Anatomic distribution of encysted cyathostome larvae in the horse.

The large intestines of 6 horses were divided by length into 12 segments, and each segment was washed and weighed. At least 5% by weight of each segment was examined by mural transillumination, and encysted cyathostome larvae were counted. Total numbers of larvae in each segment were calculated. Encysted larvae (98%) were present in the proximal 7 segments of the large intestine (cecum and proximal 75% of the ventral colon), and 2% were present in the distal 25% of the ventral colon and entire dorsal colon. Encysted larvae (6%) were located in the dorsal colon of 1 heavily infected horse. Larval density was greatest in the cecum, which harbored 57% of encysted worms yet contributed only 27% to the total weight of the large intestine. Larvae dissected from tissue cysts consisted of species that were highly prevalent as adult worms. However, there were few Cylicostephanus longibursatus, probably because of the small size and fragility of its 4th-stage larvae.