of this study reveal several key findings. Participating Kingwede residents had high prevalence of infection with S. haematobium
spp. parasites. Co-infection was also common and was associated with both individual- and household-level factors including young age and low socio-economic position. Interestingly, co-infection was more common than single S. haematobium
infection in children, after controlling for other variables. Similarly, intense infections with both parasites appeared to cluster in a subset of children.
While non-participation may have biased some of estimates of association, we believe that because individual per-household participation was reasonably good (62%), our investigation illustrates the importance of context in studies of infectious diseases. We found that contextual factors (household SEP) contributed significantly to variation in an individual's parasite infection profile, including, for children, proximity to the main local water source. These findings compare with other studies of single-species infection studies in which risk of malaria and schistosomiasis was higher in households proximate to water bodies 
and are in accord with those that associate these infections with lower SEP 
. Our results also support recent efforts to reconsider broader social and ecological contexts in studies of infectious diseases in lieu of traditional biomedical models 
. Finally, the identification of risk factors common to infection by both parasites argues for a broader, integrated approach to control and prevention 
and suggests potential targets for these efforts.
Aside from contextual variables, age was found to be the most important shared risk factor of infection, with both higher prevalence and higher intensity of single and concomitant infection in school-aged children as compared to adults. The observed negative associations between age and co-infection are consistent with other published reports from endemic regions on clinical malaria 
and S. haematobium
. Higher prevalence at younger ages is likely due to immunological naïveté governing susceptibility to both infections 
, a variable which was not measured in this study. The strength and consistency of the age effect on infection led to stratification of all analyses, with separate models for children and for adults. Our results support current trends in disease prevention and treatment efforts focusing on school-aged children. A key caveat may be that the many asymptomatic Plasmodium
spp. infections in adults could be an important source for continued transmission to children and should therefore not be ignored in intervention efforts.
Age-stratified, multivariable logistic GEE analyses of Plasmodium
spp. and S. haematobium
prevalence revealed no evidence of positive association between two parasites for the combined presence
of infection (measured as we did, by PCR and urine filtration) after adjustment for relevant SEP, behavioral and environmental variables ( and ). The apparent lack of association between these two parasite species using prevalence data has been supported by previous research 
and could be explained by dissimilar risk factors, distributions, and modes of transmission for the two infections. Comparability of our results with those of other malaria-helminth polyparasitism studies is limited by differences in outcome measures (asymptomatic infection vs. clinical disease) 
and helminth species of interest (most other co-infection studies investigate a variety of soil-transmitted helminth (STH) species rather than S. haematobium
) [reviewed in 41]
. However, our cross-sectional design may have limited our estimation of continuing infection risk, particularly for malaria, given annual weather related fluctuations in exposure.
In analyses of infection intensities, positive associations were observed between the intensities
of the two parasites among co-infected children ( and ), thus highlighting the importance of host factors in determining parasite loads, and also suggesting a synergistic relationship. Although some studies have reported antagonistic relationships between these parasites (i.e. lower P. falciparum
intensities in children harboring light S. haematobium
infections vis-à-vis those not co-infected 
), many other investigations support our findings (reviewed in 
). The potential of a synergistic relationship between these parasites is important from a clinical perspective, as well as epidemiologically. Clinically, the morbidity associated with co-infection is likely to depend on parasite loads 
as seen in single-species Plasmodium
and S. haematobium
. If heavy infections cluster in a subset of individuals, as suggested by our results, identification and targeting of this subset for multi-disease prevention and treatment interventions could be highly cost effective in that it would serve to reduce overall pathogen burdens.
Age-stratified adjusted odds of heavy S. haematobium infections compared to light infections among infected individuals.
We hypothesize that immunological parameters, unmeasured in this study, are partially responsible for the observed positive association between parasite intensities. In that case, an improved understanding of the complex immunological milieu governing infection profiles would be needed to evaluate this possibility. Failure to differentiate between co-infection and co-morbidity outcomes has confounded discussion of results from other similar studies. To illustrate, data suggest that P. falciparum
may protect against S. haematobium
infection by promoting protective antibody development, but Plasmodium
infection can also increase inflammatory factors associated with morbidity 
It is also possible that seasonal or climatic variations or other unmeasured variables influenced our results. This study was cross-sectional, thus natural temporal variations in infection prevalence and intensity were not considered. Due to limited resources, we did not control for other infections such as STH or HIV that could affect susceptibility to, or intensity of Plasmodium
spp. or S. haematobium
infections. Concomitant infections also could have effects on host immune response, thereby influencing S. haematobium
spp. associations. Although STH are likely to be ubiquitous, S. mansoni
is not endemic to this region and HIV rates are fairly low (7% prevalence in rural Kenya and 7.9% prevalence in Coast Province overall 
), suggesting that confounding likely was minimal. Genetic factors and nutritional status represent other potentially important unmeasured confounders. Our use of marginal models should have accounted for or attenuated any genetic effects shared within families by assessing clustering of individuals within households. Furthermore, another study of S. haematobium
from the same geographic region concluded that heritability in host susceptibility is low and unlikely to play a major role in determining individual risk for infection 
. Potential confounding effects of nutritional status should not be large assuming similar access to nutrition within households. This also should have been partially accounted for by the inclusion of household SEP variables in analyses. Of particular note, in our study, outdoor night-time activity was associated with lower
odds of co-infection or malaria infection among children, and with reduced odds of heavy Plasmodium
infection in adults. This finding may be related to the endophilic feeding preference of anophelines within houses, with reduced risk among those who are out of the house for part of the night.
Future research on the epidemiology of polyparasitism could benefit from the inclusion of several key parameters not fully evaluated in this study: i) A wider range of spatial and temporal scales and study sites could reduce potential chance associations and improve understanding of the climatic, seasonal and environmental factors that influence parasite distributions and interactions; ii) Malaria investigations would greatly benefit from more sensitive, standardized diagnostic tools for quantification of species-specific Plasmodium
parasites. Although work of other researchers has shown good correlation between median fluorescent intensity (MFI) values and parasitemia, the relationship between the two is not linear toward the upper and lower limits 
. Validation of our use of MFI as a quantitative measure is needed; iii) finally, in order to fully understand the implications of observed associations between parasitic species in human hosts, a better understanding is needed of relevant immunological mechanisms 
. However, the need for improved scientific knowledge about the biology, and epidemiology of polyparasitism should not take precedence over what is already known; parasites cause significant morbidity and more accessible, effective treatment and prevention is urgently required.