The principal stages of the avian plasmodium closely resemble those of the malarial parasites of man. In its earliest stage P. relictum is unpigmented, but soon the trophozoite grows and becomes pigmented, meanwhile displacing the nucleus of the avian red-blood corpuscle, a characteristic feature, distinguishing it from Halteridium. Schizonts are formed, each of which gives rise to about nine merozoites in the circulating blood. Sexual forms or gametocytes also occur in the blood. These develop in Culex fatigans, C. pipiens and C. nemorosus. Oökinetes or vermicules are formed in twelve to fifteen hours in the stomach of the mosquito, and in one to two days well-developed round oöcysts may be seen. In three to four days sporoblasts have formed within the oöcysts and young sporozoites begin to develop. In nine to ten days the oöcysts are mature, being filled with sporozoites. The oöcysts then burst and the sporozoites travel through the thoracic muscles to the salivary glands of the Culicine.
Neumann, experimenting with canaries, found that Stegomyia fasciata could transmit the infection, but less efficiently than species of Culex.
The Cultivation of Malarial Parasites.
The successful cultivation of malarial parasites in vitro was first recorded by C. C. Bass and by Bass and Johns (1912).[206] Since then, J. G. and D. Thomson,[207] and McLellan (1912–13), Ziemann[208] and others have repeated the experiments.
DIFFERENTIAL CHARACTERS OF THE HUMAN MALARIAL PARASITES.
| Character | Plasmodium malariæ (Quartan) | Plasmodium vivax (Benign tertian) | Laverania malariæ = Plasmodium falciparum (Malignant tertian) |
|---|---|---|---|
| Schizogony | Complete in seventy-two hours | Complete in forty-eight hours | Complete in forty-eight hours or less |
| Trophozoite | Smaller than P. vivax, larger than L. malariæ | Young trophozoite large. | Young trophozoite small |
| Pseudopodia not marked or long | Long pseudopodia | ||
| Movements | Rather slow in immature forms | Active amœboid movements | Sometimes actively motile |
| Pigment | Coarse granules, peripherally arranged, little movement | Fine granules, with active movement | Granules fine and scanty, movement oscillatory |
| Schizont | Smaller than red corpuscle | Larger than red blood corpuscle | Smaller than red corpuscle |
| Merozoites | 6 to 12 forming rosette | 15 to 20 regularly arranged | 8 to 32 (according to different authors) arranged irregularly |
| Gametocytes | Spherical | Spherical | Crescentic |
| Distribution of parasites in vertebrate host | About equal number in peripheral and visceral blood | Larger numbers in visceral blood | Scanty in peripheral blood compared with the enormous numbers in the internal organs. The latter part of the cycle (schizogony) may occur in the internal organs only |
| Alterations in erythrocytes | Almost normal | Pale and hypertrophied. | Corpuscle may be shrunken and dark, or may be colourless. Maurer’s coarse dots sometimes seen |
| Schüffner’s dots seen in deeply stained specimens |
Essentially the method of cultivation, as used by Thomson, is as follows: 10 c.c. of infected blood are drawn from a vein and transferred to a sterile test tube, in which is a thick wire leading to the bottom of the tube. One-tenth of a cubic centimetre of a 50 per cent. aqueous solution of glucose or dextrose is placed in the test tube, preferably before adding the blood. The blood is defibrinated by stirring gently with the wire. When defibrination is complete the wire and the clot are removed, and the glucose-blood is transferred, in portions, to several smaller sterile tubes, each containing a column of blood about one inch in height. The tubes are plugged and capped and then transferred, standing upright, to an incubator kept at a temperature of 37° C. to 41° C. The blood corpuscles soon settle, leaving a column of serum at the top, to the extent of about half an inch in each tube. The leucocytes need not be removed by centrifugalization. J. G. Thomson (1913) and his collaborators did not find it necessary to destroy the complement in the serum, and they found that the malarial parasites developed at all levels in the column of corpuscles, and not merely on the surface layer of the corpuscles as first stated by Bass and Johns.
So far only the asexual generation of the malarial parasites has been grown in vitro. Thomson rarely observed hæmolysis in the cultures. Clumping of the malignant tertian parasites occurred. In cultures of the benign tertian parasite (Plasmodium vivax) clumping was not observed. J. G. and D. Thomson consider that this difference as regards clumping explains why only young forms of malignant tertian are found in peripheral blood, as the clumping tendency of the larger forms causes them to be arrested in the finer capillaries of the internal organs. It also explains the tendency to pernicious symptoms, such as coma, in malignant tertian malaria. Further it was found from cultures that P. falciparum was capable of producing thirty-two spores (merozoites) in maximum segmentation, while P. vivax produced sixteen spores (merozoites) as a rule, though the number might be greater than sixteen. (Quartan parasites produce eight spores or merozoites in schizogony.)
It may also be mentioned here that Babesia (Piroplasma) canis has been successfully cultivated in vitro by Bass’s method. This has been accomplished by Thomson and Fantham,[209] Ziemann, and Toyoda in 1913. J. G. Thomson and Fantham used the simplified Bass technique recorded above, namely, infected blood and glucose, incubating at 37° C. In one of the B. canis cultures, starting with heart blood of a dog containing corpuscles infected with one, two, or, exceptionally, four piroplasmata, Thomson and Fantham succeeded in obtaining a maximum of thirty-two merozoites in a corpuscle. The cultures are infective to dogs and sub-cultures have been obtained.