LONDON, Oct 3 (Reuters) - Sewage systems do not break down Tamiflu, which means the main weapon against bird flu could seep into natural waters and make certain viruses resistant to the drug during a pandemic, Swedish researchers said on Wednesday. Because of this, doctors should take care to not overprescribe Roche Holding AG's <ROG.VX> market-leading antiviral drug, they said in a study published in the Public Library of Science. "Antiviral medicines such as Tamiflu must be used with care and only when the medical situation justifies it," Bjorn Olsen, a researcher at Uppsala University and the University of Kalmar said in a statement. "Otherwise there is a risk that they will be ineffective when most needed, such as during the next influenza pandemic." Roche, which the researchers said donated the drug for their study, said it was unlikely such resistance would arise. "In the highly unlikely event that such resistance was generated, this must be balanced against the fact that influenza viruses with the associated mutational changes have been shown to have lower transmissibility," the company said. Tamiflu, known generically as oseltamivir, was having lacklustre sales as a drug to prevent and treat seasonal flu until it was the first treatment to show real efficacy in helping people with bird flu. Health experts agree that a pandemic of something is inevitable. They cannot specify the disease, but the H5N1 avian flu virus currently wiping out flocks from Indonesia to Africa and parts of Europe is the main suspect. It rarely infects people but has killed 201 out of 329 people sickened since the virus re-emerged in Hong Kong in 2003, according to the World Health Organisation. In their study, the Swedish team said low levels of oseltamivir, the active substance in Tamiflu, passed virtually unchanged through basic sewage treatment processes. People had long suspected that Tamiflu would not break apart during such treatment but this is the first time researchers have actually shown this, the researchers said. In certain countries, the level discharged through these outlets may be so high that influenza viruses in nature could develop resistance to Tamiflu, they said. "Use of Tamiflu is low in most countries, but there are some exceptions such as Japan where a third of all influenza patients are treated with Tamiflu," Jerker Fick, a researcher at Umea University who led the study, said in a statement. The biggest threat is from waterfowl such as ducks that often forage near sewage outlets, Fick said. These birds could encounter oseltamivir in high enough concentrations to develop resistance to flu viruses they carry, the researchers said. In turn, the viruses could combine with other viruses that make humans sick and mutate into strains resistant to currently available antiviral drugs like Tamiflu, they added.
