Malaria incidence and prevalence on Pemba island before the onset of the successful control intervention on the Zanzibar archipelago

Abstract

Background: Malaria incidence has been reported to decrease substantially in parts of sub-Saharan Africa, including the Zanzibar Archipelago in East Africa. A cohort study with an intensive follow-up on Pemba Island just before the onset of the highly successful malaria control intervention was conducted. The reported estimates of parasite prevalence and incidence can serve as a robust baseline to evaluate the effect size of the successful interventions and the potential contribution of quality controls and other factors associated with research studies in the decreased estimate of transmission.

Methods: In a rural clinic, two successive cohorts of 537 children total aged 2-23 months were followed for six months each with an intensive visitation schedule of bi-weekly follow-up. Robust estimates of incidence and prevalence according to four different malaria definitions were obtained.

Results: Malaria incidence and prevalence placed Pemba Island in a hyperendemic rather than holoendemic setting for the years 2003-2005. Overall parasite prevalence was estimated to be 39% - with monthly estimates varying between 30% and 50%. Incidence of malaria varied between 2.3 and 3.8 malaria episodes per year based on a diagnosis of fever and various microscopy-based parasite thresholds and between 4.8 and 5.7 based on a diagnosis of fever and 100 parasites/microliter analogous to detection by rapid diagnostic tests. Both parasite densities and malaria incidence increased with age and rainy season. Malaria incidence also varied substantially between the individual villages within the study area.

Conclusions: Pemba Island was previously considered holo-endemic for Malaria. The data suggest that the transmission situation on Pemba Island was significantly lower in 2003-2005 suggesting a hyper-endemic or meso-endemic transmission environment. The figures were obtained just before the onset of the highly successful malaria control intervention by impregnated bed nets and IRS on the Zanzibar Archipelago and provide robust estimates of the malaria transmission situation prior to the control programme. Together with other published data, the results suggest that malaria transmission had started to decrease before the onset of the control programme. The local heterogeneity in malaria incidence highlights the importance of a micro-epidemiological approach in the context of malaria control and elimination.

Figure 1

Rainfall and parasite prevalence by microscopic densities per microliter per 2-week period in two cohorts totaling 537 children on Pemba Island. Any microscope positive bloodfilm (red circle); greater than 1,000 parasites/microliter (green circle); greater than 5,000 parasites/microliter (orange circle); and greater than 10,000 parasites/microliter (grey circle).

Figure 2

Yearly incidence per month per three age groups in a cohort of 537 children on Pemba Island by 4 malaria definitions. Any fever (green); fever and confirmed microscopy with greater than 100 parasites/microliter (blue); documented fever and greater than 1,000 parasites or any fever and greater than 3,000 parasites or greater than 8,000 parasites (orange); and any fever and parasites greater than 5,000/microliter (brown). Age at start of study A) ≤ 6 months; B) 7-12 months; and C) 13-22 months. Cells with less than 500 days of risk in the denominator were omitted from the analysis.

Figure 3

Yearly incidence by age in months according to 4 different definitions of malaria in two cohorts of 537 children total. Any fever (green); fever and confirmed microscopy with greater than 100 parasites/microliter (blue); documented fever and greater than 1,000 parasites or any fever and greater than 3,000 parasites or greater than 8,000 parasites (orange); and any fever and parasites greater than 5,000/microliter (brown). A) Estimate per month of age; B) Smoothing by lowess regression. The 95% confidence level was within +/- 40% of the value plotted.

Figure 4

Map of the area on Pemba Island with geographical locations of 537 children in 17 sub-villages.

Figure 5

Local heterogeneity in yearly malaria incidence: incidence is presented per subvillage (N = 17) including 95% confidence intervals. A) malaria 5,000 definition; B) IMCI definition; C) density-dependant definition; D) RDT definition.