Publications

Mitochondrial DNA (mtDNA) is finding increasing usage as a tool for studying the systematics, population genetics and epidemiology of parasitic helminths, and is generally assumed to be inherited maternally. Yet two features of Ascaris biology–fertilization by large amoeboid sperm and some novel aspects of sperm mitochondria–suggest a paternal component to mitochondrial inheritance in this organism. In this study, we compare mtDNA restriction patterns of parental worms with those of their progeny but find no evidence for paternal inheritance. We suggest that sperm-derived mitochondria are actively destroyed or outcompeted by maternal organelles in the zygote.

Cases of human Ascaris infection occur sporadically in areas such as N. America and Western Europe, where this parasite is thought to be non-endemic. Clinical case histories suggest that many of these cases may be cross-infections from pigs. I describe patterns of variation in the ribosomal DNA of Ascaris from 9 such cases. For comparative purposes, I also describe patterns of variation in parasites obtained from pigs and humans from worldwide locations. A Hae III restriction site distinguishes two classes of rDNA repeats; repeats bearing this restriction site were found in > 96% of parasites from pig populations worldwide and in all 9 worms from humans in N. America. In contrast, repeats bearing this restriction site were detected in < 2% of parasites from humans in endemic areas. The molecular data clearly incriminate pigs as the source of infection in the N. American cases. I discuss evolutionary and public health implications of incomplete host fidelity in Ascaris.

Patterns of genetic subdivision in parasite populations can provide important insights into transmission processes and complement information obtained using traditional epidemiological techniques. We describe mitochondrial sequence variation in 265 Ascaris collected from 62 individual hosts (humans and pigs) from 35 households in 3 Guatemalan locations. Restriction mapping of individual worms revealed 42 distinct mitochondrial genotypes. We ask whether the mitochondrial genotypes found in worms from individual hosts, from families of hosts and from villages represent random samples from the total Ascaris population. Patterns of genetic subdivision were quantified using F-statistics, while deviations from the null hypothesis of randomness were evaluated by a simple resampling procedure. The analysis revealed significant deviations from panmixia. Parasite populations were strongly structured at the level of the individual host in both humans and pigs: parasites bearing the same mitochondrial genotype were found more frequently than would be expected by chance within hosts. Significant heterogeneity was also observed among populations from different villages, but not from different families within a village. The clustering of related parasites within hosts suggests a similar clustering of related infective stages in the environment and may explain why sex ratios in Ascaris are female-biased. We discuss aspects of Ascaris biology which may lead to the observed patterns.

Fecal egg count scores were used to investigate the distribution and abundance of intestinal helminths in the population of a rural village. Prevalences of the major helminths were 41% with Ascaris lumbricoides, 60% with Trichuris trichiura and 50% with Necator americanus. All three parasites showed a highly aggregated distribution among hosts. Age/prevalence and age/intensity profiles were typical for both A. lumbricoides and T. trichiura with the highest worm burdens in the 5-10 year old children. For hookworm both prevalence and intensity curves were convex in shape with maximum infection levels in the 30-40 year old age class. Infected females had higher burdens of T. trichiura than infected males in all age classes of the population; there were no other effects of host gender. Analysis of associations between parasites within hosts revealed strong correlations between A. lumbricoides and T. trichiura. Individuals with heavy infections of A. lumbricoides and T. trichiura showed highly significant aggregation within households. Associations between a variety of household features and heavy infections with A. lumbricoides and T. trichiura are described.

In Guatemalan villages people commonly rear pigs, and both hosts may be infected with Ascaris. 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 of Ascaris was measured using enzyme electrophoresis, while mitochondrial DNA (mtDNA) sequence variation was quantified using restriction mapping. Low levels of enzyme polymorphism were found in Ascaris, 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 'pig Ascaris' haplotype cluster, while some parasites collected from pigs have mtDNA characteristic of the 'human Ascaris' 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-associated Ascaris populations. The results clearly demonstrate that Ascaris from humans and pigs are involved in separate transmission cycles in Guatemala.

Thin blood smears were collected from 126 mammals representing four genera of marsupials and six genera of murid rodents. A species of Hepatozoon was discovered in the New Guinea spiny bandicoot (Echymipera kalubu), trypanosome infections were found in three genera of rodent hosts and the prevalence of a rickettsial parasite of the genus Grahamella was recorded in rodents from the genera Rattus and Melomys. Dried blood samples also were taken and screened serologically for antibodies to arenavirus infection but with negative results.