Mark Siemens is a third-generation egg farmer in B.C.'s Fraser Valley and he recalls his grandfather sharing a story about fighting an unknown disease that raced through the farm decades ago, forcing him to cull the entire flock.Siemens didn't expect to be facing a similar fight so many years later.He noticed some birds seemed agitated a few weeks ago, showing symptoms of itchy eyes, and said he immediately called the Canadian Food Inspection Agency.The verdict was in by the end of the day: his chickens were infected with the highly pathogenic H5N1 variant of avian influenza."It's super sad, and it's a tough thing to go through when you know you care about these animals and you do everything you can to keep them healthy and make sure they're looked after," Siemens said in an interview.His business is one of about four dozen flocks, most of them commercial, that have been infected with avian flu in British Columbia this fall. Infections flair during migratory seasons, as wild birds are considered the chief cause…