We must be able to understand the
population dynamics of parasites in order to
improve control strategies, but studying this
has ethical complications in human systems.
Studying animal
(reservoir) hosts can
help alleviate this
problem
Theron et al. Studied Rattus Rattus reservoir
hosts from 1983 to 1990
•The island of Guadeloupe
•A variety of study sites were used in an environment
changing from marshy forests, through cattle graze land into
mildly and then heavier human populated areas.
•All study sites flood and dry out seasonally
•Captured rats during October and November each year
•Worms were removed from rats by hepatic perfusion and
careful examination of the liver and lungs
•Snails were sampled using scoops mounted on wooden
handles and sampled for cercarial shedding, although
infection dynamics in the snails were not analysed
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

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Prevalence: The percentage of the host population
that is infected
Mean Intensity: The number of parasites per
infected host
Mean Abundance: The number of parasites per
host for the entire host population
Over Dispersion: When there is greater variability
in the data set than the predicted results
The removal of
rats for
examination
had no effect on
the rat
population
dynamics
- Trapping
efficiency had
low annual
variation
There was a strongly clumped worm distribution with
strong spatial heterogeneity of parasite transmission
•50% of the parasites can be found in 4% of the available hosts
•9% of the infected hosts harbour ½ of the parasite population
The over-dispersion shown in Fig. 3 can be
explained through things such as genetic,
ecological or behavioural aspects of the parasite.
•The patchiness of habitats that include standing water,
mixed with a low infection rate of snails (25%) limits the
dispersal ability of the cercariae
•Only three of the eight study sites showed infected
snails
•This focuses the infection of rats to isolated areas, leading
to unequal infection of hosts
•Territoriality of rats, along with high population
densities reduces host movement, further reducing
parasite transmission.
•For the whole population, variance in
abundance was low and infection levels
remained at an equilibrium
•Local populations were much more
variable and were in a patchy
environments characteristic nonequilibrium state
•In strongly clumped distributions, very
low variation in prevalence may induce a
very high variation in abundance
•A drought hit the region in 1983 and
1984, which can be considered the
reasoning for the overall low abundance
during 1984 and 1985

A genetic divergence in cercarial
emergence timing
Some population sites had peak emergence
in the late morning while others had peak
emergence closer to late afternoon
 Those that emerged later in the day were
more likely to encounter, and therefore infect
their rat host

•Human participation, due to treatment for infection, in parasite
transmission has been decreasing
•Selection pressures on Schistosoma by their rat hosts can create microevolutionary adaptations in the local populations of the worm
•This leads to an increased risk of extinction of these local
populations, especially those which have not adapted to the life style
of their rat host
•The populations will be under stronger selection pressure for any
traits, such as later cercariae emergence, which help the parasite
infect the rat hosts
Due to the isolated nature of these sub-populations,
adaptations in one group may make them more amenable to
host infection than others, possibly explaining the significant
differences from one study site to another.

A. Théron, J. P. Pointier, S. Morand, D. ImbertEstablet and G. Borel. 1992. Long-term
dynamics of natural populations of
Schistosoma mansoni among Rattus rattus in a
patchy environment. Parasitology 104: 291-298.