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PLoS One: Viremia In The 2009 H1N1 Pandemic Influenza

 

 

# 5868

 

 

Today, a fascinating study  that associates viremia, and a specific mutation (D222G/N) in the 2009 H1N1 virus, to more severe disease presentation.

 

Viremia simply refers to the presence of viruses in the blood stream.

 

While many viruses cause viremia (ie. Dengue, Chikungunya, WNV) – seasonal influenza, being primarily a respiratory disease, isn’t usually one of them.

 

But as we’ve seen demonstrated over the past several years, the pathogenesis of the 2009 H1N1 virus sometimes deviated from what one normally sees with seasonal flu.

 

Recently, in mBio: Lethal Synergism of H1N1 Pandemic Influenza & Bacterial Pneumonia we saw how novel H1N1 infection exacerbated lung damage due to bacterial co-infection, when seasonal flu did not.

 

In April of 2010, in There’s No Flu Like A New Flu, I listed many of the other differences observed between seasonal flu and the novel H1N1 virus, including:

 

 

  • Research out of Hong Kong that discovered that the novel H1N1 virus – unlike seasonal flu – easily infect and replicate in the conjunctival tissues of the eye  (see I Only Have Eyes For Flu).

 

 

 

 

Although the H1N1 pandemic virus of 2009 proved to be relatively mild for the vast majority of those infected, for a very small percentage, it produced serious and sometimes life-threatening illness.

 

Add to this the fact that this flu, unlike seasonal flu, was also frequently detected in companion animals, and it certainly appears that something was inherently different about the 2009 H1N1 virus.

 

Today, a new study appears in PLoS One that adds more weight to that argument. The open access study is called:

 

 

Clinical and Virological Factors Associated with Viremia in Pandemic Influenza A/H1N1/2009 Virus Infection

 

Herman Tse, Kelvin K. W. To, Xi Wen, Honglin Chen, Kwok-Hung Chan, Hoi-Wah Tsoi, Iris W. S. Li, Kwok-Yung Yuen

Positive detection of viral RNA in blood and other non-respiratory specimens occurs in severe human influenza A/H5N1 viral infection but is not known to occur commonly in seasonal human influenza infection.

Recently, viral RNA was detected in the blood of patients suffering from severe pandemic influenza A/H1N1/2009 viral infection, although the significance of viremia had not been previously studied. Our study aims to explore the clinical and virological factors associated with pandemic influenza A/H1N1/2009 viremia and to determine its clinical significance.

Methodology/Principal Findings

Clinical data of patients admitted to hospitals in Hong Kong between May 2009 and April 2010 and tested positive for pandemic influenza A/H1N1/2009 was collected. Viral RNA was detected by reverse-transcription polymerase chain reactions (RT-PCR) targeting the matrix (M) and HA genes of pandemic influenza A/H1N1/2009 virus from the following specimens: nasopharyngeal aspirate (NPA), endotracheal aspirate (ETA), blood, stool and rectal swab.

Stool and/ or rectal swab was obtained only if the patient complained of any gastrointestinal symptoms. A total of 139 patients were included in the study, with viral RNA being detected in the blood of 14 patients by RT-PCR.

The occurrence of viremia was strongly associated with a severe clinical presentation and a higher mortality rate, although the latter association was not statistically significant. D222G/N quasispecies were observed in 90% of the blood samples.

Conclusion

Presence of pandemic influenza A/H1N1/2009 viremia is an indicator of disease severity and strongly associated with D222G/N mutation in the viral hemagglutinin protein.

 

The authors propose several theories as to why the virus was detected in the bloodstream, and gastrointestinal tract.

 

The detected viral RNA in blood could either reflect extensive pulmonary damage with phagocytic uptake of virus-infected cells or true infection of monocyte-derived dendritic cells and macrophages [26].

On the contrary, viruses in the stool may originate from swallowed respiratory secretions, although viral replication in the epithelial tissue along the gastrointestinal tract cannot be ruled out entirely.

 

 

The significance of the H222G/N mutation in the 2009 H1N1 virus has been vigorously debated for nearly two years.

 

The `Norway’ or D222G/N (D225G/N in influenza H3 Numbering) mutation cited in this study was first linked to more severe disease by Norwegian Scientists in November 2009, although patients carrying these strains can have mild illness as well. 

 

While we’ve covered this territory a number of times over the past year, a brief (and hopefully simple) review is in order. If you are up to speed on receptor binding, and the history of the D222G/N variant, feel free to skip the next section.

 

This mutation involves a single amino acid change in the HA1 gene at position 222 from aspartic acid (D) to glycine (G) (or asparagine (N)).

 

The pdmH1N1 virus carrying this mutation appears to bind more readily to receptor cells (α2-3) found deeper in the lungs, whereas unmutated seasonal flu strains bind preferentially to the (α2-6) receptor cells found in the upper airway.

 

A virus’s ability to bind to specific cells is controlled by its RBD or Receptor Binding Domain; an area of its genetic code that allows it to attach to, and infect, specific types of host cells.

image

(A Very Simplified Illustration of RBDs)

Like a key into a padlock, the RBD must `fit’ in order to open the cell to infection.

 

The evidence for the D222G/N  amino acid substitution driving increased virulence has been mixed, with the World Health Organization, the CDC, and the HPA continuing to investigate. 

 

Complicating matters - viruses can have multiple amino acid changes – and it may be the combination of these changes can unpredictably (at least for now) alter the virus’s behavior. 

 

Since the D222G/N mutation has been found in patients showing mild disease, it may be that a second (or third) mutation elsewhere in the virus – in concert with D222G/N – is required to produce greater virulence.

 

There is simply a lot we don’t know yet.

 

During the first week of January, Eurosurveillance  printed a study looking at fatal and non-fatal cases of influenza in the UK (see Eurosurveillance: Analysis Of Fatal H1N1 Cases In The UK). 

 

Ellis et al. reported that almost all of the virus samples tested in fatal and non-fatal cases during the early wave of the 2010/11 influenza season showed aspartic acid (D) at position 222.

 

In other words, no `Norway’ mutation.

Towards the end of January 2011, Eurosurveillance published a letter from an Italian researcher who had found a high percentage of D222G/N mutations in severely ill patients (43%)  – particularly when taking virus samples from the lower respiratory tract (lungs).

 

In a reply, the authors of the original study concede that in many cases, only upper respiratory swabs were available for this analysis, and that when possible, samples from the lower respiratory system would be useful.

 

This scholarly debate wasn’t over, as Ellis et al. state in their reply:

 

The selection and emergence of the D222G mutation as a cause or consequence of more severe lower respiratory tract infection is still to be resolved.

 

Emergence of this mutant is likely to exacerbate severity of disease, but by itself, may be neither necessary nor sufficient to account for a severe disease outcome, which is invariably a balance between virus virulence factors and host immune response capability.

 

And so the debate has continued, with some scientists believing the `Norway’ mutation causes more severe illness, while others are less certain.

 

It will take more samples, more research, and more time to determine the truth in the matter.

 

Still, this study is another step forward in our understanding of the unusual pathogenesis, and genetic evolution, of the pandemic H1N1 virus.

 

And since we’ve seen similar severe lung damage, and scattered reports of viremia, among the small number of H5N1 `bird flu’ cases that have been examined, what we can learn about the 2009 H1N1 virus may provide clues on how to tackle a more severe pandemic in the future.

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