Parasitological screening of vector mosquitoes and molecular biological identification of larval filarial parasites among the wildcaught Mansonia mosquito species at selected areas in the district of Gampaha, Sri Lanka, a re-emerging focus of Brugian filariasis

Abstract

Brugian filariasis, a disease caused by the Protozoan parasiteBrugia malayi has re-emerged in Sri Lanka after nearly four decades of quiescence. The Brugia malayi that prevailed in Sri Lanka in the past was the nocturnal periodic human strain transmitted by mosquitoes of the genus Mansonia. The objective of the present study was the precise identification of vector mosquitoes and parasites of the current onset of the disease. Entomological surveys were performed during September/October 2021 in Ragama Medical Officer of Health area using cattle-baited net traps. Mansonia sp. mosquitoes were dissected to detect the presence of larvae of the parasite. The lysate of dissected mosquitoes positive for larvae was used for the extraction of genomic DNA of the parasite, which was subjected to Polymerase Chain Reactions (PCR) aimed at molecular speciation using pan-filarial primers specific for the internal transcribed spacer region two (ITS2) of the ribosomal DNA. A total of 1060 mosquitoes were tested, and that included seven mosquito species belonged to four genera. Culex gelidus (n=602; 56.8%) was detected as the predominant mosquito species followed by Armigeres subalbatus (n=420; 39.6%) Cx. tritaeniorynchus (n=2; 0.2%) and Anopheles nigerrimus (n=4; 0.4%). Mansonia spp. accounted for 2,7% of the total mosquito sample and among them, the presence of Mansonia annulifera was 1,2% of the total (n=20), Ma. uniformis was 0.9% (n= 10) and Ma. Indiana was 0.2% (n= 2). About 18.7% (n=6) of Mansonia mosquito collection was positive for filarial larvae. Among them, 15.6% (n=5) was Mansonia annulifera while (3.1%; n=1) was Ma. uniformis. The PCR products of all tested samples corresponded to the band size of 625 bp, specific to B. malayi confirming the identity of the parasite. Mansonia annulifera and Ma. uniformis were confirmed as vectors of the re-emerged B. malayi (nocturnally sub-periodic) in Gampaha district. The role of other mosquito vector species would require investigation by vector incrimination and xenomonitoring-based approaches.