Biology Reference
In-Depth Information
LANDSCAPE GENETICS AS A MEANS TO
INFER
ASCARIS
TRANSMISSION WITHIN
A HOST POPULATION
Effective Ascaris control will require detailed knowledge of parasite
dispersal to fully evaluate transmission patterns among individual
human hosts. The extent of parasite dispersal, however, is difficult to
ascertain with data based solely on infection intensities (i.e. number of
worms per infected host or a surrogate such as eggs per gram of feces).
This is because direct observation of parasite offspring leaving one host
and subsequently infecting the same or a new host is nearly impossible. 31
Thus, while intensity data are necessary to explore factors that explain the
variation in the distribution of parasites among individual hosts, 32 they
do little to answer the question of where did an individual acquire their
infection (i.e. are there different foci of infection in the single human
population).
Identification of population subdivision via population genetics anal-
yses of multilocus genotypic data provides a powerful means to infer
macroparasite dispersal among subdivided units such as individuals or
groups of hosts (e.g. households). 33 e 36 When using genetic data to infer
transmission among individual hosts, the sampling unit should be the
parasite stage that infects that host. 6 In the case of Ascaris , adult worms
would be genotyped from human hosts. If, for example, expelled Ascaris
eggs from humans were used, then measures of genetic differentiation
could be inflated due to the possible sampling of sibling parasites. I refer
readers to Steinauer and colleagues 37 for a more thorough discussion of
this type of sampling. Additional insight into what controls the trans-
mission process can be gained by using landscape genetic statistical
approaches to test if epidemiological variables correlate with the observed
parasite genetic structure. Landscape genetics is a multidisciplinary field
that incorporates spatial statistics, landscape ecology, and population
genetics to evaluate the role of landscape variables (e.g. altitude, ground
cover) in shaping genetic differentiation among populations. 38 In this
regard, landscape genetics has parallel goals with the field of spatial
epidemiology, which examines the correlates of spatial variation in
infection intensity patterns. 39 As landscape genetics is still a developing
field where several methodologies are being explored, I refer readers to
a special issue in Molecular Ecology that highlights this field in more
detail. 40 Here, I demonstrate the application of landscape genetics to the
epidemiology of A.
lumbricoides from an endemic population in Jiri,
Nepal. 41
The goals of the study by Criscione and colleagues 41 were to deter-
mine if there was more than one source pool of infection (i.e. foci of
infection) and, if so, to examine epidemiological variables that may
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