A 2018 outbreak of Shiga toxin-producing E. coli O121 sickened seven people. Raw milk Gouda-like cheese produced in British Columbia caused the illnesses.
The current Canada Communicable Disease Report (CCDR) describes the outbreak investigation and recommends greater control measures for raw milk Gouda-like cheese.
A peer-reviewed journal on infectious diseases, the CCDR is published on the Government of Canada website by the Public Health Agency of Canada.
“This outbreak provides further evidence that raw milk Gouda and Gouda-like cheese processed according to regulations in North America is at risk of containing STEC, which contributes to foodborne illnesses,” authors of the report concluded. “It is recommended implementing additional control measures for raw milk Gouda and Gouda-like cheese production to minimize the risk to the public.”
The report acknowledges there were some limitations that investigators encountered. Neither the health of the cows nor the quality of the milk being produced on the farmstead involved was examined at when the outbreak occurred.
That meant it could not be definitely determined that E. coli O121 stx2 was present in the herd at the time of the outbreak. Retail records were faulty, limiting traceability. Also, none of the product was available for healthy population controls to be useful.
The journal article reports that the Public Health Laboratory tested 41 cheese samples from 24 batches between April 27, 2018, and Nov. 2, 2018, as well as three spice samples, one meat sample, and 11 environmental samples from “dairy plant A.” Thirty-eight cheese samples were collected from dairy plant A, one sample of “cheese A” was collected from a retailer in Health Region 2, and two unopened packages of different dairy plant A cheeses were collected from a patient’s home. One cheese sample tested positive for stx2 and two for stx1. The stx2-positive sample grew E. coli O121, whereas the two stx1 samples were unable to grow. All other samples, including the environmental swabs, tested negative.
The dairy plant discontinued production of cheese A on Nov. 9, 2018, and all lots of cheese A at the dairy plant were placed on hold. All lots of cheese A were recalled on Nov. 12 that year and a public health notice was issued on November 13. No additional cases occurred after those actions were taken. By March 2019, all the detained cheese had been destroyed.
In the discussion of their study into the 2018 incident, the CCDR authors made these observations:
- An investigation of an STEC outbreak involving seven cases was conducted in BC between August and November 2018. The outbreak was associated with the consumption of a raw milk Gouda-like cheese product and was due to raw milk contamination.
- This STEC outbreak was the second in BC, the third in Canada and the fourth in North America to be caused by raw milk Gouda or Gouda-like cheese since 2002. It was the first documented to be caused by E. coli O121. This investigation adds further evidence to the series of calls to action by public health professionals to improve control measures in the production of raw milk Gouda and Gouda-like cheeses.
- Epidemiologic, laboratory, and food safety investigations confirmed raw milk Gouda-like cheese to be the source of this outbreak. All seven confirmed outbreak patients reported consuming cheese, with five reporting consuming cheese from the same BC dairy plant and four reporting consuming the same cheese product. A sample of this cheese product tested positive for the same strain of E. coli O121 as the sick people.
- A single batch of this cheese could explain all the illnesses; cheese from this batch was the only one that tested positive for the outbreak strain among the 16 tested and the implicated batch was available to all cases for consumption. All other cheese products and environment swabs tested negative for STEC. Furthermore, the specific cheese product contained no pasteurization and no-kill step for the raw protein, which is a known vehicle for transmission of pathogens. Therefore, contaminated raw milk is believed to be the source of cheese contamination. Cattle are the primary reservoir of STEC, and infected cows are asymptomatic and shed sporadically.
- This outbreak was solved and controlled very rapidly. The outbreak investigation was launched on Nov. 1, 2018. The reporting of cheese consumption by four patients on Nov. 5 led to the hypothesis that cheese was the source. Following re-interviews, cheese A was hypothesized as the source of the outbreak on Nov. 9. The food safety investigation started on Nov. 9; a cheese A sample tested positive on Nov. 11; and the product was recalled on Nov. 12. The duration of the outbreak investigation was 11 days, which is much shorter than the median of 39 days for BC outbreak investigations. The rapidity of the investigation and actions taken by investigators and the dairy plant minimized the impact on the population.
- The dairy plant was compliant with current Canadian regulatory requirements and aged its raw milk Gouda-like cheese for more than 60 days. Nevertheless, three separate batches were found to be contaminated with STEC.
- This was the third reported STEC outbreak caused by raw milk Gouda or Gouda-like cheese aged longer than the 60-day minimum. Several studies have shown that 60 days of aging is insufficient to inactivate pathogenic bacteria in Gouda cheese
- Gouda and Gouda-like cheese production involve a curd-washing step to reduce the amount of lactose in the cheese curds. The combined effects from the addition of hot water to the curds dilute out the lactose in the whey, shrinks the curds to expel moisture, and creates an osmotic gradient across the curd membrane to draw out lactose while reabsorbing water. This new state decreases the formation of lactic acid, thus increasing the pH and moisture of the curd. Higher pH and moisture increase the risk of survival and growth of microbial contaminants, including E. coli that is dangerous to human health.
According to the report, this outbreak provides further evidence of the inherent risk of raw milk Gouda and Gouda-like cheeses. This is the fourth call to strengthen the regulatory requirements for such cheeses. At a minimum, the investigators recommend enhancing milk and cheese-processing controls and increasing consumer awareness.
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