Our results of the soil analysis and the behavioural and epidemiological data do not support geophagia as a vector for Trichuris or hookworm infection among adults in Pemba. The study is not sufficiently powered to conclude about the role of geophagia in the transmission of Ascaris.
Parasitological analyses of the acceptable earth samples revealed no parasitic nematode stages. Two of the rejected earth samples contained geohelminth eggs. It appears that in Zanzibar, consumers of pica substances tend to consume earth that is parasite-free, which is in contrast to previous studies where the earth consumed by Jamaican and Kenyan children was contaminated (Geissler et al., 1998b
; Wong et al., 1991
). It is worth noting that two acceptable charcoal samples each contained a single parasitic nematode stage. Of these two samples, only one contained a helminth considered potentially detrimental to human health (Strongyloides
sp. Larvae). This is the first time that non-soil pica substances have been shown to contain geohelminths.
Observations of sample collection indicate Pembans who eat earth are aware that eating earth can be ‘dirty’ and take precautions (with sample selection and preparation) as they consume pica materials. These practices (e.g. pan heating, sun drying, brushing off loose material on exterior) reduce the likelihood of helminth transmission. The caustic substances, with the harshest environments for geohelminths, were not prepared at all prior to consumption. Preparation of pica substances by consumers has not been discussed in any of the other epidemiological studies of geophagic soils; it is our hope that future studies will include such descriptions.
The epidemiological data supported these findings. There was no significant difference between the prevalence of Ascaris, Trichuris or hookworm infection at baseline by geophagia status (). In the multivariate models of helminth infection, geophagy had no predictive power, even after controlling for potential confounders (). It is possible that some Ascaris and Trichuris infections may not have been detected if women had very recently been infected, since the period of time between infection and egg-shedding is several weeks.
Strikingly, in the multivariate model for hookworm infection (), geophagia was nearly significantly positively correlated (odds ratio (OR) 1.74; P
= 0.066). Because the physiological basis for geophagia causing hookworm is not strong (Geissler et al., 1998a
; Gelfand, 1945
; Saathoff et al., 2002
), we looked for other explanations of a relationship. One such explanation comes from the primate world, where parasitic infection stimulates animals to eat earth. Regular ingestion of soil counteracted endoparasitic infections in rhesus macaques and howler monkeys, although the physiological mechanism is not fully understood (Krishnamani and Mahaney, 2000
). Humans have also reported consuming clay to treat hookworm infection (Bateson and Lebroy, 1978
We explored statistically the relationship between hookworm and geophagia further and found that inclusion of the women’s baseline haemoglobin mitigates the magnitude and significance of geophagia in the hookworm infection model (OR 1.5; P
= 0.127, full model not shown). Thus, geophagia is associated both with hookworm and haemoglobin level. When haemoglobin and geophagia are included in the same model, the effect of hookworm but not haemoglobin falls away, leading us to believe that haemoglobin may be mediating the association between hookworm and geophagia, i.e. it is in the causal pathway. Haemoglobin status may be on the causal pathway from hookworm infection to geophagia if, as many argue, geophagia is undertaken to obtain needed micronutrients caused by hookworm-induced blood loss. Because of the likely low bioavailability of micronutrients in soils, this explanation is only modestly plausible (Young, 2007
There are several differences between the present study and previous studies of the relationship between geophagia and geohelminth infection that could account for different findings. The prevalences of pica and helminth infection are both lower in Pemba than they were in other study sites. The prevalence of geophagia among pregnant women at baseline in Pemba was only 5.6% (increasing slightly to 7.0% if the definition expands to earth consumption at any point in the pregnancy), whilst in all the other studies of geophagia and helminth infection the prevalence of geophagia at baseline was ≥46%. The baseline prevalence of Ascaris, Trichuris and hookworm was slightly lower in the present study than in other studies. (A table detailing the differences in all study methodologies and results is available upon request from the corresponding author.). This lower prevalence means that this study has less power than the others.
The participants in the present study were all adult consumers of pica. In contrast, the previous two studies testing the parasitological profile of earth involved earth that children consumed. It is likely that children are less discriminate in their selection of pica substances than adults and thus in child-preferred samples there may be a more significant presence of infectious parasitic nematode stages, although the Luo children in Geissler’s (2000)
study avoided ‘contaminated’ earth. A further difference is that our technique for identification of nematodes was different than those previously used in other studies. We do not expect it to be less sensitive than previous methods used and it was sensitive enough to be able to detect nematodes in the rejected samples. Finally, the quantity of sample that we analysed for each substance (approximately 15 g) is much greater than the amounts analysed by Geissler et al. (1998a)
or Wong and Bundy (1990)
, but the number of samples analysed is much smaller, limiting our interpretation.