From the journal “Nature”:
Scientists studying virus samples from the human outbreak of avian influenza in Turkey have identified three mutations in the virus’s sequence. They say that at least two of these look likely to make the virus better adapted to humans.
The Turkey outbreak is unusual, because of the large family clusters of cases; the fact that many of those infected have only mild symptoms; and the speed with which infections have arisen — 20 cases, including four deaths, in less than two weeks.
So scientists are urgently trying to establish whether the virus is behaving differently in this outbreak from previous ones in Asia. In particular, international teams are investigating the possibility that the virus is moving between people.
“With such a large number of cases within such a short period of time, human-to-human transmission is something that we’ve had to consider,” says Maria Cheng, a spokeswoman at World Health Organization (WHO) headquarters in Geneva.
One of the mutations has been observed twice before — in a father and son in Hong Kong in 2003, and in one fatal case in Viet Nam during 2005. It increases the virus’s ability to bind to human receptors, and decreases its affinity for poultry receptors, making strains with this mutation better adapted to infecting humans.
The Turkey strains are the first in which polymerase and
receptor-binding mutations have been found together. They could make it easier for humans to catch the virus from poultry. But they might also favor human-to-human transmission. This is because the polymerase change helps the virus to survive in the cooler nasal regions of the respiratory
tract, and the haemagglutinin mutation encourages the virus to target receptors in the nose and throat, rather than lower down in the lungs. The virus is thought to be more likely to spread through droplets coughed from the nose and throat than from infections lower down.
Hay points out, however, that it is difficult to predict how the mutations will actually influence the virus’s behaviour. He adds that just two changes are unlikely to create efficient human-to-human transmission on their own.
Establishing what effects these changes are having on the epidemiology of the current outbreak is a top priority for research teams working in Turkey.
“We must learn more about the mild cases and be absolutely sure of whether these viruses are behaving differently from those we have seen elsewhere,” says Hay. “It is early days in terms of what we know about the
viruses causing these infections.”
Researchers are sequencing more strains from the Turkish cases, to see whether they share the mutations and to check for further changes. Samples were expected to arrive in London on Wed 18 Jan 2006, after being held up for more than a week in Turkey because of the Eid-ul-Adha holiday period.
