The threats caused by the H7 sub-type of the highly pathogenic avian influenza (HPAI) virus have been overlooked recently as the result of our preoccupation with the H5N1 virus.
South Korea announced its now ‘traditional’ first winter outbreak of avian flu on 23 November 2007, almost exactly one year after H5N1 sub-type was confirmed in 2006 in North Cholla province, South Korea’s biggest supplier of poultry products. Indeed, it was the Christmas 2003 (24th December) outbreak in South Korea that officially ‘kicked off’ the ensuing H5N1 pandemic in Asian poultry although the disease was already raging across the region by then.
This time around, the culprit virus is not H5N1 but a low pathogenic H7 influenza A virus (further sub-typing not specified), but this did not stop South Korea acting characteristically quickly by culling thousands of ducks at four southern farms. Officials say the virus turned up on a farm near the city of Gwangju, about 250km south of Seoul. They destroyed 3800 ducks on that farm and over 12,000 birds at three other farms in the area as a precaution.
There has been little reaction from the international press apparently pre-occupied by H5N1. They appear to dismiss the danger of these H7 virus sub-types to poultry and people. H7 cropping up in South Korea should come as no surprise. The very first H7 outbreak in Asia occurred in North Korea in March 2005, killing over 200,000 birds.
The H7 virus – including H7N7, H7N3, H7N2 and H7N1 sub-types as both low pathogenic and high pathogenic forms – has been responsible for large losses in the world poultry industry over the last ten years.
A 2003 outbreak of H7N7 HPAI in Western Europe resulted in 30 million birds culled in the Netherlands with 2.7 million and 420,000 more in neighbouring Belgium and Germany, respectively
H7N3 has caused big losses in a number of countries including Pakistan, Chile and Italy since 2000. The single biggest outbreak was probably in 2004 along the Fraser Valley in British Columbia, Canada, when the virus changed from low pathogenic to high pathogenic form during course of the outbreak and led to loss of 19 million birds.
Eastern United States appears to be favourite stomping ground for H7N2. Most influenza A viruses originating in birds usually occur as low pathogenic (LP) strains but all have the potential to mutate into highly pathogenic (HP) types during the course of an outbreak. Even nominally LP strains of H7N2 may result in moderate mortality in poultry.
H7N2 appeared in Pennsylvania in 1997/8, destroying 2.5 million birds during an ensuing depopulation management strategy before it was brought under control. H7N2 also caused an outbreak in Virginia, West Virginia and North Carolina in 2002. Originally characterised as LP, 20% of the area’s poultry farms tested positive for H7N2 before it was contained, and 4.7 million chickens and turkeys were destroyed. Another 56 million birds were at risk. A 2003 outbreak in Connecticut was potentially devastating too but on this occasion, poultry vaccination was used as an alternative to protect the state’s huge layer flocks.
One of the biggest single H7N1 events was in Italy (1999-2001) when a combination of LP and HP forms affected 17 million birds, many of which were culled.
Following the latest H7 outbreak in South Korea, the international press appeared to play down or even deny any potential for human infection, with comments like “H7 is not contagious to humans” and “There is no evidence that H7 affects humans.” Documented evidence shows otherwise.
The Dutch outbreak of H7N7 in 2003 killed a 57-year-old veterinarian. According to CIDRAP, follow-up research indicated high rates of transmission of the virus from chickens to people and secondary transmission from person to person. At least 50% of people exposed to infected poultry in the outbreak were later found to have H7 antibodies, according to a 2004 report by the Dutch National Institute for Public Health and the Environment (RIVM).
Furthermore, 59% of people who had no direct contact with infected poultry but household contact with an infected poultry worker possessed H7 antibodies, according to the RIVM. Researchers estimated that 1000-2000 people were infected with H7N7 in that outbreak, many times more than the 89 cases officially reported. They went on to suggest the population at risk from H7N7 avian influenza was not limited to those in direct contact with infected poultry, and that person-to-person transmission may have occurred on a large scale.
H7N3 has a lesser history in the human dimension but should not be underestimated as Canadian authorities pointed out following the most recent outbreak in Saskatchewan in September 2007. Dr Stephens of the Food Inspection Agency said H7N3 is not normally associated with serious illness but, like other H7 sub-types, it has the potential to produce mild human illness. This is exactly what happened in previous H7N3 outbreaks in Canada when poultry workers suffered infection that caused inflammation of the eyes and conjunctivitis.
Considering the number of H7N2 outbreaks around the eastern United States, activity of the sub-type within the human population has been negligible, but the behaviour of the virus during a recent outbreak in the United Kingdom (UK) changed all that. In April 2007, an outbreak of H7N2 started as infection of a few backyard chickens purchased at a local market but quickly moved into the human population, spreading between people and eventually infected more people than poultry. Health authorities rushed to reassure the UK population that this virus was LP. Nevertheless, media reports suggest some of the illness caused was extremely serious. One infected couple said they thought they were going to die.Virologists and influenza experts are perpetually asked about the risk of influenza A viruses (including the avian influenza viruses) changing sufficiently to produce strains capable of causing a human pandemic. Most agree that H5N1 still poses the greatest risk but are quick to add that other subtypes including H7 influenza A viruses have the same potential.