This relationship will be complicated
This relationship will be complicated: the prevalence of malaria infection in non-pregnant adults is lower than that for children within the same setting, a finding probably related to the differing levels of immunity in the adult and child populations. Since immunity depends on previous exposure and also the longevity of infection, it will, then, vary with transmission intensity.
Pregnancy provides a second layer of complexity: around the time that maternal blood flows into the placenta, the parasite (the most prevalent and pathogenic species of plasmodia) is able to sequester within the intervillous space. In women who have not been exposed to malaria during a previous pregnancy, the parasite can replicate to very high densities. Although women in their second or subsequent pregnancies will probably be exposed to a similar risk of infection as that in their first pregnancy, acquired immunity to placental parasites will allow multigravidae to more effectively control parasite replication within the placenta than was possible in their first pregnancy. Because parasite brain metabolism is likely to be heavily linked to the level of antigen the diagnostic is designed to detect (usually histidine-rich protein 2 (HRP-2), it seems reasonable to expect that malaria prevalence in later pregnancies will also follow a distinct pattern dependent on transmission intensity.
The strong correlation between infection in pregnant women and children established by van Eijk and colleagues, using extensive large-scale cross-sectional prevalence data in the two groups, is grounds for optimism about the usefulness of these data as an indicator of transmission. Moreover, results of van Eijk and colleagues\' analysis provides insight into the next steps to follow in the search of a metric that can be readily used. Their results suggest that, in areas of low transmission, prevalence in pregnant women is likely to be very similar to that in young children, but that, as transmission reaches higher intensities, cross-sectional prevalence surveys of pregnant women will increasingly underestimate those in children.
Soil-transmitted helminths (STH) and schistosomiasis have been recognised as important diseases. Diagnostics, treatments, and understanding of these were accrued throughout the 20th century, and reached the point that control and elimination seemed to be mostly a matter of implementation of mass drug administration (MDA) programmes. However, in 2015, both STH and schistosomiasis are global health problems, so perhaps we do not have the right methods or they are not being effectively applied. To our knowledge, MDA for STH and schistosomiasis has never been reported to eliminate infection without concomitant economic development. What are we missing? Most understanding about infection and disease is from longitudinal intervention studies completed in the 1980s and 1990s, which are now ageing and rarely benefited from modern techniques, such as next-generation sequencing. Very surprising is how much is not fully understood about these infections—eg, the biological or ecological basis of predisposition to high worm burdens, the importance of household-based transmission, the causes and resulting effects of co-infection with many parasites, and how to measure the burden of disease. Measurement of the burden of disease is particularly vexing. Cochrane reviews, published in 2012 and 2015, have emphasised the paucity of information (only 42 and 45 papers were included, respectively) and reported that little evidence was available for any beneficial effect of deworming, even though people in the field were convinced that these result in a great effect on physical and mental health. By contrast, evidence of the effects in livestock and wildlife is accruing. More than 1 billion children worldwide live at risk of helminth infection, and yet the effect on their health and development still cannot be quantified. In , Nathan Lo and colleagues introduce two advances that should be highlighted. First is the inclusion of five different parasites in the same study, which is sensible, in view of the substantial overlap in diagnostic procedures and treatments for the different species. Second, the authors worked with detailed data in one setting (Côte d\'Ivoire) and made comparisons between different communities within that setting; such linkage between consistent local data and models is rare. Helminth infection and disease can be very different in different communities, but well validated models in many settings might allow for the extrapolation to others. We would like to think that this paper marks the demise for universal studies of single helminth species.