[Understanding the mode of transmission of malaria agents: an astonishing series of hypotheses, observations, research and controversy].

Abstract

In 1880, Laveran observed the causative agent of malaria. As early as 1884, he considered that mosquitoes could be responsible for the transmission of haematozoa, a hypothesis which resulted from the observation and reflection of an informed hygienist. But, as Laveran himself said, "the opinion that I defended was considered by most observers to be highly unlikely".Nearly 15 years after the discovery of the haematozoan, the elucidation of the mechanism of transmission still proved difficult to establish. A link with the existence of swamps had been established a long time before, but the true mode of transmission remained a mystery until the end of the 19th century. The implication, by Manson in 1877, of mosquitoes in the cycle of the Bancroftian filaria, then other observations of the same order, ended up attracting the attention of malariologists. Laveran himself was quickly convinced of the role of mosquitoes in carrying out the natural cycle and propagating Plasmodium, but this theory had as many detractors as supporters.In 1897, Ross showed the presence of oocysts on the stomach of mosquitoes previously gorged on a malaria patient, then in 1898, of sporozoites of bird plasmodia in mosquitoes. He was convinced that, through their bite, these insects were responsible for the transmission of human malaria agents, without being able to prove it. The results obtained by Ross were immediately confirmed in Italy by Grassi and his collaborators who, in November 1898, described the stages of Plasmodium in man and, through various experiments carried out in collaboration with British researchers, showed the role of Anopheles, a result far from being accepted by all. Skepticism persisted for a long time.An excellent protozoologist, Laveran was not an entomologist. He was however among the first defenders of the anopheline theory. He worked extensively on establishing the relationships between Anopheles mosquitoes and malaria and took a close interest in the environmental conditions of the transmission. In his mind, malaria fever should henceforth be classified as a preventable disease. An era of hope thus dawned: malaria prophylaxis, based on fight against mosquitoes, could begin.