Biology Reference
In-Depth Information
day 5 and day 7
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e
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. Slotved and colleagues
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concluded in their
important comparative work between pigs and mice that the migratory
pattern of A. suum is similar in murine and porcine hosts and on these
grounds established that mice are a suitable model organism for the study
of early Ascaris infection.
Working with mice has a number of practical advantages including
ease of manipulation and the availability of a wide range of mouse strains
and immunological reagents. Behnke et al.
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highlighted how the use of
mouse model systems can enable the dissection of the mechanism of
parasite resistance at a variety of sophisticated levels. Furthermore, the
recovery of larvae has been shown to be higher in mice when compared to
rabbits, guinea-pigs, rats,
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and even pigs.
28
Undoubtedly, the relative
host size and parasite size contribute to this higher recovery.
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Lewis
et al.
53
demonstrated increased efficiency of recovery of Ascaris larvae,
expressed as a percentage of inoculi in mice, utilizing a modified version
of the Baermann method.
THE MOUSE AS A MODEL OF
ASCARIS
AGGREGATION
One of the defining parameters that contribute to our understanding of
Ascaris epidemiology is the intensity of infection or the worm burden of
an individual host
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(see also Chapter 7). This variable influences parasite
transmission, population dynamics, and the degree of individual and
community morbidity. It also determines the most effective way in which
a helminth parasite can be controlled. The terms aggregation and
predisposition are used to describe the fact that not all human hosts are
infected equally, that some hosts carry considerably heavier burdens than
most, and that there is some degree of constancy in reinfection patterns
after chemotherapeutic treatment.
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e
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Nevertheless, our understanding
of the mechanism behind the observed patterns of Ascaris infection
remains incomplete. Whereas some progress has been made in assessing
the role of susceptibility, both immunological and genetic, and quantita-
tive trait locus (QTL) studies have even identified genetic loci associated
with susceptibility and resistance to infection in humans
58
(see also
Chapter 12), exposure to ascariasis has been difficult to measure and
when it has been attempted, it has been quantified only indirectly.
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A major incentive to exploit mice as models is the availability of a wide
range of strains with well-known genotypes. For obvious reasons, early
studies did not utilize inbred strains of mice (see
Table 5.2
) and it was only
in the 1960s and 1970s that mouse strains began to be utilized in parasi-
tology. A seminal paper by Mitchell and colleagues
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demonstrated
variability in susceptibility to Ascaris in the lungs of inbred strains of mice
but variations in infectivity of batches of eggs and doses resulted in lack of
