectively was found, CK7 and CK10-antigens were lacking.
• Under an ultrastructural examination, epithelioid mesothelioma cells displayed long microvilli, cytoplasmic tonofilaments, and desmosomes – features characteristic of epithelial cells.
• The length to diameter ratio of the 10 longest microvilli revealed a mean value of approximately 16.2, signifying the alteration in cell morphology due to cancer growth.

In conclusion, the article presents a detailed report on an unusual case of biphasic malignant mesothelioma in a horse with an atypical expression of cytokeratins, providing unique insights into the morphologic and structural characteristics of this rare form of cancer.