"If you can decrease the viral load (with drugs), you can have a good outcome. Even those who are treated late had good results," he told the conference.
Anti-virals such as Tamiflu, also known as oseltamivir, were designed to counter common human flu viruses and are thought to be superfluous after 48 hours, when the average person's immune system would have begun fighting the flu attack and blocking the flu virus from replicating.
But de Jong, head of virology at the Hospital for Tropical Diseases in Ho Chi Minh City in Vietnam, said Tamiflu could still be used to fight bird flu many days after the onset of symptoms because the avian virus would still be multiplying.
The H5N1 virus remains a scourge for birds but experts fear it can cause a pandemic and kill millions of people if it learns to pass efficiently between humans.
Many countries and even big companies have begun stockpiling Tamiflu and worked out contingency plans to tackle a pandemic.
These include stockpiling food and provisions, building more hospital isolation units and quarantine arrangements.
However, an expert at the conference cautioned that the benefits of isolation and quarantine were limited in the case of bird flu, which replicates very aggressively.
"Once you detect clinical symptoms (e.g., fever), you've already got peak viremia (when virus load in the body is at a peak) for a time and you're shedding virus and infecting people without knowing it," Roy Anderson of the department of infectious disease epidemiology at Imperial College in London told Reuters.
"So quarantine and isolation just wouldn't work for influenza A. Everything happens too quickly." H5N1 belongs to the influenza A group of flu viruses.
De Jong agreed with many experts that an early start to treatment was still best - something easier said than done in places such as Vietnam, Indonesia and China.
Most of the 258 people who have been infected with H5N1 worldwide since late 2003 lived in rural areas. Of those infected, 154 died because healthcare was unavailable, too far away or the infection was too far advanced by the time they were admitted to hospital.
"You have to get very effective treatment as early as possible because you will prevent direct viral damage (to lung and other tissues)," de Jong said.
De Jong, who treated 17 H5N1 patients in Vietnam in 2004 and 2005, of whom 12 died, said diagnostic tools were absolutely crucial in rural areas.
"If the community has a small lab or regional hospital, probably these patients would have been diagnosed earlier and treatment would have been given early," he said.
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