Baseline epidemiological data are presented from a parasitological survey conducted in Thuy Loi commune, Ha Nam province, Vietnam; a farming community where night soil is routinely used as fertilizer for crops. 177 households were visited and 543 individuals (aged 1–88 years) recruited to the study. Helminth infection intensity was assessed by Kato-Katz to determine the density of parasite eggs per gram of stool (epg). Ascaris lumbricoides, Trichuris trichiura and hookworm infections were the only species detected. 83% of individuals were infected with A. lumbricoides (mean epg = 11971), 94% with T. trichiura (mean epg = 793) and 59% with hookworm (mean epg = 302). Age-dependent patterns of infection prevalence and intensity were similar for A. lumbricoides and T. trichiura, but markedly different for hookworm infection. Similarly, age-dependency in the k-values for the three infections was due to covariance with the respective mean intensities with age rather than to independent age effects, with similar patterns for A. lumbricoides and T. trichiura, and a different pattern for hookworm. Three major conclusions can be drawn from the multiple-species analyses: There is positive interaction between A. lumbricoides and T. trichiura infections; high-intensity A. lumbricoides infections are significantly associated with high-intensity T. trichiura infections; and there is positive interaction between these two species such that infection intensity of A. lumbricoides is higher in individuals with concurrent T. trichiura infection than in individuals without and vice versa.
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