BACKGROUND: Monthly malaria cases in Finland during 1750-1850 revealed regionally different peaks. The main peak was in late spring in the whole country, but additional peaks occurred in August and December in some regions of Finland. Both primary infections and relapses caused deaths from malaria. The cause and timing of relapses are analysed. METHODS: 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. RESULTS: 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. CONCLUSION: 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.
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