The national outbreak has been linked to raw chicken and illnesses have been reported in 20 states and Puerto Rico.

According to the Centers for Disease Control, the majority of the cases, 73 per cent, have been in California.

No deaths have been reported.

The CDC is collaborating with public health and agriculture officials in many states and the Department of Agriculture's Food Safety and Inspection Service (USDA-FSIS) to investigate the outbreak, which has been narrowed down to Salmonella [enterica_ serotype] Heidelberg.

Public health investigators are using DNA "fingerprints" of Salmonella bacteria obtained through diagnostic testing with pulsed-field gel electrophoresis, or PFGE, to identify cases of illness that may be part of this outbreak.

They are using data from PulseNet, the national subtyping network made up of state and local public health laboratories and federal food regulatory laboratories that performs molecular surveillance of foodborne infections.

Seven strains of _S._ Heidelberg bacteria have been identified as being linked to this outbreak. Four of these strains are rarely reported to PulseNet. The other three strains are more common, with several ill persons infected with each strain reported to CDC monthly.

The DNA fingerprints of the _S._ Heidelberg bacteria associated with the current outbreak include the strain that was also associated with a multistate outbreak of _S._ Heidelberg linked to Foster Farms brand chicken during 2012-2013.

The CDC said: “Epidemiologic, laboratory, and traceback investigations conducted by officials in local, state, and federal public health, agriculture, and regulatory agencies indicate that consumption of Foster Farms brand chicken is the likely source of this outbreak of _S._ Heidelberg infections.

“In interviews, ill persons answered questions about foods consumed and other exposures during the week before becoming ill. Among those who had brand information available, 48 (79 per cent) of 61 ill persons reported that they had consumed Foster Farms brand chicken or another brand likely produced by Foster Farms.”

Further tests have shown that some of the strains of the Salmonella that have been discovered are antibiotic resistant although not all the strains exhibited the same antibiotic resistance patterns.

To date, isolates collected from poultry were resistant to combinations of the following antibiotics: ampicillin, chloramphenicol, gentamicin, kanamycin, streptomycin, sulfisoxazole, and tetracycline.

Foster Farms submitted a plan outlining "substantive changes to its slaughter and processing" to USDA last Thursday and the USDA has allowed the three Foster Farms plants to continue to continue operating.

The review follows Foster Farms’ implementation of several new food safety controls over the last two months and the company’s commitment to install added processes during an enhanced inspection period over the next 90 days.

“We started this process more than two months ago and this officially validates our progress, but we are not stopping here,” said Ron Foster, president and CEO of Foster Farms.

“We are putting every resource and all of our energy toward food safety with the confidence that Foster Farms plants will be the most stringent in the industry.”

USDA-FSIS inspectors are continuing to inspect and approve the safety of Foster Farms chicken daily.

Dr. Ron Chapman, director of the California Department of Public Health and state health officer, said that there was no need for a recall.

He said: “The CDPH has not requested Foster Farms to recall chickens because, with proper handling and preparation, this product is safe for consumption.

“Chicken is a raw animal protein that is expected to have some level of naturally occurring bacteria present.

“Cooking chicken fully to 165 degrees Fahrenheit will kill the bacteria present. Provided that consumers do not cross-contaminate fully cooked chicken with raw chicken juices, it is safe to consume.”

“All of us at Foster Farms regret any illness associated with our products,” said Foster.

“We have worked relentlessly to address these issues and will continue to do so as we work to regain consumer trust and confidence in the Foster Farms brand. On behalf of everyone at Foster Farms, that is my commitment to you.”

However, the move to allow the plants to remain open has come under fire from some political quarters.

Congresswoman Louise M. Slaughter said: “It is a disgrace that the USDA, a body charged with protecting the public’s food supply, has chosen to let a repeat offender like Foster Farms continue operations. Foster Farms has been cited multiple times for sanitary violations just since January of this year.

“They had ample opportunity after the July outbreak incident traced to their Washington state plant to clean up all their operations, but they chose not to. Now they are addressing three plants in California, but they – and we – should be worried about all of their plants across the United States.

“The USDA’s toothless decisions endangers public health today, and encourages bad actors in the food industry to continue to break the law tomorrow. The American people need a regulatory agency that works for them and not the food industry. Until we reach that goal, outbreaks like this will become more common and the public’s health will continue to suffer.”