The swine flu pandemic is well under way. With the WHO citing almost 60,000 laboratory-confirmed cases at the time of writing, the race is truly on to understand more about the virus. Now, two new studies have painted a fresh but partly contradictory picture about two of the virus’s most important aspects – its infectivity (its ability to spread from host to host) and its virulence (its ability to cause disease in a host). These two traits will largely determine the threat that the virus poses, especially in relation to more familiar garden varieties of seasonal flu.
Both groups, one based in the US and the other in the Netherlands, tested the virus’s behaviour in ferrets. These animals are affected by flu viruses in much the same way as humans, mimicking both the severity of our infections and ease of our viral transmission.
Both studies found that the new swine flu virus takes a slightly greater toll on its host’s health than the usual strains of seasonal flu. These strains limit their infections to a ferret’s nasal passages but the new swine flu virus makes its way into the lungs too. The Americans, led by Taronna Maines at the CDC, even found traces of the virus in the ferrets’ gut.
This helps to explain the unusual profile of symptoms associated with swine flu. Most patients experience typical mild flu symptoms but an unusually large proportion (around 40% or so) have also suffered from unusual symptoms like vomiting and diarrhoea. Some have also been hospitalised due to severe pneumonia and respiratory failure, occasionally with fatal consequences. A flu virus that is unusually good at infecting the lungs and gut certainly make sense of these cases.
However, when it came to the virus’s ability to spread, the two research teams disagree. The Americans found that swine flu is less easily transmitted from ferret to ferret than other seasonal viruses of the same H1N1 subtype. However, the Dutch team, led by Vincent Munster from the Erasmus Medical Center, found that the new virus transmits just as easily as its seasonal counterparts.
The swine flu pandemic (S-OIV) currently sweeping the world is the result of an influenza H1N1 virus that made the leap from pigs to humans. But this jump is just the latest leg of a journey that has taken over 90 years and shows no signs of finishing.
Today’s pandemic is a fourth-generation descendant of the 1918 flu virus that infected around a third of the world’s population. This original virus is an incredible survivor and one that has spawned a huge legacy of daughter viruses. By importing and exporting its genes, it has contributed to several new strains that have been responsible for at least three further pandemics, including the current one.
In an editorial in the New England Journal of Medicine, David Morens says, “We are living in a pandemic era that began around 1918.” This is one of two papers that narrate the incredible story of the 1918 virus and its descendants – a thrilling tale of survival, adaptation, extinction and resurrection.
All influenza A viruses contain 8 different genetic segments that they can freely exchange with one another. Morens beautifully compares each virus to a squad of eight players, rather than a single entity. For the viral team to be successful, its eight-person genetic team has to work together. Their individual skills become more or less useful with time and the team will often swap its members for fresh faces that add something new to the mix. In technical terms, they “reassort”.
To do that, viruses need to infect the same cell and they find communal ground in the internal passages of birds, pigs and humans. Animal bodies are essentially viral networking events where different squads can meet and exchange players.
In 1918, one such squad of players went on an infamous world tour. H1N1 influenza viruses had been around for a long time, but the story of the current “pandemic era” really begins in that year. While H1N1 was busy killing humans in our millions, pig farmers at the Cedar Rapids Swine Show in Iowa also noticed something unusual. Even though H1N1 had never been described in pigs before, their herds were suffering from an unusual respiratory illness, whose symptoms were very similar to those afflicting the world’s humans. Swine flu had landed.
In the time since the words “swine flu” first dominated the headlines, a group of scientists from three continents have been working to understand the origins of the new virus and to chart its evolutionary course. Today, they have published their timely results just as the World Health Organisation finally moved to phase six in its six-tier system, confirming what most of us already suspected – the world is facing the first global flu pandemic of the 21st century.
The team, led by Gavin Smith at the University of Hong Kong, compared over 800 viral genomes representing a broad spectrum of influenza A diversity. The viral menagerie included two samples of the current pandemic strain (the virus formerly known as swine flu and now referred to as swine-origin influenza virus (S-OIV)). Also in the mix were 15 newly sequenced swine strains from Hong Kong, 100 older swine strains, 411 from birds and 285 from humans.
The team used these genomes to build a viral family tree that shows the relationships between the strains and dates their origins. They found that S-OIV was borne of several viruses that circulate in pigs, with contributions from avian and human strains. The virus made the leap to humans several months before we twigged to its presence. It was spreading right under our noses, undetected because of our lack of surveillance of flu viruses in pigs.
This beautiful diagram (enlarge it) charts the origins of the current outbreak. Each set of eight lines and arrows represents the genome of the influenza virus, which consists of eight separate strands of RNA. The bold dots on the far right represent the strain that currently troubles us. Trace back the lines of its ancestry and you can see that every one of its eight genetic segments comes from a lineage of flu that had firmly established itself in pigs for at least a decade before the current outbreak. Go back further and you can see that some of the segments have their origins in human H3N2 subtypes and bird H1N1 subtypes in the 70s and 80s.