Changes in malaria vector bionomics in Timor Leste between 2009 and 2016; a longitudinal study

Malaria was the leading cause of morbidity in Timor Leste in the past which gradually declined from 223,002 cases in 2006 to 14 cases in 2020, with zero malaria deaths since 2013 leading to malaria-free certification in June 2025. We compare the changes in vector bionomics due to implementation of vector control methods and other related environmental modifications in the country between 2009 and 2016.

Anopheline adults were collected from four sites using seven standard sampling methods in 2009 and 2016: full- and partial-night indoor human landing catches and outdoor human landing catches; cattle-baited net trap collections (CBTC) and cattle-baited hut collections; light trap collections; indoor resting collections and outdoor resting collections; and window trap collections. Enzyme Linked Immuno Sorbent Assay techniques were used for vector incrimination studies and insecticide susceptibility testing were carried out.

Anopheles barbirostris and An. subpictus were incriminated as malaria vectors; their estimated entomological inoculation rates were 16.25 and 2.35 infectious bites per person per year, respectively. The abundance of malaria vectors decreased from 2009 to 2016; changes in their resting and biting behaviours were observed with sustained use of targeted vector control measures including long lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) in combination. The number of Anopheline species encountered reduced from 13 in 2009 to 9 in 2016. CBTC yielded the most productive collection of all vector species. The biting and resting behaviours of Anophelines changed from 2009 to 2016 with more outdoor biting in 2016 compared to 2009.

An. barbirostris and An. subpictus were incriminated as malaria vectors in Timor Leste. The abundance of Anophelines decreased significantly in 2016 compared to 2009 with sustained control methods. Outdoor biting predominated in 2016. Partial-night human landing catches may replace full-night human landing catches as their performances were similar. Adaptation of vectors by resting and biting outdoors, reducing the effectiveness of current indoor-focused interventions especially in a setting which shares a porous land border with West Timor province of Indonesia where malaria is prevalent is a challenge. An Integrated Vector Management approach is combining application of multiple control methods is advocated.