Database Reference
In-Depth Information
Fig. 4.1
the parasites reproduce in the gut of mosquitoes. Subsequently, malaria parasites
are returned through the mosquitoes' salivary glands to humans as the mosquitoes
feed on red blood cells.
The mosquito hosts cannot be just “any” mosquito but must be of the genus
Anopheles
. Many species of mosquito in the genus
Anopheles
are vectors, and we
will use information on the one that is the most widespread in South and Central
America,
A. messae
2
. Among the four species of parasites, we will concentrate on
Plasmodium vivax
; this species presents the most extensive geographical spread and
is present both in tropical and temperate zones.
The STELLA model of the malaria life cycle is divided into several interrelated
modules, which we visually separated into “sectors.” Sectors enable us to organize
parts of a model by grouping model components. They also add functionality, as
they allow entities in a sector to be moved as a whole, or run separately from the
rest of a model. To create a sector, choose the Sector Frame among STELLA's tools
(Figure 4.1), place it in the model diagram, and drag it over those components of
a model that you wish to group together. Clicking on the “lock” in the upper right
hand side of a sector fixes all elements in that sector so you can move them as a
whole. Once you have sectors specified, the Run pull-down menu gives you the
option of running an entire model or specific sectors. If you run sectors individually,
then values of parameters from other sectors, which do not run, will be assumed as
given and fixed.
All of the modules of this chapter's model are grouped into individual sectors,
mainly to keep the model organized and its structure transparent. One of the modules
is used to calculate reproduction of parasites (Figure 4.2). This module contains the
various parameters affecting the basic reproductive rate, Ro, which is modeled here
as the average number of new cases of the disease that would arise from a single
infectious host introduced into a population of susceptible hosts. It is a function of
the probability of transmission of parasites from infected vertebrates to uninfected
vertebrates (TR), the proportion of vectors to human hosts (M) and the vector daily
survival rate
3
,
4
:
Ro
=(
TR
∗
M
)
/
(
−
LOGN
(
DAILY SURVIVAL VECTOR
))
(4.1)
2
Jetten, T.H. and W. Takken. 1994. Anophelism with malaria in Europe: a review of the ecology
and distribution of the genus.
Anopheles
in Europe. Wageningen Agricultural University Press,
Wageningen.
3
Anderson, R.M. and R.M. May. 1991. Infectious diseases of humans: dynamics and control.
Oxford University Press, Oxford.
4
Begon, M., J. Harper, and C.R. Townsend. 1996. ECOLOGY. 3rd edition.
