Swine flu reaches into the lungs and gut
Studies of ferrets reveal details of disease.
The swine flu virus can reach deep into the respiratory system and even as far as the intestines — findings which could explain why the disease's symptoms are different from those of seasonal flu.
Two separate groups have been using ferrets to investigate how harmful A(H1N1) influenza virus is and how easily it is transmitted. One of the studies was by Terrence Tumpey at the Centers for Disease Control and Prevention in Atlanta, Georgia, and his colleagues, and is published in Science1. Tumpey's team put droplets of three different swine flu viruses, and one 'seasonal' flu virus into the noses of ferrets. Some ferrets shared cages with other uninfected ferrets and some were placed in cages next to other ferrets, sharing nothing but the air they breathed.
The experiments showed that the ferrets with swine flu strains lost more weight than those with normal flu, and that the swine flu reached lower down into the lungs of some of the ferrets than normal seasonal flu, penetrating the intestines in some cases. This tallies with observations in humans that some patients suffered vomiting and diarrhoea. A second study by Ron Fouchier at the Erasmus University Medical Center, Rotterdam, the Netherlands, and his colleagues also showed that the virus penetrated the lungs. "This is the first indication of how pathogenic [swine flu] really is," says Fouchier. "In the field that conclusion is hard to draw."
Ferrets have long been used as an animal model for flu because they show similar symptoms to humans, and symptoms tend to last the same amounts of time in both species.
Tumpey's studies showed that the virus wasn't transmitted between animals as efficiently as the seasonal flu, but Fouchier's results, also published in Science2 suggest that the virus was transmitted just as efficiently as seasonal flu.
The disagreement could be because Fouchier used a different sample of swine flu, or that the ferrets are slightly different, says Fouchier — his ferrets sneezed a lot whereas Tumpey's didn't. Fouchier's experiments were also a little different — he didn't have any ferrets in direct contact with other ferrets, for example.
Tumpey says that the virus's failure to transmit 100% of the time shows that it is still changing to suit its new hosts. "We don't think it's fully adapted to humans yet," he says.
The virus has not caused serious illness in the majority of cases but this might change. Both studies emphasize the need to keep an eye on swine flu, particularly into the Northern Hemisphere's winter, says Tumpey. "We're worried that the virus could increase its disease-causing ability," he says.
Flu viruses in different species are continually swapping genes among themselves in a process called reassortment. Fouchier says that the swine flu virus he tested has the avian version of a particular flu gene, which may mean the virus can currently only thrive in warm conditions. If that gene mutates to a more cold-tolerant human version, the virus could grow in the nasal passage and so spread more easily. He has now made a version of the virus to include this mutation and is using his ferret model to see how it affects the animals.
These two studies will help scientists to monitor the swine flu virus in future, says John McCauley a virologist from the National Institute for Medical Research in London. "It's a very useful thing to provide the baselines," he says, to compare mutated versions against as they emerge. "We need to keep doing these studies," McCauley adds.
- Maines, T. R. et al. Science advance online publication doi:10.1126/science.1177238 (2009).
- Munster V. J. et al. Science advance online publication doi:10.1126/science.1177127 (2009).