Genetic structure and epidemiology ofAscarispopulations: patterns of host affiliation in Guatemala

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Abstract
Summary: In Guatemalan villages people commonly rear pigs, and both hosts may be infected withAscaris. This study was designed to ask whether both humans and pigs are potential hosts in a single parasite transmission cycle in such villages, or alternatively, if there are two separate transmission cycles, one involving pigs and one involving human hosts. Parasites were collected from both host species from locations in the north and south of Guatemala. Allelic variation in the nuclear genome ofAscariswas measured using enzyme electrophoresis, while mitochondrial DNA (mtDNA) sequence variation was quantified using restriction mapping. Low levels of enzyme polymorphism were found inAscaris, but allele frequencies at two loci, mannose phosphate isomerase and esterase, suggest that there is little gene exchange between parasite populations from humans and pigs. MtDNA haplotypes fall into two distinct clusters which differ in sequence by 3–4%; the two clusters broadly correspond to worms collected from humans and those collected from pigs. However, some parasites collected from humans have mtDNA characteristic of the ‘pigAscaris’ haplotype cluster, while some parasites collected from pigs have mtDNA characteristic of the ’ haplotype cluster. These shared haplotypes are unlikely to represent contemporary cross-infection events. Patterns of phylogenetic similarity and geographical distribution of these haplotypes suggest, instead, that they are the result of two historical introgressions of mtDNA between the two host-associatedAscarispopulations. The results clearly demonstrate thatAscarisfrom humans and pigs are involved in separate transmission cycles in Guatemala.