Ongoing mpox outbreak in Kamituga, South Kivu province, associated with monkeypox virus of a novel Clade I sub-lineage, Democratic Republic of the Congo, 2024

A virus is spreading. A test exists to catch it. The test doesn't work, not because it's broken, but because the virus quietly deleted the piece of its own genome that the test was designed to find. That's the situation Leandre Murhula Masirika and colleagues uncovered in eastern Democratic Republic of the Congo in early 2024. A novel sub-lineage of monkeypox virus Clade I, circulating in the town of Kamituga, had lost the exact genomic target that the most widely used rapid diagnostic was built to detect. The outbreak itself, on the surface, looked like a familiar problem. The Democratic Republic of the Congo has lived with mpox, or monkeypox, for decades. But since the start of 2023, something shifted. By November 12th of that year, national surveillance had recorded twelve thousand five hundred sixty-nine suspected mpox cases countrywide, the highest annual total on record, with a case fatality rate of four point six percent. New cases were appearing in parts of the country where the disease had not been seen before. Kamituga, in the Mwenga territory of South Kivu province, started reporting cases from September 2023 onward. Masirika and colleagues investigated ten patients hospitalized at Kamituga Hospital. All were young adults, late teens to mid-twenties, with an even split between male and female, and the majority were sex workers. Samples were sent to the Institut National de la Recherche Biomédicale in Goma for laboratory confirmation. All ten tested positive for monkeypox virus by polymerase chain reaction, or PCR. Standard case definitions were used throughout: a suspected case meant acute fever, headache, muscle pain, and back pain followed by a spreading rash; a confirmed case required PCR laboratory confirmation. On paper, this looked like routine mpox surveillance catching a local cluster. It wasn't. To find out what was actually circulating, the team sequenced the virus directly. Samples from January 2024 were processed through a targeted amplicon approach, a method that uses pre-designed PCR fragments, each about two thousand five hundred base pairs long, to enrich specifically for viral DNA before sequencing. This matters in a resource-limited setting. You're not drowning in human DNA; you're pulling out the pathogen. Libraries were prepared and run on an Oxford Nanopore sequencer, chosen for its portability and lower infrastructure demands. Raw reads were basecalled, trimmed, and aligned to a reference genome, and consensus sequences were called and quality-checked using NextClade. Six of the ten patients yielded near-complete monkeypox virus genomes, with coverage ranging from ninety-three point five to one hundred percent and an average of ninety-five point two percent. Those six genomes were then placed into a family tree alongside one hundred thirteen African reference monkeypox virus genomes from GISAID, plus two European sequences from the 2022 global outbreak. Trees were built with IQ-TREE and confirmed through the NextStrain pipeline. Every analysis told the same story. The six Kamituga sequences clustered together on a single branch emerging near the ancestral root of Clade I, distinct from all previously described Clade I lineages. They were not interspersed among known Clade I clusters from prior outbreaks. They sat apart, together, as their own monophyletic group. Clade I is the more severe, historically Central African form of the virus, while Clade II is the strain behind the 2022 global outbreak. Finding a novel sub-lineage within Clade I isn't just a taxonomic footnote. It signals a separate evolutionary history and a distinct introduction. Masirika and colleagues interpret the placement as most consistent with a new spillover event from a zoonotic reservoir into the Kamituga population. The six genomes also weren't identical to each other. Several single nucleotide polymorphism differences existed among them, suggesting the strain had been circulating locally for some time before sampling. Buried within those genomes was a structural change that would turn out to matter enormously for public health: a deletion of one thousand one hundred fourteen nucleotides that removes an entire gene called OPG032. That deletion is where the diagnostic story begins in earnest. Reverse transcription quantitative polymerase chain reaction, or RT-qPCR, the standard molecular test, works by having short primer and probe sequences bind to a specific genomic target. If the target isn't there, the test returns negative. The United States Centers for Disease Control and Prevention developed a widely used Clade I-specific real-time PCR assay that targets the C3L region, which corresponds to the OPG032 gene. In the Kamituga genomes, that region is entirely gone. Masirika and colleagues aligned their six sequences to a Clade I reference using an in-house Primer Check Tool. What the alignment shows is unambiguous. The generic monkeypox virus assay, designed to detect any monkeypox virus, still mostly works. There's a single nucleotide substitution in one generic reverse primer site, the G2R_G reverse primer. The Clade I-specific assay is a different story entirely. The Centers for Disease Control and Prevention's Clade I forward primer, reverse primer, and probe all target the C3L region. In all six Kamituga genomes, that region is absent. Not mutated. Gone. Read depth across the deleted region ranged from seventy-six fold to nine hundred forty-one fold coverage depending on the sample, confirming the deletion is genuine and not a sequencing artifact. Their conclusion is direct. The Centers for Disease Control and Prevention Clade I-specific assay "would lead to failure of the Clade I-specific real-time PCR recommended by the US CDC" and is "most likely not reliable for detection of the novel sub-lineage identified in the current study." In plain terms, a lab running the standard Centers for Disease Control and Prevention clade-typing test on a sample from a Kamituga patient would get a negative result. An active Clade I outbreak, invisible to one of the most trusted diagnostic tools in the field. There's an added layer of irony here. The same OPG032 deletion is present in Clade II viruses, the 2022 global outbreak strain. The Centers for Disease Control and Prevention assay was designed to distinguish Clade I from Clade II precisely by detecting that region, which Clade I was supposed to have and Clade II didn't. Now there's a Clade I lineage that also lacks it, sitting near the base of the Clade I tree, with a deletion that would cause the test to misclassify or miss it entirely. The conclusion Masirika and colleagues draw from all of this is concrete: broader whole-genome sequencing of monkeypox virus cases across the Democratic Republic of the Congo, combined with rapid public sharing of genomic data, is the only way to stay ahead of a virus that can evolve out from under the tests designed to catch it. The Kamituga investigation started with ten patients and produced six genomes. That was enough to identify a novel sub-lineage, detect a diagnostic-breaking deletion, and flag a surveillance gap that could affect laboratories worldwide running the standard assay. Imagine what systematic sequencing across the Democratic Republic of the Congo would reveal. The team deposited their consensus sequences to GISAID and uploaded raw data to the European Nucleotide Archive under study identification ERP one five seven four three nine. That act of rapid data sharing is part of the argument the paper makes. A diagnostic test is only as good as the genomes used to design it. If the only sequences shaping global diagnostic tools come from older outbreaks in better-resourced settings, those tools will be calibrated to yesterday's virus. The Kamituga sequences showed that Clade I, the more severe lineage, is not static. It's moving. It's diversifying. And at least one of its new branches has quietly erased the target that the world's most widely recommended test was counting on finding. Whole-genome sequencing in the field isn't just an academic exercise; it's surveillance infrastructure. The Kamituga outbreak is the case study for why that infrastructure needs to exist before the outbreak, not after the test fails. This lecture was created by ennepō. Go to https://ennepo.ai to Discover, Create and Follow the latest research in your field. Read when you can. Listen when you want to.