MALARIA ELIMINATION IN ZANZIBAR - Soper Strategies

(Figure 15). The previously reported statistics for IRS and LLIN
coverage mask the variability in these measures across the islands.
Looking at individual districts, ITN coverage ranges from a high
of 82% in Mkoani down to only 44% in North B. Two of the
three districts with prevalence greater than 1% in the last ZMCP
survey also reported less than 60% of the population sleeping
under treated nets (Wete with 54% and Micheweni with 49%).
These districts are at high risk of reversing the gains made against
malaria if high IRS coverage is not maintained to compensate for
the low net coverage. More specific modeling using shehia-specific
prevalence information will result in more precise estimates.
It should be noted that the picture of malaria in Zanzibar is more
complicated than illustrated in these relatively simple models.
The model outputs described here assume a closed system–that is,
they do not yet take into account the number of imported cases
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Box 2: Primaquine
ACTs and other first line malaria drugs kill the asexual bloodstage
parasites that cause fever and chronic infections but
they leave the mature gametocytes, the sexual stages of the
parasite that are infectious to mosquitoes. Patients treated
with ACTs can therefore still transmit malaria (Reeder et al.,
2009). In some situations, first-line treatment alone, even
if high fractions of new infections are detected and treated,
might not be sufficient to achieve elimination necessitating
additional treatment with primaquine to cure gametocyte
infections.
Primaquine has been used safely and effectively in thousands
of patients, but it should be used with great caution because
there is a risk of hemolysis in persons with the genetic defect
glucose-6-phosphate dehydrogenase (G6PD) deficiency
(Capellini and Fiorelli, 2008). Symptoms are rare in those
with modest G6PD deficiency but in those with severe
deficiencies there is a risk of major hemolytic episodes that
can on occasion lead to severe anemia and acute renal failure
(Reeve et al., 1992). All individuals who are treated with
primaquine should therefore be tested for G6PD deficiency
first, and since it is not possible to test a fetus, pregnant
women should not be treated at all.
Primaquine’s main benefit is that it kills mature gametocytes
but the drug has a very short half-life and is cleared from
the bloodstream after a few days. Gametocytes on the other
hand take about 8 days to mature, and effect of primaquine
on maturing gametocytes is not known. Primaquine given
at the time of treatment might therefore not kill mature
gametocytes unless it is given about 7 days after the clinical
episode of malaria. To be effective, primaquine might require
a follow-up treatment 7 days after the primary episode.
We recommend that Zanzibar considers adding G6PD
testing + primaquine on a 7-day follow-up visit to
their outbreak response regimen and to reduce malaria
transmission in residual transmission foci in the end phases
of malaria elimination.
occurring each year. The initial estimates described above suggest
that there are a considerable, although still unknown, number
of parasitaemic individuals traveling to the islands. Inclusion of
these cases could substantially increase the time required to reach
elimination. As such, it is clear that a robust surveillance system
will be essential to promptly identifying imported cases, treating
them, and sustaining elimination in the current environment. If
imported cases are identified and treated promptly, before they
can lead to infected mosquitoes, they will not affect the overall
malaria risk in Zanzibar. In the absence of such a system or a
dramatic natural decline in imported infections (e.g., due to
control measures on the mainland), it will not be possible to
achieve elimination.
In summary, mathematical modeling indicates that malaria
elimination in Zanzibar is feasible given the currently existing
tools. However, achieving it will require maintaining high
effective coverage of the vector control interventions across the
island. At present, coverage by bed nets alone is too low to achieve
elimination. Coverage by IRS is quite high at present, and the
redundancy of coverage by both interventions provides a safety
net. However, the expense of maintaining both interventions over
the next decade may be prohibitive, and IRS can be discontinued
if ITNs are scaled-up to a high proportion of the population,
ideally universal coverage with > 80% usage. Finally, given the
constant risk of importation, a strong surveillance system must
be developed.
CAN ELIMINATION BE SUSTAINED?
Elimination will not mean the end of malaria in Zanzibar. As
detailed in the section on importation risk above, infected
individuals will continue traveling into the country, although it
is hoped that these numbers will decrease over time as control
measures on the mainland succeed in decreasing prevalence
there. Mosquitoes will bite some of these individuals, and some
measure of transmission will likely result. The WHO defines
an “introduced” case as the product of “first-generation local
transmission; epidemiologically linked to a proven imported
case,” (WHO, 2007) and indicates that both they and secondgeneration
transmission (an imported case leads to infection of
an introduced case, which in turn leads to the infection of a
second-generation case) may occur even in countries certified as
having eliminated malaria:
“Because certification is the recognition of a considerable
operational achievement, countries will remain listed as having
achieved malaria elimination even if they subsequently suffer a
temporary occurrence of local transmission. An indication of
the re-establishment of transmission would be the occurrence of
three or more introduced and/or indigenous malaria infections
linked in space and time to local mosquito-borne transmission
in the same geographical focus, for two consecutive years for P.
falciparum, and for three consecutive years for P. vivax.” (WHO,
2007).
The goal of preventing reemergence of malaria following
elimination is thus not to prevent every single case 6 of malaria,
6 A case is defined as a person infected with the malaria parasite whether
symptomatic or not.