Assessment of natural filariae infections in the mosquitoes of Karen state, Myanmar: a cross-sectional PCR survey in six villages

Background Filarial nematodes transmitted by mosquitos cause significant burdens and important stigma in the affected communities. Focal transmission of Wuchereria bancrofti and Brugia malayi was detected incidentally during blood surveys conducted for a program of malaria elimination in Karen state, Myanmar. The objective of this study was to assess filariae infections in selected mosquito species collected in these villages striving to identify the potential vectors, characterise their biting behaviours and estimate the entomological indices of transmission. Methods Cross-sectional entomological surveys were conducted in September 2019 in six villages. Mosquitos were captured over 24-h diel cycles using the human-landing catch and cow-baited trap collection methods, and identified with dichotomic morphological identification keys. A subset of the specimens of potential vector species was randomly selected and tested with a filariasis PCR assay targeting the parasites 5S rRNA gene. For positive Aedes specimens, correct identification of the filariae and vector species was confirmed with DNA sequencing of the filariasis and mosquito cytochrome c oxidase subunit 1 PCR products. Results Overall 5.2% (40/772) mosquito specimens had a positive filariasis PCR result. W. bancrofti DNA was found in half of the positive specimens. Three other zoonotic Brugia and Dirofilaria filariae species were also identified. W. bancrofti DNA was detected in known vector species including Anopheles barbirostris , An. hyrcanus , Downsiomyia harinasutai , Finlaya poicilia and Culex sitiens . Furthermore, it was also found in Aedes albopictus / pseudoalbopictus and Armigeres flavus . W. bancrofti entomological inoculation rate estimate was 0.094 infective bites/person-day (95%CI 0.049 to 0.164). Only 7.5% (3/40) of the positive specimens were captured indoors with the human-landing catch collection method between 9 PM and 5 AM. Specimens infected with zoonotic filariae were captured with the human-landing catch collection method highlighting the potential for zoonotic filariasis transmission in this area. Furthermore, infected specimens were collected in the cow-baited trap suggesting veterinary-based vector control interventions targeting livestock could be used to tackle filariasis transmission in this setting. Conclusions This study identified potential vectors of Wuchereria bancrofti and three zoonotic filariae species in Karen state. Mosquito bed nets may have only a limited efficacy against filariasis vectors in this area. Trials of vector control interventions preventing outdoor and early transmission are warranted.