MITOCHONDRIAL-DNA AND ASCARIS MICROEPIDEMIOLOGY - THE COMPOSITION OF PARASITE POPULATIONS FROM INDIVIDUAL HOSTS, FAMILIES AND VILLAGES

Patterns of genetic subdivision in parasite populations can provide im portant insights into transmission processes and complement informatio n obtained using traditional epidemiological techniques. We describe m itochondrial sequence Variation in 265 Ascaris collected from 62 indiv idual hosts (humans and pigs) from 35 households in 3 Guatemalan locat ions. Restriction mapping of individual worms revealed 42 distinct mit ochondrial genotypes. We ask whether the mitochondrial genotypes found in worms from individual hosts, from families of hosts and from villa ges represent random samples from the total Ascaris population. Patter ns of genetic subdivision were quantified using F-statistics, while de viations from the null hypothesis of randomness were evaluated by a si mple resampling procedure. The analysis revealed significant deviation s from panmixia. Parasite populations were strongly structured at the level of the individual host in both humans and pigs: parasites bearin g the same mitochondrial genotype were found more frequently than woul d be expected by chance within hosts. Significant heterogeneity was al so observed among populations from different villages, but not from di fferent families within a village. The clustering of related parasites within hosts suggests a similar clustering of related infective stage s in the environment and may explain why sex ratios in Ascaris are fem ale-biased. We discuss aspects of Ascaris biology which may lead to th e observed patterns.