Monitoring mosquito vector populations in Djibouti City: evaluation of a new sampling method based on FTA-card

Background: Early detection of pathogens in arthropods is highly important to prevent vector-borne disease outbreaks. However, entomological surveillance methods need constant monitoring, which is expensive in time, cost and labor. In previous years, we modified a BG sentinel trap to keep alive collected mosquitoes through a sugar delivery system. During sugar feeding, potentially infected specimens can release pathogens on a substrate (FTA card) able to preserve nucleic acids for several days in field conditions. This approach simplifies vector surveillance reducing the laboratory procedures for molecular pathogen detection avoiding cool chain. Methods: In this study the modified BG sentinel trap was tested in 12 sites of Djibouti City (Djibouti). In each site, captures were conducted outdoors for 3-4 days per week from 28th January to 25th February 2020. All collected mosquitoes were morphologically identified and stored, together with FTA cards, in silica gel for subsequent transport to laboratory in Italy. RNA was extracted from FTA cards and currently screened for the presence of Rift Valley Fever Virus (RVFV) by Real Time-PCR. Results: In total 98.8% of 13,300 collected mosquitoes were successfully identified as follows: 95.6% Culex pipiens quinquefasciatus, 2.7% Aedes aegypti, 0.35% Anopheles stephensi, 0.11% Culex sitiens and 0.05% Anopheles dthali. FTA cards resulted negative for the presence of RVFV. Discussion: In this area, the Asian malaria vector An. stephensi was detected, confirming its establishment to Horn of Africa despite the vector control efforts taken over the years. Previous studies conducted in 2017 indicated An. stephensi as the only Anopheles in the study area. However, we here show for the first time also the presence of An. dthali, a secondary malaria vector, which is known to be present in Obock region, in the Northern side of the country. Large predominance of Cx. p. quinquefasciatus may be a consequence of insecticide resistance mechanisms in this species, considering the periodic adulticidal treatments conducted in the study area, which will be explored in further analysis. Finally, according to rare human cases reported, no RVFV was detected. However, due to the presence of potential vectors, the FTA card samples will be tested for the presence of other pathogens, particularly those transmitted by Cx. p. quinquefasciatus and Ae. aegypti, such as West Nile, Dengue, Zika and Chikungunya viruses.