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, a worrying prospect. It also suggests the viral strain in northern Vietnam may be more infectious but less virulent than elsewhere, i.e. more easily spread but less likely to cause death in sufferers.
“While 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,” 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 influenza or evolve to create a deadly and easily transmissible pandemic strain that could sweep across the globe, killing millions.
“Things are getting more and more worrisome,” says flu expert John Oxford, at Queen Mary University of London, UK. “It does look as though the virus is moving towards being less virulent but more infectious – if that’s the way it’s going, that’s not good news at all.”
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 “suggesting that milder H5N1 infections are occurring”.
“A virus with a mortality rate of 70% is never going to get very far,” 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’s 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.
Analysis also revealed that drug resistance to the flu drug oseltamivir had partially evolved in one patient treated with the medicine. “The community emergence and spread of viruses resistant to oseltamivir, if it were to occur, would have significant implications for prevention and control,” warns the document.
However, Oxford believes this is less concerning than the other changes. He says full drug resistance in human flu strains somewhat “cripples” their effectiveness in spreading.
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 “still hasn’t exploded”, notes Oxford. “We have more time to prepare ourselves,” he told New Scientist. But countries without a pandemic preparedness plan “should wake up”, he says, “otherwise it’ll be like a tsunami wave and we’ll all be caught in it”.
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2005 Flu Pandemic Start Signal Recombinomics Commentary May 19, 2005 >> During 15 - 25 April, 2005, a WHO team of expert consultants was invited by the authorities in Viet Nam to assess the current H5N1 situation, especially in Northern Viet Nam. Based on the epidemiological information presented, this team concluded that the epidemiology of H5N1 could be changing in that part of the country and that the risk for pandemic influenza could have risen. Therefore a second urgent Expert Consultation was held involving a group of epidemiologists, virologists, public health and animal health experts with international experience, and country representatives from Cambodia, Thailand, and Viet Nam. << The above sequence of events appears to confirm the start of the 2005 bird flu pandemic. The dates above coincide with media reports indicating that the Institute of Hygiene and Epidemiology in Hanoi had collected 1000 serum samples from people and animals in northern Vietnam. These samples would seem to have included samples from the outbreaks in Quang Binh in the central highlands, as well as the northern Vietnam clusters in Thai Binh, Haiphong, and Quang Ninh. Media reports of these outbreaks strongly suggested that these samples would be positive. The samples were sent to the CDC in Atlanta for analysis, and the holding of a second urgent Expert Consultation strongly implies that the number of positives in the 1000 samples was significant. These epidemiological data were then supported by H5N1 sequence data which further signaled significant genetic changes, leading to the disclosure of the information in the report on the May 6-7 Manila meeting. The data are certainly consistent with the start of the 2005 pandemic. http://www.recombinomics.com/News/05190504/H5N1_Pandemic_Signal.html
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