Influenza kills hundreds of thousands around
the world each year while the threat of a global pandemic remains
omnipresent Contracting influenza, or the ‘flu’, as
it is often referred to, sounds fairly innocuous in comparison to
certain other more exotic sounding infectious disease. True enough,
the majority of persons that contract influenza make a complete
recovery. However, influenza epidemics occur yearly during winter
in temperate regions and result in approximately 3 to 5 million
cases of severe illness, and about 250,000 to 500,000 deaths.[1]
Most of these deaths occur in the elderly. So what exactly is
influenza and how concerned should we be about global influenza
pandemics? Influenza is an infectious disease caused by RNA viruses
of the family Orthomyxoviridae. There are
three virus types (Types A, B and C) and many subtypes. Influenza A
viruses all circulate within, or are derived from, an avian
reservoir, but can also infect a variety of mammals. By
contrast, types B and C viruses are almost exclusively isolated
from humans. The influenza A virus is subdivided by serologic or
genetic characteristics of the haemagglutinin (H) and neuraminidase
(N) surface glycoproteins that project from the virion. Sixteen H
and 9 N subtypes are known, abbreviated H1-H16 and N1-N9,
respectively.[2] Since 1977, H1N1 and H3N2 viruses have
co-circulated globally to produce seasonal epidemics. Popular fears
about a global viral pandemic have been reflected in several
Hollywood doomsday flicks, most recently the movie ‘Contagion’,
starring Matt Damon, Kate Winslet and Jude Law. But are such
fears of global death and panic on such a rapid and massive scale
warranted? In a nutshell, yes. Reviews of Contagion have praised
the “the depth of its research and the plausibility of its
scenario” – perhaps not surprising, given that the producers took
advice from a Professor of Epidemiology, Neurology and Pathology at
Columbia University.[3] There is no doubt that high volume air
travel facilitates the rapid global spread of a pandemic influenza
virus. Yet even before the age of commercial air travel, influenza
has proved adept at spreading around the world and causing death on
a massive scale.
src="http://www.pointofcare.dk/poc2012/engine/wp-content/uploads/2012/07/Flu-300x300.jpg"
alt="" width="300" height="300" />The 1918 to 1919
“Spanish flu” killed an estimated 50 to 100 million persons,[4]
earning it the label of “the greatest medical holocaust in
history”.[5] While most of the deaths were a result of secondary
bacterial pneumonia, the causative virus was an avian-descended
influenza A virus of the H1N1 subtype, and a direct progenitor of
all the influenza A viruses circulating in humans today. Later
influenza pandemics including the Asian influenza (1957-1958) and
the Hong Kong influenza (1969-1969) have not been so devastating.
The so-called ‘swine-flu’ pandemic was first detected in Mexico in
March-April 2009, though it may have been circulating in late 2008.
The virus, an H1N1 subtype, was derived by the re-assortment of
genetic material between two pre-existing swine influenza viruses.
Fortunately, most cases have been self-limited and appear similar
to seasonal influenza, even though by March 2010 the WHO had
reported millions of cases and at least 16,813 documented deaths.
In August 2010, the WHO announced that the H1N1 2009 virus had
moved into a post-pandemic phase, though localized outbreaks were
likely to continue. In 2003, the public health community confronted
the challenge of so-called ‘avian flu’ – an H5N1 virus in poultry
and wild bird populations that led to repeated human spillover
infections with a high case fatality rate. Though this virus
continues to spread in avian populations, it has not (yet!) adapted
to be able to make efficient human-to-human transmission, though as
the virus evolves, this remains a constant possibility. While
pandemic preparedness is therefore of crucial importance, the most
effective way to prevent seasonal influenza is vaccination.
Matching vaccines to the most representative viral strains in
circulation remains a challenge. As such, the development of truly
universal influenza vaccines to provide broad protection is a
priority. Two classes of antiviral drugs (adamantanes and
neuraminidase inhibitors) have been used against most influenza
viruses but the clinical benefits are modest.[6] Antiviral
resistance has also developed in both seasonal and pandemic
viruses. In 1931, an editorial in the Journal of the American
Medical Association stated, “…it does not seem possible, with our
present knowledge, to make any prediction as to whether or not an
epidemic might be expected in the near future.” Unfortunately,
despite advances in surveillance, epidemiological modeling and
cross governmental collaboration, we are still in essentially the
same predicament some 80 years on. With respect to the next global
influenza pandemic, only one thing is certain: it is a question of
when, not if. References
[1] World Health Organization. Influenza
(Seasonal). Fact sheet No 211. 2009. Retrieved on 02/05/2012
from
target="_blank">www.who.int/mediacentre/factsheets/fs211/en/
[2] Taubenberger J.K., Morens D.M. Influenza: the once
and future pandemic. Public Health Rep 2010;
125 Suppl 3:16-26. [3]
Ellis, R. Contagion film is not far from the truth, warns virus
scientist. The Guardian. 22-10-2011. Retrieved on 02/05/2012
from
target="_blank">www.guardian.co.uk/science/2011/oct/22/contagion-film-truth-viral-pandemic
[4] Johnson N.P., Mueller J. Updating the accounts:
global mortality of the 1918-1920 “Spanish” influenza pandemic.
Bull Hist Med 2002;
76:105-115. [5] Potter
C.W. A history of influenza. J Appl Microbiol
2001; 91:572-579. [6]
Morens D.M., Taubenberger J.K., Harvey H.A., Memoli M.J. The 1918
influenza pandemic: lessons for 2009 and the future. Crit
Care Med 2010; 38:e10-e20.
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