Investigators are now testing for a range of pathogens including Rift Valley fever, dengue, yellow fever, Crimean-Congo haemorrhagic fever and leptospirosis, the WHO said.
Investigators are hoping to clarify whether the virus behind the outbreak is a mutation of a known disease or an emerging threat.
Test results will determine whether current containment measures are sufficient or whether cross-border co-ordination will be needed to prevent further spread to other regions, the WHO said.
The cases, first reported on March 30, have so far been limited to members of the same household and their close contacts.
The WHO said it is supporting the Ministry for Health in Burundi to “strengthen disease surveillance, field investigation, clinical care, and laboratory diagnosis”.
A spokesperson for Gavi, the organisation which provides vaccines for the developing world, said it was working with the WHO to support the investigation.
“Outbreaks such as these underscore the critical importance of preparedness and investing in the systems that enable rapid response,” the spokesperson said.
A joint United Nations-supported team of experts has been deployed to contain the spread and co-ordinate a response.
It is not the first time Burundi has reported an outbreak of an unidentified pathogen.
In 2023, at least three people died in an outbreak of a disease which killed rapidly, with symptoms including nose bleeds, abdominal pain, high fever, headache, dizziness and vomiting.
The cases led to the town of Baziro, in Burundi’s northwest, being quarantined and surveillance measures being introduced. It is still not clear what disease caused the outbreak.
Burundi remains at risk of infectious disease outbreaks despite recent improvements in sanitation.
The country is currently facing a resurgence of cholera, with over 3500 cases reported last year and continued transmission this year.
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