Viruses from 25 of 184 patients in the Newcastle area of eastern Australia infected with the pandemic germ had a genetic mutation that reduces the potency of Tamiflu, scientists said in a report distributed today by the International Society for Infectious Diseases’ ProMED-mail program.
The cases, first reported in May, raise concern that the resistant strain may spread, leaving doctors without their preferred treatment for influenza. GlaxoSmithKline Plc (GSK)’s Relenza drug is effective against the mutant strain, which is still found in the Newcastle area, according to the World Health Organization’s Collaborating Centre for Influenza in Melbourne.
“As long as it’s isolated to Newcastle and that region, it’s not so much of a problem,” said Ian Barr, the center’s deputy director, in a telephone interview today. “If it spreads further, it might be a concern. Fortunately, we’re heading toward the end of our flu season.”
The cluster of cases with the mutant virus is the largest reported outbreak globally to date, and shows the new variant is capable of being transmitted efficiently in the absence of drug pressure, said Jennifer McKimm-Breschkin, a virologist at the Commonwealth Science and Industrial Research Organization in Melbourne, who studies antiviral resistance.
‘Fit and Transmissible’
“It’s clearly fit and transmissible,” she said in a telephone interview today. “It would be concerning having seen such a large cluster like that.”
None of those infected had taken Tamiflu prior to being tested for flu. Of 16 patients interviewed by the scientists, none had a history of immune suppression, which contributes to drug resistance. None of the patients was admitted to an intensive care unit or died from the infection, the report said.
Further interviews with cases and virological analyses are ongoing, the scientists said.
The H1N1 virus that emerged in 2009 known as swine flu was reported in more than 214 countries and caused about 18,450 deaths worldwide through August 2010, when the WHO declared an end to the pandemic. It’s since become one of three seasonal flu strains circulating worldwide, causing infections mostly during the winter months.
Studies have shown that Tamiflu-resistant bugs develop sporadically in 0.4 percent to 4 percent of adults and children treated for seasonal influenza, according to Basel, Switzerland- based Roche.
The outbreak of resistant H1N1 in Newcastle probably emerged in a treated patient who passed the infection onto others, McKimm-Breschkin said.
Ten to 15 percent of H1N1 infections in the Hunter Valley area of New South Wales state, which includes Newcastle, are resistant to Tamiflu, WHO’s Barr said. The seasonal flu vaccine protects against the new flu variant, as well as influenza type B, the main strain circulating in the Newcastle area, he said.
Tamiflu and Relenza, an inhaled powder, reduce the severity and the duration of flu symptoms by 24 to 30 hours if treatment is started within the first two days of illness, according to the companies. Both drugs work by blocking a protein on the surface of influenza particles called neuraminidase, which allows the virus to spread from infected cells to other cells.
Mutant H1N1 viruses evade Tamiflu through a single genetic change known as the H275Y mutation which prevents the medicine from clinging to neuraminidase, enabling the pathogen to spread.
An outbreak of cases of seasonal flu with the H275Y mutation was first recorded by Norway in January 2008 and by the following August widespread resistance was reported in 40 countries in Europe, North and South America, Africa, Asia and Australia.
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