Population biology of hookworms in West Bengal: analysis of numbers of infective larvae recovered from damp pads applied to the soil surface at defaecation sites

Abstract

The aims of this study were to analyze the seasonal distribution of infective larvae on the soil surface, to determine whether numbers of infective larvae near faeces were related to the faecal egg count of individuals, and to relate the distribution of larvae to environmental characteristics. Larvae were recovered from damp pads, applied to the soil surface in an annulus around fresh, identified stools of individuals who were participating in a larger epidemiological study. This provided an estimate of exposure to infection at the time of defaecation. Transmission was restricted to the rainy season and large aggregations of larvae were encountered earlier rather than later in the rainy season. Frequency distributions for the number of larvae extracted from each pad showed a high degree of aggregation, with most monthly counts showing good fits to the negative binomial probability distribution. Despite variations in monthly sampling means, the degree of aggregation in the population of larvae was remarkably stable over the 18 month sampling period ( k of negative binomial = 0 · 01 to 0 · 08). Estimates of the degree of aggregation of the parasites in the human population were also available, and comparisons suggest that the infective larvae were much more aggregated than the parasitic stages. There was no relationship between the mean daily egg output of individuals and the number of larvae which developed and were recovered from the soil surface near the faeces. Thus, people who contribute large numbers of eggs to the environment are not necessarily those who are the greatest source of infection for others. Males under 19 years of age, who defaecated at sites positive for hookworm larvae, tended to have higher mean faecal egg counts than those who defaecated at negative sites. This observation suggests that some male youths may acquire their worm burdens more quickly than others due to sociobehavioural factors. Soil compaction was the only factor of those recorded for the environment to have a significant effect on recovery of larvae. More hookworm-positive sites had loose, friable soil than would be expected by chance. Positive sites had a narrower range of soil temperatures than negative sites, and the mean moisture content was higher at positive compared to negative sites. These findings are discussed in relation to the population biology of hookworms in Bengal.