Abstract
In 2023, a new yellow fever virus (YFV) lineage was introduced in São Paulo state, Brazil. During July 2024–June 2025, nine Callithrix penicillata marmosets tested YFV–positive, showing high viral loads and characteristic organ lesions. Those results highlight the need to include these animals in multispecies surveillance strategies for early YFV detection.
Keywords: Yellow fever, yellow fever virus, viruses, zoonoses, vector-borne infections, nonhuman primates, epizootic, marmosets, high viral load, Councilman–Rocha Lima bodies, amplifier host, Brazil
Yellow fever virus (YFV; Orthoflavivirus flavi), is an arbovirus of the Flaviviridae family transmitted by Aedes spp. mosquitoes in the urban cycle and Haemagogus/Sabethes spp. mosquitoes in the sylvatic cycle (1,2). Nonhuman primates (NHPs) are key sentinel animals for early YFV detection and a core component of the Brazilian Yellow Fever Surveillance Program (3). However, YFV pathogenicity varies among species, and the role of Brazil’s diverse NHP populations in continued YFV circulation remains poorly understood (4). Alouatta spp. howler monkeys are the most susceptible to YFV infection (5–7). Infected NHPs develop clinical and pathologic manifestations similar to those in humans (4,8,9), who exhibit high viral loads and pronounced liver damage, including areas of necrosis and apoptosis, Councilman–Rocha Lima (CRL) bodies, steatosis, and inflammatory infiltrates (10,11).
During 2016–2018, Brazil experienced its worst wild yellow fever outbreak in 70 years, in which most human and NHP cases occurred in the southeastern region (1,2). In that outbreak, most infected NHPs were Callithrix spp. marmosets, revealing 2 infection patterns: one with a high viral load and characteristic histopathologic hepatic lesions and another with low viral loads, absence of clinical disease, and minimal or absent hepatic lesions (4,12).
In 2023, a new YFV lineage emerged in eastern São Paulo state (13), then spread throughout most of the state. The first confirmed NHP deaths attributed to YFV in the region of Ribeirão Preto city occurred in late December 2024. During the following months, the outbreak intensified, and more confirmed NHP infections and 1 human case occurred in the surrounding municipalities (Figure 1). We investigated clinical and laboratory findings of YFV infection among NHPs during that epizootic outbreak.
Figure 1.
Location and example species for yellow fever virus (YFV) surveillance in Callithrix spp. marmosets during epizootic outbreak, Brazil, 2024–2025. A) C. penicillata marmoset. Source: Miguelrangeljr, CC BY-SA 3.0, via Wikimedia Commons, https://creativecommons.org/licenses/by-sa/3.0. B) São Paulo state map with dark lines highlighting the municipalities of the regional Epidemiologic Surveillance Group; red indicates areas affected by yellow fever. Map created by Raphael Lorenzeto de Abreu on WikiMedia (https://commons.wikimedia.org/w/index.php), modified by the authors and used according to permissions. C) Partial aerial view of the Ribeirão Preto region. White outlines represent borders of municipalities where YFV was detected during July 2024–June 2025. Orange dots indicate locations where YFV-positive C. penicillata marmosets were found; yellow dot indicates confirmed human yellow fever case; white arrowheads indicate areas without Alouatta spp. howler monkeys. Map from Google Earth, 2025 (https://earth.google.com).
The Study
During July 2024–June 2025, the regional Epidemiologic Surveillance Group reported 233 NHP deaths, of which 79% (184/233) were Callithrix penicillata black-tufted marmosets, 18% (42/233) Alouatta caraya black howler monkeys, 2.15% (5/233) Sapajus nigritus black capuchin monkeys, and 0.85% (2/233) Callicebus nigrifrons black fronted titi monkeys. Among the 233 NHPs, we necropsied and collected biologic samples from 154 (66.1%), 88.9% (n = 137) of which were Callitrichid marmosets.
We confirmed YFV infection in a total of 54 NHPs: 22 by laboratory testing on organ samples collected during necropsy and 32 by epidemiologic criteria. We could not obtain samples from the 32 NHPs identified by epidemiologic criteria because the carcasses were in an advanced state of decomposition. However, we included those animals because they were found in the same epidemiologic context and area in which YFV was detected in other animals.
Among 22 NHPs with laboratory-confirmed YFV infection, most were A. caraya monkeys (50%; 11/22), followed by C. penicillata (40.9%; 9/22) marmosets and C. nigrifrons (9.1%; 2/22) monkeys. As expected, samples from Alouatta and Callicebus spp. NHPs had low cycle threshold (Ct) values and typical organ lesions, including extensive hepatocellular necrosis, CRL bodies, hepatic steatosis, varying degrees of inflammatory infiltrates, and YFV antigens detectable by immunohistochemistry (IHC) (Figure 2). The YFV-positive marmosets exhibited a consistent pattern: low Ct values (Ct 11.14–20.08), typical histopathologic lesions, and detectable YFV antigens. One marmoset, stored frozen for a month before necropsy, showed a higher Ct value of 28.9.
Figure 2.
Laboratory analysis of liver samples from nonhuman primates collected for yellow fever virus (YFV) surveillance in Callithrix spp. marmosets during epizootic outbreak, Brazil, 2024–2025. A, B) Samples from Callicebus nigrifrons black fronted titi monkeys; C, D) samples from Alouatta caraya black howler monkeys. A) Hematoxylin and eosin stain of liver showing acute and severe hepatic damage characterized by diffusely individual cellular apoptosis and necrosis with Councilman–Rocha Lima bodies (arrow). B) Immunohistochemistry stain of hepatocytes; brown stain indicates cells positive for YFV antigen. C) Immunohistochemistry stain showing acute and severe hepatic damage characterized by individual cellular apoptosis and necrosis with Councilman–Rocha Lima bodies (arrow); brown indicates hepatocytes positive by YFV antigen. D) Immunohistochemistry stain of hepatocytes positive for YFV antigen (red). Scale bars indicate 20 µm.
All 9 C. penicillata marmosets were YFV-positive by IHC (Figure 3); 2 (22%) showed advanced autolysis, which precluded evaluation of histopathologic features, but we did detect CRL bodies in 1 of those 2 marmosets. Histopathologic findings in the other 7 animals showed all had hepatic necrosis, 3 (42%) with multifocal panlobular distribution and 4 (57%) with widespread panlobular involvement. Necrosis severity ranged from moderate in 2 (29%) to marked in the other 5 (71%) animals. All 7 animals had CRL bodies, and 3 (43%) had inflammatory infiltrates, consisting of mixed cellular contents in 1 (33.3%) and mononuclear cells in the other 2 (66.7%); we observed multifocal distribution and moderate severity in 2 (66.7%) cases and discrete distribution in 1 (33.3%). We identified hemorrhage in 4 (57%) of those animals, characterized by diffuse distribution in most (75%; 3/4); 3 (75%) exhibited moderate severity and 1 (25%) marked severity. We observed steatosis in 6 (86%) animals and panlobular macrovesicular and microvesicular patterns of moderate severity in 2 (34%) and marked severity in 4 (66%).
Figure 3.
Laboratory analysis of organ samples collected from Callithrix penicillata marmosets with severe hepatic lesions due to yellow fever during epizootic yellow fever outbreak, Brazil, 2024–2025. A) Hematoxylin and eosin stain of liver showing acute and severe hepatic damage characterized by diffuse macrovesicular steatosis and focal hemorrhage (star). Original magnification ×10. B) Hematoxylin and eosin stain of liver showing acute and severe hepatic damage with macrovesicular steatosis (black arrowhead), focal hemorrhage (star), and a Councilman–Rocha Lima body (elongated arrow). Original magnification ×20. C) Immunohistochemisty stain showing hepatocytes positive for yellow fever virus antigen (brown staining). Original magnification ×20. D) Immunohistochemistry stain of renal tubular epithelial cells; brown stained cells are positive for yellow fever virus antigen. Original magnification ×10.
During the 2016–2018 outbreak, 7 Callithrix spp. marmosets were YFV-positive, 5 of which were from Ribeirão Preto and neighboring cities, but none had hepatic lesions or YFV antigens detected (14). In 2020, an urban C. penicillata marmoset from midwestern Brazil showed the same pattern, YFV-positive by quantitative reverse transcription PCR but no liver lesions nor YFV antigen detection (15).
The IHC and molecular findings of this study, representing the data from the 2024–2025 epizootic outbreak in São Paulo state demonstrate a consistent pattern of severe acute hepatitis in neotropical primates infected with YFV. All 22 confirmed NHP cases revealed characteristic hepatic lesions, with hepatocellular necrosis in all evaluated specimens, predominantly showing diffuse and multifocal to coalescing patterns. Those findings corroborate previous reports describing panlobular necrosis as a hallmark of fatal YFV infection in primates (4).
We identified CRL bodies, characteristic of YFV lesions, in 80% (18/22) of cases, and noted a much higher frequency in A. caraya and C. nigrifrons monkeys and C. penicillata marmosets. Even in specimens with marked autolysis (typically observed 1–3 days after death) CRL bodies remained detectable in 3 NHPs, reinforcing the strong association of CRL bodies with YFV infections, even under suboptimal tissue preservation conditions. Inflammatory infiltrates were predominantly mixed (polymorphonuclear and mononuclear) or mononuclear, with panlobular or periportal and portal distribution. We observed macrovesicular, microvesicular, or mixed steatosis with diffuse or multifocal distribution and consistent panlobular zonation. Severity was marked to moderate in 72.2% (13/18) of affected animals.
The high prevalence and severity of lesions combined with limited inflammatory response, despite extensive necrosis and steatosis, might indicate a YFV-induced metabolic dysfunction and support YFV as the main mechanism of liver injury. The histopathologic data we present need further investigation because they point to a possible association with the new YFV lineage circulating in southern Brazil. A relevant finding that supports that hypothesis is IHC confirmation of active YFV infection in 100% of cases, including all analyzed C. penicillata marmoset specimens. That contrasts with prior observations during 2017–2018, when Callithrix spp. marmosets often exhibited lower viral loads and milder hepatic lesions (4,14,15). In the NHPs evaluated here, marmosets had severe liver damage comparable to that seen in howler monkeys, with extensive necrosis and unequivocal IHC positivity. The severe lesions observed in Callithrix spp. marmosets underscore this primate’s relevance in NHP-based YFV surveillance, particularly in areas where Alouatta spp. monkey populations have been greatly reduced.
Conclusions
Our results confirm the classic histopathologic profile of YFV infection in neotropical primates, with striking severity in C. penicillata marmosets during an outbreak in an area with reduced Alouatta spp. monkey populations. The combination of hepatocellular necrosis, CRL bodies, steatosis, and 100% IHC positivity provides robust evidence for YFV pathogenicity across multiple primate species and reinforces the need for continuous surveillance in NHPs, placing Callithrix genus NHPs under the public health spotlight in this and in future epidemics. Its ability to adapt to urbanized environments, its proximity to humans, and the high Ae. aegypti mosquito population raise concerns for yellow fever reurbanization.
Our findings underscore the need for establishing a new NHP surveillance system based on Callithrix spp. marmosets in areas where Alouatta spp. monkey populations have been reduced or extinguished. Therefore, local zoonotic surveillance services in high-risk areas must be strengthened to detect early YFV circulation in the face of current NHP deaths, especially in view of a possible change in the yellow fever epidemiologic pattern.
Acknowledgments
This article was preprinted at https://www.medrxiv.org/content/10.1101/2025.09.07.25335112v2.
Acknowledgments
We thank all the municipal zoonotic surveillance services of XXIII Health Region of São Paulo state, particularly the municipalities of Ribeirão Preto, Cravinhos, and Luís Antônio, for their prompt response and for providing the necessary samples for official diagnostic confirmation.
Animal samples were collected by the zoonotic surveillance unit of each municipality as part of national yellow fever surveillance program and sent to the Molecular Virology Laboratory of Ribeirão Preto Medical School, University of São Paulo, for initial detection of YFV and to Adolfo Lutz Institute to confirm the diagnosis. Because yellow fever is a disease with a high impact to the public health system, every detection of YFV needs to be confirmed by a national reference laboratory that, in São Paulo state, is the Adolfo Lutz Institute. All animals were found dead. This study did not require ethics committee approval according to Brazilian normative resolution no. 30, of February 2, 2016, of the National Animal Experimentation Control Council–CONCEA.
This work was funded, in part, by a grant of the FESIMA (São Paulo State Health Secretariat)–Ofício CAF no. 003/2016 and SEI process no. 024.00036429/2025-14.
Author contributions: M.J.L.S. conceptualized, devised methodology, conducted investigations, curated data, co-wrote the first draft and reviewed and edited subsequent drafts, and was responsible for project administration and acquiring funding; J.C.A.D. conducted investigations and processed samples; E.F.M. curated data, co-wrote the first draft, and reviewed and edited subsequent drafts; M.S.C., N.C.C.A.F., and J.M.G. conducted investigations, processed samples, and reviewed and edited manuscript drafts; L.B. and A.B.S. participated in investigations and sample collection; K.W., K.P.B., A.A.B.C., and D.M. reviewed and edited manuscript drafts. B.A.L.F. supervised, conceptualized, devised methodology, and provided resources for the study, reviewed and edited manuscript drafts, served as project administrator, and acquired funding. All authors read, reviewed, and approved the final version of the manuscript.
About the Author
Dr. Siconelli is veterinarian specialized in zoonotic diseases at Unidade de Vigilância de Zoonoses da Divisão de Vigilância Ambiental em Saúde, Brazil. His research interests focus on surveillance programs with emphasis on arboviruses, particularly yellow fever virus and West Nile virus.
Footnotes
Suggested citation for this article: Siconelli MJL, de Almeida Dias JC, Machado EF, Cunha MS, de Azevedo Fernandes NCC, Guerra JM, et al. Yellow fever virus surveillance in Callithrix spp. marmosets during epizootic outbreak, Brazil, 2024–2025. Emerg Infect Dis. 2026 Jun [date cited]. https://doi.org/10.3201/eid3206.251388
References
- 1.Silva NIO, Sacchetto L, de Rezende IM, Trindade GS, LaBeaud AD, de Thoisy B, et al. Recent sylvatic yellow fever virus transmission in Brazil: the news from an old disease. Virol J. 2020;17:9. 10.1186/s12985-019-1277-7 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.Possas C, Lourenço-de-Oliveira R, Tauil PL, Pinheiro FP, Pissinatti A, Cunha RVD, et al. Yellow fever outbreak in Brazil: the puzzle of rapid viral spread and challenges for immunisation. Mem Inst Oswaldo Cruz. 2018;113:e180278. 10.1590/0074-02760180278 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 3.Brazil Ministry of Health. Guide to epizootic surveillance in non-human primates and entomology applied to yellow fever surveillance, 2nd ed. Brasília (Brazil): The Ministry; 2017. [Google Scholar]
- 4.de Azevedo Fernandes NCC, Guerra JM, Díaz-Delgado J, Cunha MS, Saad LD, Iglezias SD, et al. Differential yellow fever susceptibility in New World nonhuman primates, comparison with humans, and implications for surveillance. Emerg Infect Dis. 2021;27:47–56. 10.3201/eid2701.191220 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 5.de Almeida MA, dos Santos E, da Cruz Cardoso J, da Fonseca DF, Noll CA, Silveira VR, et al. Yellow fever outbreak affecting Alouatta populations in southern Brazil (Rio Grande do Sul State), 2008–2009. Am J Primatol. 2012;74:68–76. 10.1002/ajp.21010 [DOI] [PubMed] [Google Scholar]
- 6.Moreno ES, Agostini I, Holzmann I, Di Bitetti MS, Oklander LI, Kowalewski MM, et al. Yellow fever impact on brown howler monkeys (Alouatta guariba clamitans) in Argentina: a metamodelling approach based on population viability analysis and epidemiological dynamics. Mem Inst Oswaldo Cruz. 2015;110:865–76. 10.1590/0074-02760150075 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 7.Bicca-Marques JC, Freitas DS. The role of monkeys, mosquitoes, and humans in the occurrence of a yellow fever outbreak in a fragmented landscape in south Brazil: protecting howler monkeys is a matter of public health. Trop Conserv Sci. 2010;3:78–89. 10.1177/194008291000300107 [DOI] [Google Scholar]
- 8.Fernandes NCCA, Cunha MS, Guerra JM, Réssio RA, Cirqueira CDS, Iglezias SD, et al. Outbreak of yellow fever among nonhuman primates, Espírito Santo, Brazil, 2017. Emerg Infect Dis. 2017;23:2038–41. 10.3201/eid2312.170685 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 9.Moreno ES, Spinola R, Tengan CH, Brasil RA, Siciliano MM, Coimbra TL, et al. Yellow fever epizootics in non-human primates, São Paulo State, Brazil, 2008–2009. Rev Inst Med Trop São Paulo. 2013;55:45–50. 10.1590/S0036-46652013000100008 [DOI] [PubMed] [Google Scholar]
- 10.Vieira WT, Gayotto LC, de Lima CP, de Brito T. Histopathology of the human liver in yellow fever with special emphasis on the diagnostic role of the Councilman body. Histopathology. 1983;7:195–208. 10.1111/j.1365-2559.1983.tb02235.x [DOI] [PubMed] [Google Scholar]
- 11.Quaresma JA, Barros VL, Pagliari C, Fernandes ER, Guedes F, Takakura CF, et al. Revisiting the liver in human yellow fever: virus-induced apoptosis in hepatocytes associated with TGF-β, TNF-α and NK cells activity. Virology. 2006;345:22–30. 10.1016/j.virol.2005.09.058 [DOI] [PubMed] [Google Scholar]
- 12.Santos DOD, de Oliveira AR, de Lucena FP, de Mattos SA, de Carvalho TP, Costa FB, et al. Histopathologic patterns and susceptibility of neotropical primates naturally infected with yellow fever virus. Vet Pathol. 2020;57:681–6. 10.1177/0300985820941271 [DOI] [PubMed] [Google Scholar]
- 13.Fernandes NCCA, Cunha MS, Suarez PEN, Machado EF, Garcia JM, De Carvalho ACSR, et al. Phylogenetic analysis reveals a new introduction of yellow fever virus in São Paulo State, Brazil, 2023. Acta Trop. 2024;251:107110. 10.1016/j.actatropica.2023.107110 [DOI] [PubMed] [Google Scholar]
- 14.Cunha MS, da Costa AC, de Azevedo Fernandes NCC, Guerra JM, Dos Santos FCP, Nogueira JS, et al. Epizootics due to yellow fever virus in São Paulo State, Brazil: viral dissemination to new areas (2016–2017). Sci Rep. 2019;9:5474. 10.1038/s41598-019-41950-3 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 15.Sousa DER, Wilson TM, Macêdo IL, Romano APM, Ramos DG, Passos PHO, et al. Case report: urbanized non-human primates as sentinels for human zoonotic diseases: a case of acute fatal toxoplasmosis in a free-ranging marmoset in coinfection with yellow fever virus. Front Public Health. 2023;11:1236384. 10.3389/fpubh.2023.1236384 [DOI] [PMC free article] [PubMed] [Google Scholar]