Fig. 220.—Dipylidium caninum: development of embryo. 1, solid hexacanth embryo; 2, primitive lacuna (a) in the embryo; 3, elongation of hinder part, rudiments of sucker and rostellum appearing; 4, “body” and “tail” distinct, (b) and (c) excretory system; 5, fore-body invaginates into hind-body, excretory bladder has a pore; 6, tail has dropped off; scolex growing up into secondary cavity formed by fore-body; the primitive cavity has been absorbed at stage 4. (After Benham, Grassi and Rovelli.)
Dipylidium caninum is a common intestinal parasite of dogs, in which it grows larger (Tænia cucumerina, Bloch) than in cats (T. elliptica, Batsch); it has, however, also been found in jackals, as well as in human beings, though in the latter it is of comparatively rare occurrence (twenty-four cases), and almost always affects children, generally of tender age. One-third of all the cases in children were sucklings, about a quarter of all the cases recorded were adults, and these occurred throughout all Europe with the exception of Spain and Italy.
The proglottids, which leave the intestine spontaneously, are recognizable by the naked eye on account of their form and reddish colour, as well as their two genital pores. As a rule, the presence of this parasite sets up no marked symptom in the patient.
Fig. 221.—Larva (cysticercoid) of Dipylidium caninum, consisting of body and tail. The latter is solid and bears on it the embryonal spines. The bladder, which was only slightly developed, has disappeared, and the fore-part of the body bearing the rostellum is now seen invaginated into the hind portion. The hooklets are shown in front of the excretory system which has now developed. At a further stage the tail drops off; the head now evaginates, but is still enclosed in a double-walled sac formed by the prolongation upwards on each side of the topmost parts of the body shown in the figure. Cf. fig. 220, 6. Enlarged. (After Grassi and Rovelli.)
The corresponding larval form (cysticercoid) lives in the louse of the dog (Trichodectes canis), a fact that was first established by Melnikow and Leuckart; according to Grassi and Rovelli, as well as Sonsino, it also lives in the flea of the dog (Ctenocephalus canis) and in the flea of man (Pulex irritans), but not in its larva. The adult segments, which also leave the rectum of dogs and cats spontaneously, creep about around the anus and get into the hair, and are thus partly dried and disintegrated. Part of the segments, or the oncospheres released by disintegration, are then taken up by lice and fleas, within which they develop into larvæ (cysticercoids). Dogs and cats are thus infected by their own skin parasites, which they bite and swallow whilst gnawing at their fur. The infection of human beings must occur in an analogous manner, by transmission of the cysticercoids present on the lips or tongue of dogs when the latter lick them, or it may be that the vermin of cats and dogs harbouring cysticercoids are accidentally and directly swallowed by human beings.
Family. Hymenolepididæ, Railliet and Henry, 1909.
Genus. Hymenolepis,[282] Weinland, 1858.