MALARIA ELIMINATION IN ZANZIBAR - Soper Strategies

CHAPTER 1: TECHNICAL FEASIBILITY
INTRODUCTION
The belief that insecticides and effective treatment made
eradicating malaria parasites technically feasible was the basis of
the Global Malaria Eradication Program (WHO, 1956). The
initial design of the GMEP was based largely on a successful
indoor residual spraying program using DDT in Greece, where
transmission had been interrupted by the end of 1949. In 1951,
spraying was stopped on the island of Crete because of DDT
shortages, but malaria did not return (WHO, 1967). In Greece
that same year, DDT resistance was detected in the Anopheles
sacharovi (Livadas and Georgopoulos, 1953). These experiences
led the technical advisory committee on eradication to conclude
that a time-limited program of indoor residual spraying was
possible (because it had worked in Crete) and necessary (because
DDT resistance had evolved), and it led the GMEP to adopt a
3-4 year attack phase (WHO, 1956).
The GMEP experienced some spectacular successes with this
strategy in the early going, especially in many European countries
which may be considered the “lowest-hanging fruit” (Gramiccia
and Beales, 1988). Unfortunately, other programs did not
experience the same level of success, especially in sub-Saharan
Africa. The most notable early failure occurred in the Pare-
Taveta Malaria Scheme, on the Kenyan-Tanzanian border,
where transmission was not interrupted after three and a half
years of spraying. The influence of these failures on awareness
of the importance of rigorously assessing the technical feasibility
of malaria elimination was reflected in the insistence of Emilio
Pampana, one of the fathers of malaria eradication, that technical
feasibility be determined before embarking on an elimination
program. He defined technical feasibility as “evidence that
conditions in a country are such that a particular technique
[…] will succeed in an acceptable period of time and that,
once obtained, absence of transmission could be maintained”
(Pampana, 1963).
Since that time, there has been relatively little further work
done to define conditions where it is technically feasible to
eliminate malaria and maintain its absence. Currently, WHO
guidance states that a prerequisite for an elimination attempt
is “demonstrated technical feasibility of malaria elimination in
similar eco-epidemiological settings in the recent past” (WHO,
2007). Mauritius, another island with conditions and populations
that are similar to Zanzibar, reported its last malaria case in
1997. Based on this criterion, elimination in Zanzibar may be
possible. However, the substantial differences between Zanzibar
and Mauritius, including proximity to the mainland, clearly
require a much more nuanced approach to technical feasibility
to better define the potential for achieving and maintaining zero
transmission.
This chapter seeks to define a new framework for assessing the
technical feasibility of sustainable elimination of malaria in
Zanzibar through application of mathematical transmission
modeling and detailed analysis of human migration patterns.
Using mathematical modeling techniques, the potential
for currently available interventions and tools to interrupt
transmission in the particular context of Zanzibar are evaluated
to decide whether elimination can be achieved and what level
of intervention would be required to do so. Just as importantly,
this chapter assesses the level of effort that would be required
to prevent the reintroduction of malaria transmission once
elimination has been achieved.
ESTIMATING ZANZIBAR’S MALARIOGENIC
POTENTIAL
An assessment of the feasibility of malaria elimination from the
islands of Zanzibar must consider two principle dimensions
of risk: risk of importation (often termed “vulnerability”)
and risk of transmission (termed “receptivity”) (Moonen et al.,
2009). Together, these two dimensions interact to comprise the
malariogenic potential, or overall malaria risk, for the islands of
Zanzibar. An assessment of malariogenic potential considering
both risks simultaneously is central to determining the feasibility
of elimination. For example, if Zanzibar’s importation risk can
be reduced to very low levels, meaning that almost no infected
individuals travel to Zanzibar from the mainland, control
measures that suppress the transmission risk might be relaxed
without creating an unacceptable risk of malaria resurgence.
Conversely, if a high degree of control is maintained to reduce
malaria transmission risk, such as a large majority of individuals
sleeping under insecticide-treated bed nets, malariogenic
potential may be kept at acceptable levels even if a large number
of infected individuals are traveling to the islands from the
mainland. If both importation and transmission risk are very
high, however, elimination is likely impossible. Figure 4 provides
an overview of this concept.
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