Helminth infection and eosinophilia and the risk of Plasmodium falciparum malaria in 1- to 6-year-old children in a malaria endemic area

Abstract

Background: Helminth infection is common in malaria endemic areas, and an interaction between the two would be of considerable public health importance. Animal models suggest that helminth infections may increase susceptibility to malaria, but epidemiological data has been limited and contradictory.

Methodology/principal findings: In a vaccine trial, we studied 387 one- to six-year-old children for the effect of helminth infections on febrile Plasmodium falciparum malaria episodes. Gastrointestinal helminth infection and eosinophilia were prevalent (25% and 50% respectively), but did not influence susceptibility to malaria. Hazard ratios were 1 for gastrointestinal helminth infection (95% CI 0.6-1.6) and 0.85 and 0.85 for mild and marked eosinophilia, respectively (95% CI 0.56-1.76 and 0.69-1.96). Incident rate ratios for multiple episodes were 0.83 for gastro-intestinal helminth infection (95% CI 0.5-1.33) and 0.86 and 0.98 for mild and marked eosinophilia (95% CI 0.5-1.4 and 0.6-1.5).

Conclusions/significance: There was no evidence that infection with gastrointestinal helminths or urinary schistosomiasis increased susceptibility to Plasmodium falciparum malaria in this study. Larger studies including populations with a greater prevalence of helminth infection should be undertaken.

Figure 1. The probability of remaining free of clinical malaria is plotted over the 9 months of monitoring.

Numbers of children at risk are given below the Kaplan Meier plots. Both plots use an endpoint of >2,500 parasites per microlitre and fever. Plot a) compares the probability of remaining free of clinical malaria for children with gastrointestinal helminth infection (GI Helm.) and uninfected children. Plot b) compares the probability of remaining free of clinical malaria for children with normal eosinophil counts (below 0.5×10⁶ per ml), mild eosinophilia (0.5–1×10⁶/ml) or high eosinophilia (above 1×10⁶/ml). The survival curves are not significantly different by unadjusted log-rank testing (p = 0.98 for gastrointestinal worm infection, p = 0.71 for eosinophilia).

Figure 2. The coefficients from cox regression models (left) and the coefficients from poisson regression models (right) adjusted for the same covariates are displayed for the effect of gastrointestinal helminth infection (GI Hel), A. lumbricoides infection (A. l.), S. haematobium (Sch), mild eosinophilia (Mild Eos) and marked eosinophilia (High Eos), adjusted for age, village and ITN use.