We assessed the overall socioeconomic status using a WI of household assets weighted using PCA. Unlike income or expenditure data, this information can be more easily and reliably obtained, especially in settings where income varies temporally, may not always be in currency, and is reluctantly reported.16
This method of weighing assets has been validated in studies that show that it is a better measure of socioeconomic status than income or education.16,17
Furthermore, the method of PCA has been well-described and is becoming the standard tool for constructing wealth indices.11,15,18
In this study, PCA allowed us to assess if differences in wealth within a low-income community had any effects on the rates of diarrhea and parasitic infections, which have been associated with lower socioeconomic status. Although diarrhea is more prevalent in economically disadvantaged groups when studied across populations, within our study, population relative differences in wealth were not associated with differential rates of diarrhea. This finding may be explained by the relative homogeneity of our study population compared with larger studies with more diverse populations that may be subject to confounding. Alternatively, it may suggest that only differences in wealth larger than that found in a low-income setting are associated with diarrhea.
Relative greater wealth was protective for Giardia infections in general as well as diarrhea-associated giardiasis and persistent infections with Giardia, even after adjusting for established risk factors. Wealth was in fact two times as protective for Giardia, because it not only was associated with a lower risk of infection but also for an infection to become persistent (data not shown). Similarly, greater wealth was also protective against microsporidia infections. Interestingly, this analysis showed no association between wealth and infections with Cyclospora or Cryptosporidium.
The analysis of covariates confirmed previous observations that age plays a role in the frequency of infections with Cryptosporidium, Cyclospora, and microsporidia but was not associated with the frequency of giardiasis. Although these differences have been previously reported, the exact mechanisms for these differences by age are not yet fully understood, and studies with strong epidemiological and immunological designs are needed to properly address this question.
These results may have important implications on our understanding of the epidemiology and pathogenesis of these enteric parasites. It may well be that wealth as a measure in our study was also a surrogate for other factors, such as nutritional status. This is well-established risk factor for illness in general and was not part of this study. Other factors not measured were health education and sanitation practices within the household,19
which also have an effect on the frequency of infectious diseases. Additionally, wealth could be a marker for a family's ability to respond to new hardships. A less poor family may be better able to procure alternative sources of water if it becomes scarce or still afford basic necessities (water, food, soap, etc.) if there were abrupt changes that could temporarily affect the economic status of the household. This may not happen with families in the lowest tertile.
Our study has limitations worth considering. Although wealth indices have become an accepted tool in the literature, they also have limitations.20
Indices vary with the assets included and therefore, are subject to study design or bias. In our study, we used a large and varied group of assets and ran the analysis multiple times using different indices (data not shown) to verify the consistency of results. Also, the explanatory power of the wealth index may be confounded by the inclusion of direct and indirect determinants of health; although access to running water suggests increased wealth, it may also reduce the transmission of infectious diseases. We took this fact into account and included running water and other potential confounders in the multivariate analysis. Additionally and as mentioned in the introduction, the constructed wealth indexes are dimensionless and do not have extrinsic values; thus, wealth indices from two different communities or even studies within the same community may not be compared or combined.
Our study may also be affected by case definitions. We used generally accepted definitions for episodes of diarrhea and enteric infections. However, we used a study-specific definition to determine an episode of Giardia infection, primarily because of the lack of a standardized definition. Our definition might have under- or overrepresented the real infection rates in this study. This analysis also highlights the existing challenges inherent to determining the endpoint of an episode of Giardia infection in endemic settings, where most infections are asymptomatic. In addition, we found a large proportion of cases of parasitic infection without diarrhea, suggesting that our case definition of diarrhea may have lacked sensitivity.
To our knowledge, this is the first study to explore the association of wealth with specific enteric causes of infectious diarrhea. Our analysis suggests that, beyond the known risk factors, poorer individuals within already impoverished settings face a different burden of infections; Giardia and microsporidia are more frequent among the poorest, whereas Cryptosporidium and Cyclospora were not affected by these variations in socioeconomic status.
Our study provides further evidence that, within already impoverished settings, relative differences in wealth can have important implications on health. This suggests that, in the face of limited resources, targeted interventions focused on poorer subpopulations within low-income settings may have more favorable results. Assessment of relative wealth using household characteristics, which can be done quickly and reliably, may provide an impartial method for such targeting efforts.