In July 2022, the WHO declared a public health emergency of international concern regarding monkeypox — a viral disease caused by monkeypox virus (MPXV). Interestingly, the 2022 MPXV outbreak seems to be particularly focused in men who have sex with men (MSM) and is displaying characteristics of a sexually transmitted infection (STI), with symptoms often confined to the anogenital region.

The scale of this outbreak is much larger than previous ones; one hypothesis for this is that the outbreak is driven by undiagnosed cases, which might be due to asymptomatic infections (which are, therefore, spread unknowingly) or to unrecognized infection, whereby the signs of MPXV were attributed to another pathogen.

In order to investigate this hypothesis, stored samples, which had been collected for routine oropharyngeal and anorectal gonorrhoea and/or chlamydia tests at the Institute of Tropical Medicine in Antwerp, Belgium, during May 2022, were retrospectively analysed for MPXV DNA. The samples came from 224 men who had symptoms of these STIs or who were at high risk, including MSM with HIV, MSM using HIV pre-exposure prophylaxis and men who were notified of a diagnosis of gonorrhoea and/or chlamydia in a recent partner.

PCR analysis identified four samples that were positive for MPXV; three of these came from men who were asymptomatic and the other from a patient who had reported a rash that had been attributed to a herpes flare-up. The diagnosis of the asymptomatic men was confirmed using a number of further techniques, including whole-genome sequencing and viral isolation.

The detection of previously undiagnosed monkeypox suggests that case finding must be intensified in order to reduce the spread of the virus. In particular, the symptoms and signs of MPXV infection should be better communicated to health-care workers to prevent misdiagnosis, and populations at high risk should be encouraged to keep records for contact tracing and to understand that transmission can be possible even when asymptomatic.