In 1869, Melnikoff found in a dog louse, Trichodectes canis, some peculiar bodies which Leuckart identified as the larval form of this tapeworm. The worm is, however, much more common in dogs and cats than is the skin parasite, and hence it appears that the Trichodectes could not be the only intermediate host. In 1888, Grassi found that it could also develop in the cat and dog fleas, Ctenocephalus felis and C. canis, and in the human flea, Pulex irritans.
The eggs, scattered among the hairs of the dog or cat, are ingested by the insect host and in its body cavity they develop into pyriform bodies, about 300µ in length, almost entirely destitute of a bladder, but in the immature stage provided with a caudal appendage ([fig. 115]). Within the pear-shaped body ([fig. 116]) are the invaginated head and suckers of the future tapeworm. This larval form is known as a cysticercoid, in contradistinction to the bladder-like cysticercus of many other cestodes. It is often referred to in literature as Cryptocystis trichodectis Villot.
As many as fifty of the cysticercoids have been found in the body cavity of a single flea. When the dog takes up an infested flea or louse, by biting itself, or when the cat licks them up, the larvæ quickly develop into tapeworms, reaching sexual maturity in about twenty days in the intestine of their host. Puppies and kittens are quickly infested when suckling a flea-infested mother, the developing worms having been found in the intestines of puppies not more than five or six days old.
Infestation of human beings occurs only through accidental ingestion of an infested flea. It is natural that such cases should occur largely in children, where they may come about in some such way as illustrated in the accompanying figures [117] and [118].
Hymenolepis diminuta, very commonly living in the intestine of mice and rats, is also known to occur in man. Its cysticercoid develops in the body cavity of a surprising range of meal-infesting insects. Grassi and Rovelli (abstract in Ransom, 1904) found it in the larvæ and adult of a moth, Asopia farinalis, in the earwig, Anisolabis annulipes, the Tenebrionid beetles Akis spinosa and Scaurus striatus. Grassi considers that the lepidopter is the normal intermediate host. The insect takes up the eggs scattered by rats and mice. It has been experimentally demonstrated that man may develop the tapeworm by swallowing infested insects. Natural infection probably occurs by ingesting such insects with cereals, or imperfectly cooked foods.