Yellow fever virus (YFV) is the agent of yellow fever (YF), which affects both humans and non-human primates (NHP). Neotropical NHP are highly susceptible to YFV and considered sentinels for YFV circulation. Brazil faced a significant YF outbreak in 2017–2018, with over 2000 human cases and 2000 epizootics cases, mainly in the State of Minas Gerais, Brazil. This study aimed to investigate whether YFV circulation persisted in NHP after the human outbreak had subsided. To this end, NHP carcass samples collected in Minas Gerais from 2021 to 2023 were screened for YFV. RNA was extracted from tissue fragments and used in RT-qPCR targeting the YFV 5’UTR. Liver and lung samples from 166 animals were tested, and the detection of the β-actin mRNA was used to ensure adequacy of RNA isolation. YFV RNA was detected in the liver of 18 NHP carcasses collected mainly from urban areas in 2021 and 2022. YFV positive NHP were mostly represented by Callithrix, from 5 out of the 12 grouped municipalities (mesoregions) in Minas Gerais state. These findings reveal the continued YFV circulation in NHP in urban areas of Minas Gerais during 2021 and 2022, with the attendant risk of re-establishing the urban YFV cycle.

The yellow fever (YF) vaccine consists of an attenuated virus, and despite its relative safety, some adverse events following YF vaccination have been described. At the end of 2016, Brazil experienced the most massive sylvatic yellow fever outbreak over the last 70 years and an intense campaign of YF vaccination occurred in Minas Gerais state in Southeast Brazil from 2016 to 2018. The present study aimed to develop a genotyping tool and investigate 21 cases of suspected adverse events following YF vaccination. Initial in silico analyses were performed using partial NS5 nucleotide sequences to verify the discriminatory potential between wild-type and vaccine viruses. Samples from patients were screened for the presence of the YFV RNA, using 5′UTR as the target, and then used for amplification of partial NS5 gene amplification, sequencing, and phylogenetic analysis. Genotyping indicated that 17 suspected cases were infected by the wild-type yellow fever virus, but four cases remained inconclusive. The genotyping tool was efficient in distinguishing the vaccine from wild-type virus, and it has the potential to be used for the differentiation of all yellow fever virus genotypes.

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Human activity in sylvatic environments and resulting contact with wildlife, such as non-human primates (NHPs), can lead to pathogen spillover or spillback. Both NHPs and humans host a variety of herpesviruses. While these viruses typically cause asymptomatic infections in their natural hosts, they can lead to severe disease or even death when they move into novel hosts. In early 2024, deaths of Callithrix penicillata, the black-tufted marmoset, were reported in an urban park in Belo Horizonte, Minas Gerais, Brazil. The epizootic was investigated in collaboration with CETAS/IBAMA and the Zoonoses Department of Belo Horizonte. Nine marmoset carcasses and four sick marmosets were found in the park; the latter exhibited severe neurological symptoms and systemic illness before succumbing within 48 h. Carcasses were tested for rabies virus and were all negative, and necropsy findings revealed widespread organ damage. In addition, the samples were tested for yellow fever virus, with negative results. Finally, molecular testing, viral isolation, and phylogenetic analysis demonstrated human herpesvirus 1 (HHV-1) as the causative agent. The likely source of infection was human-to-marmoset transmission, facilitated by close interactions such as feeding and handling. This study highlights the risks of pathogen spillover between humans and nonhuman primates, emphasizing the need for enhanced surveillance and public awareness to mitigate future epizootics.

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.