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

Multiple non-endemic countries were recently hit by a monkeypox virus outbreak. While most cases present with lesions in the anogenital region, it is unclear whether asymptomatic or otherwise undiagnosed infections are fuelling this epidemic.

Monkeypox is a viral disease that is endemic in several African countries. Based on what is known about its natural history stemming from outbreaks in the general population in those countries, monkeypox virus (MPXV) is thought to be transmitted through close contact with symptomatic cases and all those infected are assumed to develop symptoms. This implies that an outbreak in the general population tends towards extinction with relatively minor hygienic interventions.

The current outbreak in non-endemic countries has surpassed any preceding outbreak and was recently declared as a public health emergency of international concern by the World Health Organization. It differs from previous outbreaks in that it primarily affects men who have sex with men (MSM) and is linked with sexual contact, that many of them present with symptoms that are largely limited to the anogenital region, and that viral DNA cannot only be found in lesions and saliva but also in anogenital samples.

A hypothetical explanation for this extraordinary surge in cases is that a proportion of monkeypox infections remains undiagnosed because the patient’s signs and symptoms are not attributed to a possible MPXV infection or because patients don’t experience any symptoms. Asymptomatic patients may unintentionally spread the virus and asymptomatic infections may be difficult to contain due to lack of healthcare seeking of infected individuals.

In the Institute of Tropical Medicine in Antwerp, Belgium, hosting a large sexual health clinic, low threshold sexually transmitted infection screening is offered to high-risk groups such as men who have sex with men (MSM) living with HIV, and MSM using HIV pre-exposure prophylaxis (PrEP). Indications for oropharyngeal and anorectal gonorrhoea/chlamydia testing are symptoms compatible with gonorrhoea or chlamydia, notification by a recent sex partner with gonorrhoea or chlamydia, or regular gonorrhoea/chlamydia screening in asymptomatic men at risk. In line with ITM’s common clinical practice, men who deny having symptoms are not clinically examined and self-swab anorectal samples. Oropharyngeal swabs are taken by a clinician and in case of symptoms, examination by a clinician is performed

In this study we retrospectively looked for the presence of MPXV-DNA in stored samples that had been collected for routine oropharyngeal and anorectal gonorrhoea/chlamydia testing from the month of May 2022 to assess whether MPXV infections had remained undiagnosed.

Throughout May 2022, 237 men underwent sampling for anorectal or oropharyngeal gonorrhoea/chlamydia testing. Left-over DNA extracts from samples of 224 men were available for testing by MPXV-PCR. These included two oropharyngeal swabs, 60 anorectal swabs, and 162 pooled samples (the combination of a patient’s first-void urine, oropharyngeal swab, and anorectal swab).

MPXV-PCR was positive on four DNA extracts from four men: three from anorectal swabs, and one from a pooled sample. At the time of sampling, one man suffered from a painful vesicular perianal rash, which was misdiagnosed as a flare-up of herpes simplex and three did not report any symptoms. The four men, between 30 and 50 years old, had a well-controlled HIV infection under antiretroviral therapy, a history of multiple STIs and had not been previously vaccinated against smallpox.

All men were contacted as soon as their retrospective diagnosis was made, and recalled to the clinic for additional case investigation. The three men that had not reported symptoms, returned between 21 and 37 days after sample collection. They were thoroughly questioned about potential monkeypox-related and other symptoms and clinically examined for signs of monkeypox. All denied having noticed any symptoms during the two months prior to the day of the sample collection and up till their return visit. No signs of monkeypox were observed during clinical examination. All reported condomless sexual intercourse with at least one male partner within a few days to one month before and all had sex with at least one partner after the day of the sample collection. According to the men, none of their main partners had reported symptoms of monkeypox. Casual partners could not be traced. Interestingly, one case predated the first detected symptomatic case in Belgium by several days and did not report international travel or participation in mass gatherings, indicating that MPXV had been circulating in Belgium before the first cases were formally detected.

The retrospective diagnosis of monkeypox in the three men with an asymptomatic infection was confirmed by a positive PCR on new DNA extracts of the stored original patient samples and a positive PCR targeting a wider range of orthopox viruses in all three cases, whole genome sequencing recovering 98% of the MPXV genome in the anorectal swab of one case, viral isolation revealing the presence of replication-competent MPXV in the anorectal swab of two cases, and seroconversion of orthopox-directed IgG antibodies in a convalescent serum from the day of their return to the clinic.

While it cannot be excluded that the three asymptomatic men had unnoticed signs of monkeypox at the time of infection, the significance of these cases lies in the fact that they would not have sought medical care if it were not for a scheduled visit for routine HIV follow-up and STI screening, did not self-isolate and had sexual contacts around the time of detectable MPXV DNA.

Replication-competent virus was demonstrated in two of the asymptomatic cases. If prospective studies reveal that patients without (recognized) symptoms may be able to transmit the virus, this implies that undiagnosed infections in the current outbreak may play a much more significant role in terms of overall disease transmission compared to previous orthopox epidemics because of the dense sexual network including anonymous contacts of some MSM, which hampers efficient contact tracing.

The finding of monkeypox cases that remained undiagnosed at the beginning of the epidemic, urges for (1) intensification of case finding and contact tracing, (2) increasing awareness among healthcare workers and individuals at risk that monkeypox symptoms may overlap with those of other diseases and that not all individuals with monkeypox infection notice symptoms and seek medical attention, (3) access to low-threshold testing, and (4) evaluation of the usefulness of pre- or post-exposure vaccination of individuals at highest risk of infection. Testing and quarantining of individuals reporting symptoms may not suffice to contain the outbreak.

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