Studies onTrypanosoma grayi. III. Life-Cycle in the Tsetse-fly and in the Crocodile

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Abstract
An account is given of the life cycle and morphology ofT. grayiin the crocodile (Crocodilus niloticus) and in the tsetse-fly (Glossina palpalis). The immunological relations of this trypanosome to its hosts and its affinities to other species are also dealt with.T. grayibelongs to the group of trypanosomes developing in the posteriorstation of the invertebrate host and transmitted by the contaminative method (“lewisigroup”).T. grayioccurs in very small numbers in the crocodile and is concentrated chiefly in the peripheral circulation of the skin (about 200 parasites per 1 c.c. of blood). It is one of the largest trypanosomes, the blood forms measuring up to 91μWhen fed on an infected crocodile the tsetse-fiy takes up not more than about six trypanosomes. These commence their development in the mid-gut, giving rise to crithidial and trypanosome forms, and later extend to the hind-gut where their evolution is completed and the infective metacyclic trypanosomes are produced. These, when voided with the faeces, serve to infect the crocodileper os.Since the incubation period ofT. grayiin the crocodile is about four days, this is the time required for the small metacyclic trypanosomes to develop into the large blood forms.The distribution ofT. grayiin the gut of the fly in the course of its development is determined by the presence of the peritrophic membrane and involves three successive waves of migration in opposite directions: (1) From the intraperitrophic space backwards, into the colon, thence (2) forwards into the extraperitrophic space up to and including the mid-gut; and finally (3) they again migrate backwards, to the hind-gut. (Diagrams of the life cycle and distribution ofT. grayiinGlossinaare given in Text-figs. 2 and 3.)Evidence is produced to show that the so-called “cysts” ofT. grayidescribed by previour authors are really artifacts.Apparently the majority of crocodiles in Victoria Nyanza harbourT. grayi. The trypanosome has no harmful efleet upon the crocodile and it was proved that the infection can persist for more than two years.The crocodile appears to possess anaturalpartial or tolerance immunity againstT. grayi, which does not protect it from invasion by the parasite, but maintains its numbers at a constantly low level.The infection rate in the experimental tsetse-ffies is very high (average 61 per cent.) in the early days of infection, but later f ails to an average of 17·3 per cent. The majority of the flies, originally fully susceptible to infection, appear toacquirean immunity after the fifth day.The average degree of infection in “wild” flies is 11·2 per cent. This compares closely with that in the experimental flies. Hence it is inferred that the majority of these flies had fed in nature on infected crocodiles.T. grayiis closely allied to trypanosomes of the “lewisigroup” and to some of the trypanosomes of land reptiles and amphibia. On the strength of the clifterence existing between these two groups of trypanosomes, some modification in the accepted classification of these species is introduced.A name,T. theodorisp.n., is proposed for the goat trypanosome described by Theodor (1928).

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