Uncovering Hidden Transmission: Active Surveillance Reveals Cryptic Circulation of Yellow Fever Virus in Urban Marmosets in Belo Horizonte, Brazil, 2024.

Between 2016 and 2018, the state of Minas Gerais, Brazil, experienced its most significant yellow fever (YF) outbreak in 80 years. Yellow fever virus (YFV) circulation persisted afterward, with continued non-human primate (NHP) epizootics and, recently, human cases. In June 2024, YFV RNA was detected in a dead marmoset (Callithrix penicillata) in an urban square in Belo Horizonte (BH), prompting a field investigation in an adjacent park to assess infection in potential mosquito vectors and NHPs. A total of 250 mosquitoes representing nine species were collected at ground and canopy level, of which Aedes fluviatilis and Aedes scapularis comprised 78.8% of the specimens. Haemagogus spp. and Sabethes spp. mosquitoes were not collected, possibly due to the short sampling window during the dry season. No active YFV infection was detected in any of the mosquito pools tested. Eight marmosets (Callithrix penicillata) were captured and tested for arboviral infections. Five out of eight sera, representing both adult and juvenile (less than 17 months old) animals, tested positive for anti-YFV IgM. Interestingly, two adults recaptured in later expeditions revealed seroconversion. One was IgM-positive in July 2024 but negative by September 2024, consistent with the expected decline in IgM levels. The other, initially IgM-negative (as of July 2024), tested positive in April 2025, indicating recent exposure to YFV. These findings provide evidence for the ongoing, low-level circulation of YFV among urban NHPs, posing a continued risk of viral spillover to humans. Moreover, these results highlight the importance of active surveillance in detecting recent infections that would likely be missed by passive monitoring. This integrated approach enhances our understanding of local YF epidemiology and supports early, evidence-based public health interventions to prevent future human outbreaks.