Computer artwork of an H1N1 influenza A (flu) virus
Out of the blue, a novel influenza virus has emerged in Mexico. The virus seems to have been born from the combination of at least two pig viruses, that themselves carried gene segments that originated in viruses of other species such as birds or humans. The combination of elements from three species is called "triple reassortment".
This new virus, although dubbed "swine flu", has not been identified from pigs in Mexico, nor is it caught directly from pigs, but has the ability to infect and transmit efficiently between humans. This led the World Health Organization to increase its pandemic alert to level 4.
So far there seem to have been around 159 fatalities among perhaps 2000 cases in Mexico, although only a small number of these have been confirmed in the laboratory as influenza infections. Although this suggests a scary fatality rate of around 10%, there has so far been just one death outside of Mexico – of a 23-month-old Mexican child in Texas, who had recently been in Mexico. The "case fatality" of the virus is not yet known.
This is the key point that scientists are now investigating. Travellers returning to their home countries after acquiring the virus in Mexico have experienced mild influenza-like illness. What might explain this apparent discrepancy in disease outcome?
A matter of numbers
One simple answer is that there have been many more mild cases in Mexico than counted so far and that the true case fatality rate is more like 1 to 2%. This would be akin to that seen during previous pandemics such as the 1957 (H2N2) Asian influenza or the 1968 (H3N2) Hong Kong flu. Even with the 1918 Spanish influenza, which killed tens of millions, the case fatality rate is estimated at between only 2.5 and 5%.
In other words, as cases outside of Mexico increase, we may expect a small number of fatalities from what is otherwise a rather mild virus. Increased epidemiology – for example, monitoring the spread of the virus and the strain type in the affected areas in Mexico – will shed light on this issue.
Another possibility is that the Mexican fatal cases have been due to co-infections with other pathogens that together lead to a worse outcome, or that somehow the indigenous population of Mexico is genetically predisposed to fare badly with this particular influenza strain. There is no evidence to support either of these possibilities, but then so far we have few clinical details of any of the cases.
Something about Mexico?
Neither does the limited data available suggest that the viruses in Mexico differ genetically from those that have caused mild symptoms in the US. The outcome of any influenza infection likely depends on the dose to which an individual was exposed. It may be that travellers unwittingly exposed to swine influenza have experienced a lower dose than some of the Mexican victims whose circumstances have not as yet been described.
Moreover in Europe and North America we are well outside the influenza season and the climatic conditions may be unfavourable for high-dose transmission. Contacts of returning travellers may have acquired the infection at a lower dose than individuals in Mexico. What is certain is that, as the number of cases outside of Mexico rise, we will have a much better idea of what sort of influenza virus this is.
Analysis of the viral genome indicates that it does not carry any of the genetic determinants associated with high virulence in other influenza viruses like H5N1. Genetic analysis suggests that the virus is unable to grow in areas beyond the respiratory tract, although it is already well adapted for binding to the upper respiratory tract of humans.
Evolution of a virus
However, until real biology begins in earnest with this virus in relevant model systems, coupled with careful descriptions of clinical courses in those infected, we cannot be sure what to expect.
Furthermore, this is a novel influenza virus, fresh from a cross-species jump. The virus might be expected to change within the coming months in ways that adapt it to its new host. If the virus persists until the advent of the next influenza season, or takes hold in the southern hemisphere where influenza season is getting underway right now, we might also have to look out for further reassortment events between this virus and the currently circulating human influenza A H3N2 or H1N1 strains.
The biological consequences of such changes are unpredictable at present, but for many reasons the "second wave" of this virus may not follow the characteristics of the first.
Out of the blue, a novel influenza virus has emerged in Mexico. The virus seems to have been born from the combination of at least two pig viruses, that themselves carried gene segments that originated in viruses of other species such as birds or humans. The combination of elements from three species is called "triple reassortment".
This new virus, although dubbed "swine flu", has not been identified from pigs in Mexico, nor is it caught directly from pigs, but has the ability to infect and transmit efficiently between humans. This led the World Health Organization to increase its pandemic alert to level 4.
So far there seem to have been around 159 fatalities among perhaps 2000 cases in Mexico, although only a small number of these have been confirmed in the laboratory as influenza infections. Although this suggests a scary fatality rate of around 10%, there has so far been just one death outside of Mexico – of a 23-month-old Mexican child in Texas, who had recently been in Mexico. The "case fatality" of the virus is not yet known.
This is the key point that scientists are now investigating. Travellers returning to their home countries after acquiring the virus in Mexico have experienced mild influenza-like illness. What might explain this apparent discrepancy in disease outcome?
A matter of numbers
One simple answer is that there have been many more mild cases in Mexico than counted so far and that the true case fatality rate is more like 1 to 2%. This would be akin to that seen during previous pandemics such as the 1957 (H2N2) Asian influenza or the 1968 (H3N2) Hong Kong flu. Even with the 1918 Spanish influenza, which killed tens of millions, the case fatality rate is estimated at between only 2.5 and 5%.
In other words, as cases outside of Mexico increase, we may expect a small number of fatalities from what is otherwise a rather mild virus. Increased epidemiology – for example, monitoring the spread of the virus and the strain type in the affected areas in Mexico – will shed light on this issue.
Another possibility is that the Mexican fatal cases have been due to co-infections with other pathogens that together lead to a worse outcome, or that somehow the indigenous population of Mexico is genetically predisposed to fare badly with this particular influenza strain. There is no evidence to support either of these possibilities, but then so far we have few clinical details of any of the cases.
Something about Mexico?
Neither does the limited data available suggest that the viruses in Mexico differ genetically from those that have caused mild symptoms in the US. The outcome of any influenza infection likely depends on the dose to which an individual was exposed. It may be that travellers unwittingly exposed to swine influenza have experienced a lower dose than some of the Mexican victims whose circumstances have not as yet been described.
Moreover in Europe and North America we are well outside the influenza season and the climatic conditions may be unfavourable for high-dose transmission. Contacts of returning travellers may have acquired the infection at a lower dose than individuals in Mexico. What is certain is that, as the number of cases outside of Mexico rise, we will have a much better idea of what sort of influenza virus this is.
Analysis of the viral genome indicates that it does not carry any of the genetic determinants associated with high virulence in other influenza viruses like H5N1. Genetic analysis suggests that the virus is unable to grow in areas beyond the respiratory tract, although it is already well adapted for binding to the upper respiratory tract of humans.
Evolution of a virus
However, until real biology begins in earnest with this virus in relevant model systems, coupled with careful descriptions of clinical courses in those infected, we cannot be sure what to expect.
Furthermore, this is a novel influenza virus, fresh from a cross-species jump. The virus might be expected to change within the coming months in ways that adapt it to its new host. If the virus persists until the advent of the next influenza season, or takes hold in the southern hemisphere where influenza season is getting underway right now, we might also have to look out for further reassortment events between this virus and the currently circulating human influenza A H3N2 or H1N1 strains.
The biological consequences of such changes are unpredictable at present, but for many reasons the "second wave" of this virus may not follow the characteristics of the first.
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