Biology Reference
In-Depth Information
the concept of a basic reproductive number, R
0
, to measure transmission
success, following on from Ross's studies of malaria which had been
expanded on by Macdonald.
12,13
The basic reproductive number for
helminth macroparasites is essentially the number of successful offspring
that a parasite is capable of producing.
14,15
More precisely, it is the
average number of female offspring produced throughout the lifetime of
a mature female worm, which themselves achieve reproductive maturity
in the human host in the absence of any density-dependent constraints.
16
If R
0
is less than unity the parasite is unable to sustain transmission. If it is
greater than or equal to unity the parasite persists in the defined human
community.
Macdonald also turned to the question of mating success in a dioecious
worm where the presence of female and male worms in an infected
person is essential for the production of viable eggs to continue the life-
cycle of the parasite. He defined the concept of a “breakpoint” in trans-
mission, where transmission is halted due to mating success falling below
the value required to sustain transmission. This concept has been elabo-
rated on in more recent work to include various assumptions about both
the distribution of worm numbers person and parasite reproductive
biology.
10,17
It was not until the early 1980s that attention returned to the trans-
mission dynamics and population biology of helminth parasites of
humans, largely stimulated by a rapid growth in the use of mathematical
models in the study of ecology and population biology of free-living
species.
15,16
The first papers published in this period of renewed interest
introduced the concepts and terminologies of population ecology to the
epidemiological study of infectious agents. These concepts included
population regulation by density-dependent fecundity and mortality,
frequency distributions of organisms per spatial location or, in the case of
parasites, per host, non-linear processes in transmission and population
stability, and the notion of multiple population steady states or equilibria.
Mathematical formulation was essential in mapping out the impact of
various biological and epidemiological processes (often highly nonlinear
in form) on population abundance and population behavior following
perturbation induced by activities such as control interventions.
This chapter describes the more recent developments in this field and
their role in the study of the epidemiology and control of A. lumbricoides,
with a focus on the various uses of mathematical models, the questions
that can be addressed by their use, and future needs in model develop-
ment and application. Throughout, our emphasis is on making the
methods transparent to epidemiologists, parasitologists and public health
professionals with a minimum of technical detail. For those who “get their
kicks” from equations we provide the key references to formulations and
derivations.
