For the past two months, Blue Bell Creameries has grappled with an outbreak of listeria that has been linked to three deaths and at least 10 illness in four states. While the source of the contamination has yet to be determined, federal investigators now believe the issue has been ongoing for at least five years.

NBC News reports that genetic tests performed by the Centers for Disease Control and Prevention link the Listeria bacteria from two Blue Bell factories to at least six cases of listeriosis that took place as far back as 2010.

Prior to linking the six older cases, the CDC said the outbreak had sickened five people – three of whom later died – in Kansas, three in Texas and one each in Arizona and Oklahoma.

The CDC says that the outbreak involves two distinct strains of bacteria that have been making consumers sick across the nation.

“This is a complex and ongoing multistate outbreak investigation of listeriosis illnesses occurring over several years,” CDC said in a statement. “Several strains of Listeria monocytogenes are involved in this outbreak. Information indicates that various Blue Bell brand products are the source of this outbreak.”

CDC experts say testing of recently made Blue Bell products has provided a link to six cases of listeria that occurred in Texas, Oklahoma and Arizona starting in 2010.

Dr. Robert Tauxe, an expert on food borne disease with the CDC, says that detectives first discovered the relationship between the past listeriosis cases when further investigating the recent deaths in Kansas.

Investigators had taken samples from a freezer at a Kansas hospital where the three patients died. While some of the ice cream products matched the bacteria found in those patients, one sample did not. However, Tauxe tells NBC News that the genetic sequence of that sample matched the six old cases of listeria.

The discovery led the CDC to determine that there were two strains of Listeria from two Blue Bell factories, one in Texas and another in Oklahoma.

An outbreak of this length is unusual, Tauxe says, but it’s possible that few people were actually sickened by the bacteria.

“It is not the biggest outbreak, not by any means,” he said. “But it is probably the longest outbreak of listeria.”

As a response to the new findings, Blue Bell recalled all of its products from retailers and shut down all four of its operating factories on Monday.

The massive voluntary recall was initiated after two chocolate chip cookie dough ice cream samples tested positive for the potentially deadly bacteria.

The final recall includes frozen yogurt, sherbet and frozen snacks distributed in 23 states as well as internationally, because those items “have the potential to be contaminated,” the company said.

On Tuesday, the company announced it would step up efforts to track down the source of the listeria contamination by hiring microbiologists to work with CDC investigators and by expanding current cleaning and sanitation systems.

“As each day passes, we are getting closer and closer to figuring out how this listeria was introduced into our facilities. … It’s a matter of doing the work and not making excuses,” a representative for Blue Bell said.

Blue Bell Listeria Outbreak Has Been Going on For Five Years, CDC Says [NBC News]

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