Retrospective detection of asymptomatic monkeypox virus infections among male sexual health clinic attendees in Belgium

Abstract

The magnitude of the 2022 multi-country monkeypox virus (MPXV) outbreak has surpassed any preceding outbreak. It is unclear whether asymptomatic or otherwise undiagnosed infections are fuelling this epidemic. In this study, we aimed to assess whether undiagnosed infections occurred among men attending a Belgian sexual health clinic in May 2022. We retrospectively screened 224 samples collected for gonorrhea and chlamydia testing using an MPXV PCR assay and identified MPXV-DNA-positive samples from four men. At the time of sampling, one man had a painful rash, and three men had reported no symptoms. Upon clinical examination 21-37 days later, these three men were free of clinical signs, and they reported not having experienced any symptoms. Serology confirmed MPXV exposure in all three men, and MPXV was cultured from two cases. These findings show that certain cases of monkeypox remain undiagnosed and suggest that testing and quarantining of individuals reporting symptoms may not suffice to contain the outbreak.

Fig. 1. Phylogenetic tree of the MPXV genome of case 2.

Phylogeny of the MPXV genome of case 2 (ITM_pt31, in blue), in the context of MPXV genomes collected from seven recent cases with monkeypox symptoms at the same institution (submitted to GenBank, in green), a range of MPXV genomes from samples collected in non-endemic countries between 1 April 2022 and 1 July 2022 (downloaded from GISAID, https://www.gisaid.org, in black) and a reference genome from the 2018–2019 outbreak in Israel (MN648051.1, in red). The phylogenetic tree was created by parsnp (default parameters). Branches containing no samples from our institute were collapsed for simplicity. The full version of the tree can be found in Extended Data Fig. 3.

Extended Data Fig. 1. Visualisation of MPXV-PCR template sizes.

Tapestation 4150 (Agilent, Santa Clara, US) was used and the experiment was only performed once. PC: Positive control; NC: Negative control; C: cases 1 to 4, respectively.

Extended Data Fig. 2. Single nucleotide variations in the monkeypox virus genome of asymptomatic case 2.

Single nucleotide variations in the monkeypox virus genome of asymptomatic case 2 (ITM_pt31), as compared to a reference genome from the 2018 – 2019 outbreak in Israel (MN648051.1). Numbers in the top row denote the alignment coordinates of each position. Visualisation by snipit (https://github.com/aineniamh/snipit).

Extended Data Fig. 3. Full version of Fig. 1.

Phylogeny of the monkeypox virus genome of asymptomatic case 2 in the current study (ITM_pt31, in blue), in the context of monkeypox virus genomes collected from seven recent symptomatic cases at the same institution, submitted to Genbank, in green), a range of monkeypox virus genomes from samples collected in non-endemic countries between April 1 and July 1, 2022 (downloaded from GISAID, https://www.gisaid.org, in black), and a reference genome from the 2018 – 2019 outbreak in Israel (MN648051.1, in red). Phylogenetic tree created by parsnp (default parameters).