Fig. 109.—Spores of Sarcocystis tenella, Raill. a, fresh, showing the polar capsule; b, stained, showing metachromatic granules and nucleus. × 1,000. (After Laveran and Mesnil.)
The gymnospores of Sarcocystis muris, from the mouse, show active boring movements when kept in saline solution warmed to 35° or 37° C. S. muris is very deadly to its host. From their structure the spores do not appear to have great powers of resistance to external conditions. They measure 12 µ by 3 µ to 4 µ or less.
Laveran and Mesnil (1899) isolated a toxin from S. tenella of the sheep and called it sarcocystin. This substance is especially pathogenic to experimental rabbits.
The duration of life of the Sarcosporidia is a comparatively long one. The affected muscular fibres may remain intact and capable of performing their functions for a long time, but at last they perish, if the host lives long enough. Thus the Sarcosporidia of the muscles are then enveloped only by sarcolemma, and finally, when this likewise disappears, they fall into the intra-muscular connective tissue. In many cases the Sarcosporidia die off within their hosts, this, according to Bertram, being brought about by a disintegration of the spores in the central chambers. In other cases the leucocytes play a part in the destruction of the Sarcosporidia, and sometimes it happens that lime salts are deposited in and around the vacant cylinders.
In some places pigs, sheep, mice and rats are infected with sarcosporidiosis to a remarkable extent, in certain cases almost reaching 100 per cent. Young animals also are infected, and perhaps infection only takes place during youth.
Although the natural mode of transmission of the Sarcosporidia remains to be determined, yet various experimental researches on the problem are of interest and importance. Theobald Smith (1901) found that mice could be experimentally infected with S. muris by feeding them with the flesh of other infected mice. The incubation period was a long one, namely forty to fifty days. Thus, on the forty-fifth day after feeding young Sarcosporidia were found, and seventy days after feeding spore formation began. Ripe spores were found two and a half to three months after the commencement of these experiments. This mode of infection—a cannibalistic one—hardly seems likely to be the natural method for the infection of sheep and ruminants generally. Smith’s researches have been confirmed. Nègre[225] (1910) found that the fæces of mice fed on infected muscular tissue were infective to other mice when ingested by them. Negri[226] infected guinea-pigs with S. muris by feeding them on infected mouse flesh, and found that the parasite in guinea-pigs showed different characters from those exhibited by it in mice. Darling[227] also succeeded in infecting guinea-pigs with S. muris, and Erdmann infected mice with S. tenella (from the sheep).
According to Erdmann[228] (1910) the Sarcosporidian spore germinates in the intestine of the host, which has recently ingested infected material. The spore liberates its contained toxin—sarcocystin—which acts upon the adjacent intestinal epithelium, whereby the latter is shed, and an amœbula creeps out of the spore. The amœbula is able to penetrate the denuded area and get directly into the lymph-spaces of the submucous coat of the intestine. The first period of development, lasting some twenty-eight to thirty days, is said to be passed in the lymph-spaces of the intestine. Later the amœbula reaches a muscle fibre. Writing in May, 1914, Erdmann[229] records the appearance of small amœboid and schizogony forms six days after infection of the host. Crawley[230] (1913) controverts some of these statements and considers that the Sarcosporidian spore, still sickle-shaped, bores its way into the epithelial cells of the intestine and comes to rest there. The spore then becomes round or elliptical, and peripheral masses of chromatin appear within it, suggesting schizogony. This happens about twelve hours after feeding, and in twenty-four hours the spores appear to have left the intestine. More recently (May, 1914), Crawley[231] considers that there is sexual differentiation among the Sarcosporidian spores, a few hours after their ingestion by the host.
Interesting discussions have occurred as to the site of the toxic sarcocystin within the spore. Metachromatic granules occur in the middle of the Sarcosporidian spore (fig. [109]), and the toxin may be contained in these grains, as they disappear, according to Erdmann, before the amœbula penetrates the denuded intestinal wall. However, a polar capsule, containing a polar filament, may be present at one end of a Sarcosporidian spore. Laveran and Mesnil described a striated area at the more pointed end of the spore of S. tenella, which area they consider to represent a polar capsule. Fantham[232] (1913) found a vacuole-like, polar capsule area in the spores of S. colii from the African mouse-bird. The sarcocystin may be contained in the polar capsule. The nucleus of the spore is generally at the opposite, blunter end.