Researchers have provided more detail on one of the largest Salmonella Dublin outbreaks in France in recent years linked to raw milk cheese.

In January 2016, the National Reference Center for Salmonella reported to Santé Publique France, the national public health agency, an increase in Salmonella Dublin infections across the country, with 37 isolates identified between mid-November 2015 and mid-January 2016, compared with 10 during the same period in the previous two years. Between November 2015 and March 2016, 83 cases were identified.

Two different bovine raw milk kinds of cheese, Morbier and Vacherin Mont d’Or, were the most likely vehicles of transmission for the outbreak, according to researchers writing in the Eurosurveillance journal.

Results suggested that at least two outbreaks of Salmonella Dublin occurred during the same period and potentially originated from different sources.

The investigation led to a reinforced control plan for processing plants of raw milk cheeses to prevent future outbreaks.

An increase in salmonellosis incidence was seen in cattle at the end of summer 2015, according to Santé Publique France which could explain the rise of contaminated cheese batches in autumn and winter 2015.

Researchers said probable low levels of contamination in the implicated cheeses may have led to false negative test results, possibly allowing some tainted batches to enter the market.

The median age of cases was 70 years (range: 1 to 94), 44 were female and respondents came from 12 of the 13 regions in mainland France, with 19 cases from the Bourgogne-Franche-Comté region. Ten deaths were reported with no confirmation that Salmonella Dublin infection was the cause. Among cases with available data, 41 of 60 were hospitalized.

Whole genome sequencing (WGS) and multilocus variable-number tandem repeat analysis (MLVA) were performed to identify microbiological clusters and links among cases, animal and food sources. Use of MLVA and WGS subtyping methods allowed identification of different clusters and potential vehicles of infection.

Researchers compared different clusters of cases with other cases (case-case study) and controls recruited from a web-based cohort (case-control study) in terms of food consumption.

They interviewed 63 of 83 cases and 2,914 controls completed a questionnaire. Both studies’ indicated successive Salmonella Dublin outbreaks from different sources between November 2015 and March 2016.

Twelve producers were identified as the potential origin of cheeses consumed by cases. The trace-back investigations linked one Morbier maker and three Vacherin Mont d’Or producers, to 11, five, four and three cases, respectively. All these firms were located in Bourgogne-Franche-Comté, the Eastern part of France.

In the case-control study, cases of distinct WGS clusters were more likely to have consumed Morbier or Vacherin Mont d’Or.

Based on the results, the Ministry of Agriculture launched the reinforced control plan.

The group of producers of Morbier and Vacherin Mont d’Or cheeses implemented an action plan, including testing for Salmonella of batches of cheese sold since February 2016, more regular farm visits by veterinarians and detection and containment of infected cattle.

It also involved a mission from the Ministry of Agriculture and Food to support milk industry professionals in the Bourgogne-Franche-Comté region to identify and recommend better practices for detection and management of contaminated raw milk products and creation of a working group with experts on Salmonella issues from different organizations. The Morbier processing plants union reinforced sanitary protocols, including more frequent testing of milk.

(To sign up for a free subscription to Food Safety News, click here.)