The U.S. Department of Agriculture is threatening to close three California poultry plants operated by Foster Farms blamed for an outbreak of salmonella poisoning that has sickened at least 278 people nationwide.

In a letter sent Monday to Foster Farms, the USDA said sanitary conditions at the facilities were so poor that they posed a “serious ongoing threat to public health.”

The agency has ordered Foster Farms, one of the nation’s largest privately owned poultry producers, to develop a plan by Thursday to clean up the plants. Two of those facilities are in Fresno and one is in Livingston, Calif., where the company is based.

The CDC reports as of October 7, 2013, a total of 278 individuals infected with the outbreak strains of Salmonella Heidelberg have been reported from 17 states. Most of the ill persons (77%) have been reported from California. The number of ill persons identified in each state is as follows: Alaska (2), Arkansas (1), Arizona (11), California (213), Colorado (4), Connecticut (1), Florida (1), Hawaii (1), Idaho (2), Michigan (2), North Carolina (1), Nevada (8), Oregon (8), Texas (5), Utah (2), Washington (15) and Wisconsin (1).

42% of ill persons have been hospitalized.

Epidemiologic, laboratory, and traceback investigations conducted by local, state, and federal officials indicate that consumption of Foster Farms brand chicken is the likely source of this outbreak of Salmonella Heidelberg infections.

The CDC reported in July of 2013, a total of 134 persons infected with the outbreak strain of Salmonella Heidelberg reported from 13 states. 31% of ill persons were hospitalized, and no deaths were reported.  Most of the ill persons were reported from two states, Oregon (40) and Washington (57).  Collaborative investigative efforts of local, state, and federal public health and regulatory agencies indicated that Foster Farms brand chicken was the most likely source of this outbreak. Testing conducted by the Washington State Public Health Laboratories identified the outbreak strain of Salmonella Heidelberg in four intact samples of chicken collected from three ill persons’ homes in Washington, which were traced back to two Foster Farms slaughter establishments.

As of October 7, 2013, U.S. Department of Agriculture’s Food Safety and Inspection Service (USDA-FSIS) issued a Public Health Alert due to concerns that illness caused by Salmonella Heidelberg is associated with chicken products produced by Foster Farms at three facilities in California.

This investigation is ongoing. USDA-FSIS is prepared to take additional actions or expand the investigation based on new evidence.

The outbreak strains of Salmonella Heidelberg are resistant to several commonly prescribed antibiotics. This antibiotic resistance may be associated with an increased risk of hospitalization or possible treatment failure in infected individuals.