WASHINGTON (Reuters) - A strain of bird flu which is devastating to commercial chicken flocks but not harmful to humans has spread into Pennsylvania and more cases may be found in Delaware, agriculture officials in both states said on Thursday.

The disease was also confirmed in four small poultry markets in northeastern New Jersey, a state which has had the disease for a dozen years and does not have any large, commercial flocks, according to its state officials.

The H7 avian influenza in the three states is a different strain than the H5N1 virus that has devastated flocks in 10 nations in Asia. Some 19 people have died in that outbreak.

"The single take-home message is that the form of avian influenza in New Jersey is not a human health threat," said state health commissioner Clifton Lacy.

Because avian influenza is spread easily, authorities often quarantine infected farms and destroy their birds. A dozen nations have banned all or some U.S. poultry shipments since the first infected flock was found last week in Delaware.

Chicken accounts for one-third of U.S. meat consumption. The government estimates each American will consume 82.5 pounds of chicken this year.

The Pennsylvania infection was found in a flock of nearly 500,000 egg-producing hens, which was immediately quarantined. The birds, which have not shown any physical signs of illness, may be destroyed depending on further testing.

"We have a flock that has tested positive in Lancaster County," Pennsylvania Agriculture Secretary Dennis Wolff told Reuters. "It is still undecided whether to depopulate."

Some 16 poultry flocks within a two-mile radius of the infected Pennsylvania farm will undergo testing for the virus.

Wolff said the strain of bird flu in the Pennsylvania chickens was not yet determined but he thought it was likely to be the same as that which was found in Delaware. Avian influenza was last found in Pennsylvania in 2002, he said.

Farmers fear the disease because it can rapidly spread throughout a flock, cutting egg production and producing misshapen or soft-shelled eggs. Bird flu is spread through the birds' feces or mouth secretions and can infect turkeys, pheasants, quail, ducks, geese, and migratory birds.

The disease is so contagious that Delaware officials have asked farmers to post "keep out" signs on their property, to stop taking tractors and farm equipment to auction and to have essential visitors wear disposable biosecurity clothing.

In Delaware, a major producer of chickens for companies such as Tyson Foods Inc., officials said they hoped the disease was contained but could not rule out more cases.

No new cases have been found in tests of 36 of the 80 or so poultry houses within six miles of the two infected flocks.

"We are cautiously optimistic because no new cases of Avian Influenza have been discovered, said Delaware Agriculture Secretary Michael Scuse, adding, "We realize that due to the fact that the virus is easily spread among chickens, it is entirely possible that we may see more cases."

Scuse asked the public to stay away from poultry farms in order to reduce the chance of the virus spreading.

"We have made arrangements with state and local enforcement agencies to maintain strict biosecurity on all poultry farms," he said.

New Jersey officials said they discovered avian influenza during annual testing of the markets within miles of New York City. At the live markets, buyers select chickens to be killed and then take them home to cook.