Quantitative assessment of geophagous behaviour as a potential source of exposure to geohelminth infection

The common form of pica, geophagy, has direct adverse nutritional effects and also exposes children to soil-borne infection. Existing methods for assessing geophagy are either inappropriate for field use (radiology) or unreliable (reporting). A new method is described, based on the measurement of soil-derived silica in stools. More than 90 percent of silica is excreted within one gut transit period of ingestion. The amount excreted is proportional to the amount ingested. Faecal level of dietary silica (<2 percent dry wt stool) can be distinguished from levels due to geophagy (up to 25 percent dry wt stool). Studies in 2 children's homes in Jamaica showed that 33 percent and 66 percent of children were geophagous, ingesting up to 10g soil day-1. The geophagy of <20 percent of the children accounted for >60 percent of the total soil ingested. This overdispersion of exposure to soil-borne infection may contribute to the observed aggregation of geohelminth infection.(AU)

The role of environmental and host behavioural factors in determining exposure to infection with Ascaris lumbricoides and Trichuris trichiura

This study examines the contribution of environmental and host behavioural factors to the rate of reinfection with geohelminths in children. Preceeding the field study, two practical procedures for estimating these factors were developed and standardised: first, a method, based on existing procedures, for extracting parasite eggs from soil samples; and second, an original method, based on the assessment of soil-derived silica from faeces, for quantifying the rate of soil ingestion (geophagia) by the study children In the field study, exposure of Ascaris lumbricoides and Trichuris trichiura was examined longitudinally at two Places-of-Safety (childrens' homes) in urban Kingston, Jamaica. At the start of the study, existing helminth infections were chemotherapeutically removed from the study populations who then naturally reacquired infection during a three month exposure period. At the end of this period the infection intensity was determined. Exposure to infection was quantified by estimating the rate of ingestion of geohelminth eggs throughout the study period. This was achieved by determining the density of eggs in the soil and the rate of soil ingestion. The eggs of both geohelminths were recovered from the soil at both localities. The mean egg densities ranged from 0.05 to 4.0 epg-soil. The eggs were overdispersed at the other. The estimated rate of egg ingestion (of each species) was overdispersed among the two populations. At the home with young children of relatively uniform age, there was a significant correlation between the rate of egg ingestion and the reacquired infection intensity: subjects who have a high rate of egg ingestion have high worm burdens. This correlation was not significant for the population of older children who were heterogeneous in age. It is suggested that the older subjects may have shown more restrained geophagic behaviour. Additionally, those who were more homogeneous in age, and perhaps susceptible to infection, tended to show a more direct relationship between the rates of egg ingestion and parasite establishment. The study demonstrated that the number of parasites established in the host was of the same order of magnitude as the number of eggs ingested from soil. This implies that for the study populations, soil ingestion is a major source of geohelminth infection (AU)