Disseminated lipid-rich peritoneal mesothelioma in a horse.

The study presents a unique case of a horse diagnosed with a lipid-rich form of mesothelioma, a rare type of cancer, highlighting the diversity of the disease’s manifestation.

Study Objective and Animal Details

• The main objective of this study was to investigate and document a unique medical case involving a 9-year-old Haflinger mare, which was presented to the Liphook Equine Hospital. The horse had shown symptoms of weight loss, azotemia (high levels of nitrogen-containing compounds in the blood), and repeated episodes of ascites (accumulation of fluid in the abdominal space) over the span of 10 days.

Clinical Examination and Surgery

• As the mare’s condition did not improve and showed alarming symptoms, the medical team performed an exploratory laparotomy, a surgical procedure that involves opening the abdominal cavity to examine the organs inside for disease.
• During the surgical exploration, numerous and differently-sized masses were found scattered throughout the peritoneal cavity, prompting serious concern about the mare’s health. As a result, the horse was euthanized.

Macroscopic and Histological Findings

• The masses found in the peritoneal cavity varied macroscopically. Some had papillary formations (small, nipple-like projections), while others were nodular (knob-like).
• The histological examination, which involves studying tissues under a microscope, revealed that these masses were composed of abnormally large cells, with some being multinucleated (having more than one nucleus). These cells showed significant anisokaryosis (variable nuclear sizes) and anisocytosis (variable cell sizes), coupled with high cell division rates.

Lipid Vacuoles and Immunohistochemical Findings

• A notable symptom in these abnormal cells was the presence of clear, circular or ovoid vacuoles or pockets, which stained positive with oil red O, indicating they contained large amounts of lipids.
• Immunohistochemical testing, a process used to help identify the type of cancer by testing for certain antigens in a sample of tissue, revealed that the cells expressed both vimentin and cytokeratin proteins. These proteins are common in mesothelioma, confirming the hypothesis of the disease.

Electron Microscopy and Final Diagnosis

• Further examination using electron microscopy showcased cancerous cells with distinct features like tight junctions, microvilli (small projections on the surface of some cells), and a high quantity of lipid droplets in the cytoplasm.
• Based on these combined observations, this particular case was diagnosed as a lipid-rich form of mesothelioma. This form of the disease should be considered when lipid vacuoles are identified in suspected cancerous cells from horse’s abdominal samples.