Zika cases found in Guineau-Bissau do not stem from the virus strain linked to a surge in birth defects in Latin America, the World Health Organization said.
When Guinea-Bissau announced in early July that it had recorded several cases of Zika, it was believed to be the second country in West Africa hit by the so-called Asian strain of the virus after Cape Verde.
That strain has been spreading like wildfire in Latin America since 2015, and has more recently taken hold in Asia, with researchers warning Friday that 2.6 billion people worldwide were in danger of infection.
But in a report published late Thursday, WHO said that "in Guinea-Bissau, the gene sequencing results of the four confirmed Zika cases sent in July have preliminarily confirmed that the cases are of the African lineage."
This, it said, means that the cases were "not (from) the predominant global outbreak Asian lineage."
The African strain of the Zika virus, which takes its name from Uganda's tropical Zika forest where it was first discovered in 1947, has been widespread on the continent since then.
But until recently, Zika caused little concern, as it usually led only to mild, flu-like symptoms, with many Africans appearing to have built up immunity against the virus.
It remains a mystery whether immunity to the African Zika strain can offer protection against the Asian strain.
Benign in most people, Zika has been linked to a form of severe birth defect called microcephaly which causes newborns' heads to be abnormally small.
It can also cause rare adult-onset neurological problems such as Guillain-Barre Syndrome (GBS), which can result in paralysis and even death.
'Further surveillance needed'
In its report, WHO said "investigation of five reported cases of microcephaly" was going on in Guinea-Bissau.
While the African strain of the virus found in the country had "not been associated with microcephaly and other neurologic complications, further surveillance is needed," the UN health agency said.
Since the spread of Zika did not spark concern before the current outbreak began last year, WHO pointed out that little work had been done previously to track the virus and there had been only "very few confirmed cases of the African lineage."
Peter Salama, the WHO's chief on outbreaks and health emergencies, told reporters Friday that the microcephaly cases in Guinea-Bissau had been detected even earlier than the Zika cases, stressing that it remained unclear if they had any connection with the virus.
"At the moment, we don't know the answer," he said.
In an outbreak that started mid-2015, more than 1.5 million people have been infected with Zika in Brazil, and more than 1,600 babies born with abnormally small heads and brains.
Seventy countries and territories have reported local mosquito-borne Zika transmission, with Brazil by far the hardest hit.
WHO declared the outbreak an international public health emergency last February, and the UN agency said Friday that that assessment still stands.
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