After cleaning up the latest avian flu outbreak in the Fraser Valley, scientists still don't know for sure why the valley's poultry farms are so susceptible to the bird bug.

Three of the four most recent avian flu outbreaks in Canada have hit Fraser Valley chicken and turkey operations — the latest one forcing a cull of 60,000 turkeys on an Abbotsford, B.C., farm last week.

Tests so far indicate the H5 virus has not spread to any other poultry producers within a three-kilometre quarantine zone.

However, the Canadian Food Inspection Agency said Sunday it has quarantined seven premises outside the zone and two more inside, based on an analysis of the movement of people, products and equipment.

In all, there are 36 premises under quarantine but the agency said that so far all of those flocks have tested negative for the virus.

Experts have difficulty explaining why the Fraser Valley has been hit so often but there are theories.

Chief B.C. veterinarian Ron Lewis points out that the Fraser Valley is underneath the busy Pacific flyway, which leaves its billion-dollar poultry industry vulnerable to migrating wild waterfowl carrying avian flu.

Scientists say the high concentration of commercial poultry producers in the Fraser Valley could also be a factor in the bird flu spread.

This is the third avian flu outbreak in the Fraser Valley in the last few years.

In 2004, an H7-type flu transformed into a highly contagious strain.

Farm after farm was quarantined until finally about 15 million birds — almost the entire valley poultry population — were destroyed.

In the second Fraser Valley outbreak in November 2005, two duck farms were infected with the H5N2 strain of the virus.