Tweet
ECDC Health Content. Pathogenicity and transmissibility of pandemic influenza A(H1N1)v ? results from an animal model (July 4, 2009, edited)
Scientific Advances - Influenza A(H1N1)
Pathogenicity and transmissibility of pandemic influenza A(H1N1)v ? results from an animal model
Two studies conducted in the US and Europe have been published this week in the journal Science both investigating how pandemic influenza A(H1N1)v virus behaves in an animal model - ferrets. Their results of the studies have important implications for humans since ferrets show transmissibility and disease severity features for seasonal influenza A viruses (H1N1 and H3N2 subtypes) and animal influenza viruses (H5 and H7 types) very similar to what is seen in humans. They are considered by many the best available animal model for how influenza behaves in humans.
Transmission and Pathogenesis of Swine-Origin 2009 A(H1N1) Influenza Viruses in Ferrets and Mice (Maines et al.) ? United States
Maines TR et al. Transmission and Pathogenesis of Swine-Origin 2009 A(H1N1) Influenza Viruses in Ferrets and Mice. Science 2009. Published online July 2 2009; Abstract available at: http://www.scienceonline.org/cgi/con...stract/1177238
This investigated the virulence and transmissibility of three different isolates of the pandemic virus. These were compared with a representative 2007 season influenza A(H1N1) virus ? the A/Brisbane/59/2007. The ferrets received intranasal inoculations of the three pandemic influenza virus isolates. After 24 h, one lot of animals was distributed into different cages with communication holes pierced in the walls of the cages leading to uninnoculated ferrets - to assess the efficiency of droplet transmission - and another lot of animals was placed sharing the same environment - to check for direct contact transmission. Inoculated and contact animals were assessed for clinical signs and for detection by titration of the virus in convalescent sera over two-weeks. Clinical signs of influenza was associated two of the three pandemic virus inoculations and a marked weight loss occurred for all three. Ferrets inoculated with the pandemic viruses displayed high titres of infectious virus in nasal washes at 1 day post-infection and these were maintained for 5 days. This was similar to what happened with the seasonal influenza strain. However unlike the seasonal influenza strain, the pandemic viruses were detected in the intestinal tract and in the lower respiratory tract at higher titres. However reassuringly there was no viraemia or infectious virus in the brain, kidney, liver and spleen tissues of the ferrets with the pandemic viruses
Although direct contact transmission was effective with all three pandemic viruses, the respiratory droplet transmission was significantly reduced compared to that of the seasonal influenza virus. The authors further corroborated this by characterizing the binding specificity of the viral haemagglutinin to the sialylated glycan receptors in a human host using dose-dependent direct receptor binding and human lung tissue binding assays techniques which this group are familiar with.
To further evaluate the pathogenicity of influenza A(H1N1)v mice were inoculated with the three same isolates of the pandemic virus before determining the virus replication, the morbidity (measured by weight loss), the 50% mouse infection dose and the 50% lethal dose. This found that the pandemic virus was not lethal, but still infectious in mice replicating efficiently in their lungs, without prior host adaptation, like the 1918-9 Spanish influenza virus (which this groups has worked with). Unlike avian influenza A(H5N1) viruses however the pandemic viruses did not replicate systemically in the mice.
The authors commented that the lack of efficient respiratory droplet transmission suggests that additional virus adaptation in mammals may be required to reach the transmissible phenotypes observed with seasonal influenza or with the 1918 pandemic virus. They suggested that further adaptation of the polymerase basic protein 2 (PB2) of any influenza A(H1N1) viruses would be critical for efficient droplet respiratory transmission but that this might only need a simple mutant selection or re-assortment of the pandemic virus.
Pathogenesis and Transmission of Swine-Origin 2009 A/H1N1 Influenza Virus in Ferrets (Munster et al.) ? The Netherlands
Munster VJ et al. Pathogenesis and Transmission of Swine-Origin 2009 A(H1N1) Influenza Virus in Ferrets. Science 2009. Published online July 2 2009; abstract available at: http://www.scienceonline.org/cgi/con...stract/1177127
This second study also used the ferret model in order to investigate clinical signs, viral shedding, tissue distribution, pathology and airborne transmission of the influenza A(H1N1)v virus, comparing it with a seasonal influenza virus. For this study the authors used only one strain of influenza A(H1N1)v virus, isolated from a pandemic virus isolated in The Netherlands, and the comparison seasonal influenza strain of choice for this study was also from the 2007 season ? A/Netherlands/26/2007. Two groups of ferrets were inoculated with the pandemic virus and the animals were observed for clinical signs and weighed daily. Both viruses caused typical influenza illness for ferrets but the maximum weight loss for animals inoculated with the seasonal influenza strain was somewhat lower than for the pandemic strain. The recovery of the ferrets inoculated with the pandemic virus was also slower by 2 days when compared to the seasonal influenza group. On days 2 and 3 after inoculation virus titres in both nose and throat swabs were significantly higher for animals inoculated with the influenza A(H1N1)v virus than for seasonal influenza and the total viral shedding from the throat at the end of the study period was 1.5-fold higher in animals inoculated with the pandemic virus. Larger areas of the lungs were affected for animals inoculated with the pandemic virus, compared to the seasonal influenza strain. As in the American study the pandemic virus did not show many signs of involving organs outside the respiratory tract, though somewhat more than for the seasonal influenza strain. Unlike the American studies it proved possible to demonstrate short-distance airborne ferret to ferret transmission for both seasonal and the pandemic virus.
The authors of the study conclude that these results match observations in humans, where generally mild disease but efficient human-to-human transmission has been observed. The influenza A(H1N1)v virus in this model did not cause mortality in the animals and did not replicate systemically, these being effects more typical of infections with avian influenza A(H5N1) viruses or with the Spanish 1918 pandemic virus. Reportedly, these results prove that, whilst the pandemic virus may be a relatively mild pathogen in humans, it is somewhat more pathogenic than seasonal influenza virus in ferrets, with wider distribution of virus replication and associated lesions in the respiratory tract of these animals. Since the pandemic virus replicates in the same sites as seasonal A(H1N1) and A(H3N2) influenza viruses, the possibility of re-assortment of this virus with seasonal influenza viruses, and more importantly with avian A(H5N1) viruses, is a serious concern. This re-assortment could also imply the apparition of antiviral resistant strains of the pandemic virus. The possibility of an isolated mutation cannot be ruled out either.
ECDC Comment (3/07/2009):
Understanding the pathogenesis and transmission of the influenza A/H1N1v is essential for planning of appropriate Public Health responses to the current pandemic. In the two pandemicly published articles different influenza A/H1N1v isolates from patients with varying severity of their clinical symptoms have been compared with a contemporary seasonal H1N1 virus for their ability to cause disease in mice and ferrets. In addition, their ability to transmit to na?ve ferrets has also been assessed.
To date, these are the first studies that have tried to analyse the pathogenicity and transmissibility characteristics of the influenza A(H1N1)v virus. The results of the first study (Maines et al.) might be considered more representative since they used three different strains of the pandemic virus as opposed to only one strain used in the second study (Munster et al.). Additionally, the first study was carried out over a longer period (two versus one week). However there are important common findings for the pandemic virus:
It is possible to speculate that droplet respiratory transmission was diminished in ferrets infected with the influenza A(H1N1)v virus when compared to the normal seasonal influenza viruses in the US study at least might be encouraging. However ferrets are ferrets and humans are humans and this study is not a replacement for human studies of viral expression. A conclusion from the Netherlands study that pandemic A(H1N1)v virus is more pathogenic than seasonal A(H1N1) needs some interpretation before extending to humans. The control used seasonal A(H1N1) which on a case by case basis is less pathogenic in humans than seasonal A(H3N2). Also the picture in humans is coloured by the welcome absence of infections in older people. More experience and data are needed before similar conclusions can be drawn from humans. What finally is evident, and we need to be vigilant of is that the pandemic virus is here to stay, co-circulating with seasonal influenza strains, maybe re-assorting with them or with more pathogenic strains like the avian influenza viruses, or even mutating by giving origin to (yet again) a pandemic influenza virus. These publications confirm the clinical picture observed in humans with the pandemic strain and is suggestive that we will continue to see severe disease in some individuals even without any further genetic changes in the circulating influenza A/H1N1v strains. Should a more significant genetic change to the circulating viral strains occur, the ferret and the mouse models may serve as appropriate models to assess any changed pathogenicity. A baseline has been established by these research groups.
-
<cite cite="http://www.ecdc.europa.eu/en/health_content/sciadv/090704_sciadv.aspx">ECDC Health Content</cite>
Pathogenicity and transmissibility of pandemic influenza A(H1N1)v ? results from an animal model
Two studies conducted in the US and Europe have been published this week in the journal Science both investigating how pandemic influenza A(H1N1)v virus behaves in an animal model - ferrets. Their results of the studies have important implications for humans since ferrets show transmissibility and disease severity features for seasonal influenza A viruses (H1N1 and H3N2 subtypes) and animal influenza viruses (H5 and H7 types) very similar to what is seen in humans. They are considered by many the best available animal model for how influenza behaves in humans.
Transmission and Pathogenesis of Swine-Origin 2009 A(H1N1) Influenza Viruses in Ferrets and Mice (Maines et al.) ? United States
Maines TR et al. Transmission and Pathogenesis of Swine-Origin 2009 A(H1N1) Influenza Viruses in Ferrets and Mice. Science 2009. Published online July 2 2009; Abstract available at: http://www.scienceonline.org/cgi/con...stract/1177238
This investigated the virulence and transmissibility of three different isolates of the pandemic virus. These were compared with a representative 2007 season influenza A(H1N1) virus ? the A/Brisbane/59/2007. The ferrets received intranasal inoculations of the three pandemic influenza virus isolates. After 24 h, one lot of animals was distributed into different cages with communication holes pierced in the walls of the cages leading to uninnoculated ferrets - to assess the efficiency of droplet transmission - and another lot of animals was placed sharing the same environment - to check for direct contact transmission. Inoculated and contact animals were assessed for clinical signs and for detection by titration of the virus in convalescent sera over two-weeks. Clinical signs of influenza was associated two of the three pandemic virus inoculations and a marked weight loss occurred for all three. Ferrets inoculated with the pandemic viruses displayed high titres of infectious virus in nasal washes at 1 day post-infection and these were maintained for 5 days. This was similar to what happened with the seasonal influenza strain. However unlike the seasonal influenza strain, the pandemic viruses were detected in the intestinal tract and in the lower respiratory tract at higher titres. However reassuringly there was no viraemia or infectious virus in the brain, kidney, liver and spleen tissues of the ferrets with the pandemic viruses
Although direct contact transmission was effective with all three pandemic viruses, the respiratory droplet transmission was significantly reduced compared to that of the seasonal influenza virus. The authors further corroborated this by characterizing the binding specificity of the viral haemagglutinin to the sialylated glycan receptors in a human host using dose-dependent direct receptor binding and human lung tissue binding assays techniques which this group are familiar with.
To further evaluate the pathogenicity of influenza A(H1N1)v mice were inoculated with the three same isolates of the pandemic virus before determining the virus replication, the morbidity (measured by weight loss), the 50% mouse infection dose and the 50% lethal dose. This found that the pandemic virus was not lethal, but still infectious in mice replicating efficiently in their lungs, without prior host adaptation, like the 1918-9 Spanish influenza virus (which this groups has worked with). Unlike avian influenza A(H5N1) viruses however the pandemic viruses did not replicate systemically in the mice.
The authors commented that the lack of efficient respiratory droplet transmission suggests that additional virus adaptation in mammals may be required to reach the transmissible phenotypes observed with seasonal influenza or with the 1918 pandemic virus. They suggested that further adaptation of the polymerase basic protein 2 (PB2) of any influenza A(H1N1) viruses would be critical for efficient droplet respiratory transmission but that this might only need a simple mutant selection or re-assortment of the pandemic virus.
Pathogenesis and Transmission of Swine-Origin 2009 A/H1N1 Influenza Virus in Ferrets (Munster et al.) ? The Netherlands
Munster VJ et al. Pathogenesis and Transmission of Swine-Origin 2009 A(H1N1) Influenza Virus in Ferrets. Science 2009. Published online July 2 2009; abstract available at: http://www.scienceonline.org/cgi/con...stract/1177127
This second study also used the ferret model in order to investigate clinical signs, viral shedding, tissue distribution, pathology and airborne transmission of the influenza A(H1N1)v virus, comparing it with a seasonal influenza virus. For this study the authors used only one strain of influenza A(H1N1)v virus, isolated from a pandemic virus isolated in The Netherlands, and the comparison seasonal influenza strain of choice for this study was also from the 2007 season ? A/Netherlands/26/2007. Two groups of ferrets were inoculated with the pandemic virus and the animals were observed for clinical signs and weighed daily. Both viruses caused typical influenza illness for ferrets but the maximum weight loss for animals inoculated with the seasonal influenza strain was somewhat lower than for the pandemic strain. The recovery of the ferrets inoculated with the pandemic virus was also slower by 2 days when compared to the seasonal influenza group. On days 2 and 3 after inoculation virus titres in both nose and throat swabs were significantly higher for animals inoculated with the influenza A(H1N1)v virus than for seasonal influenza and the total viral shedding from the throat at the end of the study period was 1.5-fold higher in animals inoculated with the pandemic virus. Larger areas of the lungs were affected for animals inoculated with the pandemic virus, compared to the seasonal influenza strain. As in the American study the pandemic virus did not show many signs of involving organs outside the respiratory tract, though somewhat more than for the seasonal influenza strain. Unlike the American studies it proved possible to demonstrate short-distance airborne ferret to ferret transmission for both seasonal and the pandemic virus.
The authors of the study conclude that these results match observations in humans, where generally mild disease but efficient human-to-human transmission has been observed. The influenza A(H1N1)v virus in this model did not cause mortality in the animals and did not replicate systemically, these being effects more typical of infections with avian influenza A(H5N1) viruses or with the Spanish 1918 pandemic virus. Reportedly, these results prove that, whilst the pandemic virus may be a relatively mild pathogen in humans, it is somewhat more pathogenic than seasonal influenza virus in ferrets, with wider distribution of virus replication and associated lesions in the respiratory tract of these animals. Since the pandemic virus replicates in the same sites as seasonal A(H1N1) and A(H3N2) influenza viruses, the possibility of re-assortment of this virus with seasonal influenza viruses, and more importantly with avian A(H5N1) viruses, is a serious concern. This re-assortment could also imply the apparition of antiviral resistant strains of the pandemic virus. The possibility of an isolated mutation cannot be ruled out either.
ECDC Comment (3/07/2009):
Understanding the pathogenesis and transmission of the influenza A/H1N1v is essential for planning of appropriate Public Health responses to the current pandemic. In the two pandemicly published articles different influenza A/H1N1v isolates from patients with varying severity of their clinical symptoms have been compared with a contemporary seasonal H1N1 virus for their ability to cause disease in mice and ferrets. In addition, their ability to transmit to na?ve ferrets has also been assessed.
To date, these are the first studies that have tried to analyse the pathogenicity and transmissibility characteristics of the influenza A(H1N1)v virus. The results of the first study (Maines et al.) might be considered more representative since they used three different strains of the pandemic virus as opposed to only one strain used in the second study (Munster et al.). Additionally, the first study was carried out over a longer period (two versus one week). However there are important common findings for the pandemic virus:
- Significant but not lethal pathogenicity from the pandemic A(H1N1)v ? somewhat more than for seasonal A(H1N1) but considerably less than that seen for A(H5N1) (bird flu) in ferrets.
- In contrast to the seasonal influenza A H1N1 confined to the nasal cavity the pandemic influenza A/H1N1v isolates also replicated in the trachea, bronchi and bronchioles. No ferrets died from their infection,
- Droplet respiratory expression (but not transmission) occurring with the pandemic virus was higher than with seasonal influenza though not reaching the levels when using a 1918 pandemic virus.
- Successful and efficient ferret to ferret transmission but,
- Respiratory droplet transmission was significantly reduced compared to respiratory droplet transmission of the seasonal influenza virus.
- No systemic replication of the A(H1N1)v
- Pandemic viruses being found at higher levels in the lower respiratory tract of the infected individuals,
- Pandemic viruses in the intestinal tracts of the animals, which is consistent with some report of a explains the higher frequency of gastrointestinal symptoms observed in individuals affected by the pandemic virus.
- An ability of both viruses to infect the same cells in the respiratory tract (upper and lower) so increasing the risk of reassortment when co-infection occurs in humans.
It is possible to speculate that droplet respiratory transmission was diminished in ferrets infected with the influenza A(H1N1)v virus when compared to the normal seasonal influenza viruses in the US study at least might be encouraging. However ferrets are ferrets and humans are humans and this study is not a replacement for human studies of viral expression. A conclusion from the Netherlands study that pandemic A(H1N1)v virus is more pathogenic than seasonal A(H1N1) needs some interpretation before extending to humans. The control used seasonal A(H1N1) which on a case by case basis is less pathogenic in humans than seasonal A(H3N2). Also the picture in humans is coloured by the welcome absence of infections in older people. More experience and data are needed before similar conclusions can be drawn from humans. What finally is evident, and we need to be vigilant of is that the pandemic virus is here to stay, co-circulating with seasonal influenza strains, maybe re-assorting with them or with more pathogenic strains like the avian influenza viruses, or even mutating by giving origin to (yet again) a pandemic influenza virus. These publications confirm the clinical picture observed in humans with the pandemic strain and is suggestive that we will continue to see severe disease in some individuals even without any further genetic changes in the circulating influenza A/H1N1v strains. Should a more significant genetic change to the circulating viral strains occur, the ferret and the mouse models may serve as appropriate models to assess any changed pathogenicity. A baseline has been established by these research groups.
-
Comment