Of this species only two specimens have been recorded; they occurred in human beings and were sent at different times to the first describer of them by a doctor in Lincoln (Nebr.). Perhaps Tænia solium, var. abietina, Weinld., which was found in a Chipeway Indian, is of the same species in spite of the shorter segments.
Tænia echinococcus, v. Sieb., 1853.
Syn.: Tænia nana, v. Ben., 1861 (nec v. Sieb., 1853); Echinococcifer echinococcus, Weinld., 1861.
Fig. 250.—Tænia echinococcus: the cirrus sac, the vagina, uterus, ovary, shell gland and vitellarium, and the testicles at the sides are recognizable in the second proglottis; the posterior proglottis shows the uterus partly filled with eggs, as well as the cirrus sac and the vagina. 50/1.
Tænia echinococcus measures 2·5 to 5 or 6 mm. in length; the head is 0·3 mm. in breadth, and has a double row of twenty-eight to fifty hooklets (on an average thirty-six to thirty-eight) on the rostellum.
The size and form of these hooklets vary (the larger ones are 0·040 to 0·045 mm. in length, the smaller ones are 0·030 to 0·038 mm. in length). The suckers measure 0·13 mm. in diameter; the neck is short; there are only three or four segments, the posterior segment being about 2 mm. in length and 0·5 mm. in breadth. The genital pores alternate; there are forty to fifty testicles; the vas deferens is spirally coiled; the cirrus pouch is pyriform. The ovary is horseshoe-shaped with the concavity directed backwards; the vitellarium double, each half almost bean-shaped, at right angles to the plane of the segment; the shell gland is round. The median trunk of the uterus is dilated when filled with eggs and (instead of lateral branches) has lateral diverticula. It is not unusual for the eggs to form local heaps. The embryonal shell (embryophore) is moderately thin, with radial striæ, almost globular, 30 µ to 36 µ in diameter.
When mature this parasite lives in the small intestine of the domestic dog, the jackal, and the wolf, and apparently also in Felis concolor, and is usually present in great numbers; it can also be transmitted experimentally to the domestic cat, one successful result out of seven (Dévé).[292] The larval stage (Echinococcus polymorphus) lives in various organs—chiefly in the liver and lungs—of numerous species of mammals (twenty-seven), especially in sheep, ox and pig, and it is even not uncommon in man, though the Tænia itself has never been found in a human being; accordingly man can only acquire the echinococcus by ingesting the eggs of the “dog worm.” The dogs disseminate the eggs of Tænia echinococcus wherever they go, or carry them to their mouths and coats by biting up the evacuated segments, and are thus able to transmit them directly to human beings (by licking them or making use of the same crockery, etc.). In other cases the oncospheres, enclosed in the embryophores, must withstand desiccation for a time and then (as when the dogs are “kissed” or otherwise caressed) are transmitted into or on to man. As echinococcus disease in man is always very dangerous, it would be a matter of general interest to prevent dogs being infected by destroying the echinococci,[293] and all measures would be justifiable which would diminish the superfluous number of house-dogs (for instance, high taxes); measures should also be adopted to limit the association of men with dogs, particularly in such frequented places as restaurants, railway carriages and tram-cars.