Effects of impregnated bednets
Using mathematical models of malaria transmission, we examined the effect of insecticide-treaded bednets on the transmission of malaria. We obtained the apparently paradox result that the use of bednets can deteriorate the malaria situation. This result originates partly from the way in which malaria is transmitted and partly from the complex interplay between malaria infection and the human immune system.
For the sake of simplicity, let us start with untreated bednets. Mosquitoes that are prevented from obtaining a blood meal because of a bednet will sooner or later give up and look for some other host. This concentrates the vector flies on those people who do not use bednets. Thus the incidence of infection increases among people without bednets whereas it decreases for people who use bednets.
This apparent succes does not necessarily mean that people who are infected less often also become sick less often. The repeated infection of people who grow up in endemic regions gradually built up some immune protection (premunity) against malaria disese, but this protection can be lost within a few years in the absence of infection. Mosquitoes are concentrated on people without bednets if many people use bednets. This may lead to an increase in the fraction of infected mosquitoes in some regions. Bednet users gradually loose their premunity, but they cannot trust that they will escape infection forever Whether the combined effect of these features leads to more or less disease among people with or without bednets depends crucially on the epidemiologic parameters in the afflicted region and cannot be predicted without knowledge of these parameters.
Insecticide-impregnated bednets aim to kill as many mosquitoes as possible. In addition to their toxic effect, they also act as insect repellents which increases the individual protection, but decreases their effect on a population level. Combined with the interplay between infection and premunity, the longterm effects of the wide-spread use of treated bednets become virtually unpredictable unless all important immunologic and epidemiologic parameters are known.
The series of lectures and computer exercises, initiated in 2000 at the Université de Yaoundé I, Cameroon, for students of mathematics, medicine and natural sciences, was continued in 2003 and is planned to take place at the University of Brazzaville, Congo, in summer 2004. A group of students and post-doctoral researchers has formed to further explore this issue.
Lecture notes (in German) (PDF, 1.3 MB)