More than 300,000 chickens on Maryland's Eastern Shore were destroyed yesterday to stem a fresh outbreak of avian influenza. The flu has infected birds in Delaware, Pennsylvania and New Jersey in recent weeks and is threatening the heart of the tri-state Delmarva economy.

The most recent outbreak - discovered Saturday in Pocomoke City near the Virginia line - is the first on a commercial farm in Maryland, where poultry is responsible for about a third of the state's $1.4 billion-a-year agriculture industry.

The flu strain is not considered harmful to humans. But authorities fear it could mutate into a more dangerous variant, such as the one that caused a number of recent human deaths in Asia.

The form of the disease found Saturday is same as that found about 50 miles north on two Delaware farms early last month. That discovery spurred an unprecedented containment effort that has included testing, quarantining and cleaning to prevent spread of the disease.

Since then, the flu has been found on a farm in Pennsylvania and at live bird markets in New Jersey. A more harmful strain also was discovered in Texas.

The continuing occurrences represent a serious setback to the poultry industry in Maryland and beyond.

Poultry farmers have been living with restrictions on movement and use of manure needed to fertilize corn and soybean crops that are grown to feed chickens. Auctions on farm equipment have been suspended, and farmers face an import ban on U.S. poultry products by more than 30 countries.

Efforts by U.S. agriculture officials to persuade nations to lift their bans have been unsuccessful, and it is unclear how the new cases will affect negotiations.

"We're wondering what more restrictions they're going to impose on us," Vance Phillips, a Sussex County, Del., councilman and poultry farmer said yesterday. "This has taken a heavy toll financially, emotionally." He said he feared that control efforts will not stop the flu until it has wreaked havoc on the industry.

He noted that most Delmarva chicken is consumed domestically, but restrictions and the import ban, mostly by Asian countries coping with a highly pathogenic type of flu, could trickle down. As other states lose overseas markets, he said, they will turn to the same local markets as Eastern Shore farmers and drive down the prices.

"We have been working very closely with Delaware and Virginia on this because the virus does not know state lines," said Maryland Agriculture Secretary Lewis R. Riley at a news conference in Pocomoke City yesterday morning. "We really thought that it was contained in Delaware and that the situation was coming to a close. Obviously, there is not going to be business as usual here on the Shore for the next few weeks."

Three dozen members of a poultry industry emergency task force helped state agriculture officials from Maryland, Delaware and Virginia iron out details yesterday of a quarantine they hope will avoid the kind of devastating outbreak that forced the slaughter of 5 million birds in Virginia two years ago.

"Our concern is an economic concern," said Riley, who has raised chickens on his farm near Salisbury for more than 40 years. "This industry affects the livelihood of just about everybody on the Eastern Shore in one way or another."

State officials announced the quarantine of 71 farms in a 6-mile radius of the most recently infected farm. All of those will be tested for avian flu, and officials said testing would continue across the Delmarva peninsula until at least March 16. Officials ordered that samples be taken from each of more than 5,100 chicken houses on the peninsula 72 hours before birds are shipped for slaughter.

"It is a massive logistical task - every house will have birds tested," said Bill Satterfield, executive director of Delmarva Poultry Inc., the industry's largest trade group. "We're challenged because we cover three states, but we are working together to bring this to an end."

The flu spreads from chicken to chicken through saliva and waste that is typically spread by human shoes, cages and trucks.

Live bird markets, frequented largely by immigrants and found in urban areas, are considered particularly susceptible to the virus because it is spread through live animals. Experts believe the markets help keep the flu alive somewhere in the United States at all times.

Other animals and wild birds also can carry the virus, and that has led some to speculate that the Maryland farms were not infected by human behavior.

"This is the first time ever that the virus has affected a commercial flock," said Doug Green, a former DPI president who has been growing chickens for 25 years in Maryland's Somerset County. "We've battened down the hatches. It's frustrating because we don't know how it got here. It could have been ... a duck or some other wild bird flying into a farm."

Phillips, the Delaware poultry farmer, shared the opinion that the culprit was likely a buzzard or other wild fowl prevalent on the rural peninsula.

About 40 percent of Delmarva farms, or more than 800, have tested negative so far in an unprecedented effort among state officials and commercial farms.

In Maryland, officials signed quarantine papers late Saturday, ordering the destruction yesterday of 118,000 6-week-old birds. About a mile away, another 210,000 animals that have not shown signs of illness were killed because the operation is owned by the same farmer.

Following similar precautions employed in Delaware last month, the carcasses of the animals were composted and will remained sealed at high temperatures in chicken houses for up to six weeks.

Under the latest quarantine, samples were taken from chickens at eight farms within the 2-mile radius. Additional tests were ordered for dead birds found at 63 farms within 6 miles of the affected farm.

Farmers are prohibited from cleaning manure and litter from houses or spreading chicken waste to fertilize fields in the quarantine area, which stretches from U.S. 50 to the Virginia border just south of Pocomoke. Some are concerned that the ban on fertilizing fields will delay spring planting or force grain farmers to use more expensive commercial fertilizer.

Like their counterparts in Delaware, Maryland farmers are being discouraged from gathering at meetings or equipment auctions until the virus is controlled.

Sales of live poultry have been stopped at the state's only two live-auction operations in Hagerstown and Westminster.

Riley and other officials refused to give the name of the contract grower whose farm was infected. Nor would they identify which of four major poultry processors on the peninsula owns the birds. The virus was detected when the grower became alarmed by an unusually high mortality rate in his flock and alerted the company.