The deadly H5N1 bird flu strain that has been plaguing southeast Asia appears to be evolving in a way that may signal a greater threat of a human flu pandemic, according to a World Health Organization report.
Human cases of avian influenza in northern Vietnam in 2005 are showing an epidemiological pattern and virological features which differ from those seen in the 2004 cases. They also differ from those currently seen in human cases in southern Vietnam and other Asian countries.
In particular, though not yet proven, the differences indicate the virus may have already been transmitted from human-to-human 鈥� a worrying prospect. It also suggests the viral strain in northern Vietnam may be more infectious but less virulent than elsewhere; more easily spread but less likely to cause death in sufferers.
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鈥淲hile the implications of these epidemiological and virological findings are not fully clear, they demonstrate that the viruses are continuing to evolve and pose a continuing and potentially growing pandemic threat,鈥� says the WHO report on influenza A/H5N1 in humans in Asia. The report is based on its consultation meeting of experts in Manila, Philippines on 6 and 7 May.
The great fear is that bird flu could hybridise with human flu or evolve to create a deadly and easily transmissible pandemic strain that could sweep across the globe, killing millions.
鈥淭hings are getting more and more worrisome,鈥� says flu expert John Oxford, at Queen Mary University of London, UK. 鈥淚t does look as though the virus is moving towards being less virulent but more infectious 鈥� if that鈥檚 the way it鈥檚 going, that鈥檚 not good news at all.鈥�
Symptomless cases
The WHO document notes that while 83% of human cases in southern Vietnam die, and 71% of cases in Thailand in 2004. In northern Vietnam, the strain is felling 34% of sufferers, though that would still be devastating if millions of people became infected.
Furthermore, three cases of symptomless infections have also been seen in people who had close contact with confirmed cases in Vietnam 鈥渟uggesting that milder H5N1 infections are occurring鈥�.
鈥淎 virus with a mortality rate of 70% is never going to get very far,鈥� says Oxford, as too many hosts die before infecting large numbers of other people. But he believes that if the virus really has changed in virulence, human-to-human transmission may be the next step.
Other evidence pointing to a change in the virus鈥檚 nature includes more clusters of human cases in north Vietnam compared with the south. The ages of victims has also changed in northern Vietnam, with the average age rising from 17 in 2004 to 31 in 2005. The range of ages affected is also much wider, from babies to people over 80.
Lab observations indicate the virus has changed. Specific gene changes have been pinpointed in the Vietnam strains from 2005, although it is unclear what their significance is.
Crippled virus
Analysis also revealed that drug resistance to the flu drug oseltamivir had partially evolved in one patient treated with the medicine. 鈥淭he community emergence and spread of viruses resistant to oseltamivir, if it were to occur, would have significant implications for prevention and control,鈥� warns the document.
However, Oxford believes this is less concerning than the other changes. He says a beneficial side effect of human flu developing full drug resistance is that, in becoming resistant, the strains become 鈥渃rippled鈥� in terms of their ability to spread.
The WHO recommends that countries ramp up their flu pandemic preparedness plans, stockpile antiviral drugs, and explore ways of making human H5N1 vaccine available to affected Asian countries before a pandemic develops.
Though the situation is concerning, it 鈥渟till hasn鈥檛 exploded鈥�, notes Oxford. 鈥淲e have more time to prepare ourselves,鈥� he told New 杏吧原创. But countries without a pandemic preparedness plan 鈥渟hould wake up鈥�, he says, 鈥渙therwise it鈥檒l be like a tsunami wave and we鈥檒l all be caught in it鈥�.
