Septic peritonitis due to colonic perforation associated with aberrant migration of a Gasterophilus intestinalis larva in a horse.

An adult quarter horse mare was presented with acute colic and fever. Physical examination and abdominocentesis showed septic peritonitis, and the mare was euthanatized. Necropsy and histopathologic examination revealed a focal partial perforation of the right ventral colon, which contained a single bot fly larva, identified as a third-instar larva of Gasterophilus intestinalis. This larva was embedded deep within the muscularis and the submucosa. Although bot fly larvae are known to attach to aberrant sites within the digestive tract, this is the first known report of deep penetration of the colon by a gasterophilus larva, with the resulting leakage of intestinal content leading to septic peritonitis.