MALARIA ELIMINATION IN ZANZIBAR - Soper Strategies
MALARIA ELIMINATION IN ZANZIBAR - Soper Strategies
MALARIA ELIMINATION IN ZANZIBAR - Soper Strategies
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to below 1% with IRS (Bhattarai et al., 2007). These low levels of<br />
malaria prevalence in Zanzibar today suggest that elimination is<br />
feasible, although it may be challenging to go the last mile.<br />
History has demonstrated numerous times that regions with high<br />
innate risk of malaria will see rapid resurgence of transmission if<br />
control measures are weakened or removed. For example, after<br />
Madagascar instituted a control program of DDT spraying and<br />
case detection and treatment in the 1940s and ‘50s, malaria<br />
was nearly absent from the highlands for many years. But when<br />
control activities were halted in the late 1970s, an explosive<br />
epidemic caused an estimated 40,000 deaths over five years<br />
(Mouchet et al., 1998). There are numerous similar examples<br />
from diverse environments and areas of the world. In contrast,<br />
countries that have effectively sustained interventions have<br />
successfully maintained a high level of control or elimination.<br />
In sub-Saharan Africa, examples of such success include South<br />
Africa (control) and Mauritius (elimination), both of which had<br />
to adapt their programs when changing conditions led to modest<br />
resurgence (Julvez et al., 1990; Mabaso et al., 2004).<br />
Zanzibar itself has already experienced such resurgence. Malaria<br />
in Zanzibar reached a previous low forty years ago at the end of<br />
the GMEP. In June 1968, with prevalence reduced over 10-fold<br />
in Unguja and 40-fold in Pemba, malaria had been reduced to<br />
a “minor health problem” (Tavrow et al., 1988). The program<br />
was then halted for “political and economic reasons,” a trend that<br />
was occurring around the world due to disappointment that full<br />
elimination had not been achieved. Without control measures,<br />
the resurgence of malaria was rapid (Schwartz et al., 1997). A<br />
survey conducted five years after the end of the program found<br />
that prevalence had rebounded six-fold to 54% prevalence<br />
in Unguja and 10% in Pemba (Minjas et al., 1988). Should<br />
history repeat itself and the current control measures be relaxed,<br />
it is certain that malaria in Zanzibar will once again return to<br />
hyperendemic levels within a few years.<br />
CURRENT TRANSMISSION RISK<br />
Actual levels of malaria transmission in Zanzibar at a given time<br />
depend upon not only the historic, or innate levels of malaria, but<br />
also the levels of control activities that will decrease transmission<br />
from that baseline. Malaria transmission can be measured in<br />
several ways, but an extremely useful measure is the number of<br />
human malaria cases resulting from each human malaria case,<br />
called the basic reproductive number R0 (Box 1), pronounced<br />
“R naught.” R0 measures transmission under the conditions that<br />
maximize transmission–no control and no immunity. To define<br />
transmission risk under different levels of control, we use the<br />
controlled reproductive number, RC, which is the number of new<br />
infections that will result from each infection at a certain level<br />
of control activities. Understanding both R0 and RC is critically<br />
important for elimination and post-elimination planning, telling<br />
us what interventions are needed to get to zero transmission and<br />
how fast new imported cases might develop into outbreaks.<br />
Box 1<br />
The basic reproductive number R 0<br />
1 | Technical Feasibility<br />
Zanzibar’s high innate level of malaria–that is, the level of<br />
the disease that would naturally occur if all control measures<br />
were removed–is determined by the presence of efficient<br />
vectors, including Anopheles gambiae, appropriate climate,<br />
geography, and socioeconomic environment for malaria<br />
parasites to spread efficiently. Together, these factors produce<br />
a high basic reproductive rate of malaria (R0)-the number of<br />
new infections an infected person would generate if there<br />
were no control measures in place.<br />
The controlled reproductive number R C<br />
The value of R C indicates the number of new infections<br />
that each infected person will generate under a given level<br />
of control activities. If that value is greater than one, malaria<br />
levels are increasing, while if it is less than one, they are<br />
decreasing. The current R C level in Zanzibar depends on<br />
the effective coverage of control activities. For example, if<br />
around 60% of the population is fully protected by LL<strong>IN</strong>s or<br />
IRS, RC will be approximately equal to one, while if 75% of<br />
the population is protected, it will drop to about 0.5.<br />
To put the historical trends in malaria endemicity into a context<br />
that is useful for assessing the technical feasibility of elimination,<br />
it is necessary to estimate R0 and RC. Estimates of R0 for malaria<br />
are based on the prevalence of malaria in children aged 2-10 and a<br />
mathematical model of malaria transmission (Smith et al., 2007).<br />
Zanzibar’s past hyperendemic malaria, with prevalence of around<br />
60%, corresponds to estimates of R0 around 100, but with wide<br />
confidence limits that suggest it was greater than 30 but could<br />
have been as high as a thousand. More recent data suggest that<br />
prevalence remained around 35-40% even in the absence of a<br />
formal control program. This lower (though still high) level of<br />
innate malaria suggests that R0 had fallen to around 10 or 15.<br />
R C is likely to depend upon a diversity of spatially varying<br />
factors including coverage of interventions like bednets or IRS,<br />
socioeconomic conditions, and urban development. As such,<br />
current levels of malaria transmission are not homogenous<br />
across Zanzibar (Figure 5). Survey data indicate that Central<br />
and North B on Unguja and Micheweni in north Pemba<br />
appear to have higher malaria risk than elsewhere. These three<br />
districts also were three of the four districts with the lowest<br />
percentages of individuals reporting sleeping under treated nets<br />
in a 2007 ZMCP survey; all reported figures lower than 50%.<br />
Determining the relative importance of other factors will require<br />
regular and detailed data on interventions, socioeconomic<br />
status, development, and parasite prevalence across Zanzibar.<br />
Nevertheless, recent years of transmission data indicate that R C<br />
is likely around one on average, and it is probably lower than<br />
one across much of Zanzibar, particularly in the high population<br />
context of Stone Town and in surrounding urban areas.<br />
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