There is a significant risk associated with drinking raw milk from Norwegian cows, according to a study.

Lene Idland, of the Norwegian University of Life Sciences (NMBU) faculty of veterinary medicine, provided updated figures on the prevalence of some pathogens that may be present in raw milk in three published studies.

Idland collected milk and environmental samples from 18 dairy farms in Eastern Norway. Sites were visited six times from August 2019 to July 2020.

Pathogens found at farms
Listeria monocytogenes, Campylobacter, and Shiga toxin-producing E. coli (STEC) were found in environmental and milk filter samples from farms. Around 3 percent of the bulk tank and teat milk samples were contaminated by Campylobacter and one type of STEC was isolated from bulk tank milk.

A total of 7 percent of milk filters were contaminated by STEC, 13 percent by Listeria monocytogenes, and 4 percent by Campylobacter. Four STEC isolates were eae positive, which is a gene associated with the ability to cause severe human disease.

The three pathogens are commonly found in Norwegian dairy farms, and it is difficult to avoid transmission to raw milk. Good on-farm hygiene can reduce the risk of milk being contaminated but it does not eliminate it, said Idland.

The second study showed the same Listeria clone can persist in a cattle herd over time, and that clones detected in the farm environment can contaminate milk filters and bulk tank milk.

Isolates from farm environments and raw milk were compared with those from other environmental habitats and listeriosis patients. Findings revealed that clusters of isolates with no likely association were indistinguishable using different types of analysis. This indicates the need to improve surveillance systems and not rely only on DNA analyses, found the study.

Drinking milk that has not been pasteurized is gaining popularity. In Norway, it is mandatory to pasteurize milk sold commercially but this does not apply to the random sale of raw milk directly from farms.

Related outbreaks in Norway are relatively rare but 17 children were infected by Campylobacter after drinking raw milk or Cryptosporidium after contact with animals during a farm visit in 2021.

Impact of changing practices
Dairy cattle production in the country is in transition from tie-stall housing with conventional pipeline milking systems, to modern loose housing systems with robotic milking. The occurrence of the three pathogens was higher in samples collected from farms with loose housing compared to those with tie-stall housing.

“New farm technologies may create novel niches for microbes to survive or grow in which can cause food safety challenges. Good hygienic measures seem to reduce the risk of zoonotic pathogens entering the milk production chain,” found the study.

The third study highlighted the importance of storing raw milk at low temperatures between milking and consumption, including during transportation.

An experiment showed that storage at abusive temperatures may lead to rapid propagation of STEC, which increases the risk of infection.

People who have a preference for drinking raw milk or giving it to their children should be made aware of the risks associated with consumption, particularly for young children, the elderly, and immunocompromised individuals.