Number of malaria cases
Over the ten year period between 2010 and 2019, 833 confirmed malaria cases were reported in Cabo Verde, of which 570 (68.4%) were classified as indigenous, 263 (31.5%) as imported, and two as introduced, one each in 2018 and 2019 (Table 2). However, 423 (74.2%) of the locally acquired cases occurred during the outbreak between July to October, 2017. Indegeneous cases have been reported from almost all municipalities, with the last reported locally acuired case in January 2018 (Fig. 1B, Fig. 2A). Imported infections consisted of 58.1% of all confirmed cases in the pre-epidemic, and 93.4% of cases in the post-epidemic period and occur in the majority of municipalities (Fig. 1C; Fig. 2B). Malaria incidence per 1,000 population was estimated to be less than 0.09 for all years except for during the 2017 epidemic (0.80/1,000 people). Malaria mortality was very low throughout the study period, with a total of nine deaths (range 0 to 3; the maximum case fatality rate was 8.33%) and a corresponding mortality rate ranging from 0 to 0.60 per 100,000 population at risk (Fig. 2C).
Figure 2: The total number of malaria cases (A), imported infections (B) and deaths (C) per year reported in Cabo Verde between 2010 and 2019. The different colours in the stacked bar chart represents the location the case was reported
Demographic characteristics of malaria cases
Overall, there was good routine reporting of demographic information with only two individuals missing the sex classification and 16 missing age. Across the ten-year period, 74.1% of cases were male, and was consistent across time and according to the case classification (Table 2). Similarly, the median age of cases was 33 years (IQR = 20–43 years). However, individuals with imported malaria were older compared to locally acquired infections with a median age of 35 and 24, respectively. Only 2.6% and 6.0% of reported cases were under 5 years, and 6–19 years old, respectively. Locally acquired cases were consistently reported between August and October (75.4%) whereas imported infections were reported throughout the year, ranging from 3.4% in February to 13.3% of infections in October. However, the highest risk period for imported infections was between August (9.5%) and January (8.4%), coinciding with the main transmission period in west African countries.
Table 2: Demographic characteristics of confirmed malaria infections reported in Cabo Verde between 2010 and 2019, stratified by pre-epidemic, epidemic, and post-epidemic years as well as locally acquired and imported infections as classified by routine malaria programs. The total (N), percentages and corresponding 95 Confidence Intervals (95% CI) are included.
Malaria cases were reported from the majority of municipalities in the country (14/22) at some point during the study period (Fig. 1A). The majority of cases were reported in Praia, on Santiago Island, the capital of the country (686; 83.4%), followed by São Vicente (33; 4.0%), Assomada (22; 2.7%), Sal (20; 2.4%) and Boa Vista (17; 2.1%). The municipalities reporting the greatest proportion of imported infections were Praia (163; 62.0%), São Vicente (34; 12.9%), and Santa Caterina (21; 8.0%) (Fig. 1B). The likely origin of imported infections based on travel history spanned 22 countries including Brazil (the single case of P. vivax), Philippines, and multiple African countries (Fig. 3). The majority of imported malaria infections had reported travel to Portuguese speaking countries, including 24.3% and 22.4% of cases recently travelling to Guinea Bissau and Angola, respectively. Other main countries where imported infections reported traveling to include Senegal (30; 11.4%), Equatorial Guinea (20; 7.6%), Nigeria and Guinea Conakry (both with 15; 5.7%) and Cote d´Ivoire (10; 3.8). The ratio of imported to locally acquired infections suggests that the estimated R0 was likely below 1 in 6 of the 10 years of analysis (Fig. 4A). According to the specific estimates for Santiago, where there was sufficient cases to determine the island specific estimates, the pattern was similar, except in 2018 where it lingered around 1 for that year (Fig. 4B ).
Figure 3: Global map showing the likely origin of imported infections that were reported in Cabo Verde between 2010 and 2019. The size of the circle is scaled according to the number of cases likely originating in that country, wth the different colors to differentiate the different countries for visualisation. The locatin of Cabo Verde is shown as the black circles and connector lines shown in light grey.
Figure 4: Estimated annual R0 according to the ratio of imported to local cases resported for Cabo Verde (A) and Santiago Island only (B) where there were sufficient cases (min 10 cases required for models) per year to obtain estimates. The y-axis presents the maximum estimate of R0 that is plausible based on the data with year presented on the x-axis. The red dashed line shows where R0 equals 1 whereby above this line transmission is increasing and below, transmission is expected to die out.