TIMELINE-Tracing the bird flu outbreak in N. American poultry flocks

Exports, which totaled more than $6 billion last year, have been hit as buyers, including China and Mexico, slap bans on U.S. supplies.

Below is a timeline of the spread of the viruses, according to the U.S. Department of Agriculture's Animal and Plant Health Inspection Service (APHIS), Canada's Food Inspection Agency (CFIA), and responses by the industry and trade partners.

Dec. 2, 2014 - The CFIA quarantines two turkey and chicken farms in Canada's British Columbia province after an H5 type of avian influenza is detected there, later confirmed to be the H5N2 strain.

Dec. 3 - South Korea, Hong Kong, Taiwan and Japan impose restrictions on British Columbian poultry and products.

Dec. 8 - The United States suspends imports of birds and hatching eggs, poultry meat, eggs and egg products and animal byproducts from British Columbia.

Dec. 19 - The outbreak's first U.S. case is confirmed as H5N8 avian influenza strain is found in a mixed poultry flock in Douglas County, Oregon.

Dec. 20 - South Korea, one of the top buyers of U.S. poultry, halts imports of poultry and poultry products from the United States.

Jan. 3, 2015 - The first case of the highly contagious H5N2 avian influenza strain confirmed in a backyard flock of 140 mixed birds in Benton County, Washington.

Jan. 6 - Mexico, the largest market for U.S. poultry at $1.2 billion in 2014, bans imports from states with confirmed cases.

Jan. 7 - No. 2 U.S. poultry importer Canada bans imports from affected areas.

Jan. 8 - Imports of U.S. poultry, poultry products and eggs banned by China.

Jan. 23 - H5N8 appears for the first time in a commercial turkey flock of 134,400 birds in California.

Feb. 2 - The CFIA finds the H5N1 virus in a backyard poultry flock in British Columbia.

Feb. 12 - The first commercial chicken flock is hit with H5N8. The Kings County, California, flock had 112,900 birds.

March 4 - The first instance of highly pathogenic avian influenza (HPAI) along the Mississippi migratory flyway is confirmed in a commercial flock of 26,310 turkeys in Minnesota, the top U.S. turkey producing state. The flyway runs from the Gulf of Mexico to the northern Midwest along the Mississippi River valley. The virus is thought to be traveling with wild birds as they migrate north.

April 6 - The CFIA confirms an H5 HPAI strain on a turkey farm in Ontario. A day later, Japan and Taiwan impose restrictions on poultry and products from the region.

April 11 - The H5N2 strain is confirmed for the first time in a commercial chicken operation, hitting 200,000 egg-laying hens in Jefferson County, Wisconsin.

April 20 - The biggest flock hit so far, as H5N2 is confirmed in 3.8 million egg-laying hens in Osceola County, Iowa. Mexico expands its import ban to include live birds and eggs from Iowa - the top egg-producer in the United States.

April 20 - Wisconsin declares a state of emergency.

April 23 - Minnesota declares a state of emergency.

April 29 - Saudi Arabia, the world's second-largest importer of chicken broiler meat, bans imports of poultry meat and egg products from Ontario.

April 29 - A chicken broiler breeding farm in Kossuth County, Iowa, initially tests positive for H5 bird flu, believed to be the first case at a broiler breeding farm.

May 1 - USDA confirms bird flu in nine more commercial flocks, including a 5.7 million-bird flock of egg-laying hens in Buena Vista County, Iowa, the largest finding to date. Nationwide, more than 21.6 million birds in 114 mostly commercial operations are affected, the worst outbreak in U.S. history.

May 1 - Iowa declares a state of emergency.

May 5 - U.S. government approves $330 million in emergency funds to fight bird flu spread.

May 11 - USDA confirms H5N8 avian flu in a backyard poultry flock in Indiana. (Reporting by Karl Plume and P.J. Huffstutter in Chicago; Editing by Matthew Lewis)