The deadly swine flu virus has the potential to reach deep into the respiratory system, and even as far as the intestines, according to two new studies on ferrets.
The above findings could explain why the disease's symptoms are different from those of seasonal flu.
The studies were conducted by two separate groups that have been using ferrets to investigate how harmful A(H1N1) influenza virus is, and how easily it is transmitted.
One of the studies was led by Terrence Tumpey at the Centers for Disease Control and Prevention in Atlanta, Georgia, whose colleagues 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.
It was found 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.
In a second study, led by Ron Fouchier at the Erasmus University Medical Center, Rotterdam, the Netherlands, it was found that the virus could penetrate the lungs.
"This is the first indication of how pathogenic [swine flu] really is. In the field that conclusion is hard to draw," Nature magazine quoted Fouchier as saying.
Ferrets have long been used as an animal model for flu because they show similar symptoms to humans, and tend to last the same amounts of time in both species.
The studies showed that the virus wasn't transmitted between animals as efficiently as the seasonal flu, but Fouchier's results indicated that the virus was transmitted just as efficiently as seasonal flu.
The studies have been published in Science.