Biology Reference
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obtain a comparable measure of personal
protection (Howard
et al
., 2000); in other sites,
there was clear evidence that the mass ef ect was
absent and the observed epidemiological benefi ts
were due to personal protection only
(D'Alessandro
et al
., 1995, 1997; Quinones
et
al
., 1998; Schellenberg
et al
., 2001). Thus, when
the fi rst large-scale ITNs distributions were
targeted only to high risk groups (under-5
children and pregnant women), with no ef ort to
achieve full-coverage, it was not clear whether
or not the recipients would be protected to the
degree predicted by the trials.
Zoophagy
One particular aspect of species-specifi c
behaviour, which has received relatively little
attention, is the infl uence of zoophagy of the
vector, i.e. its preference to feed upon animals,
on ITN ef ectiveness. In principle, the degree of
personal protection given by a net to a human
occupant seems unlikely to be greatly af ected by
the proportion of bites that a mosquito
population takes on animals rather than people.
Indeed, the degree to which a net gives personal
protection depends on its ef ectiveness as a
barrier, deterring female mosquitoes that are
already committed to an attack on a human
being. On the other hand, we would expect the
mass ef ect of ITNs/LLINs to be strongly af ected
by zoophagy: the mean longevity of the vector
population is much more likely to be reduced by
the use of bed nets if the vector is anthropophilic
(human seeking) than if it is highly zoophilic.
Some confi rmation of this is available from
the contrasting results of trials against
anthropophilic and zoophilic vectors (Lines,
1996). Further evidence is provided by the
observation that in Kisumu, Kenya, intensive
vector control (especially LLINs) altered the
relative abundance of
An
.
gambiae
s.s.
and
An
.
arabiensis
(Bayoh
et al
., 2010). This is probably a
result of the fact that compared to
An
.
gambiae
s.s.,
An
.
arabiensis
tends to be more exophilic and
zoophilic. Note, however, that contrary to some
interpretations, this observation does not imply
that there has been an evolutionary change in
behaviour within any of the species, or that the
intervention is inef ective against
An
.
arabiensis
.
Instead it indicates that the intervention was
simply more ef ective against
An
.
gambiae
s.s.;
both species were suppressed, but
An
.
gambiae
s.s. was more suppressed than
An
.
arabiensis
,
resulting in a shift in the relative abundance of
the two sibling species.
Species
Each malarious region of the world has its own
local endemic
Anopheles
species, a subset of
which acts as vectors of malaria. Each of these
species has its own specifi c biological char-
acteristics, including longevity, host choice,
breeding sites and time of biting. This biological
variation is obviously relevant to, and likely to
infl uence, the ei cacy of vector control inter-
ventions. Hence, the fact that an intervention
has been shown to be ef ective against one
Anopheles
species does not necessarily imply that
it will be equally ef ective against another. Of
course, some degree of inference between
species is possible, as long as behavioural
contrasts are taken into account, but this
process of extrapolation from one species to
another should be regarded as suggestive, not
conclusive.
It is worth noting how this was done in the
case of ITNs. When ITNs were introduced, the
main focus of global attention was on their
ef ectiveness in Africa, and the question of
ef ectiveness in other regions was rather
neglected. In those other regions, however, local
malaria programmes and researchers were well
aware of the need for additional trials against
local vectors and in local conditions. Through
such trials, a reasonably complete body of
evidence was eventually generated across a
range of
Anopheles
species. The general
conclusion was that ITNs could be surprisingly
ef ective even against early-biting vector species
such as
An
.
albimanus
in Latin America,
An
.
culicifacies
in India and
An
.
sinensis
in China
(Pates and Curtis, 2005; Hill
et al
., 2006).
11.5.2 Larval source management
Landscape and nature of the breeding sites
As discussed previously (see Cameron
et al
.,
Chapter 10, this volume), larval source
management (LSM) requires good knowledge of
the larval biology of the local vectors, and is
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