Nebraska agriculture officials say another 1.8 million chickens must be killed after bird flu was found on a farm in the latest sign that the outbreak that has already prompted the slaughter of more than 50 million birds nationwide continues to spread. The Nebraska Department of Agriculture said Saturday that the state's 13th case of bird flu was found on an egg-laying farm in northeast Nebraska's Dixon County, about 120 miles north of Omaha, Nebraska. Just like on other farms where bird flu has been found this year, all the chickens on the Nebraska farm will be killed to limit the spread of the disease. The U.S. Department of Agriculture says more than 52.3 million birds in 46 states — mostly chickens and turkeys on commercial farms — have been slaughtered as part of this year's outbreak. Nebraska is second only to Iowa's 15.5 million birds killed with 6.8 million birds now affected at 13 farms. In most past bird flu outbreaks the virus largely died off during the summer, but this yea