Mystery Rays from Outer Space

Wild Geese and Rushes (Huang Chu Tsai - - Sung Dynasty)

Where did avian influenza come from?

The H5N1 avian influenza virus infects mainly birds, but there have been plenty of cases of spread into humans, where it is much more virulent than the ordinary, generic human influenza viruses that sweep around the world each year. H5N1 avian influenza is actually a group of related viruses, not just a single virus; and although it’s now mainly a virus of chickens and ducks, it was originally a pathogen of waterfowl.

When viruses jump from one species to another, they’re usually poorly adapted to their new host. Most often, the new host probably doesn’t even notice the infection. Sometimes the virus may be highly pathogenic, and may even rapidly kill the new host: Ebola virus (bat to human), Sin Nombre virus (rodent to human), and SARS virus (palm civet to human), are all examples of this. But in spite of high virulence, the virus isn’t necessarily well adapted to the new host; it doesn’t transmit effectively from one individual to another, and the infection burns itself out, only sustained by repeated jumps from the original population into the new one. That’s the phase we’re in with avian influenza.

SARS virus, though, is also an example of another viral phenomenon. The virus may be poorly adapted at first, but viruses (with their 24-hour generation time and in many cases their high mutation rates) evolve fast. What’s more, though the new host may not be a good fit in some ways, the entire population should be immunologically naïve — that is, there should be no immune resistance to the virus in the new host, in contrast to the original host, which typically would have a fair bit of resistance spread around the population. If the virus can adapt to the new host at all, there’s a smorgasbord of victims for it to infect. SARS virus jumped from palm civets into humans a few times, and then it rattled around in humans for a bit, gradually adapting and accommodating itself to humans instead of civets, until it was — well, not a very good human pathogen, but at least it was adequate at transmitting itself; it was capable of a sustained epidemic with further input from the parental civet virus.

Not much is known about how viruses acclimate to new species. Since it’s generally assumed that the next pandemic influenza outbreak of humans will come from the H5N1 virus once it’s become better adapted to humans, it’s of obvious interest to learn how it made its earlier jump to become adapted to chickens and ducks. A recent paper in PLoS Pathogens¹ tracks the virus back to its roots.

Influenza viruses have a segmented genome — that is, the RNA that makes up the viral genome is split into eight separate pieces. If two influenza viruses infect the same cell, then when the new pieces of RNA get packaged up to make a virus, you can get reassortment — the progeny viruses can contain RNA segments from two different parental viruses. Did H5N1 adapt to domestic birds through reassortment with a well-adapted chicken virus, or did it make the jump all at once, as a single virus, and later on become better adapted?

Vijaykrishna et al looked at a lot of virus sequences and concluded that the virus did originally jump to ducks from migratory waterfowl as a single already-formed virus. The common ancestor of all the present H5N1 viruses probably formed, in migratory waterfowl, around 1994 — a couple of years before H5N1 was detected as a highly virulent pathogen of domestic ducks, which was in 1996.

After it was introduced into ducks, then new reassortants did arise — probably as the virus began to acclimate to its new hosts:

Analysis of virus population dynamics revealed a rapid increase in the genetic diversity of Gs/GD lineage in poultry in China from mid-1999 to early 2000. This corresponds with the period when each of the major Gs/GD-like H5N1 variants or sublineages diverged and subsequently became widespread in poultry throughout China. It is likely that combined strong ecological and evolutionary factors led to this rapid increase in diversity, namely, the spread of the virus through large, immunologically naive poultry populations…¹

The authors suggest that China is a particularly good greenhouse for new H5N1 viruses to arise, probably because of the large and frequently connected population of domestic birds:

… transmission of these reassortant viruses within large highly connected populations of duck and other poultry species results in frequent interspecies transmission and genetic drift. Therefore, it is likely that this process selects for relatively fit viruses with a broad host range which are subsequently exported to other geographical regions. It is interesting to note that further reassortment has not been observed once those H5N1 viruses were transmitted out of China. We suggest that host population structures elsewhere may not result in the same intense multi-species transmission we observe in southern China.¹

The selection for “relatively fit viruses with a broad host range” is the main concern here, I think. The next pandemic is likely to come out of China.

1. Dhanasekaran Vijaykrishna, Justin Bahl, Steven Riley, Lian Duan, Jin Xia Zhang, Honglin Chen, J. S. Malik Peiris, Gavin J. D. Smith, Yi Guan, Ron A. M. Fouchier (2008). Evolutionary Dynamics and Emergence of Panzootic H5N1 Influenza Viruses PLoS Pathogens, 4 (9) DOI: 10.1371/journal.ppat.1000161[↩][↩][↩]

… adaptation of H9 viruses to land-based birds can lead to strains with expanded host range…. some bird species may act as a bridge in the generation of strains that can replicate more efficiently in mammals. In our particular case, quail provided an environment that improved the ability of a duck H9N2 virus to replicate in mice.

– Hossain MJ, Hickman D, Perez DR (2008) Evidence of Expanded Host Range and Mammalian-Associated Genetic Changes in a Duck H9N2 Influenza Virus Following Adaptation in Quail and Chickens. PLoS ONE 3(9): e3170. doi:10.1371/journal.pone.0003170

(My emphasis)

Avian influenza has a terribly high mortality rate in the humans it infects — perhaps as many as 80% of infected people die. Why is avian flu so lethal, while other strains of influenza rarely cause serious damage in young, healthy people?

One explanation has been cytokine storms. According to this hypothesis,¹ avian influenza causes a massive innate immune response, leading to the release of large amounts of cytokines. It’s the resulting inflammation that is lethal, not the damage that the virus causes directly.

The problem with this hypothesis was that it apparently didn’t help with treatment. If cytokine storms are responsible for mortality, then suppressing cytokine responses should reduce the death rate; and that didn’t happen, according to a paper² published last year. What’s more, although cytokine levels are associated with increased mortality, they’re also associated with virus levels. That is, cytokine levels may be an indicator of the amount of virus present, rather than a direct risk factor.

Because cytokine inhibition does not protect against death, therapies that target the virus rather than cytokines may be preferable. ²

For this reason (and others) I said at the time that “I think the evidence for this is pretty weak - and for avian flu in particular, it’s been shown that cytokines are probably not the culprits at all.”

But I’m always happy to have my mind changed, and a new paper that came out a couple of weeks ago³ has made me reconsider (even though it’s mouse work, not tested yet in humans). This is from Kwok-Yung Yuen’s group in Hong Kong. (Yuen is probably best known for his work on SARS — another disease that’s been claimed to act via cytokine storms — but he has worked on on avian influenza for a long time as well.) Their trick was to use triple therapy: a dual blockade of cytokines, plus antiviral treatment. None of these treatments worked alone. If you suppress viral replication while allowing the cytokines to persist, the mice die; if you shut down the cytokines while allowing the virus to continue replicating, the mice die. But if you block cytokines and virus — then most of the mice survive.

Actually, the antiviral alone does work pretty well if you start treatment almost immediately after infection. Of course, that’s not practical in humans; humans arrive at the hospital days after they’ve been infected, and by then viral replication and cytokine levels are already roaring ahead. The exciting part about Yuen’s triple therapy is that it gives decent survival — again, in mice, not yet humans — even if you don’t start treatment for 2 days after infection. This makes it a practical treatment for humans, and since the anti-inflammatory drugs they used are not particularly expensive or obscure, I’m sure we will see this treatment tried out in humans sooner rather than later.

1. Chan, M. C., Cheung, C. Y., Chui, W. H., Tsao, S. W., Nicholls, J. M., Chan, Y. O., Chan, R. W., Long, H. T., Poon, L. L., Guan, Y., and Peiris, J. S. (2005). Proinflammatory cytokine responses induced by influenza A (H5N1) viruses in primary human alveolar and bronchial epithelial cells. Respir Res 6, 135.
   and
   de Jong MD, Simmons CP, Thanh TT, Hien VM, Smith GJ, Chau TN, Hoang DM, Chau NV, Khanh TH, Dong VC et al. (2006) Fatal outcome of human influenza A (H5N1) is associated with high viral load and hypercytokinemia. Nat Med 12:1203-1207. doi:10.1038/nm1477[↩]
2. Salomon, R., Hoffmann, E., and Webster, R. G. (2007). Inhibition of the cytokine response does not protect against lethal H5N1 influenza infection. Proc Natl Acad Sci U S A 104, 12479-12481. [↩][↩]
3. Zheng, B., Chan, K., Lin, Y., Zhao, G., Chan, C., Zhang, H., Chen, H., Wong, S.S., Lau, S.K., Woo, P.C., Chan, K., Jin, D., Yuen, K. (2008). Delayed antiviral plus immunomodulator treatment still reduces mortality in mice infected by high inoculum of influenza A/H5N1 virus. Proceedings of the National Academy of Sciences, 105(23), 8091-8096. DOI: 10.1073/pnas.0711942105[↩]