Malaria and helminth interactions in humans: an epidemiological viewpoint

Abstract

In the tropics, helminths are among the most common chronic infections of humans and Plasmodium infections the most deadly. As these two groups of parasites have similar geographical distributions, co-infection is commonplace. It has increasingly been speculated that helminth infections may alter susceptibility to clinical malaria, and there is now increasing interest in investigating the consequences of co-infection, with studies yielding contrasting results. The immunological interactions between helminths and malarial parasites are unclear, although several hypotheses have been proposed. This review provides an epidemiological overview of the possible interactions between helminths and malarial parasites, in relation to geographical distributions and disease patterns, and provides a critical discussion of the results of the epidemiological studies that have so far been conducted to investigate the possible associations. Future studies that might be considered, in order to address the gaps in knowledge, are also considered.

Figure 1

Age–prevalence curves for (a) P.falciparum infection prevalence, severe and non-severe malaria rates from an area of high transmission (Chonyi) in Kilifi District, coastal Kenya (data from Snow et al., 1997, Mwangi et al., 2005) (b) helminth infections (taken from Bundy and Medley (1992)). For non-severe malaria, these are the proportion of children that had at least one episode of malaria in two years of follow-up while for severe malaria it is the number of malaria admissions/1,000 children/annum. For the malaria graph, the black circles represent parasite prevalence, the clear squares, mild malaria and the black triangles severe malaria while for the helminth graph, black circles represent A.lumbricoides, clear squares T.Trichuira, clear triangles Hookworm and black triangles Schistosome infections.

Figure 2

Age specific patterns of severe malaria admissions at five sites in Kenya and The Gambia (Snow et al., 1997). The cross-sectional P.falciparum parasite prevalence (given in brackets) in childhood populations from different areas are represented with clear circles for Siaya (83%) Kenya, black squares Kilifi South (74%), Kenya, black triangles Kilifi north (49%) Kenya, clear squares Sukuta (37%), The Gambia and clear triangles Bakau (2%) in The Gambia.

Figure 3

Distribution of coincidental distribution of Plasmodium falciparum and hookworm in sub-Saharan Africa (adapted from Brooker et al., 2006b). Dark green represents high hookworm and malaria infections, light blue represents high malaria, yellow represents high hookworm, grey represents no data and white, no infections.