Fig. 143.—Limnæus truncatulus, Müll., the intermediate host of Fasciola hepatica. a., natural size; b., magnified. (From Leuckart.)
The LIFE-HISTORY of the liver fluke was discovered by R. Leuckart and P. Thomas. According to these investigators the elongated miracidium (fig. [131], a) ciliated all over develops from the eggs a few weeks after the latter (fig. 142) have reached the water, and after it has become free the embryo penetrates and becomes a sporocyst (fig. [131], b) in a water-snail (Limnæus truncatulus, Müll. = L. minutus, Drap.) that is common in fresh water, and can live in the smallest collection of water as well as in fields that have been flooded. The sporocyst first of all produces rediæ, which remain in the same host (and under certain circumstances, e.g. in summer, these develop a second generation of rediæ), and these finally form cercariæ (fig. [134]). The latter become encysted on blades of grass and are taken up by the respective hosts with their food; this takes place towards the end of summer, while the sheep feeding on the pasture land in the spring spread the eggs of the fluke, and sometimes the fluke itself, by passing them with their fæces.
In districts where Limnæus truncatulus is absent, analogous species act as the intermediary hosts, of which one example according to Lutz is Limnæus oahuensis in the Sandwich Islands.
[The host in Europe is Limnæus truncatulus. This snail extends from Siberia to Sicily and Algeria, and according to Captain Hutton is a native of Afghanistan. It also occurs in Thibet, Amoor, Morocco, Tunis, Canary Islands and the Faroe Islands. It deposits its eggs or spawn upon the mud around ponds, ditches and streams. The eggs are laid in batches of thirty to a hundred, each snail laying as many as 1,500 eggs; they are united into strips of a gelatinous substance. In about two weeks young snails appear. It is amphibious, being more frequently met with out of the water than in it. It occurs in elevated spots as well as in low-lying districts. Moquin-Tandon found it at 4,000 feet in the Pyrenees. In the allied species, L. peregra, the fluke will develop up to a certain stage, but never completes all its varied phases.
[In South America the host is probably Limnæus viator, Orb., and in North America Limnæus humilis, Say.—F. V. T.]
In human beings as well as in some of the mammals quoted above, the liver fluke is only a casual parasite, and hitherto only twenty-eight cases have been observed in man; the infection was mostly a mild one and there were no symptoms, or only very trifling ones; a few isolated cases were only discovered post mortem. Occasionally, however, even when the infection was inconsiderable, severe symptoms were set up, which in isolated cases led to death. The symptoms (enlargement and painfulness of the liver, icterus) merely pointed to a disease of the liver.
Diagnosis can only be established by finding eggs in the fæces. Care should be taken not to confuse them with those of Dibothriocephalus latus.
Halzoun.