The Nigerian government, accused of failing to contain the H5N1 bird flu virus when it arrived there in February, can seek comfort in the conclusions of a study in Nature tomorrow (6 July).
Its authors say that rather than H5N1 spreading across the country from the initial outbreak, the virus arrived on at least three separate occasions, possibly carried by migratory birds.
On 7 February, Nigeria became the first African state to declare the arrival of the bird flu virus, which was initially detected on commercial poultry farms in the northern state of Kaduna.
On 20 February, the government created a biosafety zone to prevent the virus from spreading to the rest of the country. Yet on 24 February and 8 March, H5N1 appeared on two farms in the southwestern Lagos state.
Following these events, the government was criticised for failing to contain the virus.
Claude Muller of the National Public Health Laboratory, Luxembourg and his colleagues in Nigeria and the Netherlands recovered samples from infected birds in Lagos state and analysed the genetic sequences of a key viral protein.
They compared them to sequences for the same protein from virus samples collected in the north of the country.
The study shows that the southwestern and northern viruses are not closely related, and that neither are the two southwestern viruses closely related to each other.
The virus found on 24 February was most closely related to a virus found in Egypt, while the virus from the 8 March outbreak was related to viruses found in wild swans and buzzards found in Poland, Denmark and Germany.
The researchers conclude that the virus was independently introduced to these three farms.
They point out that the routes of introduction coincide with the flight paths of migratory birds, but they do not rule out the movement of commercial poultry as the source of the later outbreaks.
The H5N1 virus has now reached 14 of Nigeria's 31 states.