Oil Red O-positive lipid in peritoneal fluid from a horse with a rectal tear.

This research describes a case of a horse with a rectal tear, in whom lipid (fat) vacuoles were observed in the peritoneal fluid, likely caused by leakage of administered mineral oil. Though this occurrence’s frequency is unidentified, this first-ever reported case highlights the need for cytopathologists to understand its implications.

Initial Presentation and Preliminary Treatment

• A 4-year-old Quarter Horse mare was admitted to the Texas A&M University Veterinary Medical Teaching Hospital due to a rectal tear.
• Initial evaluation, which involved rectal palpation and colonoscopy, uncovered a grade IIIb rectal tear.
• An analysis of peritoneal fluid (fluid from the lining of the abdominal wall and organs) was performed that indicated a modified transudate (a type of fluid which has transpired through a membrane or has been secreted).
• The preliminary supportive treatment provided comprised of fluid therapy and administration of mineral oil via a nasogastric tube (a tube inserted through the nose into the stomach).

Progression of the Condition and Euthanasia Decision

• About 48 hours post presentation, another abdominocentesis (procedure to remove fluid from the abdomen) was carried out.
• The cytologic examination on the fluid indicated a marked suppurative exudate (pus formation) was noticeable.
• Material seen within neutrophils and macrophages, and in the background, turned bright pink to red with Oil Red O, a fat stain. This suggested the presence of lipid vacuoles, probably from the leakage of mineral oil through the rectal tear.
• As the mare’s condition worsened and given the poor prognosis, euthanasia was chosen.

Postmortem Findings

• Necropsy, or autopsy in animals, displayed both gross and histologic signs of peritoneal effusion (excess peritoneal fluid) and a full-thickness rectal tear.
• Histology also revealed transmural necrotizing pyogranulomatous colitis (a severe inflammatory bowel disease causing granulomas and cell death) and fibrinous peritonitis (inflammation of the peritoneum, the lining of the abdominal cavity).

Conclusion and Implications

• This case is the first reported instance of Oil Red O-positive lipid vacuoles in the peritoneal fluid of a horse likely from the leakage of mineral oil through a rectal perforation.
• The frequency or commonness of such an occurrence in horses remains unknown.
• It’s of essence for cytopathologists, medical professionals specialized in diagnosing diseases on the cellular level, to familiarize with the appearance and significance of lipid-type droplets in phagocytic cells in cytologic fluid analysis specimens.