What is H7N2 virus?
H7N2 virus is a subtype of the species Influenza A virus (sometimes called bird flu virus).
While the global health community's attention in recent years has targeted the H5N1 virus, which has killed more than 180 people worldwide since 2003, some experts worry that attention is being diverted from seemingly less dangerous bird flu subtypes like H7.
"There may be a bit of complacency when it comes to recognizing the pandemic potential of H7 viruses," World Health Organization bird flu expert Dr. Michael Perdue said recently.
One person Virginia, US in 2002 and one person in New York, US, in 2003 were found to have serologic evidence of infection from the H7N2 virus; both fully recovered.
In February 2004, an outbreak of low pathogenic avian influena (LPAI) A (H7N2) was reported on 2 chicken farms in Delaware, US and in four live bird markets in New Jersey, US, supplied by the same farms. In March 2004, surveillance samples from a flock of chickens in Maryland, US, tested positive for LPAI H7N2. It is likely that these were the same strains.
A Centres for Disease Control and Prevention (CDC) study following the 2002 outbreaks of H7N2 in commercial poultry farms in western Virginia concluded:
An important factor contributing to rapid early spread of AI virus infection among commercial poultry farms during this outbreak was disposal of dead birds via rendering off-farm. Because of the highly infectious nature of AI virus and the devastating economic impact of outbreaks, poultry farmers should consider carcass disposal techniques that do not require off-farm movement, such as burial, composting, or incineration.
On 24 May 2007, an outbreak of H7N2 was confirmed at a poultry farm near Corwen, in Wales from tests on chickens that died from the H7N2 virus. The owners of the Conwy farm bought 15 Rhode Island Red chickens two weeks prior but all died from H7N2.
The 32 other poultry at the site were slaughtered. A one kilometer restriction zone was put in force around the property in which birds and bird products cannot be moved and bird gathering can only take place under licence.
Nine people who were associated with the infected or dead poultry and reported flu-like symptoms were tested. Four tested positive for evidence of infection from H7N2 and were successfully treated for mild flu.
Officials said the number of confirmed human cases could increase. Those who are thought to have had contact with H7N2 are being offered antivirals by health authorities.
Having so many human cases at once is a potential concern. In Asia, where the H5N1 virus has circulated most widely, millions of people have been exposed to millions of infected birds, resulting in about one new infection per week.
"Here, we're talking about a small number of birds and yet we still have four cases," Perdue said. "Unless there's something unusual about the contact with birds, that suggests the virus is finding new ways of getting into humans."
The H7 subtype has previously sparked human outbreaks. In an outbreak in the Netherlands in 2003, 89 human cases were reported, mostly of conjunctivitis, as well as one death. There were also at least three likely instances of human-to-human transmission involving family members of poultry workers. In the case of the single fatality, officials noticed that particular virus had about 10 mutations.
British officials have been quick to reassure the public that the "low
pathogenic" H7N2 virus — in comparison to the "highly pathogenic"
H5N1 virus — poses little risk to the population. In fact H7N2 appears
to cause only mild symptoms such as eye infections.
But low pathogenic viruses can quickly morph into highly pathogenic ones, sometimes within weeks. Too little is known about flu viruses to predict with any certainty which ones are likely to be most lethal for humans.
"The pandemic risk from low pathogenic avian viruses is almost as bad as that from highly pathogenic avian viruses," said Dr. Angus Nicoll, an influenza expert at the European Centre of Disease Prevention and Control.
"When people say low pathogenic or highly pathogenic, that only refers to how unpleasant the disease is for birds," Nicoll explained. "That's almost irrelevant for humans."
Like all flu viruses, low pathogenic viruses mutate rapidly, and could theoretically transform into a pandemic strain without the warning signals of a more virulent strain, which would leave many dead chickens — and perhaps humans — in its wake. Experts are worried about the possibility of a bird flu virus mixing with a human flu virus to create a new pandemic strain.
"If you have an H7 virus causing mild symptoms, that might give the virus the chance to reassort into a more dangerous virus before anybody notices," Perdue said. And for health officials hoping to quash a pandemic in its emerging stages, it might be too late to contain a global outbreak without an early warning.
Most experts believe that the preoccupation with H5N1 as the most likely pandemic candidate is justified. However, "the last two flu pandemics were the result of a human flu virus recombining with low pathogenic avian viruses," said Perdue. The H7N2 recently detected in Britain would fall into that category. "Given that historical context, perhaps we should concentrate our efforts a little more in that direction."
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