GENETIC-STRUCTURE AND EPIDEMIOLOGY OF ASCARIS POPULATIONS - PATTERNS OF HOST AFFILIATION IN GUATEMALA

In Guatemalan villages people commonly rear pigs, and both hosts may b e infected with Ascaris. This study was designed to ask whether both h umans and pigs are potential hosts in a single parasite transmission c ycle in such villages, or alternatively, if there are two separate tra nsmission cycles, one involving pigs and one involving human hosts. Pa rasites were collected from both host species from locations in the no rth and south of Guatemala. Allelic variation in the nuclear genome of Ascaris was measured using enzyme electrophoresis, while mitochondria l DNA (mtDNA) sequence variation was quantified using restriction mapp ing. Low levels of enzyme polymorphism were found in Ascaris, but alle le frequencies at two loci, mannose phosphate isomerase and esterase, suggest that there is little gene exchange between parasite population s from humans and pigs. MtDNA haplotypes fall into two distinct cluste rs which differ in sequence by 3-4%; the two clusters broadly correspo nd to worms collected from humans and those collected from pigs. Howev er, some parasites collected from humans have mtDNA characteristic of the 'pig Ascaris' haplotype cluster, while some parasites collected fr om pigs have mtDNA characteristic of the 'human Ascaris' haplotype clu ster. These shared haplotypes are unlikely to represent contemporary c ross-infection events. Patterns of phylogenetic similarity and geograp hical distribution of these haplotypes suggest, instead, that they are the result of two historical introgressions of mtDNA between the two host-associated Ascaris populations. The results clearly demonstrate t hat Ascaris from humans and pigs are involved in separate transmission cycles in Guatemala.