Outbreak of human monkeypox in Nigeria in 2017-18: a clinical and epidemiological report

Abstract

Background: In September, 2017, human monkeypox re-emerged in Nigeria, 39 years after the last reported case. We aimed to describe the clinical and epidemiological features of the 2017-18 human monkeypox outbreak in Nigeria.

Methods: We reviewed the epidemiological and clinical characteristics of cases of human monkeypox that occurred between Sept 22, 2017, and Sept 16, 2018. Data were collected with a standardised case investigation form, with a case definition of human monkeypox that was based on previously established guidelines. Diagnosis was confirmed by viral identification with real-time PCR and by detection of positive anti-orthopoxvirus IgM antibodies. Whole-genome sequencing was done for seven cases. Haplotype analysis results, genetic distance data, and epidemiological data were used to infer a likely series of events for potential human-to-human transmission of the west African clade of monkeypox virus.

Findings: 122 confirmed or probable cases of human monkeypox were recorded in 17 states, including seven deaths (case fatality rate 6%). People infected with monkeypox virus were aged between 2 days and 50 years (median 29 years [IQR 14]), and 84 (69%) were male. All 122 patients had vesiculopustular rash, and fever, pruritus, headache, and lymphadenopathy were also common. The rash affected all parts of the body, with the face being most affected. The distribution of cases and contacts suggested both primary zoonotic and secondary human-to-human transmission. Two cases of health-care-associated infection were recorded. Genomic analysis suggested multiple introductions of the virus and a single introduction along with human-to-human transmission in a prison facility.

Interpretation: This study describes the largest documented human outbreak of the west African clade of the monkeypox virus. Our results suggest endemicity of monkeypox virus in Nigeria, with some evidence of human-to-human transmission. Further studies are necessary to explore animal reservoirs and risk factors for transmission of the virus in Nigeria.

Funding: None.

Figure 1

Temporal distribution of suspected, confirmed, and probable human monkeypox cases in Nigeria, 2017–18 (n=276)

Figure 2

Confirmed human monkeypox cases in Nigeria by age and sex, 2017–18 (n=122)

Figure 3

Geographical distribution of confirmed human monkepox cases in Nigeria, 2017–18 (n=122) Discrepancies between state boundaries and the Nigerian border are because ecological data come from a different source and were further georeferenced and digitised. The white area in Borno state is Lake Chad, which was not included as an ecological zone. Discrepancies in the south are caused by the presence of creeks and the shoreline, and the different versions of boundary layers that were available. FCT=Federal Capital Territory.

Figure 4

Frequency of signs and symptoms in people with confirmed monkeypox virus infection in Nigeria, 2017–18 Percentages were calculated individually for each symptom on the basis of the number of patients with available data.

Figure 5

Haplotype network of seven monkeypox samples from Rivers state, Nigeria, in 2017 Circles indicate unique haplotypes (genomic sequences), with smaller circles representing one sample and larger circles representing two samples with identical examined sequences. Numbers in parentheses along the branches show mutations between samples. The examined single-nucleotide polymorphism sequences 3018 and 3019 were identical, as were two samples (one human, one from a prairie dog) from the US outbreak in 2003, which were used as a reference. US samples have a single mutational difference, which was not identified in this analysis because of missing data at that nucleotide position for other samples.