E. coli and Campylobacter can persist on dairy farms for months and contaminate unpasteurized, bulk tank milk despite some hygiene measures, according to a thesis.
Anniina Jaakkonen’s work investigated the frequency and contributing factors of milk contamination by Shiga toxin-producing E. coli (STEC) and Campylobacter jejuni on Finnish dairy farms. It is based on three publications: one in 2017 in the journal Zoonoses and Public Health, another in 2019 in Applied Environmental Microbiology and the final one in April this year in the journal PLoS ONE.
In the first and third studies a dairy farm was first sampled due to a suspected outbreak. The first one describes an outbreak of STEC O157 with 11 cases identified in south-western Finland in June 2012. Six children were hospitalized, four with a type of kidney failure called hemolytic uremic syndrome (HUS), after drinking raw cow’s milk from a local farm. The farm was sampled during the outbreak and three months later.
The third is a follow-up study of a campylobacteriosis outbreak in western Finland in November 2012. Two children had been hospitalized with bloody diarrhea and Campylobacter jejuni infection after consuming raw cow’s milk purchased from a local farm. The farm was sampled during the six months after the outbreak.
In the second bit of research, three dairy farms were recruited after previous on-farm isolation of both STEC O157:H7 and Campylobacter jejuni. They were sampled between January 2014 and June 2015. Despite simultaneous isolation of STEC O157:H7 or Campylobacter jejuni from cattle, these bacteria were rarely isolated from milk filters and milk.
E. coli and Campylobacter isolated on farms
Hygienic measures included continual disinfection of drinking and feeding troughs and contaminated areas. Enhanced hygiene practices were applied in milking and handling of feed and manure. To decontaminate bulk tank milk on the outbreak-associated farms, either the milk room was replaced or the milking machine and tank were rinsed with acid, and components were replaced.
In Finland on-farm sales are permitted up to 2,500 kilograms per year without official approval. Farms that annually sell more than this amount of unpasteurized milk need an approved food establishment and monitoring plan for pathogens.
In study one, STEC O157:H7 was isolated from cattle feces and the farm environment, from nine samples within three months after the outbreak. In study two, STEC O157:H7 was isolated from all three farms during a one-year sampling period. STEC contamination occurs during milking when cattle are shedding the bacterium, despite strict on-farm hygiene. STEC O157:H7 persisted in two herds for up to 12 months.
Campylobacter jejuni was isolated from all dairy farms in all three studies, but only in the third bit of research were isolates recovered from bulk tank milk. It was found to persistently contaminate bulk tank milk for seven months despite hygiene measures. Only the outbreak-causing strain with sequence type (ST) 883 was isolated from milk, although other types were isolated from the farm.
Finding and reducing contamination
STEC was rarely isolated from bulk tank milk and milk filters and only simultaneously with fecal isolation. Higher detection rates came from milk filters than milk by culture methods and real-time PCR. So, milk filters are more reliable sampling targets for monitoring of STEC than milk.
Milk contamination by STEC bacteria can be reduced, but not prevented, by on-farm practices. The effect of nine risk factors on stx contamination of bulk tank milk, as an indicator for STEC contamination, was studied. Reduced milk contamination was associated with culling of dairy cows, major cleansing in the barn, and pasturing of dairy cows.
Higher average outdoor temperature was associated with increased milk contamination. No effect was observed for five risk factors: abnormalities in feed, maintenance and breaks of the milking equipment, number of rainy days, total bacterial counts, and total cell counts.
Jaakkonen concluded that all farms producing raw drinking milk should apply cost-effective hygienic measures to reduce contamination risk. These steps cannot totally prevent milk contamination, and heat treatment of raw milk before consumption is recommended for safety.
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