Last week USDA confirmed discovery of a highly pathogenic H7N8 avian flu virus in a turkey flock in Indiana. Subsequently, eight more turkey flocks tested positive for H7N8 but were found to have a low pathogenic variant of the virus. While the virus genome is being analyzed, 400,000 infected birds, both turkeys and chickens, are being euthanized in efforts to stamp out the outbreak before it reaches the proportions seen in the 2015 epidemic when over 48 million birds were killed.
Dr. Jürgen Richt, Director, Department of Homeland Security Center of Excellence for Emerging and Zoonotic Animal Diseases, Kansas State University (webpage):
Expertise: Emerging diseases of livestock focusing mainly on viral and prion diseases; influenza viruses, especially swine and avian influenza viruses.
“Not much is known about the H7N8 virus so far. The genetic sequence of the virus is not yet publicly available and therefore it is not possible to conclude how this new virus originated. However, it is known that low pathogenic H7 subtype influenza viruses are common in wild birds in the U.S.
“Since low pathogenic H7N8 influenza viruses were found in poultry flocks in the area of the initial outbreak, it is possible that a low pathogenic H7N8 was introduced from wild birds into poultry flocks which subsequently mutated into a highly pathogenic avian influenza virus (HPAIV) during replication in turkeys. The limited information from USDA supports this possibility. Highly pathogenic avian influenza viruses have been documented to arise from low pathogenic viruses through mutations in the haemagglutinin (H) surface protein.
“If the initial infection did come from wild birds, many of whom may be carrying H7N8 viruses, we can expect more cases of H7N8 infection of poultry. It will be critical to recognize flocks which are infected with the low pathogenic variant and stamp these flocks out before the virus can mutate into a highly pathogenic variant. Sampling of wild birds in the affected areas in Indiana will most likely help in determining the source of the infection. Every time low pathogenic H5- or H7-subtype influenza viruses are introduced into highly susceptible poultry species, these low pathogenic H5- and H7- subtype influenza viruses may undergo a series of mutations resulting in HPAIVs; these events arise de novo and are unpredictable.
“Vaccines should only be used if they are highly efficacious against circulating strains of HPAIVs. Most vaccine formulations will not allow ‘Differentiating Infected from Vaccinated Animals’ (DIVA), which is a problem for poultry export markets because vaccinated poultry will have an antibody profile similar to infected poultry, so foreign countries may be unwilling to accept vaccinated birds. In order to avoid this issue, DIVA compatible vaccines against HPAIV need to be developed.”
Dr. Rodrigo Gallardo, Assistant Professor in Avian Medicine, Department of Population Health & Reproduction, University of California, Davis (webpage):
Expertise: avian virology, specifically in RNA viruses and their capabilities of mutation, recombination and variation.
“The recently detected H7N8 virus in Indiana is a unique strain for North America, and its lineage suggests North American origin. This information is only preliminary and more information in this context will be available when the results of the sequencing and further characterization are advanced.
“Up to this point we do not know if this H7N8 will be as difficult to control as the H5N2 and H5N8 that affected the U.S. last winter and spring. The good news is that the industry has been working since the last outbreak towards raising biosecurity measures and response controls. This has been corroborated with the fast depopulation measures taken in the current outbreak. As an example a flock of 155,000 egg laying hens is being depopulated as a preventative measure because of the proximity to the infected turkey flocks.
“So far USDA/APHIS has confirmed eight flocks with low pathogenic H7N8 and one flock with highly pathogenic H7N8. Apparently, a low pathogenic strain H7N8 mutated to be highly pathogenic after infecting turkeys, causing high mortality in just one of the positive turkey flocks.
“Commonly in order to have an HPAI outbreak there are three situations that need to occur:
- Proximity to water bodies and waterfowl (ponds, lakes, etc.)
- Biosecurity breaks (allows the introduction of AI to the farms)
- Mutations: a low pathogenic virus infects a flock and mutates to become highly pathogenic
“This has happened several times e.g. the H7N3 virus in Chile, 2002.
“Even though the virus has not been detected in wild birds yet, contact with wild birds is one of the ways in which the virus might have been introduced.
“The US has the strongest avian influenza surveillance system in the world, and more surveillance in backyard flocks, waterfowl and commercial birds is being done. Control area testing continues to come back negative. In the last 24 hours 221 tests have been submitted.
“Vaccine use is controversial and sometimes the decision to use vaccines in order to control an outbreak is more economical than scientific. Having that in mind, vaccination is an alternative but not the best route if we would like to eradicate this outbreak, the reason being that vaccination may create a population of birds that might be infected but do not show signs. Since these vaccines do not provide sterilizing immunity, the virus replicates and is shed, infecting birds outside of the vaccination/control zone and enlarging the outbreak.
“This virus has not been associated with infections in humans, the Indiana Department of Health with the Dubois County Health Department are monitoring workers that were exposed to this birds and verify that H7N8 has not been transmitted to people.“
Dr. Carol Cardona, Pomeroy Endowed Chair, Veterinary Biomedical Sciences Department, University of Minnesota (webpage):
Expertise: avian influenza and other viral diseases in poultry; viral and host factors that determine transmissibility and viral adaptation.
“This is the first time the highly pathogenic avian influenza virus H7N8 has ever been seen. Many highly pathogenic avian flu viruses have a preceding low pathogenic strain that can arise from two different viruses reassorting; like an H7N2 reassorting with an H2N8, for example, to give rise to an H7N8. When a low pathogenic virus infects poultry farms, like the turkey farm in Indiana, passage in those birds can cause the virus to become highly pathogenic. When an H5 or H7 virus goes from low to highly pathogenic we sit up and take notice.
“For a virus to become infective, the hemagluttinin (H) has to be cleaved – cut in half – by an enzyme. Normally that can only happen in the digestive, respiratory and sometimes the reproductive tract. But once that cleavage site becomes different and is recognized by enzymes that are prevalent in different parts of the body, the infection can become systemic. That’s what happens when a virus converts from being low to highly pathogenic, and so far it has only happened in H5 and H7 viruses.
“Birds infected with a low pathogenic variant tend to shed less of the virus (because not every cell is infected) which improves the ability to control it. However, a similar situation to that in Indiana happened in Pennsylvania in 1983 when there was a low pathogenic precursor to H5N2 that eventually became highly pathogenic after being passed through chickens for quite a while, and that virus spread far and wide.
“Even though this is the first time H7N8 has been seen, it may come from a long line of H7 viruses that have been in wild birds for decades, though it’s not possible to know that for sure. It is unusual to see new combinations but not super-unusual, and if there’s anything we can say about flu it’s that it’s always different. Wild birds are a natural reservoir of these viruses and poultry farmers raise birds knowing that and putting appropriate biosecurity measures in place.”
Latest USDA/APHIS update: http://1.usa.gov/1WrNeGm
Declared interests (see GENeS register of interests policy):
Dr. Jürgen Richt: “I have no commercial conflict of interest in avian flu vaccine, however part of my scientific work deals with DIVA compatible vaccines for avian flu.”
No further interests declared