Nigeria has been infected multiple times with H5N1 bird flu probably carried by migratory birds from southern Russia and northern Europe or introduced by imported chickens, scientists said on Wednesday.

After analysing samples from infected birds on two farms in south-western Nigeria they found the viruses were genetically distinct from each other and from H5N1 found in the north of the country.

So rather than one strain spreading through the country, different strains had been introduced on separate occasions, which could make controlling the spread of the virus more difficult.

“It strongly suggests that the virus in the north and the virus in these two southern farms have been independently introduced into Nigeria,” Claude Muller, of the National Public Health Laboratory in Luxembourg, said in an interview.

The analysed samples were similar to strains found in southern Russia and northern Europe but not from southeast Asia, which has been hardest hit by avian flu.

Muller added that the H5N1 virus has infected nearly half of the 31 federal states in Nigeria so measures such as mass culling and vaccination, as well as international assistance, will be needed to contain or eradicate the virus.

“For the countries in western Africa I think it will be very difficult for them, just on their own, to efficiently combat these viruses,” he said.

Nigeria was the first country in Africa to be hit by bird flu when ducks tested positive for H5N1 in February. Since then Niger, Egypt, Burkina Faso and Cameroon have also had outbreaks.

There have been no human cases of the infection in Nigeria but Muller said it is not easy in Africa to monitor humans in an efficient way to exclude the virus. Each strain of the virus poses an added risk to people.

Muller said the poultry industry is second only to oil production in Nigeria and is particularly vulnerable to avian virus because chickens are imported from all over the world.

The country also has several bird sanctuaries along the flight paths that link Nigeria with southern Russia and Europe and western Asia.

“These migratory birds could be a source of introduction but this we cannot prove it,” said Muller, adding that the other possibility is trade.

Scientists fear H5N1, which has infected 229 people since late 2003 and killed at least 131, could mutate into an easily transmissible form in humans that could cause a pandemic.

Muller and his team, who reported their findings in the journal Nature, work with scientists at the University of Ibadan in Nigeria and the World Health Organisation to monitor poultry viruses.

Within four days of the first case being reported in the African nation they had set up a laboratory to analyse samples.