The virus, which makes regular appearances at markets with live poultry, is less troubling for New Jersey because the state does not have a commercial broiler chicken industry.
But officials in Delaware - which has the top broiler-producing county in the nation, Sussex - have wondered if the Kent County, Del., operation that first tested positive for the flu last week picked up the virus while delivering chickens to a New York live market.
"Because the population at live bird markets is a fluid population, birds coming in and birds going out and changing hands, it's an ideal area to spread the disease if it's in the bird population," said Ed Wengryn, a field representative with the New Jersey Farm Bureau.
A U.S. Department of Agriculture epidemiologist yesterday began the task of trying to figure out how the virus found its way to the Kent County farm and to a Sussex County operation five miles away, according to the trade group Delmarva Poultry Industry Inc.
The strain discovered there and at the live markets is not harmful to humans - unlike the avian flu that appeared in Southeast Asia - but it's bad for business. More than a dozen countries, mostly in Asia, have banned poultry from the United States.
U.S. health officials, concerned that the Asian strain could develop into a disease easily transmittable between people, told a congressional panel yesterday that drug companies need only a few months to produce 270 million doses of a vaccine for humans if the need arises.
In Delaware, the situation remained an economic rather than a health risk.
"We are cautiously optimistic because no new cases of avian influenza have been discovered," Michael T. Scuse, Delaware's agriculture secretary, said in a statement yesterday. "However, 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. We will continue testing."
Results came back yesterday for four chicken houses, all of which tested negative for avian flu. Thirty-six chicken houses have been tested and found clean, though there are still many to check - roughly 80 farms are within six miles of the infected sites.
New Jersey tests its approximately 30 live bird markets once a month, a process that takes three to four weeks. Officials said the four markets that tested positive during the last round will be allowed to sell their existing stock of birds before they're closed for a day or two of cleaning.
In the past week, Delaware killed about 85,000 birds and quarantined all farms within six miles of the infected sites. Among other measures, the state has canceled all farm meetings to try to cut down on points where the disease could spread. Maryland followed suit, also banning live poultry auctions.
"We've responded quickly because this has the potential to devastate Delmarva's chicken industry," said Bill Satterfield, executive director of the Delmarva Poultry Industry Inc., who called avian influenza the disease that chicken farmers fear most.
He said the $1.5 billion broiler industry on the three-state peninsula has a leg up on the situation because poultry companies and state agencies worked out a payment plan about a decade ago for covering the costs of containment - from disposal of carcasses and disinfection of farms to reimbursement for lost chickens.
"It allows decisive action immediately," Satterfield said. "If we have a case, we can make the decision within the hour, 'OK, those birds are going to go down,' instead of waiting days or months to figure out 'Are we going to get reimbursed for it?'"
Sun staff writer Meredith Cohn and the Associated Press contributed to this article.