MALARIA ELIMINATION IN ZANZIBAR - Soper Strategies

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VERY HIGH IMPORTATION RISK VERY LOW FIGURE 4: VARYING CONTROL MEASURE REQUIREMENTS FOR SUSTAIN- ABLE ELIMINATION ACCORDING TO RELATIVE LEVELS OF OUTBREAK RISK AND IMPORTATION RISK (ADAPTED FROM COHEN ET AL., 2009) Rigorous quantitative assessments of importation and transmission risk have never been done, including during the GMEP (Hay et al., 2008). Useful guidelines for defining and containing transmission and importation risk have recently been released (Malaria Elimination Group, 2009). However, translating these recommendations into operational practice requires explicit methods to calculate these essential values. For importation risk, such methods must take advantage of new opportunities offered by geographical information systems (GIS), spatial statistics, and the increasing ability to quantify and evaluate the epidemiological impact of human transport systems (Tatem et al., 2006a; Tatem et al., 2006b), while rigorous assessment of transmission risk requires mathematical modeling. In the following sections, the two dimensions of malaria risk are considered in turn. First, available historical data for Zanzibar are analyzed in order to quantitatively assess the “innate” risk of malaria that exists there–that is, the level of transmission that would be expected in Zanzibar if no control measures existed. Second, information on current control measures in Zanzibar is additional and is considered here in order to estimate presentday risk of transmission. Third, importation risk is assessed using GIS, mathematical models, and spatial analysis. Understanding the feasibility of sustainable malaria elimination requires not only computing the risk of importation and transmission, but also determining whether those levels of risk are sufficiently low to permit elimination and maintain it. As such, the chapter concludes with applications of mathematical models to evaluate the prospects for elimination under present circumstances as well as the requirements for maintaining malaria-free status. 16 SCREENING, SOURCE REDUCTION TARGETED SCREENING SURVEILLANCE AND CASE RESPONSE ALONE VERY LOW SCREENING, SOURCE REDUCTION, FOCAL IRS, LLINS TARGETED SCREENNG, FOCAL IRS, LLINS FOCAL IRS, LLINS TRANSMISSION RISK SUSTAINABLE ELIMINATION NOT FEASIBLE TARGETED SCREENNG, COMPREHENSIVE RECEPTIVITY-LOWERING INTERVENTIONS COMPREHENSIVE RECEPTIVITY-LOWERING INTERVENTIONS VERY HIGH INNATE TRANSMISSION RISK Assessing the feasibility of malaria elimination in Zanzibar requires understanding how current or potential interventions will affect transmission. To do so, we must consider not just the presentday amount of malaria–which is a product of all of the successful interventions implemented by the ZMCP–but also the “innate” amount of malaria that would occur eventually if all interventions were removed. Zanzibar’s current parasite prevalence of
to below 1% with IRS (Bhattarai et al., 2007). These low levels of malaria prevalence in Zanzibar today suggest that elimination is feasible, although it may be challenging to go the last mile. History has demonstrated numerous times that regions with high innate risk of malaria will see rapid resurgence of transmission if control measures are weakened or removed. For example, after Madagascar instituted a control program of DDT spraying and case detection and treatment in the 1940s and ‘50s, malaria was nearly absent from the highlands for many years. But when control activities were halted in the late 1970s, an explosive epidemic caused an estimated 40,000 deaths over five years (Mouchet et al., 1998). There are numerous similar examples from diverse environments and areas of the world. In contrast, countries that have effectively sustained interventions have successfully maintained a high level of control or elimination. In sub-Saharan Africa, examples of such success include South Africa (control) and Mauritius (elimination), both of which had to adapt their programs when changing conditions led to modest resurgence (Julvez et al., 1990; Mabaso et al., 2004). Zanzibar itself has already experienced such resurgence. Malaria in Zanzibar reached a previous low forty years ago at the end of the GMEP. In June 1968, with prevalence reduced over 10-fold in Unguja and 40-fold in Pemba, malaria had been reduced to a “minor health problem” (Tavrow et al., 1988). The program was then halted for “political and economic reasons,” a trend that was occurring around the world due to disappointment that full elimination had not been achieved. Without control measures, the resurgence of malaria was rapid (Schwartz et al., 1997). A survey conducted five years after the end of the program found that prevalence had rebounded six-fold to 54% prevalence in Unguja and 10% in Pemba (Minjas et al., 1988). Should history repeat itself and the current control measures be relaxed, it is certain that malaria in Zanzibar will once again return to hyperendemic levels within a few years. CURRENT TRANSMISSION RISK Actual levels of malaria transmission in Zanzibar at a given time depend upon not only the historic, or innate levels of malaria, but also the levels of control activities that will decrease transmission from that baseline. Malaria transmission can be measured in several ways, but an extremely useful measure is the number of human malaria cases resulting from each human malaria case, called the basic reproductive number R0 (Box 1), pronounced “R naught.” R0 measures transmission under the conditions that maximize transmission–no control and no immunity. To define transmission risk under different levels of control, we use the controlled reproductive number, RC, which is the number of new infections that will result from each infection at a certain level of control activities. Understanding both R0 and RC is critically important for elimination and post-elimination planning, telling us what interventions are needed to get to zero transmission and how fast new imported cases might develop into outbreaks. Box 1 The basic reproductive number R 0 1 | Technical Feasibility Zanzibar’s high innate level of malaria–that is, the level of the disease that would naturally occur if all control measures were removed–is determined by the presence of efficient vectors, including Anopheles gambiae, appropriate climate, geography, and socioeconomic environment for malaria parasites to spread efficiently. Together, these factors produce a high basic reproductive rate of malaria (R0)-the number of new infections an infected person would generate if there were no control measures in place. The controlled reproductive number R C The value of R C indicates the number of new infections that each infected person will generate under a given level of control activities. If that value is greater than one, malaria levels are increasing, while if it is less than one, they are decreasing. The current R C level in Zanzibar depends on the effective coverage of control activities. For example, if around 60% of the population is fully protected by LLINs or IRS, RC will be approximately equal to one, while if 75% of the population is protected, it will drop to about 0.5. To put the historical trends in malaria endemicity into a context that is useful for assessing the technical feasibility of elimination, it is necessary to estimate R0 and RC. Estimates of R0 for malaria are based on the prevalence of malaria in children aged 2-10 and a mathematical model of malaria transmission (Smith et al., 2007). Zanzibar’s past hyperendemic malaria, with prevalence of around 60%, corresponds to estimates of R0 around 100, but with wide confidence limits that suggest it was greater than 30 but could have been as high as a thousand. More recent data suggest that prevalence remained around 35-40% even in the absence of a formal control program. This lower (though still high) level of innate malaria suggests that R0 had fallen to around 10 or 15. R C is likely to depend upon a diversity of spatially varying factors including coverage of interventions like bednets or IRS, socioeconomic conditions, and urban development. As such, current levels of malaria transmission are not homogenous across Zanzibar (Figure 5). Survey data indicate that Central and North B on Unguja and Micheweni in north Pemba appear to have higher malaria risk than elsewhere. These three districts also were three of the four districts with the lowest percentages of individuals reporting sleeping under treated nets in a 2007 ZMCP survey; all reported figures lower than 50%. Determining the relative importance of other factors will require regular and detailed data on interventions, socioeconomic status, development, and parasite prevalence across Zanzibar. Nevertheless, recent years of transmission data indicate that R C is likely around one on average, and it is probably lower than one across much of Zanzibar, particularly in the high population context of Stone Town and in surrounding urban areas. 17
Page 1: PHOTO: © SASJA VAN VECHGEL MALARIA
Page 5: ACKNOWLEDGEMENTS The feasibility as
Page 9 and 10: EXECUTIVE SUMMARY Over the past sev
Page 11 and 12: Financial Feasibility The optimal m
Page 13 and 14: INTRODUCTION BACKGROUND In October
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Page 67: CONCLUSIONS This chapter considers
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elimination will raise two major fi
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INDOOR RESIDUAL SPRAYING As noted a
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analysis be conducted. Moreover, gi
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FIGURE 30: AVERAGE COST PER YEAR FO
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which are being explored by the glo
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portion of those costs). Before thi
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Jacobs, Lesley A (9/2007). Rights a
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Sandberg, Ingstad K, Bjune G (2007)
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World Health Organization, Global M
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