off events. Scientists were still in the dark about what sort of genetic changes were needed for the virus to become easily transmissible between humans, underlining the need for close surveillance and thorough investigation of all cases. Nevertheless, there was so far "no evidence ... from any outbreak site that the virus has increased its ability to spread easily from one person to another," the WHO said. While there was no sign yet that the virus was becoming a greater human threat, there were indications that changes in H5N1 were facilitating its spread amongst animals. Studies had shown that the viruses were becoming progressively more lethal in experimentally infected chickens and mice and could last some three times as long -- six days instead of two -- in the environment at warm temperatures. There were also indications that some of these changes were becoming fixed, making the virus genetically more stable and increasing its ability to travel and be re-introduced by migratory birds returning to breeding grounds. The virus seen in Nigeria, Iraq, Turkey, Russia, Mongolia and Kazakhstan was very similar to the one found in China's Qinghai Lake nature reserve in April 2005, it said. --SP 21 44 Local Time 18 44 GMT
