Nigeria was the first African nation to report H5N1 avian influenza, and now scientists have found evidence that the virus was introduced into the country at least three separate times, possibly by migratory birds.

Researchers compared the genetic makeup of H5N1 virus samples from three poultry farms: one in the north and two near Lagos in the southwest. The say their analysis of the samples, reported in the Jul 6 issue of Nature, shows three distinct genetic lineages.

"We . . . find that three H5N1 lineages were independently introduced through routes that coincide with the flight paths of migratory birds, although independent trade imports cannot be excluded," says the report by M. F. Ducatez of Luxembourg and eight colleagues from Luxembourg, Nigeria, and the Netherlands.

The amino acid sequence at the cleavage site of the hemagglutinin protein of the two Lagos H5N1 strains matched that of strains found in Europe, Russia, and central Asia, the reports says.

Further, analysis of the complete hemagglutinin gene of the two Lagos strains showed they most closely resembled strains from Astrakhan in Russia and from Egypt. The sample from northern Nigeria most closely matched a strain from Kurgan in Russia.

Also, the virus from one of the Lagos area farms was more closely related to the northern Nigeria sample than to the virus from the other Lagos area farm, less than 50 kilometers away, the scientists report. The data ruled out a Southeast Asian origin for any of the viruses.

"Our results indicate . . . that the viruses in the southwest [the Lagos area] were not introduced from the north, but that instead they all arrived independently, perhaps at inland waters and key bird areas or through unprotected trade," the report says.

Highly pathogenic H5N1 was first reported in Nigeria at a large commercial poultry farm in February. The authors note that the country is at particular risk because chickens are imported from all over the world without rigorous biosafety standards. Poultry farming is second only to oil production in Nigeria. They also believe the country is at increased risk because it has many bird sanctuaries along two flight paths that connect with southern Russia and Europe and with western Asia.

The potential role of wild birds in spreading avian flu has been controversial. At a conference organized by the United Nations Food and Agriculture Organization (FAO) in May, scientists agreed that wild birds have spread the virus to new areas in some cases, but said poultry trade has been the main transmission vehicle.

The Nature authors say that the H5N1 virus has reached 14 of 31 Nigerian states. "It is difficult to see how the virus might still be contained without depopulation measures, combined with surveillance and large-scale vaccination," they write.

Authorities have expressed deep concern about the impact of avian influenza on Nigeria because many rural families depend on backyard poultry flocks for food and income. Also, efforts to contain avian influenza in Nigeria and other African nations must compete with other major public health concerns such as hunger, HIV, and malaria (see Feb 8 CIDRAP News article).

Ducatez MF, Olinger CM, Owoade AA, et al. Avian flu: multiple introductions of H5N1 in Nigeria. Nature 2006 Jul 6;442(37) [Abstract]