In cultures on the N.N.N. medium the leishmania increase in size, and develop a flagellum, finally assuming an elongated leptomonas form. It is thought that this latter represents a stage of the development of the parasite in the invertebrate host; such a development, however, has not yet been followed. Patton believes that in the case of L. donovani the parasite completes its developmental cycle in Cimex rotundatus, and in the Mediterranean area there is a certain amount of evidence to incriminate Ceratophyllus fasciatus, the dog-flea, as being the definitive host. In the case of L. tropica, it has been pointed out that since the sores, as they occur in man, are situated in some uncovered part of the body, the invertebrate host is more likely to be some biting fly or mosquito, or even the sand-fly (Phlebotomus).
Morphologically the leishmania is a leptomonas, but the fact that it occurs in a vertebrate host should justify its inclusion in a separate genus, Leishmania.
Fig. 239.—Diagram of types of flagellates. (After Wenyon.)
a, Trypanosome; b, Crithidia; c, Leptomonas; d, Leishmania.
Nomenclature of the hæmoflagellates.—The generic terms given to these different parasites are also utilized to denote the different stages in their life cycles. Of this cycle the leishmania is taken to represent the most primitive, the trypanosome of vertebrates the most highly developed form (Fig. 239).
One and all these stages occur in the developmental forms of certain trypanosomes in culture, and in the gut of the definitive host the glossina, and even in the vertebrate, as in T. lewisi. The generic titles Trypanosoma, Herpetomonas, Crithidia and Leptomonas should, strictly speaking, be reserved for the parasite according to the highest developmental form it is capable of assuming. The accompanying diagram, taken from Wenyon (Fig. 240), illustrates this idea.
