In Papua New Guinea where malaria is endemic, nationwide vector control campaign based on free distribution of insecticidal bed nets is presently ongoing to alleviate the burden of the disease. In the north coast of Madang province, prevalence of malaria infection in human populations in 2006 was 42.1% Plasmodium falciparum and 41.7% Plasmodium vivax. After the commencement of the bed net campaign in this region in 2008, infection prevalence dropped for both P. falciparum (9.0% in 2014) and P. vivax (12.7% in 2010). Human biting rates, the number of Anopheles bites per person-night, dropped from a range of 10-60 in 2009 to 3.0-18 in 2011. Infectious biting rates, the number of Plasmodium spp. sporozoite-infective Anopheles bites per person-year, dropped from a range of 20-350 in 2009 to 0-15 in 2011. The research described in this dissertation investigated if the ongoing efforts to intensify the bed net program has succeeded in reducing malaria transmission further or whether further reduction is hampered by vector-related ecological factors which enable transmission to persist. Barrier screen and human landing catch mosquito sampling, active-case malaria detection, and bed net survey were conducted in 2016-2017 in the north coast villages of Madang. PCR methods were used to identify Anopheles species and their blood-meal host, and to test for malaria infection in humans and mosquitoes. Human blood meals were fingerprinted by microsatellite genotyping to identify the individuals bitten. The results show that despite high bed net usage (> 80% of villagers reportedly slept under a net) in all villages, the overall infection prevalence rebounded to 18% P. falciparum and 14% P. vivax in 2016-2017. The human biting rates increased to a range of 9.0-31 bites per person-night. The infectious biting rates also increased to a range of 6.5-159.7 infective bites per person-year. Anopheles species richness ranged from four to six species per village, but relative abundance was highly uneven within and between villages, and community composition was generally dissimilar among villages. The vectors did not exhibit strict anthropophagy but were rather opportunistic in their host selection and utilized non-human hosts when access to humans was limited by the bed nets. The frequency of blood meals obtained from different human individuals was not random but rather clustered, and a few individual villagers were utilized as hosts more frequently than others. The human biting frequency was also clustered spatially and was high in few locations and low in most locations, causing the basic reproduction rate of malaria to increase by multiplicative factors 2265 1.6. Most (29-50%) of the bites taken on humans occurred before 22:00 hr when 75-90% of people were awake and unprotected by the bed nets and 67-83% of the bites occurred before nearly everyone retired to bed at 2:00 hr. Most (> 50%) of the bites taken on humans occurred outdoors where the risk of exposure to the bed nets was low. These results show that in the north coast of Madang, malaria transmission has increased in recent years and continues to persist at high levels despite ongoing efforts to intensify the vector control program. The diverse Anopheles species, their high abundance and great dissimilarity in species composition among villages indicate that the bed net is failing to affect the vectors, allowing vector populations to revert to pre-bed net condition. The opportunistic host utilization and outdoor biting behavior of the local vectors enable them to evade the indoor-deployed bed nets by feeding on alternative hosts to supports their population size and continue to transmit malaria by feeding on humans outdoors and early in the evening when people are active and unprotected by the bed nets. By increasing the basic reproduction rate, the heterogeneity of bites on humans causes transmission to persist rather than dissipate.
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