ISLAMABAD, May 25 (Reuters) - Pakistani authorities have detected the H5N1 strain of bird flu at two more poultry farms on the outskirts of Islamabad, taking to five the number of outbreaks found this week, a government official said on Friday.

Several outbreaks of the H5N1 strain of bird flu have been found in poultry this year in Pakistan, where the deadly virus first appeared in early 2006. Pakistan has had no human cases.

"We have discovered two more cases in the same area where the virus was found in three poultry farms early this week," said Food and Agriculture Ministry official Rafiq-ul-Hassan Usmani.

Samples were gathered on Wednesday after the death of more than 3,000 birds on the two farms in the Chak Shezad area. The samples tested positive for the H5N1 strain and authorities culled the remaining 4,000 chickens on Thursday.

Usmani said authorities had stepped up surveillance in the area, which is known as "poultry pocket" because of its more than 40 commercial chicken farms.

"My hunch is that birds on many of these farms are not getting the vaccination, and the free movement of workers and birds within the area is also causing the spread," he said.

Health officials had been sent to examine farm workers, he said.

Poultry was culled on several small farms in early 2007 after detection of the virus and authorities briefly shut Islamabad Zoo in February after four peacocks and a goose died of H5N1.

The WHO says 186 people worldwide have died of bird flu since the H5N1 virus resurfaced in Southeast Asia in 2007. The virus has since spread throughout much of Asia, parts of Europe, the Middle East and Africa.

Indonesia has the highest number of confirmed human deaths from the virus, with 77.

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