H5N1 bird flu has invaded Nigeria on at least three separate occasions, the first genetic analysis of the virus shows. Each time, wild birds were probably responsible, which means that its continued spread across Africa is likely to be difficult to halt.
After spreading west across Asia, the highly pathogenic virus was identified in Africa in February, in the northern Nigerian state of Kaduna. Outbreaks multiplied quickly, reaching Lagos in April. Last week Taraba became the 14th of Nigeria’s 36 states to report an outbreak.
This inexorable spread has been blamed on the transport of poultry across the country, but Claude Muller of the National Public Health Laboratory in Luxembourg and colleagues have found that this was not always the cause. The team took samples from infected birds at two farms less than 50 kilometres apart in Lagos state. The samples were sequenced at the Erasmus Medical Centre in Rotterdam, the Netherlands, and compared with gene sequences of other samples of H5N1, including the virus from Kaduna. This revealed that the Nigerian samples were three distinct strains, which were too distantly related to have evolved from one another while in Nigeria (Nature, vol 442, p 37).
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“We were surprised to see such wide divergence over such a short time,” says team member Albert Osterhaus of the Erasmus Medical Centre. Finding these three strains means the virus probably reached Nigeria on separate occasions.
This does not completely rule out the possibility that H5N1 arrived in poultry imports, as Nigerian officials and bird conservationists have claimed. “But then you would not expect to see so many different lineages,” says Osterhaus.
“The analysis revealed three strains of the virus, which were too distantly related to have evolved while in the country”
The Nigerian strains were not descended from the strains of the virus common in east Asia, despite a sizeable poultry trade between Nigeria and China. They evolved from the strain that was first observed in Qinghai, China, and then spread across Asia. One Lagos strain is most closely related to H5N1 found in a buzzard in Denmark and swans in Germany, where it was initially found only in wild birds. The other Lagos strain is closest to one from Egypt, which lies on the migration route from Siberia, while the Kaduna strain is closest to H5N1 from central Asia.
“We really have to get our act together on human and animal surveillance,” Osterhaus says. If H5N1 is repeatedly invading African birds, it will be hard to stop its spread. Osterhaus fears people in Africa may already have been infected as a consequence.
