Malaria outbreak in Riaba district, Bioko Island: lessons learned

Abstract

At the beginning of 2019, a sudden surge of malaria cases was observed in the district of Riaba, Bioko Island. Between January and April, confirmed malaria cases increased 3.8-fold compared to the same period in 2018. Concurrently, anopheline human biting rate (HBR) increased 2.1-fold. During the outbreak, 82.2% of the district population was tested for malaria with a rapid diagnostic test; 37.2% of those tested had a detectable infection and were treated according to national guidelines. Vector control interventions, including indoor residual spraying and larval source management were scaled-up. After the interventions, the number of confirmed cases decreased by 70% and the overall parasite prevalence in the communities by 43.8%. Observed prevalence in a follow up malaria indicator survey, however, was significantly higher than elsewhere on the island, and higher than in previous years. There was no significant reduction in HBR, which remained high for the rest of the year. The surge was attributed to various factors, chiefly increased rainfall and a large number of anthropogenic anopheline breeding sites created by construction works. This case study highlights the need for sustained vector control interventions and multi-sector participation, particularly in malaria control and elimination settings with persistently high local malaria receptivity.

Fig. 1

Bioko Island and Riaba district. a Location of Bioko Island in the Gulf of Guinea, off the coast of Cameroon and to the Northwest of the continental region of Equatorial Guinea (Río Muni). b Administrative divisions of Bioko showing Riaba district in darker grey. The red points indicate communities within the district. The green areas are nature reserves. Ureka is part of Luba district

Fig. 2

Confirmed malaria cases, malaria incidence and HBR on Bioko Island, by district, 2015–2019. The light-yellow shading highlights the period between January to April 2019. a Ratio of confirmed malaria cases to all outpatient consultations. b Malaria incidence by district and age-group. Solid, dashed and dotted lines represent the 2–12, 13–17 and 18–50 years old age groups. c HBR estimates from the 12 entomological monitoring sentinel sites and rainfall data for the whole of Bioko Island. Entomological monitoring did not take place in January and February 2019, hence the missing data for these months. Rainfall data were digitized from https://www.worldweatheronline.com

Fig. 3

Malaria prevalence estimates from MIS and from the outbreak survey. Data from MIS were plotted separately for Riaba and for the rest of the island. Error bars illustrate the 95% confidence intervals. Individuals with history of off-island travel in the previous 8 weeks were excluded from the MIS data to control for the confounding effects of malaria importation [22]

Fig. 4

Malaria prevalence by age group. The grey shaded areas illustrate the 95% confidence intervals. Individuals with history of off-island travel in the previous 8 weeks were excluded from the MIS data to control for the confounding effects of malaria importation [22]

Fig. 5

Location of communities and mapped anopheline breeding habitats in Riaba district. a Riaba is shaded in lighter grey. Breeding sites are marked according to type as natural or anthropogenic (i.e. construction sites and tyre tracks). Patio López and Patio Balboa are highlighted as the two longitudinal entomological monitoring sites in Riaba district. The numbers identify the other communities. b Detail of the area around the construction sites (red box in a)

Fig. 6

Malaria prevalence estimates by community in Riaba district. Riaba is shaded in lighter grey. a Outbreak survey. b 2019 MIS. The shaded areas around the mapped points are proportional to the 95% confidence intervals of the prevalence estimates

Fig. 7

Prevalence estimates in Riaba district during the outbreak against the 2019 MIS. Horizontal and vertical lines illustrate the 95% confidence intervals of the estimates. Sites where less than 10 people were sampled are not shown. Numbers correspond to the communities shown in Fig. 5. Patio Balboa and Patio López are named specifically

Fig. 8

Construction site in Riaba district. Large water collections left unattended became active mosquito breeding habitats. In the picture, an entomologist is sampling the site for anopheline larvae. This was one of many such anthropogenic breeding habitats found in Riaba district in 2019