Asymptomatic Orthopoxvirus Circulation in Humans in the Wake of a Monkeypox Outbreak among Chimpanzees in Cameroon

Abstract

Monkeypox virus is a zoonotic Orthopoxvirus (OPXV) that causes smallpox-like illness in humans. In Cameroon, human monkeypox cases were confirmed in 2018, and outbreaks in captive chimpanzees occurred in 2014 and 2016. We investigated the OPXV serological status among staff at a primate sanctuary (where the 2016 chimpanzee outbreak occurred) and residents from nearby villages, and describe contact with possible monkeypox reservoirs. We focused specifically on Gambian rats (Cricetomys spp.) because they are recognized possible reservoirs and because contact with Gambian rats was common enough to render sufficient statistical power. We collected one 5-mL whole blood specimen from each participant to perform a generic anti-OPXV ELISA test for IgG and IgM antibodies and administered a questionnaire about prior symptoms of monkeypox-like illness and contact with possible reservoirs. Our results showed evidence of OPXV exposures (IgG positive, 6.3%; IgM positive, 1.6%) among some of those too young to have received smallpox vaccination (born after 1980, n = 63). No participants reported prior symptoms consistent with monkeypox. After adjusting for education level, participants who frequently visited the forest were more likely to have recently eaten Gambian rats (OR: 3.36, 95% CI: 1.91-5.92, P < 0.001) and primate sanctuary staff were less likely to have touched or sold Gambian rats (OR: 0.23, 95% CI: 0.19-0.28, P < 0.001). The asymptomatic or undetected circulation of OPXVs in humans in Cameroon is likely, and contact with monkeypox reservoirs is common, raising the need for continued surveillance for human and animal disease.

Figure 1.

Map of study sites and historically confirmed human and chimpanzee monkeypox cases. Confirmed human and chimpanzee monkeypox cases in Cameroon are shown, with the year of confirmation noted. Human monkeypox cases were last reported in Mfou district in 1979 in Ekidmekoe village, and an outbreak of monkeypox occurred in captive chimpanzees in 2016. Participants were recruited from the villages of Metet, Nzdefidi, Ndangueng I, and Nkilzok I. Employees of the nearby primate sanctuary were also invited to participate.

Figure 2.

Summary of univariable modeling results. The sampling location refers to the location from which the participant was recruited, either the primate sanctuary or one of the four villages included in the study. The level of education refers to whether the participant had no education or a primary, secondary, or superior level of education. This figure appears in color at www.ajtmh.org.