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.)
Again, Bruce and his colleagues on the Royal Society Commission investigating sleeping sickness in Nyasaland, have stated (April, 1913) that “evidence is accumulating that T. rhodesiense and T. brucei (Plimmer and Bradford) are identical.” The exact identity of trypanosomes showing posterior nuclei is, then, far from settled, although Laveran by cross immunity tests has declared that T. brucei is distinct from T. rhodesiense. No one has yet seen posterior nuclei in T. gambiense.
Trypanosoma cruzi, Chagas, 1909.
Syn.: Schizotrypanum cruzi, Chagas, 1909.
The trypanosome was discovered by Chagas87 in the intestine of the bug, Triatoma (Conorhinus) megista, in Brazil, and then in the blood of a small monkey bitten by the bug. A little later it was found in the blood of a child, aged two years, suffering from irregular fever, extreme anæmia and enlarged glands in the State of Minas Geraes, Brazil. Chagas found that he was able to infect many of the usual laboratory animals with the trypanosome, by allowing the bug to bite them. He was also able to culture the parasite on blood agar.
Chagas found the Reduviid bug, Triatoma megista, in the houses of the poorer inhabitants of the Brazilian mining State, and that it attacked the people, more especially the children, at night, biting the face. On this account the insect is called “barbeiro” by the inhabitants. The bite is somewhat painful. The disease has since been found in other parts of Brazil, e.g., Matta de São João in Bahia province, Goyaz, Matto Grosso and São Paulo provinces, as well as in Minas Geraes.
Morphology.—The trypanosome has a large blepharoplast or kinetic nucleus. It is stated to occur both free and in the red blood corpuscles in the peripheral blood. It is about 20 µ long, on an average.
Two forms of the parasite (fig. 33, 6, 7) are described in the human blood. In one free form there is a large egg-shaped blepharoplast and the posterior (aflagellar) end of the parasite is drawn out. The blepharoplast (kinetic nucleus) may have a chromatin appendage. The nucleus is oval or band-like, containing a karyosome. The flagellum, starting close to the blepharoplast or its appendage, has a free portion of variable length. The other free form in the blood has a more or less round, terminal blepharoplast, smaller than in the first form, without a chromatin appendage as a rule. The body of this second form is decidedly broader than that of the first mentioned.
Fig. 33.—Trypanosoma cruzi. Schizogony. 1, merozoite in red blood corpuscle; 2, parasite totally enclosed in red cell, no flagellum or undulating membrane; 3-5, parasites partially enclosed in red cell; 6, 7, parasites in human blood; 8-11, parasites in lungs of the monkey, Callithrix; 12, 13, initial forms of schizogony; 14, 15, schizogony in the lungs of Callithrix. (After Chagas.)
The dimorphism has been interpreted sexually, the first mentioned forms being termed males, the second ones females. The correctness of this interpretation is very doubtful.
No sign of longitudinal division was ever seen in the peripheral blood or in the internal organs. The “endocorpuscular” forms may be completely or partially enclosed in the red cell or only attached thereto (fig. 33, 1-5). At the beginning of infection the endocorpuscular forms are the more numerous. Some authorities, however, doubt these stages.
Life-history in the Vertebrate Host.—Chagas found fluctuations in the number of the parasites in the peripheral blood. He believes the increase of the parasites to be periodic.
The investigations of Chagas and of Hartmann have revealed two types of multiplication which take place in the internal organs of the vertebrate host.
(a) The first type—which possibly belongs to another organism, Pneumocystis carinii, see p. 90—occurs in the capillaries of the lungs. The flagellate parasite entering the lung capillaries loses its flagellum and undulating membrane. Its body becomes curved, and the two ends fuse, and so an oval mass is formed (fig. 33, 8-11). In some cases the blepharoplast disappears, in other cases it blends or fuses with the nucleus. The nucleus of the rounded parasite then divides into eight by successive divisions (fig. 33, 12-15). Next the body, which is surrounded by its own periplast, also divides, giving rise to eight tiny daughter individuals or merozoites (fig. 33, 15). The merozoites lie inside the periplast, which acts as a sort of “cyst wall.” The merozoites are said to exhibit dimorphism, and Chagas has interpreted the dimorphism in terms of sex. The daughter forms, produced by the parent trypanosomes which kept their blepharoplasts, themselves have blepharoplasts as well as nuclei, and have been termed “males” or “microgametes.” The merozoites, arising from parent trypanosomes which lost their blepharoplasts, have themselves only nuclei, and have been called “females” or “macrogametes.” In the case of the so-called “female” forms the single nucleus divides into two unequal parts, of which the smaller becomes the blepharoplast, and a flagellum is formed later. The so-called “males” possess early a rudiment of a flagellum. Both kinds of merozoites escape from the parent periplast wall, and enter red blood corpuscles. They grow into flagellates within the corpuscles, and then become free as adult trypanosomes in the blood-stream.
Fig. 34.—Trypanosoma cruzi. Transverse section of a striated muscle containing rounded forms of the parasite in the central portion. × 1,000 approx. (After Vianna.)
(b) The second mode of multiplication is one of asexual reproduction (schizogony or agamogony). It was first described by Hartmann from hypertrophied endothelial cells of the lungs. It has since been found in the cardiac muscle, in the neuroglia of the central nervous system, and in striped muscle (fig. 34). In laboratory animals it has also been found in the testicle and suprarenal capsules. In these tissues the parasite is intracellular, appearing as a small rounded body with nucleus and blepharoplast, without flagellum or undulating membrane. In other words the parasite is Leishmania-like in the body tissues, and recalls the organism of kala-azar.
Chagas considers this second mode of multiplication to be strictly asexual. By this means the number of parasites in the vertebrate host is increased, and symptoms are produced. On the other hand the first mode of multiplication, seen in the lung capillaries, is considered by Chagas to be a process of gametogony, in which sexual forms are differentiated. He finds that (1) the adult trypanosomes exhibit a dimorphism in human blood rarely seen in artificially infected guinea-pigs.