The discovery of avian influenza (search) in Maryland compelled agriculture officials to order the slaughter of 328,000 birds on two huge commercial chicken farms, nearly four times the number killed when two Delaware farms were infected last month.

The cases in Delaware and on Maryland's Eastern Shore are from the same H7 strain (search), which is not harmful to humans, authorities said Sunday.

Maryland agriculture officials confirmed Saturday that a farm with about 118,000 chickens was infected. They ordered the slaughter of those birds, as well as 210,000 others at another farm about a mile away under the same ownership. The slaughter began Sunday and was expected to take until Tuesday.

Agriculture officials also ordered a quarantine that covers eight farms within a two-mile radius of the infected farm, which grows chickens on a contract basis for Mountaire Farms of Selbyville, Del. The department also began testing 79 poultry farms within a six-mile radius.

The infected farm in Worcester County is about 45 miles from the nearest infected farm in Delaware. Authorities said they had not discovered a connection between the cases, but could not rule out any relation.

Officials emphasized the strain of the virus is not a threat to humans, but it can wipe out poultry farms, especially if it turns up in a highly pathogenic form.

"It is discouraging, and it's surprising to us," Maryland Agriculture Secretary Lewis Riley, said Sunday at a news conference near the infected farm in Pocomoke City (search). "We're ready, and we're prepared to address it and to handle it."

The flu case dimmed hopes that international markets banning U.S. poultry would drop their embargoes. Those with a block on imports include the 15-nation European Union, China, Japan, Mexico, Russia and South Korea.

A different, more dangerous avian influenza strain has killed some 100 million birds in Asia and has been blamed for the deaths of more than 20 people in Vietnam and Thailand.

"It's got everyone's palms sweating right now. It's a very serious situation," said Jeff Green, who works for a fertilizer distributor and owns a chicken farm in nearby Marion. "We hope the quarantine will get it under control."

Also on Sunday, Riley tightened a statewide ban on moving, gathering or selling live birds.

Industry leaders and state officials declined to identify the infected farm to keep away visitors who might spread the disease. But it is visible from a nearby state road, and workers in white plastic bio-security suits were seen Sunday cleaning out the chicken houses as state troopers blocked off the area.

Mountaire Farms officials did not immediately return a call Sunday.

Officials said they discovered the flu after a grower reported many of his chickens were dying. The state ordered the slaughter of birds on that farm as well as in houses about a mile away that are under the same ownership. A third farm owned by the grower is two miles away and will be observed this week, officials said.

A more dangerous strain of avian influenza was found in Texas last month.