[Paul-Louis Simond and coccidia]

In 1880, during a sojourn at Constantine, Alphonse Laveran discovered the etiological agent of human malaria. During his microscopic observations of the parasite in freshly collected blood, Laveran's attention was attracted to the movement of flagellar bodies in the preparations. For Laveran these flagellar bodies corresponded to living organisms, in fact he considered them the most characteristic stage of the parasite; perhaps the sexual form of the parasite. In 1884, back in Paris at the military hospital Val de Grâce, A. Laveran showed these flagellated bodies to Pasteur, Roux and Chamberland who all thought that is was impossible not to recognize a living body in this mass of protoplasm displacing the surrounding red blood cells with its protruding flagella. As early as 1890, Elie Metchnikoff established a link between the flagellar bodies of Laveran and the stealthy stage of parasitic Coccidia infecting the intestinal epithelium of salamanders. It was the Pasteuriens and a few others scientists like Danilewski and Pfeiffer who firmly believed that the flagellar forms were indeed a normal stage during the hematozoan life cycle. On the contrary, Grassi and the Italian school, as well as the French protozoologist Labbé were convinced that the flagellar bodies corresponded to a degenerative form of the parasite provoked by the exposure of the parasites to air during slide preparation. In early 1896, P-L Simond joined Metchnikoff's laboratory and was assigned to study salamander coccidia in order to clarify the nature of the flagellar bodies. In a very short period of time, Simond clearly demonstrated that the coccidia had two types of life cycles; one of which resulted in the formation of flagellar bodies. Simond called the flagellar bodies "chromatozoites" due to the important quantities of chromatin twisted around the flagellum. From these observations and the sperm-like movement of the chromatozoites, Simond put forth the hypothesis that the chromatozoites were the male sexual forms of the parasite. He noted that they were found in all species of coccidia, as well as in the different species of malaria causing hematozoans of man and birds. Simond went even further in his interpretation to suggest that malaria parasites undergo sexual reproduction. This was indeed true, but Simond thought erroneously that sexual reproduction in hematozoans would result in the formation of resistant spores like in the Coccidia. The sexual reproduction hypothesis of malaria parasites was also formulated independently a year later by W. G. MacCallum (1898) working in the United States on Halteridium, a hematozoan infecting crows. Initially MacCallum was not aware of Simond's work, but later gave Simond full credit for his work on Coccidia. Furthermore, MacCallum observed for the first time the chromatozoite (microgamete) enter a female element (macrogamete) to form a mobile worm-like stage of the parasite know today as the ookinete. In his writings Simond is very modest, giving most of the credit to his mentors Metchnikoff and Laveran, but Laveran in an article which appeared in 1899 recognizes P.-L. Simond as the first person having put forth the hypothesis that the flagellar bodies of hematozoans causing human and bird malaria were the male forms of the parasite destined to fertilize the female elements.