Population Structure and Dynamics of Helminthic Infection: Schistosomiasis

Abstract

While disease and outbreaks are mainly clonal for bacteria and other asexually reproducing organisms, sexual reproduction in schistosomes and other helminths usually results in unique individuals. For sexually reproducing organisms, the traits conserved in clones will instead be conserved in the group of organisms that tends to breed together, the population. While the same tools are applied to characterize DNA, how results are interpreted can be quite different at times (see another article in this collection, http://www.asmscience.org/content/journal/microbiolspec/10.1128/microbiolspec.AME-0002-2018). It is difficult to know what the real effect any control program has on the parasite population without assessing the health of this population, how they respond to the control measure, and how they recover, if they do. This review, part of the Microbiology Spectrum Curated Collection: Advances in Molecular Epidemiology of Infectious Diseases, concentrates on one approach using pooled samples to study schistosome populations and shows how this and other approaches have contributed to our understanding of this parasite family's biology and epidemiology. *This article is part of a curated collection.

FIGURE 1

Schistosome life cycle. Shown are developmental stages for the three major species, the three intermediate host snail genera, and the human host. Eggs in urine (H, S. haematobium) or stool (M, S. mansoni; J, S. japonicum) hatch on contact with freshwater. The newly hatched miracidium finds the appropriate snail host, in which asexual reproduction occurs to produce cercariae. Cercariae are released from snails, penetrate the skin of the human host (also other animals for S. japonicum), and migrate to the hepatic portal vein system. Mated worms take up residence in veins near the intestines (S. mansoni and S. japonicum) or the urinary system (S. haematobium). Adult worms reproduce sexually and make eggs that continue the cycle. (Adapted with permission from reference .)

FIGURE 2

Schistosome subpopulation structures. For infrapopulations, each small box represents the multilocus individual schistosomes (colored dots) from an infected person. For the component population, the large box represents the combined multilocus individual parasites from all humans in the community. New infections conceptually are taken from the human component population, since this over time returns to the snail component population where human infections actually arise.

FIGURE 3

Electropherograms for multiplexed amplifications of S. mansoni eggs from stool. DNA from infected individuals was first amplified using primers for one marker, and then up to four amplifications from different markers were combined and processed for microsatellite genotyping on an automated sequencer. (a) Results for one infrapopulation and a set of four microsatellite markers. (Top) All markers combined; (middle) two markers with labeled peak characteristics for SMDA23, including size (base pairs), peak height, and peak area; (bottom) three markers showing only trimeric marker SM13-410, yielding peaks with 3-bp spacing. Peaks with spacing not a multiple of three are not recorded. (b) Results for the same marker set as for panel a from three different individuals with different intensities of infection. Allele frequencies were calculated as the peak height or area divided by the sum for the peaks for that allele. Using height or area gave comparable results. Tabs under each peak in the electropherogram identify height, area, and fragment size.

FIGURE 4

Assessing for selection by population differentiation after treatment with praziquantel. Schistosome infrapopulations are highly variable. If persistent populations arise from stochastic processes (e.g., refractory juvenile parasites or parasites in a protected location), the day after a selective sweep (praziquantel treatment) will leave a residual largely derived from the parent population (small pre-post Di). Assuming that resistance arises on one or a few multilocus backgrounds, the day following a selective sweep will leave a population that is more similar to itself than to the parent population (large pre-post Di). Colored discs represent different classes of multilocus genotypes.