Nigerian child holds up a chicken outside the family home in Todo village

Scientists say they have found evidence from Nigeria that the deadly H5N1 bird flu strain has entered the country multiple times.

Experts report in Thursday's issue of the journal Nature they believe that separate strains of the virus have entered the country multiple times instead of one strain spreading through the nation.

The scientists say the new findings could make controlling the spread of the virus more difficult.

Scientists analyzed samples from infected birds on several farms in the country and found multiple strands of the virus that were genetically distinct from each other.

The findings do not reveal whether the virus was carried in to Nigeria by migratory birds or by imported birds.

Nigeria was the first country in Africa to be hit by bird flu when it was discovered there in February.

Several other west African countries are also grappling with outbreaks of the disease, including Nigeria's neighbors, Niger and Cameroon.

No human cases have been detected in the region, although the disease is believed to have killed more than 120 people around the world since 2003.