Biology Reference
In-Depth Information
of this chapter proposes the novel integration of the effective population
size (
N
e
) parameter into population monitoring and epidemiological
studies of parasites. Using microsatellite data from a metapopulation of
A. lumbricoides
in Nepal, I demonstrate the utility of estimating
N
e
with
single-sample, contemporary estimators. I also discuss assumptions and
provide some guidelines for estimating
N
e
. My goal is to emphasize the
importance of the above topics in epidemiological research, highlight the
population genetic methodologies that have been used, and point to new
directions that may aid the development or monitoring of
Ascaris
(and
metazoan parasites in general) control programs.
A species' life history and the way samples are collected can influence
interpretation of some of the genetic analyses I discuss. Thus, I first
provide a brief summary of the biological characteristics of
Ascaris
.
Sampling will be addressed in the context of each study that is discussed
below and just note here that genotypes were always obtained from adult
worms.
Ascaris
has a direct life-cycle where mature male and female
adult worms reside in the lumen of the small intestine.
8
The mating
system has not been extensively studied. However, recent paternity
analyses indicate there is polyandry in pig
Ascaris
9
and Hardy
Weinberg
equilibrium, indicating random mating, has been observed on very local
scales (i.e. within people in a single village).
10
A female can produce
millions of eggs over her lifetime, which is about 1 year.
11
Eggs are
released into the external environment where they can persist for 6 to 9
years.
12
Infection occurs by ingestion of eggs via fecal contaminated
material. Larvae hatch in the small intestine, penetrate the intestinal wall,
migrate to the lung to become fourth-stage larvae, and then migrate up
the trachea back into the esophagus and ultimately the small intestine. In
about 60 days from the point of infection, females will start to produce
eggs.
8
Key life history aspects in terms of population genetics are that
breeding worms are transiently separated into groups (i.e. hosts)
13
and
that the long-lived eggs can lead to overlapping generations. As will be
discussed, the latter is of significance because breeding worms that end
up in the same host may be of different offspring cohorts (i.e. there is
overlapping of generations).
e
ASCARIS
CROSS-TRANSMISSION BETWEEN
HUMANS AND PIGS
The subject of whether
Ascaris
in humans and pigs is one or two species
(
A. lumbricoides
and
A. suum
, respectively) is still being discussed
7,14,15
and really points to an underlying question that is central for many
human parasites: are there reservoir hosts (i.e. is there zoonotic trans-
mission)? The answer to this question would clearly impact control
