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It has recently been shown using genetic markers that Ascaris
in humans and pigs in Central America comprise
reproductively isolated populations. We present a similar analysis for
region of China in which close association between
pigs and humans has been the norm for thousands of years, and agricultural
practices will result in frequent exposure to
eggs from both sources. DNA fragments from selected regions of mitochondrial
and ribosomal DNA were amplified by
PCR and allelic forms identified following digestion with a panel of
restriction enzymes, using DNA from a total of 115
individual worms from both people and pigs from 2 neighbouring villages.
Significant frequency differences in both
mtDNA haplotypes and the rDNA spacer were found between the 2 host-associated
populations, indicating that they
represented reproductively isolated populations. Mitochondrial haplotype
frequencies were different from those observed
in Guatemala and also from other Asian Ascaris populations,
suggesting low levels of gene flow between populations.
However, we found no evidence for significant heterogeneity in the
genetic composition of Ascaris infrapopulations in
either humans or pigs, possibly indicative of agricultural practices
in China which have resulted in a random distribution
of alleles within the parasite populations.
A longitudinal investigation on natural populations of Ascaris in humans and pigs and an investigation of soil contamination with Ascaris eggs were carried out from June 1993 to June 1994 in 2 villages, Manhu area, Xinjian County, Jiangxi Province, China. Results from these studies indicate that although human ascariasis is endemic there is significant fluctuation in both prevalence and the mean number of eggs/g faeces (epg) of the communities. Fluctuation of age-stratified prevalence and mean epg was detected in children but not in most adult groups. Most cases of human ascariasis were judged to involve low intensities of infection and a typical overdispersion distribution pattern was observed through the year. It was estimated that during the year, nearly half of the eggs discharged in the environment came from infections in children aged between 2 and 15 years which accounted for about 30% of the total population. Soil in and around houses and in vegetable gardens was found to be contaminated by Ascaris eggs and this situation remained relatively stable throughout the year. Monthly developmental rate of Ascaris eggs in soil was detected and the results suggest that the fluctuation in prevalence observed during the year should be directly attributed to the effect of seasonality of egg development. Features of Ascaris infection in pigs were found to be similar to those in humans except for a lower mean intensity of infection. The possibility of cross-infection of Ascaris between human and pig hosts is discussed.
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