Cardoso, S. F.; Kikuti, L. A. O.; Yoshikawa, A. A. G.; Pinheiro, I. C.; Guesser, J. V. C.; Neves, M. S. A. S.; Couto-Lima, D.; Gatti, R. R.; Prophiro, J. S.; Pitaluga, A. N.; Rona, L. D. P.

Yellow fever (YF) is an infectious disease caused by the yellow fever virus (YFV), an arbovirus of the Flaviviridae family. It is transmitted through the bite of infected mosquitoes belonging to the Culicidae family and affects both humans and non-human primates (NHPs). This study aimed to investigate the sylvatic Culicidae fauna and the occurrence of natural YFV infection in a microregion of southern Santa Catarina, Brazil, an area recently affected by a sylvatic YF outbreak. Entomological collections were carried out between January and February 2023 in five municipalities with confirmed viral circulation. Natural YFV infection was assessed using RT-LAMP. A total of 4,352 female culicids were collected, representing at least 32 species, including several key sylvatic YFV vectors. Haemagogus leucocelaenus was identified in all sampled municipalities, whereas Haemagogus janthinomys, the primary vector of sylvatic YFV in Brazil, was not detected. Mosquitoes from the genera Aedes, Haemagogus, Psorophora, and Sabethes were tested for YFV. Only one pool, composed of Sabethes albiprivus, tested positive, yielding a minimum infection rate (MIR) of 11.6. This is the first record of natural YFV infection in Sa. albiprivus in southern Brazil, and only the third reported case globally, highlighting its potential role as a secondary vector in maintaining viral circulation in sylvatic environments. Based on species presence and abundance, Hg. leucocelaenus is likely to have acted as the primary YFV vector in the study area. The composition of the culicid fauna, coupled with the detection of YFV in sylvatic vectors, indicates an ongoing epidemiological risk. These findings underscore the need to strengthen entomological surveillance and expand YF vaccination coverage in affected and neighbouring regions.