The recent emergence and rapid spread of several new HPAI H5 viruses (H5N8, H5N2, H5N3, and a `new’ H5N1), and the infection of a woman in Taiwan in 2013 with H6N1, has elevated concerns that one of these avian viruses could someday adapt well enough to humans to pose a public health threat.
The good news is, thus far, none have shown an ability to infect and sicken humans. But most our reassurances about their poor infectivity in humans is based on a lack of symptomatic infections among those exposed.
In time we’ll undoubtedly see studies testing for avian strain specific antibodies - that would be suggestive of asymptomatic infection - but for now these viruses appear to present only a minor human health threat.
There is precedent when it comes to human infection with LPAI H5 avian flu viruses, and a few studies we’ve looked at in the past include:
- In 2012, in Taiwan: Three Poultry Workers Show H5N2 Antibodies, we looked at a report that three poultry workers and officials working in animal quarantine have tested positive for antibodies for the H5N2, but all remained healthy and asymptomatic (note: refer to article for other possible causes of seropositivity).
- Some earlier H5N2 studies have been suggestive of prior H5N2 human infection – particularly among poultry workers – including: Human H5N2 avian influenza infection in Japan and the factors associated with high H5N2-neutralizing antibody titer & Serological survey of avian H5N2-subtype influenza virus infections in human populations.
- And in 2009, two H5N2 viruses were isolated in South Korean pigs (see Isolation and genetic characterization of H5N2 influenza viruses from pigs in Korea).
Still, we haven’t seen any solid evidence that H5N2 has produced significant or serious human illness, although the possibility was explored in the J. Chinese Medical Association in 2012 in The threat of highly pathogenic avian influenza H5: will H5N2 infections occur in humans?.
We’ve a new study – conducted in Taiwan in 2012 before the new crop of HPAI H5 viruses emerged - that examined blood samples from hundreds of Poultry Farmers (PFs), Live Poultry Vendors (LPVs), and the general public (NPVs), looking for antibodies to three avian viruses circulating in poultry at that time (H5N2, H7N3, and H6N1).
While they found a fairly low incidence of antibodies overall (max 2.99% for H5N2 in poultry vendors), poultry workers were 9 to 10 times more likely to carry avian flu antibodies than the general public.
H6N1 was the least prevalent of the three, with only 1 participant (a poultry farmer) showing antibodies. No H7N9 antibodies were detected – not entirely unexpected, as the virus did not emerge in China until 2013. Antibodies for H7N3 were roughly half as common as H5N2.
While we can’t assume that the new H5N2 virus will behave in the exactly the same way as the H5N2 virus that was circulating in Taiwan three years ago, this does provide us with some useful (and fairly reassuring) baseline data.
The following study was published in the journal Epidemiology and Infection on March 12th.
In Taiwan, avian influenza virus (AIV) subtypes H5N2, H6N1 and H7N3 have been identified in domestic poultry, and several strains of these subtypes have become endemic in poultry. To evaluate the potential of avian-to-human transmission due to occupational exposure, an exploratory analysis of AIV antibody status in poultry workers was conducted. We enrolled 670 poultry workers, including 335 live poultry vendors (LPVs), 335 poultry farmers (PFs), and 577 non-poultry workers (NPWs).
Serum antibody titres against various subtypes of viruses were analysed and compared. The overall seropositivity rates in LPVs and PFs were 2·99% (10/335) and 1·79% (6/335), respectively, against H5N2; and 0·6% (2/335) and 1·19% (4/335), respectively, for H7N3 virus. Of NPWs, 0·35% (2/577) and 0·17% (1/577) were seropositive for H5N2 and H7N3, respectively.
Geographical analysis revealed that poultry workers whose workplaces were near locations where H5N2 outbreaks in poultry have been reported face greater risks of being exposed to viruses that result in elevated H5N2 antibody titres. H6N1 antibodies were detected in only one PF, and no H7N9 antibodies were found in the study subjects.
Subclinical infections caused by H5N2, H6N1 and H7N3 viruses were thus identified in poultry workers in Taiwan. Occupational exposure is associated with a high risk of AIV infection, and the seroprevalence of particular avian influenza strains in humans reflects the endemic strains in poultry in this region.