Some parasites are so strictly confined to one species of host that, even when artificially introduced into animals very closely related to their normal host, they do not thrive, but sooner or later, often very quickly, die off, and very rarely establish themselves. For example, repeated attempts have been made to rear the adult _Tænia solium_ in the dog, or to rear _Cysticercus cellulosæ_ in the ox, or the _Cysticercus_ of _Tænia saginata_ in the pig, but they have always proved unsuccessful. Only exceptionally has it been possible to transfer _Cœnurus cerebralis_, the larval stage of a tapeworm (_Tænia cœnurus_) of the dog from the brain of the sheep to that of the domestic goat. On the other hand, in the case of the Trichinellæ transference to a different host is easily accomplished.
Under natural conditions, it is not uncommon for certain kinds of specific parasites to occur occasionally in unusual hosts. Their relationship to the latter is that of INCIDENTAL PARASITES. Thus _Echinorhynchus gigas_, a specific parasite of the pig, is only an incidental parasite of man; _Fasciola hepatica_ and _Dicrocœlium lanceatum_ are specific to numerous kinds of mammals, but may be found incidentally in man. On the other hand, _Dibothriocephalus latus_, a specific parasite of man, may occasionally take up its abode in the dog, cat and fox. As a rule, all those parasites of man that are only rarely met with, notwithstanding that human beings are constantly being observed and examined by medical men, are termed INCIDENTAL PARASITES OF MAN. In many cases we are acquainted with the normal or specific host of these parasites. Thus we know the specific host of _Balantidium coli_, _Eimeria stiedæ_, _Fasciola hepatica_, _Dipylidium caninum_, etc.; in others the host is as yet unknown. In the latter case the question partly relates to such forms as have been so deficiently described that their recognition is impossible, partly to parasites of man in various regions of the earth, the Helminthes and parasites of which are totally unknown or only slightly known, or finally to early developmental stages that are difficult to identify. Animals that usually live free, and exceptionally become parasitic, may likewise be called incidental parasites. In this category are included a few _Anguillulidæ_ that have been observed in man; also _Leptodera appendiculata_, which usually lives free, but may occasionally become parasitic in black slugs (_Arion empiricorum_): when parasitic it attains a larger size, and produces far more eggs than when living a free life. In order to avoid errors, the term “incidental parasites” should be confined to true parasites which, besides living in their normal host, may also live in other hosts. Leuckart speaks of FACULTATIVE PARASITISM in such forms as _Leptodera_. L. Oerley[3] succeeded in artificially causing _Leptodera_ (_Rhabditis_) _pellio_ to assume facultative parasitism by introducing these worms into the vagina of mice, where the parasites remained alive and multiplied. _Leptodera pellio_ dies in the intestines of mammals and man; it remains alive in frogs, but always escapes into the open with the fæces.