#17,858
Starting in late November we began to see a flurry of reports from public health agencies on concerns over the discovery of sexual transmission of MPox Clade I in the DRC, and reports of a possible case originating in Belgium.
Clade I Mpox (formerly Monkeypox) - which is endemic to central Africa (see map at top of post) - is far more severe than the Clade IIb Mpox virus which began spreading internationally 2022.
Clade I appears more transmissible, can produce more disfiguring lesions, and is associated with a much higher fatality rate.
While we've seen no indication of international spread of the Clade I Mpox virus, the discovery of at least 2 clusters of sexual transmission (in MSM and sex workers) in the DRC raises concerns that - like Clade IIb - this more aggressive strain could eventually turn up outside of Africa.
Even more curious, the index case (as far as can be ascertained) was a traveler from Belgium, who developed symptoms on the day of his arrival in the DRC. The WHO explains:
The first known cases were reported when a man, resident in Belgium and with connections to the Democratic Republic of the Congo, tested positive for clade I in Kenge, Kwango province, during a visit to the Democratic Republic of the Congo. Thereafter, sexual contacts of this case in the Democratic Republic of the Congo also tested positive for clade I MPXV, with closely related viral sequences.
No Evidence for Clade I Monkeypox Virus Circulation, Belgium
To the Editor: As professionals involved in the mpox response in Belgium, we read with concern the report by Kibungu et al. on a clade I mpox outbreak linked to sexual transmission in the Democratic Republic of the Congo (DRC), in March 2023 (1). The authors and the World Health Organization (WHO) reported that the male index case had a sexual encounter with another man in Belgium before traveling to the DRC, where he developed symptoms the day he arrived and tested positive for monkeypox virus (MPXV) 8 days later (2). By that timeline, WHO suggested, the man likely was infected in Belgium. This conclusion raised concerns about clade I MPXV circulation within sexual networks in Belgium and Europe, regions highly affected by the 2022 clade IIb outbreak.
Mpox diagnoses in Belgium were mostly made by using a PCR that does not distinguish between clades (3). After the aforementioned reports, we retested stored samples from 296 mpox patients, 37% of all mpox patients in Belgium, by using a clade I–specific PCR (4). None tested positive.
In addition, from October 2022 onward, few mpox cases were reported in Belgium; none occurred in the 6 weeks before the DRC cluster started. Also, in the 9 months after the DRC outbreak, only 4 mpox cases were detected in Belgium, the earliest of which was diagnosed 12 weeks after the DRC index case (L. Liesenborghs et al., unpub. data). Moreover, during January–November 2023, we screened 2,415 men visiting our sexual health clinic using an MPXV-specific PCR as part of ongoing surveillance to detect undiagnosed or asymptomatic infections (5). We detected only 1 presymptomatic clade IIb MPXV infection.
On the basis of this information, we have no indications that clade I MPXV has been circulating in Belgium. However, sustained vigilance, clade differentiation, and timely outbreak investigations remain crucial to halting potential spread of clade I MPXV through sexual transmission.
While reassuring, this doesn't negate the possibility that Clade I is already circulating undetected in Europe or other regions outside of Africa.
The international spread of the milder Clade IIb virus wasn't discovered until May of 2022, but there is evidence that suggests it may have been circulating at low levels - unnoticed - for years (see APOBEC3 deaminase editing in mpox virus as evidence for sustained human transmission since at least 2016).
While the world reacted with surprise, the international spread of Mpox has been something scientists have been warning about for decades.
Like all viruses, Mpox (Clade I and II) continue to evolve and diversify. This was discussed in the 2014 EID Journal article Genomic Variability of Monkeypox Virus among Humans, Democratic Republic of the Congo, where the authors cautioned:
Small genetic changes could favor adaptation to a human host, and this potential is greatest for pathogens with moderate transmission rates (such as MPXV) (40). The ability to spread rapidly and efficiently from human to human could enhance spread by travelers to new regions.
A 2016 study (see EID Journal: Extended H-2-H Transmission during a Monkeypox Outbreak) looked at a large 2013 outbreak of Monkeypox in the DRC and suggested that the virus's epidemiological characteristics may be changing (possibly due to the waning smallpox vaccine derived immunity in the community).
And in February of 2022 - just months before clade IIb began its well publicized world tour - we looked at a cautionary report published in PLoS NTD: The Changing Epidemiology of Human Monkeypox—A potential threat?) that warned that`. . . monkeypox is gradually evolving to become of global relevance.'
