The Animal Parasites of Man

by Harold Benjamin Fantham; Max Braun; J. W. W. Stephens; Fred. V. Theobald

Kinghorn and Yorke, however, have shown that climatic conditions, namely, those of temperature, also affect the infectivity of the tsetse fly, as the ratio of flies capable of transmitting T. rhodesiense to those incapable of transmitting the virus is 1 : 534 in hot valley districts (e.g., Nawalia, Luangwa Valley, temperature 75° to 85° F.), while on elevated plateaux (e.g., Ngoa, on the Congo-Zambesi watershed, temperature 60° to 70° F.) the ratio falls to 1 : 1312.

Mechanical transmission by the tsetse fly does not occur, if a period of twenty-four hours has elapsed since the infecting meal.

Developmental Cycle in the Fly.—The period which elapses between the infecting feed of the flies and the date on which they become infective varies from eleven to twenty-five days in the Luangwa Valley, according to Kinghorn and Yorke. Attempts carried out at laboratory temperature on the Congo-Zambesi plateau, during the cold season, to transmit T. rhodesiense by means of G. morsitans were always unsuccessful. The developmental cycle of the trypanosome in the fly is influenced by the temperature to which the flies are subjected (as stated above). The first portion of the developmental cycle proceeds at the lower temperatures (60° to 70° F.), but higher temperatures are necessary for the completion of the development of the trypanosome. Kinghorn and Yorke found that the trypanosomes may persist in the fly, at an incomplete stage of their development, for at least sixty days when the climatic conditions were unfavourable.

The first portion of the developmental cycle of the trypanosome takes place in the gut of the fly. Invasion of the salivary glands of the tsetse is secondary to that of the intestine, but is necessary for the infectivity of the fly. A relatively high mean temperature, 75° to 85° F., is essential for the passage of the trypanosomes into the salivary glands and the completion of their development therein.

Kinghorn and Yorke[83] state that the predominant type of trypanosome in the intestine of infected G. morsitans was a large broad form, quite different from that which is most common in the salivary glands. The trypanosome in the glands resembles the short form seen in the blood of the vertebrate host. The authors quoted state that both the intestinal and salivary gland forms of infective G. morsitans are virulent when inoculated into healthy animals.

Bruce and colleagues[84] have quite recently (June, 1914) published an account of their investigations of T. rhodesiense in G. morsitans in Nyasaland. (Incidentally it may be remarked that Bruce considers T. rhodesiense to be identical with a polymorphic strain of T. brucei—see pp. [83], [94]). The development of T. rhodesiense takes place in the alimentary canal and salivary glands, not in the proboscis, of the tsetse fly. In feeding experiments with laboratory bred flies, as well as with a few wild flies, fed on infected dogs or monkeys, only 8 per cent. of the flies were found to be infected on dissection. Of such infected flies, however, only some allow of the complete development of the trypanosomes within them, in other words only about 1 per cent of the flies become infective. The length of time which elapses before a fly becomes infective varies from fourteen to thirty-one days, averaging twenty-three days, when kept at 84° F. (29° C.). The dominant intestinal type of flagellate in the fly is that seen in the proventriculus, which contains many long, slender trypanosomes. These proventricular forms find their way to the salivary glands, wherein crithidial and encysted forms are seen. They change into “blood forms,” which are short, stumpy trypanosomes and are infective. “The infective type of trypanosome in the salivary glands—corresponding to the final stage of the cycle of development—is similar to the short and stumpy form found in the blood of the vertebrate host.” The cycle is thus very similar to that of T. gambiense in G. palpalis (fig. [30]).

Culture.—J. G. Thomson (1912),[85] and subsequently Thomson and Sinton, succeeded in cultivating T. rhodesiense in a modified Novy-MacNeal medium. The development obtained resembled that of the trypanosome in the intestine of Glossina.

General Note on Trypanosomes with Posterior Nuclei.

Posteriorly placed nuclei have been found to occur not only in T. rhodesiense by Stephens and Fantham (1910), but also in T. pecaudi by Wenyon (1912), in T. brucei by Blacklock (1912), and in T. equiperdum by Yorke and Blacklock (1912).

Recently Stephens and Blacklock (1913)[86] have shown that two trypanosomes, different morphologically, have been confused under the name T. brucei. One of these is polymorphic (i.e., it exhibits long and slender as well as short and stumpy forms) and came from Uganda, while the other is monomorphic and is the original Zululand strain described by Bruce from cattle suffering from “nagana.” Bruce (1914) considers that morphological change has occurred in T. brucei in its passage through laboratory animals, and thus explains the diversity of views. The posterior nuclear forms described by Blacklock occurred in the Uganda strain of T. brucei. (See p. [95].) Similarly, a posterior nuclear form, T. equi, has been separated from T. equiperdum. (See p. [98].)