Contamination occurred although raw milk met the microbiological criteria, said the study in Eurosurveillance.
From February to April 2014, 55 people were reported in Finland with YP infection and four were hospitalised.
YP infection was associated with a single producer, with 98% of cases drinking raw milk from them.
Five people who had drunk the producer’s raw milk fell ill with campylobacteriosis in March 2014, before the YP outbreak notification, but their illness could not be associated with raw milk because Campylobacter had not been found in the product.
An identical YP serotype and genotype were found in isolates from the patients, the milk filter from producer’s farm and raw milk from a case patient’s fridge.
The concentration of YP in tank milk increased during storage at 4 °C. It was 2 colony-forming units (cfu)/mL when collected and 5 cfu/mL, 26 cfu/mL and 120 cfu/mL after one, two and five days of storage, respectively. After 12 days the concentration was 3,500 cfu/mL.
While the infectious dose of YP is believed to be 108 or more bacteria if ingested orally, ingestion of fewer than 104 cfu could was enough for infection, said the researchers.
The producer voluntarily discontinued raw milk production, withdrawing product after the trawling interviews were completed and fulfilled the legal requirements for raw milk production.
Raw milk was produced on a farm that had 90 milking cows and two regular daily employees with the majority of milk produced was delivered to a dairy for pasteurisation.
In February 2014, the farm started a packaging facility for raw milk once per week.
From 10 February to 3 April 2014, 11,835L of this raw milk was delivered to 24 shops in southern Finland. It was sold in 3L packages, with a ‘use by’ date five days from the date of packing.
In compliance with Finnish regulations, all packages had a warning label, stating the milk may contain harmful microbes and should be heat-treated before serving to risk groups.
The milk was contaminated by several organisms. The most probable route was through bovine faeces during milking.
The National Institute for Health and Welfare (THL) and the Finnish Food Safety Authority (Evira) recommend children, the elderly, pregnant women and persons with a severe primary disease should heat-treat raw milk before consumption.
An Evira study found Yersinia bacteria can multiply at refrigeration temperatures within a couple of days to a level where they can cause food poisoning to healthy people.
In the Eurosurveillance work, age was associated with illness: those under 23 years were more likely to get ill but previously diagnosed chronic diseases did not affect the probability of YP infection.
“Our results are in line with findings from a study showing that healthy adults can become infected by consuming contaminated raw milk, and we suggest that also adults should heat raw milk before consuming it,” said the researchers.
Milk filters or bulk tank milk
Researchers said the current testing regime for Campylobacter may be insufficient, because concentration of the pathogen may be below the threshold of what is detectable in milk.
“In our investigations in April 2014, an identical C. jejuni genotype was detected in samples of one YP patient, bovine faeces and the producer’s milk filter. Examination of milk filters instead of bulk tank milk samples seems to increase the probability of detection of pathogens.
“In the filter, the concentration of pathogens is higher than in bulk tank milk because large volumes of raw milk flow through it. The milk filter could therefore be used as the standard for testing.”
The outbreak was identified because a cluster of YP cases visited the same hospital. In Finland, Yersinia and Campylobacter infection notifications are not monitored in real time, and isolated strains do not need to be sent to the national reference laboratory for typing.
“Despite the early detection and rapid control measures, this outbreak was larger than previously reported outbreaks linked to raw milk consumption,” said the researchers.
“Moreover, there were probably more cases than recorded. Additional cases on the basis of symptoms were detected by us in one in four of the responding households. Persons with severe symptoms are more likely to seek medical care and thus more likely to be sampled.”
“Outbreak of Yersinia pseudotuberculosis O:1 infection associated with raw milk consumption, Finland, spring 2014”