Evidence Required for Adoption of New Vector Control Methods in Public Health - Biological and Environmental Control of Disease Vectors - page 197

much easier if the breeding sites are 'few, fi xed
and fi ndable' (WHO, 2012). When such
favourable conditions arise, it is usually because
of some combination of the specifi c biology of
the local species and a specifi c feature of the
landscape. One example is the use of environ-
mental interventions to change the salination of
coastal water bodies in order to control species
that breed in brackish water such as An .
sundaicus (Walker and Lynch, 2007). Such
specifi city, and the fact that LSM interventions
often need to be carefully adapted to local
conditions, makes it more dii cult to extrapolate
from a successful trial in one location to what
can be expected from routine implementation in
other locations.
This kind of extrapolation from one setting
to another may be favoured if, for example, some
specifi c feature of the landscape allows suitable
water to collect only in a few locations. This
would also allow generalization about the
operations needed to deliver a given antilarval
intervention. Examples include urban areas,
deserts and perhaps also very hilly areas, where
most of the land surface is steeply sloping and
potential breeding sites are confi ned to the valley
bottoms. Similarly, useful generalizations can
often be made when breeding sites are man-
made and uniform, for example, various
methods of mosquito control in rice fi elds, or the
control of An . culicifacies in abandoned gem-pits
in Sri Lanka (Yapabandara et al ., 2001).

houses. Such settings can therefore of er better
opportunities for extremely cost-ef ective LSM
interventions (Worrall and Fillinger, 2011;
WHO, 2012).

11.6 Stratifi cation for Generalization

In practice, of course, it is not possible to conduct
a full set of trials for every candidate intervention
against every vector species in every context. It is
also impossible to measure and model every
environmental variable that might infl uence the
success of vector interventions, for every species
in every setting. However, the task of practical
decision making does not have to be hopelessly
complex, simply because the number of possible
decisions is rather small. We have only a few
available interventions (and a few possible
combinations), so in any one country, it is fairly
straightforward to defi ne broad areas where (for
example) LLINs should be used alone, areas
where LLINs should be supplemented by large-
scale IRS, and those where focal IRS (preventive
or responsive) is needed to control the risk of
malaria epidemics. This form of stratifi cation
has been somewhat neglected in recent years,
but used to be regarded as a basic principle and
as the foundation of a good national malaria
control strategy - see for example the 'eco-
epidemiological types' in the Global Malaria
Control Strategy described by WHO (2006).
Since many environmental and biological
methods of vector control are likely to have a
niche role - that is, they are highly specifi c to
particular ecological settings - product
developers may well fi nd it helpful to consider
the scope of their candidate intervention, and in
their claims about the product, to defi ne the
limits of the eco-epidemiological settings where
it is likely to be ef ective. This will greatly facilitate
the task of external experts who may be asked to
assess whether these claims are justifi ed by the
available evidence.

Population density
Human population density af ects the relative
cost-ef ectiveness of house-based interventions
(IRS, LLINs) and larval source management
(LSM) interventions for malaria control. In rural
parts of the tropics, Anopheles mosquitoes can fl y
several kilometres (Gillies, 1961), and mosquito-
breeding sites are often widely scattered over the
countryside, while people are clustered in groups
of houses and villages. Therefore, larviciding
operations must be conducted over extensive
areas in order to protect small numbers of
people. Conversely in urban areas, the landscape
is covered by houses, while breeding sites are few
and fi xed in location. In city centres, there may
be only a handful of well-known breeding sites,
each surrounded by thousands of closely packed

11.7 Conclusions

In this topic, a variety of biological and
environmental vector control interventions
have been described. This chapter discussed the