The classification of trypanosomes is very difficult. Laveran (1911)54 has suggested the examination of the relative length of the flagellum as a diagnostic character, and so arranged these flagellates in mammals in three groups. The first group included those trypanosomes always having part of the flagellum free (e.g., T. evansi, T. vivax); the second group comprised forms without a part of the flagellum free (e.g., T. congolense), while the third group included forms some members of which have free flagella, while others have not (e.g., T. gambiense). Bruce55 (1914) and Yorke and Blacklock56 (1914) have also devised classifications.
Resting stages of some trypanosomes have been found in the internal organs of their vertebrate hosts. The formation of these oval, Leishmania-like bodies will be noted in individual cases later. Similar small oval bodies form an important phase in the life-history of T. cruzi, which multiplies normally by multiple fission or schizogony into these oval, daughter elements, and not by binary longitudinal fission in the circulating blood.
Polymorphism in trypanosomes (e.g., T. gambiense, T. rhodesiense) is now interpreted as a phenomenon resulting from growth and division.57 Long, thin forms are those about to divide. Fully mature forms are shorter and broader. Various intermediate types occur and represent growth forms. Formerly, polymorphism was interpreted in terms of sex, thin forms being regarded as males, broad forms as females, while the intermediate types were termed indifferent. Conjugation was not observed, and there is no evidence in support of the sexual interpretation.
The transmission of trypanosomes from one vertebrate host to another is usually accomplished by the intermediation of some biting arthropod in the case of terrestrial animals, while leeches are usually considered to act as transmitters in the case of the trypanosomes occurring in aquatic animals. Developmental phases of the life-histories of trypanosomes occur in the invertebrate transmitters, and will be considered in individual cases.
Trypanosoma gambiense, Dutton, 1902.
Syn.: Trypanosoma hominis, Manson, 1903. Trypanosoma nepveui, Sambon, 1903. Trypanosoma castellanii, Kruse, 1903. Trypanosoma ugandense, Castellani, 1903. Trypanosoma fordii, Maxwell Adams.
In vertebrate blood Trypanosoma gambiense is polymorphic, for long, thin forms may be seen in contrast with short, stumpy forms, as well as intermediate forms (fig. 29, a—c). This polymorphism has been interpreted in terms of sex, especially by German investigators, following Schaudinn (see above). However, there is no evidence of conjugation, and the polymorphic forms are more easily interpreted in terms of growth and division, for the long thin forms are potential dividing organisms, and the stumpy or short parasites, with little or no free flagellum, are the adult individuals.
Morphology of T. gambiense in the Circulating Blood.
Fig. 28.—Trypanosoma gambiense. × 1,700. (After Dutton.)
T. gambiense varies from 13 µ to 36 µ in length, its average length being 24·8 µ, as was determined in 1913 by exact biometrical methods by Stephens and Fantham.58 Three forms of parasite occur. According to Miss Robertson,59 the relatively short forms from 13 µ to 21 µ long may be regarded as the mature or “adult” type of parasite in the blood. They carry on the cycle in the vertebrate. From them intermediate forms, which are longer than the “adult” but at first have the same breadth, arise by growth. They possess a free flagellum. The intermediate forms grow into long individuals, which are those about to divide. The products of division give rise, directly or indirectly, to the adult forms.
Fig. 29.—Trypanosoma gambiense. Development in vertebrate host. a, long, slender, b, intermediate and c, short, stumpy forms, found in the blood; d, e, f, non-flagellate, latent forms from internal organs. × 2,000. (Original. From preparations by Fantham.)
The organism has an elongate body with an anterior or flagellar end and a blunter posterior or non-flagellar end. The protoplasm is finely granular, large inclusions being rare. The central nucleus is oval and large, often containing most of its chromatin concentrated as a karyosome, with small granules only scattered near or on the fine nuclear membrane. The blepharoplast is either rounded or rod-shaped. The undulating membrane is thrown into folds and is bordered by the flagellum. A small basal granule may be present near, or at the actual origin of the flagellum.
Multiplication in the vertebrate is brought about by longitudinal division. According to the recent account of division by Miss Robertson, the blepharoplast doubles, then the flagellum splits for the greater part of its length, and the daughter flagella separate, one being shorter than the parent flagellum. The nucleus often shows two well marked dark granules on the membrane at opposite poles, and these appear to act as centrosomes. Nuclear constriction occurs and the halves gradually separate. Finally the two daughter organisms become free, the aflagellar end splitting last. The products of division may be equal or unequal. Repeated division goes on in the general circulation until the blood swarms with parasites. Then the trypanosomes gradually disappear, and a period occurs when it is practically impossible to demonstrate the parasite in the blood. At such a period, trypanosomes can be obtained by puncture of the enlarged lymphatic glands or of the spinal canal, or can be found in the internal organs, more particularly in the spleen, lungs, liver and bone-marrow. In the latter organs, latent bodies are produced (fig. 29, d—f) which are capable of again becoming flagellates and entering the general circulation. Their formation was described by Fantham (1911).60 The parasite contracts, the blepharoplast migrates towards the nucleus, a very thin coat differentiates around the two nuclei and a certain amount of cytoplasm, and the parts exterior to the coat disintegrate, leaving a small, oval body behind. Fuller details are given in connection with T. rhodesiense. Laveran (1911)61 considers that latent bodies are “involution” forms, but acknowledges that they can flagellate and become infective in fresh blood.
No multiplication of the trypanosomes within the cells of the lung, liver or spleen of infected monkeys was found by Miss Robertson in her recent researches.
There appear to be negative periods in infected monkeys, since, although trypanosomes may occur in their blood at such times, they are not infective to Glossina.
Development in Glossina palpalis.—The principal accounts are those by Sir D. Bruce and his colleagues (1911),62 and by Miss Robertson63 (1912), whose results will be followed. According to the latter investigator T. gambiense never enters the body cells of the fly (G. palpalis), nor does it penetrate the gut wall into the body cavity. Practically no crithidial stage occurs in the fly’s main gut, but a trypanosome facies is retained therein.
After the trypanosomes are ingested by the fly during a meal of infected blood, sooner or later multiplication occurs. This development usually begins in the middle or posterior part of the mid gut, and trypanosomes of varying sizes are produced. After the tenth or twelfth day, many long, slender trypanosomes (fig. 30, a) are found, which gradually move forwards into the proventriculus. Such long, slender forms represent the limit of development in the lumen of the main gut. The proventricular type, developed about the eighth to the eighteenth or twentieth day, is not infective; it may occur in the crop, but is not to be found permanently there. Between the tenth and the fifteenth days multinucleate forms of trypanosomes are found, and may be styled multiple forms (fig. 30, b). Some of these latter may be degenerative.
Fig. 30.—Trypanosoma gambiense. Development in the fly, Glossina palpalis. a, slender, proventricular form; b, multinucleate