Travel risk, malaria importation and malaria transmission in Zanzibar

Abstract

The prevalence of Plasmodium falciparum malaria in Zanzibar has reached historic lows. Improving control requires quantifying malaria importation rates, identifying high-risk travelers, and assessing onwards transmission.Estimates of Zanzibar's importation rate were calculated through two independent methodologies. First, mobile phone usage data and ferry traffic between Zanzibar and mainland Tanzania were re-analyzed using a model of heterogeneous travel risk. Second, a dynamic mathematical model of importation and transmission rates was used.Zanzibar residents traveling to malaria endemic regions were estimated to contribute 1-15 times more imported cases than infected visitors. The malaria importation rate was estimated to be 1.6 incoming infections per 1,000 inhabitants per year. Local transmission was estimated too low to sustain transmission in most places.Malaria infections in Zanzibar largely result from imported malaria and subsequent transmission. Plasmodium falciparum malaria elimination appears feasible by implementing control measures based on detecting imported malaria cases and controlling onward transmission.

Figure 1. Map of Zanzibar.

(a) Zantel coverage regions in mainland Tanzania, (b) Zanzibar and its prevalence rate (%) on the two main islands: Unguja and Pemba

Figure 2. Qualitative assessment of malaria importation to Zanzibar and its influence on local malaria dynamics.

The pathogen can be introduced into the island according to four ways: 1) Zanzibar residents (R) can become infected while traveling abroad and bring it home; 2) infected visitors (V) to Zanzibar can bring the pathogen from mainland Tanzania; 3) infected immigrants (M) travel and settle permanently in Zanzibar; 4) infected vectors travel to Zanzibar either flying or passively carried out by wind or riding along human means of transportations. Once introduced, the probability for further transmission is based on the length and the receptivity of the area where the pathogen is imported, defining the net importation rate.

Figure 3. Relative importance of residents on malaria importation.

Contribution of resident travelers to the net malaria importation rate as a function of the number of imported infections from visitors versus residents (δ_v/δ_r) and the length of stay for visitors (T_v). The solid red line represents the contribution threshold below which residents contribute to more than 50% to malaria importation.

Figure 4. Quantitative assessment of malaria importation.

Vulnerability (the number of imported malaria per year per 1,000 inhabitants, δ) and receptivity (the effective reproductive number, R_c) in three different epidemiological settings in Zanzibar: (a) urban Unguja with a parasite rate of 0% (95% Confidence Interval: 0–0.2%), (b) rural Unguja with a parasite rate of 0.5% (0.3–0.9), and (c) Pemba with a parasite rate of 1.5% (1–2.1). The solid black line represents the mean annual number of imported malaria cases and the dashed black lines the upper and lower bounds of the 95% confidence interval represented by the grey area. The red dotted line represents the empirical estimation of δ in Zanzibar according to travel assessment survey, which is of 1.6 imported malaria cases per 1,000 inhabitants per year.

Figure 5. Quantitative assessment of the impact of malaria importation on local malaria dynamics.

The effect of vulnerability (the number of imported malaria per year per 1,000 inhabitants, δ) and of receptivity (the effective reproductive number, R_c) on, (a) the parasite rate (PR), and (b) the proportion of local infection among all malaria infection. The red dashed line represents the empirical estimation of δ in Zanzibar according to the travel assessment survey, which is of 1.6 imported malaria cases per 1,000 inhabitants per year. The solid red line represents the contribution threshold above which local infections contribute to more than 50% to total malaria prevalence