WASHINGTON (Reuters) - Genetic tests of samples taken from Turkish victims of the bird flu virus show it has made a small change but probably not enough to make it more dangerous yet, researchers said on Thursday. The mutation is one that would be expected in a highly changeable virus, the experts said, and is one that would be predicted to eventually allow it to cause a pandemic. H5N1 avian influenza has caused a burst of human infections in Turkey and has been found in flocks of poultry across the country. It has killed three children in Turkey and infected a total of 18 people, according to Turkish authorities. Globally it has infected just 147 people and killed 78 of them, according to the tally from the World Health Organization, which only includes four of the Turkish cases. Scientists are carefully watching the virus to see if it makes the changes needed to allow it to pass easily from human to human, which could spark a pandemic that could kill millions. Samples from two of the first Turkish victims were sent to a WHO-affiliated laboratory in Britain for analysis. There were two different strains of virus in the bodies of the teenage victims, said Dr. Ruben Donis, team leader of the molecular genetics team of the U.S. Centers for Disease Control and Prevention's Influenza branch. "One was a regular virus like we have seen in poultry in Turkey before -- no surprises there," Donis said in a telephone interview. But half the viruses had a mutation in a protein called hemagglutinin, which influenza viruses use to attach to the cells they infect. EXPANDING ITS RANGE This mutation has been found in the past to allow the virus to infect a greater range of cells via a structure known as sialic acid, Donis said. When researchers have tested flu viruses in the lab, they found this particular mutation gave the virus a better ability to attach to human-like cells. "It is unclear whether the mutation occurred in the person or whether it occurred in the chicken," said Dr. Anthony Fauci, head of the U.S. National Institute for Allergy and Infectious Diseases. "This same mutation was identified in 2003 in Hong Kong and yet did not take off in a way that led to greater transmissibility either from chicken to human or human to human," Fauci said in a telephone interview. Dr. Guenael Rodier, head of the World Health Organization mission to Turkey, said there was no evidence the virus had changed either its infection rate or its pathogenicity. "In local terms (in Turkey) it is not too worrying," Rodier said in an interview. "We are not expecting an explosion." But Rodier and Fauci said it will be important to study the virus carefully and note how and when it changes. This week researchers at the Scripps Research Institute in La Jolla, California, said they had developed a quick test, called a glycan array, that could alert scientists to when the virus changes. "For the first time in human history we have the chance and the tools to watch the virus spreading. In previous pandemics we had to start with the pandemic," Rodier said. The H5N1 virus remains largely a virus that affects birds. But all influenza viruses mutate and evolve very easily, and regularly change into what are known as pandemic strains, which spread rapidly around the world, infecting and killing unusually large numbers of people.