The frequency and distribution have, nevertheless, decreased perceptibly in places; at the commencement of the eighteenth century the broad tapeworm was very common in Paris, at the present date it only occurs when imported (Blanchard); in Geneva, also, according to Zschokke, it has become rarer (formerly 10 per cent., now only 1 per cent.).
The disturbances produced in man by the presence of broad tapeworms are, as a rule, very trifling; in other cases they produce partly gastric disorders and partly nervous symptoms; in a number of cases, again, they set up severe anæmia, apparently caused by toxins produced by the worms and absorbed by the host. There is no danger of auto-infection, as the larval stage lives only in fishes, not in warm-blooded animals. The case reported by Meschede (ova like those of Dibothriocephalus latus in the brain of a man who had suffered from epilepsy for six years) must be otherwise explained.
Fig. 209.—A piece of the body wall of the Burbot, Lota vulgaris. The tangential section has exposed the muscles of the trunk, with a plerocercoid of Dibothriocephalus latus. Natural size.
Human beings, like other hosts, can only acquire the broad tapeworm by ingesting its plerocercoids with the previously mentioned fresh-water fishes; the opportunity for such infection is afforded the more readily by the fact that not only do the lower classes not pay sufficient attention to the cooking of fish, so that all the larvæ that are present may be killed, but also in certain localities the custom exists of eating some parts of these fishes in a raw condition; even the mere handling of the usually severely infected intermediary hosts may occasionally cause infection. The plerocercoids are as well known as, but differ materially in appearance from, the cysticerci (Cysticercus cellulosæ) of pig’s flesh. In Germany the occurrence of the plerocercoids of Dibothriocephalus latus has been confirmed in the pike, miller’s thumb and perch of East Prussia, and more particularly in those taken from the Courland Lagoon.
The life of D. latus is a very long one (six to fourteen years), as is deduced from persons who have left D. latus regions after they have been infected.
According to the experiments of M. Schor, plerocercoids of D. latus placed in slowly warmed water completely lose their movement at 54° to 55° C.; they survive the death of their host for several days; they are killed by low temperatures -3° to +1° C. in two days; strong acids and salt solutions kill them at once, also high temperatures, but all the same at least ten minutes is required in boiling or frying fish in order to kill the plerocercoids with certainty.
Dibothriocephalus cordatus, R. Lkt., 1863.
Syn.: Bothriocephalus cordatus, R. Lkt.