30 research outputs found
The marginal distributions of exposure and host susceptibility, together with contours of their joint distribution.
<p>The y-axis shows exposure, <i>E</i>, and the x-axis shows host susceptibility, <i>α</i>. The line <i>w<sub>T</sub> = αE</i> describes the threshold of detectable infections: the minimum worm burden in an individual detectable by currently available assays. The shaded area depicts those combinations of <i>α</i> and <i>E</i> producing infection intensities above this lower limit of detection, w<sub>t</sub>. The fraction of the population susceptible to infection at or above this threshold is that lying to the right of the line <i>α</i> = <i>α</i>*.</p
Repeated <em>Schistosoma japonicum</em> Infection Following Treatment in Two Cohorts: Evidence for Host Susceptibility to Helminthiasis?
<div><p>Background</p><p>In light of multinational efforts to reduce helminthiasis, we evaluated whether there exist high-risk subpopulations for helminth infection. Such individuals are not only at risk of morbidity, but may be important parasite reservoirs and appropriate targets for disease control interventions.</p> <p>Methods/Principal Findings</p><p>We followed two longitudinal cohorts in Sichuan, China to determine whether there exist persistent human reservoirs for the water-borne helminth, <i>Schistosoma japonicum</i>, in areas where treatment is ongoing. Participants were tested for <i>S. japonicum</i> infection at enrollment and two follow-up points. All infections were promptly treated with praziquantel. We estimated the ratio of the observed to expected proportion of the population with two consecutive infections at follow-up. The expected proportion was estimated using a prevalence-based model and, as highly exposed individuals may be most likely to be repeatedly infected, a second model that accounted for exposure using a data adaptive, machine learning algorithm. Using the prevalence-based model, there were 1.5 and 5.8 times more individuals with two consecutive infections than expected in cohorts 1 and 2, respectively (p<0.001 in both cohorts). When we accounted for exposure, the ratio was 1.3 (p = 0.013) and 2.1 (p<0.001) in cohorts 1 and 2, respectively.</p> <p>Conclusions/Significance</p><p>We found clustering of infections within a limited number of hosts that was not fully explained by host exposure. This suggests some hosts may be particularly susceptible to <i>S. japonicum</i> infection, or that uncured infections persist despite treatment. We propose an explanatory model that suggests that as cercarial exposure declines, so too does the size of the vulnerable subpopulation. In low-prevalence settings, interventions targeting individuals with a history of <i>S. japonicum</i> infection may efficiently advance disease control efforts.</p> </div
Description of the two cohorts at enrollment (<i>T<sub>0</sub></i>).
*<p>Cohort 1 is composed of 424 residents from 10 villages in Xichang County, Sichuan, China where schistosomiasis was endemic, monitored from 2000 to 2006.</p>†<p>Cohort 2 is composed of 400 residents from 27 villages in two counties in Sichuan, China where schistosomiasis reemerged following reduction of <i>S. japonicum</i> infection prevalence below 1%, monitored from 2007 to 2010.</p>‡<p>Prevalence and infection intensity estimates include all participants in village-wide infection surveys conducted at cohort enrollment: 1,801 individuals in 10 villages in cohort 1, 1,608 individuals in 27 villages in cohort 2.</p>**<p>Participants were asked about water contact behaviors from the start of the rice planting season. In Xichang County (from which cohort 1 participants were drawn) rice planting begins in April, whereas in the two reemerging counties (from which cohort 2 participants were drawn) rice planting begins in May.</p
Distribution of incident <i>S. japonicum</i> infections by age in cohorts 1 (top) and 2 (bottom).
<p>Incident <i>S. japonicum</i> infections were measured at two follow-up points (2002 and 2006 in cohort 1, 2008 and 2010 in cohort 2). All participants were tested for <i>S. japonicum</i> at enrollment (2000 in cohort 1, 2007 in cohort 2) and all infections were promptly treated with praziquantel.</p
The observed and predicted proportion of the population with two consecutive <i>S. japonicum</i> infections.
*<p>The expected prevalence of two consecutive infections was estimated based on the prevalence of infections at <i>T<sub>1</sub></i> and <i>T<sub>2</sub></i>.</p>†<p>The expected prevalence of two consecutive infections was estimated accounting for <i>S. japonicum</i> exposure. The infection prediction model included water contact minutes by month and activity for all measures for which at least 20% of cohort participants reported exposure, age, sex, baseline village infection prevalence, county and year of infection test. Prediction models were fit separately for each cohort.</p>‡<p>P-values were estimated assuming the number of individuals with two consecutive infections follows a binomial distribution, where is equal to the expected prevalence of two consecutive infections and is equal to the number of individuals in the full population. Thus the p-value is that of a two-sided, one-sample test assuming the probability of double-infections is equal to <i>P<sub>DI</sub></i>.</p
<i>S. japonicum</i> infection prevalence and intensity at follow-up.
*<p>Cohort 1 is composed of people from 10 villages where schistosomiasis is endemic. Participants were tested for <i>S. japonicum</i> infection in 2000 (<i>T<sub>0</sub></i>), 2002 (<i>T<sub>1</sub></i>) and 2006 (<i>T<sub>2</sub></i>).</p>†<p>Cohort 2 is composed of people from 27 villages in two counties where schistosomiasis reemerged following reduction of <i>S. japonicum</i> infection prevalence below 1%. Participants were tested for <i>S. japonicum</i> infection in 2007 (<i>T<sub>0</sub></i>), 2008 (<i>T<sub>1</sub></i>) and 2010 (<i>T<sub>2</sub></i>).</p>‡<p>Infection prevalence and intensity were estimated for the source population, accounting for the stratified sampling used in enrolling cohort participants. Each individual in the cohort was assigned a weight equal to the inverse probability of being sampled.</p
Associations between Schistosomiasis and the Use of Human Waste as an Agricultural Fertilizer in China
<div><p>Background</p><p>Human waste is used as an agricultural fertilizer in China and elsewhere. Because the eggs of many helminth species can survive in environmental media, reuse of untreated or partially treated human waste, commonly called night soil, may promote transmission of human helminthiases.</p><p>Methodology/Principal Findings</p><p>We conducted an open cohort study in 36 villages to evaluate the association between night soil use and schistosomiasis in a region of China where schistosomiasis has reemerged and persisted despite control activities. We tested 2,005 residents for <i>Schistosoma japonicum</i> infection in 2007 and 1,365 residents in 2010 and interviewed heads of household about agricultural practices each study year. We used an intervention attributable ratio framework to estimate the association between night soil use and <i>S. japonicum</i> infection. Night soil use was reported by half of households (56% in 2007 and 46% in 2010). Village night soil use was strongly associated with human <i>S. japonicum</i> infection in 2007. We estimate cessation of night soil use would lead to a 49% reduction in infection prevalence in 2007 (95% CI: 12%, 71%). However, no association between night soil and schistosomiasis was observed in 2010. These inconsistent findings may be due to unmeasured confounding or temporal shifts in the importance of different sources of <i>S. japonicum</i> eggs on the margins of disease elimination.</p><p>Conclusions/Significance</p><p>The use of untreated or partially treated human waste as an agricultural fertilizer may be a barrier to permanent reductions in human helminthiases. This practice warrants further attention by the public health community.</p></div
Description of study participants in 36 villages in Sichuan, China.
<p>EPG—Eggs per gram of stool</p><p><sup>a</sup> Includes households with a working anaerobic biogas digester or a triple compartment septic tank.</p><p><sup>b</sup> Asked only in 2007.</p><p>Description of study participants in 36 villages in Sichuan, China.</p
The association between night soil use and <i>S. japonicum</i> infection in 36 villages in Sichuan, China, 2007 and 2010.
<p><sup>a</sup>Odds ratios and 95% CIs were estimated using a multi-level fixed-effect logistic regression model. All models accounted for unmeasured within-village correlation.</p><p><sup>b</sup>Adjusted for age (categorized in 10-year increments), sex and county of residence.</p><p><sup>c</sup>Adjusted for all variables in Adjustment A as well as whether anyone in the household owned bovines, village bovine density (the mean number of bovines per household), household and village SES, the area cultivated by household members in the past year and village agricultural intensity (the mean area cultivated per household in the past year).</p><p><sup>d</sup>Village-level night soil use describes the mean buckets of night soil applied per household in the village, calculated excluding the index household. It is categorized by quartiles: very low (0–19 buckets), low (20–33 buckets), medium (34–68 buckets) and high (69–245 buckets).</p><p><sup>e</sup>Tests for trend were conducted by modeling the categorical variable as ordinal.</p><p>The association between night soil use and <i>S. japonicum</i> infection in 36 villages in Sichuan, China, 2007 and 2010.</p
The relationship between <i>S. japonicum</i> infection and night soil from improved and unimproved sources.
<p>Figure shows odds ratios (points) and 95% confidence intervals (lines) estimating the association between <i>S. japonicum</i> infection and night soil application from improved sanitation systems (dashed lines) and unimproved sources (solid lines) in 2007 (A) and 2010 (B). A household was classified as having improved sanitation if they reported having a working anaerobic biogas digester or triple compartment septic tank. Models were adjusted for age (categorized in 10-year increments), sex, county of residence and household night soil use. Village night soil use was defined as the average quantity of night soil used by all households in the village excluding the index household and was categorized by quartiles, with the lowest quartile serving as the reference group. Unimproved night soil categories: very low (0–10 buckets), low (11–22 buckets), medium (23–48 buckets) and high (49–244 buckets). Improved night soil categories: very low (0 buckets), low (0.1–2 buckets), medium (3–11 buckets) and high (12–100 buckets).</p