To the Editor:
Mpox virus (MPXV) clade Ib emerged in South Kivu in the Democratic Republic of the Congo (DRC) in 2023 1. The emergence of clade Ib is characterized by rapid eastward spread from DRC into Uganda, Burundi, Rwanda, Kenya and Zambia, and travel associated cases outside Africa2. The Ministry of Public Health of the Republic of the Congo (RoC) declared a mpox epidemic on April 24, 2024. Suspected mpox cases were identified by epidemiological teams of the Ministry of Health.
Between August 2024 and March 2025, 39 mpox cases were identified, 16 MPXV clade Ia and 23 MPXV clade Ib. All clade Ib cases were from Brazzaville and Pointe-Nore, mostly adult males (16/23); median age was 32 years, with one pediatric case, unlike clade Ia which included several children (Appendix Figure S1). Clinical manifestations included fever, genital lesions, skin lesions, and lymphadenopathy. Epidemiological investigations revealed several potential transmission pathways: three cases reported prior transactional sexual activities before symptom onset, three cases had direct contact with symptomatic individuals, one case reported exposure through shared hospital bathroom facilities, and two cases had travel-related exposure.
A notable transmission event linked to Kinshasa, DRC, case MPOX-25-CG-002 traveled from Kinshasa to Pointe-Noire via Brazzaville, staying with MPOX-25-CG-005 and developing lesions by day five. He returned to Brazzaville for treatment at a hospital, where he shared a bathroom with MPOX-25-CG-018, then a further household transmission to MPOX-25-CG-020. This highlights a transmission chain originating in Kinshasa and involving multiple contacts across RoC locations (Appendix Table S1).
Whole-genome sequencing yielded eight full MPXV genomes. Phylogenetic analysis (Figure 1B) revealed that the MPXV clade Ib sequences segregate into two clusters. The first cluster is comprised of two sequences: one from Kinshasa, DRC, and one from this study (MPOX-24-CG-000), from July 2024. The second cluster consists of sequences obtained between December 2024 and March 2025, and two sequences from Kinshasa, DRC, further supporting trans-border transmission dynamics.
Figure 1:
A. Map of the Republic of the Congo (RoC), with departments with mpox positive cases in blue, and grey represent no case. Size of the circle correspond with the number of Clade Ib cases. B. Phylogenetic analysis of MPXV nucleotide sequences including historic and contemporary clade I sequences. The tree was visualized in FigTree (http://tree.bio.ed.ac.uk/software/figtree/) and the phylogenetic tree were rooted using Clade II MPXV sequences. MPXV Clade Ib sequences obtained in this study are indicated in red. Whereas, historic RoC MPXV Clade 1a sequences are indicated in purple (before 2023), blue (2023) and green (2024), and other sequences are in black. Node labels represent bootstrap values greater than 69, 1,000 replicates. Scale bar indicates nucleotide substitutions per site.
Historically, mpox in RoC has been exclusively caused by MPXV clade Ia, primarily arising from zoonotic transmissions and limited human-to-human transmission 3. Previous genomic analyses from our group indicated similarities between MPXV clade Ia sequences from RoC and DRC, suggesting transborder transmission 3. The phylogenetic clustering of MPXV clade Ib sequences from RoC and Kinshasa, alongside epidemiological evidence suggests Kinshasa as the likely origin and highlights an elevated risk of trans-border pathogen transmission. While sexual contact appears to be the predominant mode of MPXV clade Ib transmission 4, alternative transmission routes, such as nosocomial and household/community transmission, are also plausible.
This study represents evidence of clade Ib introduction in RoC on two independent occasions, where the second introduction resulted in a cluster of human-to-human transmission. The detection of MPXV clade Ib in RoC represents a westward spread and a significant increase of the geographical “at risk” area in Africa of MPXV clade Ib, emphasizing the need for enhanced cross-border surveillance and coordinated public health interventions.
Supplementary Material
Data availability and Visualization
All novel sequences reported are submitted to GISAID.
This research was supported by the Intramural Research Program of the National Institute of Allergy and Infectious Diseases (NIAID), National Institutes of Health (NIH).
The contributions of the NIH authors were made as part of their official duties as NIH federal employees, are in compliance with agency policy requirements, and are considered Works of the United States Government. However, the findings and conclusions presented in this paper are those of the authors and do not necessarily reflect the views of the NIH or the U.S. Department of Health and Human Services.
Footnotes
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Ethical approval
The collection of these samples was established through a national surveillance program initiated by the Ministry of Health and Population of the RoC, under the ethical approval from the Congolese Foundation for Medical Research Ethics committee (Avis n°053/CEI/FCRM/2024).
Disclosure forms provided by the authors are available with the full text of this letter at NEJM.org.
References
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