Indonesian bird flu officials said Tuesday they were investigating several recent avian influenza deaths where the victims were believed to have not come into contact with infected poultry.
"In the last three to four months, we have had four cases where the poultry in the victim's neighbourhoods (tested) negative for the virus," said Bayu Krisnamurthi, head of Indonesia's National Avian Influenza Committee.
"The number is significant enough for us to intensify our investigations so that we could have a more accurate explanation," he told a press briefing giving an overview of what has happened in Indonesia this year with bird flu.
"Some 20 percent of confirmed cases in 2006 were inconclusive, meaning there was no direct contact with poultry. This year (2007) the figure has been raised to 30 percent," he added.
The bird flu virus is usually transmitted to humans from infected birds, but scientists fear it could mutate into a form easily transmissible between humans, sparking a global pandemic that could kill millions.
Krisnamurthi nevertheless insisted that Indonesia had made progress in tackling bird flu, with the number of reported cases decreasing this year.
In 2007, 40 cases were confirmed with 35 fatalities, compared to figures of 55 and 45, respectively, in 2006.
But he also conceded that Indonesia needed more effective measures, particularly to combat bird flu in poultry.
According to Krisnamurthi, even though the agriculture ministry vaccinated 70 percent of the country's farmed poultry in 2007, "the quality of the vaccine must be improved," he said.
Indonesia has recorded a total of 115 confirmed bird flu cases, 93 of which resulted in death.