An outbreak of bird flu in Nigeria this year stemmed not from a single source of the virus, but rather three distinct strains that entered the country at different times, according to a new study.
Researchers, who published their findings today in the journal Nature, found the different strains most closely resembled those identified in Egypt, Russia and Mongolia.
Some experts said the findings suggested the Nigerian outbreak -- the first in Africa -- was caused by migratory birds carrying the H5N1 bird flu virus and the illegal sales of infected poultry.
"This reinforces that not only is there a natural distribution of the H5N1 virus through migratory birds, there's another intercontinental human distribution system via trade," said Dr. William Schaffner, an infection control doctor at Vanderbilt University who was not connected with the research.
Other scientists were less certain trade played a role. The authors of the study wrote that the routes the virus followed coincided with the flight paths of migratory birds. But they added that the sale of poultry could not be excluded as a cause.
Regardless of the route, scientists agreed the virus followed a more varied path into the country, thus presenting more problems in containing it.
"The fact that you have multiple introductions is a warning to all of us," said Hon Ip, a virologist at the U.S. Geological Survey's National Wildlife Health Center, who was not connected with the research. "Mother Nature is way more complicated than you might initially assume."
The Nigerian outbreak began in February with the detection of the virus at a large farm that raised chickens, geese and ostriches. Africa and Europe are the latest areas to be infected with the virus, which was first detected in China in 1997.
Bird flu rarely infects humans, but scientists fear that a mutation could eventually make it easily transmissible, leading to the possibility of a pandemic. So far, 131 people, primarily in Southeast Asia, have died from the virus, according to the World Health Organization.
The genetic analysis of the Nigerian viruses was led by the Institute of Immunology at Luxembourg's National Public Health Laboratory. The team of researchers analyzed 18 swabs from chickens at two farms in Lagos state in southwest Nigeria and compared them to previously sequenced strains from northern Nigeria, said team member Dr. Claude P. Muller of the Institute of Immunology.
The analysis found that one Lagos strain was genetically similar to a virus found in a chicken in Egypt. The other Lagos strain was most similar to a sample from a swan and duck in Russia. The northern sample was related to a virus found in swans and geese in Mongolia.
"If there are multiple introductions, that means there are several risks in the way the virus can get into Nigeria," said Juan Lubroth, senior officer for the U.N. Food and Agriculture Organization's infectious diseases group.
Lubroth said governments will have to rethink their importation restrictions, inspection methods and hygiene regulations.
"If you let up and think everything is quiet, everything is good and you've done a good job, there is no doubt in my mind, the virus will ooze out of Nigeria," he said.
The genetic analysis showed that the Nigerian strains had significantly mutated compared to strains found at the outbreak's epicenter in Southeast Asia.
"You can see how this thing can continue to rather rapidly change its genetic make-up," said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota.
"From the standpoint of the unknown human pandemic, it's troubling," said Osterholm, who was not connected with the research. "This virus is not sitting still, both genetically and how it's being moved around."