On November 27, 2007, a health department officer in central Massachusetts contacted the Massachusetts Department of Public Health (MDPH) to report a Listeria infection in an 87 year old man, later identified as John Powers. Pulsed-field gel electrophoresis (PFGE) performed on Mr. Powers’s Listeria monocytogenes stool isolate produced a pattern indistinguishable from that of isolates from three other cases identified in residents of central Massachusetts in June, October, and early November 2007. MDPH, in collaboration with local public health officials, conducted an investigation, which implicated pasteurized, flavored and non-flavored, fluid milk produced by a local dairy as the source of the outbreak. The milk was later revealed to have been produced by Whittier Farms, Inc., a family owned dairy located in Sutton, Massachusetts. In fact, it was coffee-flavored milk, produced by Whittier Farms and purchased at Shady Oaks Farm, that tested positive for Listeria and that was a PFGE-match to the strain of Listeria associated with Mr. Powers, the other victims, and the environmental samples collected from the dairy facility.
Whittier Farms operated a milk product pasteurizing, bottling, and processing facility; the dairy had operated for nearly 50 years. Raw milk was transported by tanker truck to the Whittier Farms processing facility from the company’s own farm (with nearly 300 cows) and from another, independent farm located 25 miles away. Whittier Farms produced various milk and non-milk beverage products in glass and plastic bottles, including several varieties of flavored milk. Retail outlets were located at the dairy and the farm; however, the bulk of the dairy’s milk products were sold through home delivery, and at various retail establishments in Massachusetts, including Shady Oaks Farm in Medway. The milk products were sold under the Whittier Farms name and other brand names. Bulk cream was distributed to a bakery in Rhode Island, where it was used in cooked products.
The MDPH defined a case of outbreak-associated listeriosis as illness in a Massachusetts resident with illness onset in 2007 who: (1) was culture-positive for Listeria monocytogenes with PFGE patterns that matched the outbreak patterns as established by the first case; or (2) had culture-confirmed Listeria monocytogenes and a history of consuming milk products produced by Whittier Farms during the six weeks preceding illness and for whom a bacterial isolate was not available for PFGE analysis.
Five patients had illness consistent with the case definition. All but one of the patients met the first case definition criterion. The median age of the patients was 75 years old (range: 31 to 87 years); three were male. All five patients were hospitalized. All three of the males (75 to 87 years old), including Mr. Powers, died from sepsis attributed to Listeria, and died close to the time of their acute illness onset. The first case in a female was in a 31 year old woman who had chorioamnionitis at 36 weeks gestation. She delivered a healthy but premature infant. A subsequent placental culture tested positive for Listeria. The second case in a female was in a 34 year old woman who had a fever and abdominal pain. She experienced a stillbirth at 37 weeks gestation, and cultures of her blood, fetal blood, and placental tissue all were positive for Listeria monocytogenes.
Interviews were subsequently conducted with the patients or their families. One patient, however, could not be interviewed. Of the remaining four patients, all but one had consumed products from Whittier Farms during the six weeks preceding their illness.
On December 17, evidence of Listeria growth was reported from a coffee-flavored milk sample, retrieved from the home of Mr. Powers. In response to the December 17 findings, the Massachusetts Food Protection Program (MFPP) inspected Whittier Farms and collected eleven samples of unopened, flavored and unflavored milk products for testing on December 18. On December 21, the organism from the December 17 sample was confirmed to be Listeria monocytogenes and was a match to the four clinical isolates from the other patients. Thus, the source of this particular genetic strain of Listeria was identified. The same unique PFGE pattern was identified within Whittier Farms milk products and the four PFGE confirmed cases associated with the outbreak, including Mr. Powers. From this point forward, there was no doubt that the Whittier Farms product resulted in the patients’ Listeria infections.
MFPP returned to Whittier Farms on December 26 and collected environmental swab samples from inside the processing facility. On December 27, the State Laboratory Institute (SLI) of MDPH reported a presumptive positive Listeria specimen in a sample of unopened, coffee-flavored milk that had been collected from Whittier Farms on December 19. In response to this finding, MFPP asked the dairy to voluntarily cease all operations and recall its dairy products; Whittier Farms complied with this request on December 27. On December 30, SLI confirmed that Listeria monocytogenes with PFGE patterns identical to the outbreak strain was isolated from a sample of unopened, coffee-flavored milk ingested by Mr. Powers.
From December 28, 2007, to January 3, 2008, MFPP conducted a full environmental investigation in conjunction with the Food and Drug Administration and the local board of health. The facility did not have an environmental monitoring program for Listeria monocytogenes. Although this is not required by law, is typically implemented as a best practice by other food processors of ready-to-eat foods. Contamination, as demonstrated by the positive environmental samples, was documented in close proximity to areas where hoses were used to clean equipment. On February 1, 2008, Whittier Farms decided to permanently close the milk processing facility, citing an inability to invest the money necessary to make the facility safe.
On January 2, 2008, after the closure of Whittier Farms and the recall of its dairy products, approximately 100 additional environmental and product samples were collected by MFPP from the dairy’s processing facility and adjacent retail store. One environmental swab from a floor drain in the finished product area, one skim milk sample, and seven flavored milk samples tested positive for Listeria monocytogenes and matched the outbreak strain by PFGE analysis. Two additional environmental swabs and four additional samples of milk, both flavored and non-flavored, tested positive for seven distinct strains of Listeria, including three different Listeria species and three strains of Listeria monocytogenes with PFGE patterns that differed from those of the outbreak strain.
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 PFGE is a technique that generates a specific banding pattern when applied to DNA extracted from isolates of bacteria. The DNA is cut into fragments with restriction enzymes, yielding between eight and twenty-five large bands that can be up to 600 kb long. After digestion, the fragments are separated electrophoretically by size on an agarose gel, thus creating an array of bands known as the “fingerprint.” The bands are visualized by staining with a dye that binds to the DNA and fluoresces under ultraviolet illumination for easy detection. The resulting image is a DNA fingerprint that can be recorded photographically. The fingerprint is often referred to as the PFGE “pattern.” Once a PFGE pattern has been produced, it can be stored electronically in a database and compared to other images using analytical software. One such database, PulseNet USA, serves as the national molecular subtyping network system for foodborne disease surveillance. Implemented in 1996 by scientists at the Centers for Disease Control and Prevention (CDC), PulseNet USA is comprised of 64 public health and food regulatory laboratories. Using standardized protocol, these laboratories routinely perform PFGE of foodborne pathogens. Once a PFGE pattern is generated, it is uploaded to PulseNet USA, where scientists continuously evaluate and compare submissions to detect clusters of indistinguishable patterns. Indistinguishable patterns indicate that the bacterial isolates are genetically related and might come from the same source. Epidemiologic tools are then used to determine whether or not an outbreak is underway.