The Canadian Food Inspection Agency said that more than 140,000 turkeys and chickens from the affected farms will be euthanized.
The agency said the fifth farm near Abbotsford was quarantined after higher than normal turkey deaths were reported by a farmer Friday.
B.C. chief veterinary officer Dr. Jane Pritchard says 60,000 turkeys are in the fifth farm.
Avian influenza was first detected a week ago on a broiler-breeder chicken farm in Chilliwack which housed 7,000 chickens. About 1,000 of the chickens died from avian influenza.
Officials say no people have fallen ill.
The B.C. outbreak has led at least seven countries —the United States, Japan, South Korea, Taiwan, China, South Africa and Mexico — to ban poultry products from B.C. or all of Canada.
Canada's chief veterinary officer, Dr. Harpreet Kochhar said birds from the first farms where avian influenza was detected have already been euthanized.
Kochhar said the source of the infection is not yet known, although he added that it's possible that wild or migratory birds could have infected the farms. He said the focus is on containing the infection's spread.
Pritchard said there had been movement of birds between some of the infected farms.
In 2004, health officials ordered 17 million chickens, turkeys and other domestic birds slaughtered to contain an outbreak of avian influenza at 42 poultry farms in B.C.'s Fraser Valley, costing the poultry industry hundreds of millions of dollars.
Avian influenza poses little risk to people who are consuming poultry meat if it is handled and cooked properly.
In rare cases, the virus can be transmitted to people who have had close contact with the birds, health officials said.