Scientists identify distinct Ebola lineage behind 2026 outbreak
Genome sequencing suggests that the Bundibugyo ebolavirus strain responsible for the outbreak in the Democratic Republic of the Congo and spillover to Uganda is genetically distinct (variant) from strains responsible for earlier Bundibugyo ebolavirus disease outbreaks.
An international team of scientists, including University of St Andrews’ researcher Dr Wilber Sabiiti, has identified a distinct lineage (variant) of Bundibugyo virus behind the current Ebola outbreak in Central and East Africa.
The findings indicate that the virus is not simply a re-emergence of those sampled during the 2007–08 outbreak in Uganda or the 2012 outbreak in the Democratic Republic of the Congo (DRC).
Researchers from Uganda’s Ministry of Health, the Interdisciplinary Consortium for Epidemics Research and partner institutions – National Institute of Medical Research in DRC, Dalhousie University in Canada and University of St Andrews in the UK used whole-genome sequencing and genome-analysis software to examine the virus.
Their analysis found 219 nucleotide differences between a genome from the 2026 outbreak and genomes from Uganda’s 2007–08 outbreak. It differed from the 2012 DRC genomes at between 216 and 227 positions, approximately 1.2 percent of the comparable sequence.
The changes place the 2026 virus on a distinct branch of the Bundibugyo virus family tree, according to the studies published in The Lancet and Nature Medicine.
Several changes affected viral proteins involved in processes such as entering human cells, copying viral RNA and forming new virus particles. However, the researchers cautioned that genetic differences alone do not show that the virus is more transmissible, causes more severe disease or can evade existing diagnostic tests or experimental medical countermeasures. Clinical signs reported during the current outbreak appear broadly consistent with those recorded in 2007 and 2012. Early symptoms are non-specific and may include fever, fatigue, muscle pain, headache and sore throat, followed in some patients by gastrointestinal illness, organ dysfunction and bleeding. Researchers have not yet established whether the 2026 lineage produces a clinically distinct pattern of disease. Further laboratory and epidemiological studies will be needed.
A companion evolutionary analysis published in the Journal of Infection in Developing Countries compared Bundibugyo virus with Sudan virus and Ebola virus, formerly known as Zaire ebolavirus. The researchers examined the viruses’ “molecular clocks”, the genetic changes that accumulate over time and help scientists reconstruct their evolutionary histories. While historical Bundibugyo virus genomes broadly followed the expected evolutionary pattern, genomes from the 2026 outbreak appeared to have accumulated fewer changes than predicted. This unexpected result raises questions about where and how the virus was evolving before the present human outbreak began. One possible explanation is that the virus persisted for an extended period in parts of a previously infected person’s body where the immune system has limited access. Another is that it entered the human population through a separate transmission event from an animal reservoir, where it had been exposed to different evolutionary pressures. Researchers emphasised that both explanations remain hypotheses requiring further investigation.
Dr Sabiiti, of the Division of Infection and Global Health in the University of St Andrews School of Medicine, said “The genomic evidence tells us that this is not simply one of the viruses detected in 2007 or 2012 returning unchanged. It belongs to a distinct lineage, and its unusual evolutionary pattern raises important questions about where it has been circulating. We cannot yet say whether it persisted in a previous human infection or entered the population through a new animal-to-human transmission event. Answering that question will require genomic sequencing to be combined with epidemiological, clinical and ecological investigations which underscore the importance of investing in genomic surveillance and rapid diagnostic capacity.”