Natural relapses in vivax malaria induced by Anopheles mosquitoes

Monthly data of deaths from malaria in 1750–1850 were successively correlated with mean temperatures of June and July of five years in succession forwards from the current year and through 10 years in succession backwards to identify timing of relapses in Plasmodium vivax.Malaria cases show an increasing correlation with June-July temperatures, with peaks in late summer, midwinter and late spring and then dropped gradually during 2–9 years from the first summer depending on the region. The longest incubation time identified was 8 years and 7 months.High correlations of June-July temperatures with deaths from malaria in August to September in the same year indicate a close connection to the new generation of hatching Anopheles mosquitoes. Because rapid sporogony before October is impossible in Finland, the most plausible explanation is an early induction of relapses of vivax malaria by uninfected anophelines. Malaria cases during the winter and the following spring are caused by both primary infections and induced relapses. All subsequent cases represent relapses. It is proposed that the basic relapse patterns in vivax malaria are regulated by anophelines. It is also proposed that the Plasmodium is enhancing blood sucking of Anopheles messeae, which so far has been considered a bad vector.It has long been known that vivax malaria causes relapses in a variable degree. The malarial relapse including a pre-erythrocytic stage in Plasmodium vivax was first outlined in 1948 [1,2], but the true relapse stage, the hypnozoite, was only identified in 1982 [3]. The exact cause of activation of hypnozoites has so far remained unexplained [4]. Regionally different incubation times have been recognized [5-7], suggesting a polymorphic genetic basis for the incubation time [8,9]. In general it seems that P. vivax of the temperate zone has a longer incubation time than its tropical counterpart although a long incubation time of tropical malaria is also known [10]. No convincing expla