Coffee-flavored milk taken from a cooler at a central Massachusetts dairy carried germs identical to bacteria that killed two elderly men and made two other people sick, according to state test results released yesterday that investigators said left little doubt about the dairy being the source of the infections.
Genetic fingerprinting conducted at the state laboratory has indicated that a milk sample collected at Whittier Farms dairy two weeks ago, a sample taken in November from a bottle in a victim's refrigerator, and blood drawn from the four patients all harbored exactly the same type of listeria, a striking discovery, state disease trackers said.
"The pattern is very unique," said Dr. Alfred DeMaria, the state's director of communicable disease control. "It means there's an outbreak here. There's no question there's an outbreak. And it implies that the dairy is the common source."
It is exceedingly rare, disease investigators said, to discover that cases of listeria are caused by germs with identical genetic profiles. Instead, each infection tends to be the result of a slightly different form of the bacterium.
In 19 other cases in 2007 in Massachusetts, each infection was caused by a germ with a distinctive fingerprint. Similarly, there were no genetic matches among 99 listeriosis cases in the previous five years.
That, specialists said, is why it was so telling that the samples from the dairy, the patient's refrigerator, and the four patients all matched.
The owners of Whittier Farms, a family-owned operation with its own herd of cows and milk bottled in glass, continued to decline requests for interviews yesterday.
The state investigation into how milk produced at the dairy became tainted with listeria is increasingly focused on the packaging process, officials of the Department of Public Health said.
The entire production line in Shrewsbury is being swabbed by investigators, but after evaluating the dairy's pasteurization practices, they concluded that Whittier had closely followed guidelines dictating the temperature at which milk is sterilized and for how long. That finding pointed state specialists in the direction of what happened after the milk was pasteurized, said Suzanne Condon, the top environmental health official at the Department of Public Health.
"It sounds like something toward the end of the production line and something perhaps in the bottling itself, any area where something is introduced," Condon said. "Is there something that happened to the bottle itself?"
The state has four inspectors reviewing operations at the processing plant, including how the dairy washed the glass bottles it used. Whittier earned a following in large part because its milk was home-delivered in bottles, as well as plastic.
While tests on the coffee-flavored bottle of milk, produced Dec. 17, were positive for listeria, small samples taken from nine other bottles were negative, said state disease specialists, who now intend to evaluate larger samples of milk taken from Whittier-owned stores in Shrewsbury and West Sutton. State authorities said they expect to receive lab results of the production-line samples next week.
Investigators were perplexed about the outbreak, Condon said, because Whittier had an enviable safety and quality track record. Inspection reports obtained by the Globe last week showed that a series of routine inspections last fall by the state found no health violations at the dairy.
In interviews last week, dairy industry representatives expressed surprise that listeria would be found in milk that has undergone pasteurization, a process designed to kill germs.
"This is an extremely rare situation, but obviously something we take seriously," said Jenny Bourbeau, spokeswoman for the New England Dairy & Food Council, an industry organization. "Dairy products are among the safest products on the shelf, because they are pasteurized."
Yesterday the state provided the most detailed description so far of the products sold by Whittier, which voluntarily halted production last week. Whittier makes milk products under brand names that include Whittier, Schultz, Balance Rock, Spring Brook, Model Dairy, and Maple.
The products include whole milk, 2 percent, 1 percent, skim, and heavy cream and feature such flavors as low-fat chocolate, coffee, strawberry, vanilla, and egg nog. Except for Balance Rock, which comes only in bottles, all the Whittier-produced brands are sold in plastic containers, as well as glass.
Whittier milk was sold through the dairy's two company stores and at small shops and bakeries and was delivered directly to customers' doorsteps. Large grocers did not stock it.
Stephen Smith can be reached at stsmith@globe.com.
