Post-mortem examination of 181 elk (Cervus canadensis nelsoni) from the northern Yellowstone Park herd in 1967–68 revealed that 41% were infected with Thysanosoma actinioides. Infections occurred in all age classes of animals from seven areas in the Yellowstone, Gardner and Lamar drainages in the northern section of the Park.
Prevalence of the parasite was higher in calves and yearlings than in mature elk. Infections varied from 1 to 16 worms per animal, with an average intensity of 4.3 in 75 elk. Worms were confined to the first 6.2 feet of the small intestine, with no distinct habitat preference apparent within this area. Little evidence of T. actinioides or gross lesions associated with its presence was found in the liver or bile ducts of elk examined 20 to 40 minutes after death.