Dororthy H. Crawford is Professor of Medical Microbiology and Assistant Principal for Public Understanding of Medicine at the University of Edinburgh. She is the author of several books and papers, most recently for OUP, Deadly Companions: How Microbes Shaped our History. In the post below she compares the current swine flu outbreak with previous flu pandemics, and asks why this one is apparently more serious in Mexico than elsewhere and how it might develop.
She has previously written for OUPblog on the UK Foot and Mouth Disease outbreak in 2007.
Ironically, while all those trying to predict the next flu pandemic are monitoring the antics of bird flu in Asia and North Africa, swine flu escapes from a pig farm in Mexico and goes global. This unpredictability is typical of flu virus which is constantly mutating and recombining its genes, eventually producing a ‘new’ strain that can infect and spread in humans unrecognised by our immune system.
This unruly behaviour gives an average of three pandemics (defined as a spreading infection in more than one continent at once) per century. In the 20th century we had Hong Kong flu in 1968, Asian flu in 1957, and the post WW1 Spanish flu in 1918 that killed around 40M people.
Within days of Mexico reporting a flu epidemic, now with some 985 infections and 25 deaths, cases appeared in around 20 countries, including the UK, all directly seeded from Mexico thanks to our efficient trans-global airliners. Scientists at the Centre for Disease Control, Atlanta, US, quickly identified the virus as H1N1; a common subtype in humans. But the ‘Mexican’ pig virus, which has been traced to one that has been circulating in pigs since the 1930s, is very different from the human H1N1 strain. It has occasionally jumped to humans before but until now it has never succeeded in spreading between humans.
Flu viruses have RNA genomes with eight separate gene segments that mutate and recombine rapidly. The ‘Mexican’ virus has six genes derived from North American swine flu and two from Eurasian swine flu; a combination that has never been found infecting humans before. It is not clear how or why it jumped species, but given that it is now spreading between humans, and most of us are likely to be non-immune, this is a recipe for a pandemic.
At present events are moving so fast that whatever I write will not only be out of date, but may even be proved wrong, by tomorrow, so I will restrict myself to speculating on the following: why is ‘Mexican’ flu apparently more serious in Mexico than elsewhere’? and: ‘how might the pandemic develop’?
Traditionally, respiratory infections like flu are a threat to the very young, the very old and those with chronic diseases, causing thousands of deaths in these groups in the UK every year. Generally the death rate from flu is less than 1%, so the rate of around 2.5% reported from Mexico (985 cases of flu and 25 deaths) is high. But outside Mexico cases seem to be mild, with the only death reported being a child in the US where over 160 cases have been diagnosed. So what is the explanation?
In all flu outbreaks those who seek help represent the tip of the iceberg, with many more infections being too mild to require medical attention. It is likely that the Mexican figures represent only those sufferers who consulted a doctor and are therefore distorted towards the severe end of the spectrum. More research is needed to uncover the actual prevalence of the disease in the community before a reliable death rate can be calculated.
Reports of deaths among young adults in Mexico are worrying as this is reminiscent of the 1918 H1N1 virus with its high death rate in the young. The explanation may be that the older generation are partially protected by having met a similar strain of H1N1 in the past, but again we need more details from Mexico before specific risks can be assessed.
In the Northern hemisphere flu epidemics usually strike in the winter when the virus transmits more easily between people who are huddled inside trying to keep warm. But in the UK the 1918 pandemic began with a small outbreak in the Spring which receded in the Summer only to take off again in the Autumn, causing thousands of infections and deaths. Given the present pattern, we may now be witnessing the equivalent of the 1918 Spring outbreak, heralding a full pandemic in the Autumn. If so this could be good news as it gives us a 6 month breathing space – time enough to prepare a vaccine to protect our vulnerable groups.