Mysterious U.S. Swine Flu Probe Widens as Mexico Finds Swine Flu *updated*, page 227

An American flu expert – one of many watching the way that A(H1N1) swine flu is behaving in New Zealand, Australia and South America during the local flu season – predicts it will return to the Northern Hemisphere as the dominant strain of seasonal flu.

Professor William Schaffner told the Centre for Infectious Disease Research and Policy the degree of transmission "has surprised me and frankly most of my colleagues also".

Prof Schaffner, of the department of preventive medicine at Vanderbilt University School of Medicine, added: "I think the ongoing transmission this (Northern Hemisphere) summer and what we know is happening in the Southern Hemisphere - in Argentina and in Australia and New Zealand - is going to be a harbinger of what is going to happen this next influenza season in the United States.

"We all anticipate that H1N1 will be a dominant if not the dominant influenza strain."

He commented as the journal Science published two new animal studies which show the novel H1N1 (swine) influenza now spreading in New Zealand can cause more serious lung disease than seasonal flu strains and sheds infective material from the lung and throat tissue, where it reproduces at higher rates.

The studies - in ferrets and mice - could explain autopsies and case reports of severe pneumonia as well as the virus's rapid spread.

A team from Erasmus Medical Centre in the Netherlands found that the virus transmits easily between ferrets housed in cages whose walls are 10cm apart.

But a team from the Massachusetts Institute of Technology (MIT) and the Centres for Disease Control and Prevention (CDC) found that the novel virus only transmitted well when the ferrets shared direct contact – a sign, that team said, that the new virus has not yet fully adapted to mammals.

The European team, though, warned that the new H1N1 is adapted enough to compete with seasonal flu strains for turf in humans.

It "has the ability to persist in the human population, potentially with more severe clinical consequences," they wrote.

Both studies were published online today by the journal Science. In the study from the CDC and MIT group, investigators tested the ability of three different novel H1N1 isolates to cause disease in ferrets and mice and assessed the transmissibility of the new virus in ferrets by comparing it to a seasonal H1N1 virus.

The animals that received the novel flu viruses lost more weight than the ones that were infected with the seasonal flu strain. Viral shedding patterns were similar for the novel flu groups and the seasonal flu group.

Investigators found high levels of viruses in the lower respiratory tracts of animals infected with two of the three novel H1N1 viruses but not the seasonal flu virus.
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The group also found the novel flu virus in the intestinal tracts of the novel-H1N1-infected animals, consistent with gastrointestinal symptoms, such as diarrhoea, that have been reported in humans infected with the new virus.

In the transmission comparisons, the researchers found that the seasonal H1N1 virus transmitted quickly and completely by droplet transmission and direct contact among the ferrets. But the novel H1N1 viruses did not spread by droplets to all ferrets, and transmission was delayed by five or more days in two of six infected pairs.

This lack of efficient respiratory droplet transmission suggested that additional virus adaptation in mammals may be required to reach the highly contagious behaviours of seasonal H1N1 flu or the 1918 pandemic virus, the group wrote.

Terrence Tumpey, the study's author and senior microbiologist in the CDC's influenza division, said the study shows that the new virus is transmitting, but not quite as well as seasonal strains.

"Seasonal strains transmit like clockwork as soon as we push the cages together," he added.

"We think it (the new virus) could still make additional changes to become more transmissible," Dr Tumpey said. "We need to keep a close eye on the virus and monitor for changes."