Malaria Outbreak in Farafangana District, Southeast Madagascar, 2018: Are Secondary Vectors a Threat to Current Malaria Control Approaches?

A malaria outbreak occurred in Farafangana District, Madagascar, in 2018, shortly after the implementation of insecticide-treated bed net distribution and indoor residual spraying campaigns. Entomological and epidemiological investigations were conducted to characterize disease transmission in six villages in three communes. Mosquitoes were collected using human landing catches, light traps, and pyrethrum spray catches. Vector biting behavior was described, and sporozoite indices were determined. To describe demographic and risk data, questionnaires were administered to individuals from randomly selected households, and rapid diagnostic tests (RDTs) were performed on consenting household members. Anopheles coustani (An. coustani), Anopheles gambiae s.s. (An. gambiae s.s.), Anopheles funestus (An. funestus), and Anopheles mascarensis (An. mascarensis) were the most frequently captured malaria vector species. Outdoor biting was common for all predominant Anopheles species collected (exophagy rates varied from 59.8% for An. gambiae s.s. to 100.0% for An. coustani), except for An. funestus, which exhibited an exophagy rate of less than 47.0%. Of 1,488 Anopheles mosquitoes collected, 25 (1.7%) had Plasmodium falciparum sporozoites, only one of which was collected indoors. The remaining 24 were collected outdoors, 13 (54.2%) of which were An. coustani. The other 12 specimens were An. funestus, Anopheles squamosus, An. gambiae s.s., and An. mascarensis. Of 226 individuals tested using RDTs, 71 (31.4%) had positive results. A total of 61 (85.9%) of these individuals were asymptomatic, most of whom were children. Highly infected secondary malaria vectors, in addition to primary vectors, combined with a predominance of exophagy, contributed to parasite transmission in the Farafangana District, where indoor-targeted vector control measures had been implemented. A high proportion of asymptomatic infections likely sustained transmission. Control strategies for outdoor biting should be explored.