There are about 38 species of threadworms found in sheep, swine, goat, ox, deer, camel, rabbit, primates, dogs, and cats.
Some species are parasites of birds, reptiles and amphibians.
Worms indistinguishable from S. stercoralis have been recovered from dogs and cats. The threadworm of man occurs from about 35 degrees north latitude to 30 degrees south latitude. Generally the distribution is limited to warm moist areas because such climates are favorable to the survival of the juvenile stages. The heterogonic life cycle is advantageous to the parasite as it affords the opportunity for the parasite to exist and reproduce for one or more generations in the absence of a host. Also the number of infective juveniles may be increased by reproduction of the free-living females and males.