New knowledge about transmission and persistence of E. coli O157:H7 between and within farms has been revealed by a researcher at a Swedish university.

Lena-Mari Tamminen’s doctoral dissertation at the Swedish University of Agricultural Sciences (SLU) identified possible target areas for on farm measures to reduce prevalence of the deadly pathogen.

In Sweden, domestic transmission of a highly virulent subtype of Shiga toxin-producing E. coli (STEC) O157:H7, originating in regional clusters of infected cattle farms, is increasing. A total of 40 reports of hemolytic uremic syndrome (HUS) — the highest ever number of annual cases — were recorded in 2018 and half of these occurred in children younger than 10 years old.

By reducing incidence in cattle, the spread of infection to humans could be prevented, but since the animals show no symptoms, it is not easy to know when and how to act. However, E. coli is not part of the animal’s normal intestinal flora so they can be free from it given the right conditions.

Risk factors
The risk of introduction of STEC O157:H7 on cattle farms was studied by collecting environmental samples in spring and fall from 80 sites on the island of Öland. One risk factor for introduction of the infection was purchase of animals.

Tamminen assesses a calf in terms of health and welfare to study individual differences. Photo by Karin Sjöström

By comparing farms in an area where the bacteria circulated, Tamminen and colleagues could get a picture of how the bacteria spread and what types of farms are most at risk of getting it.

On four farms, environmental sampling with analysis of strains was carried out during summer, between the spring and fall sampling. Animals picked up the neighbor’s bacteria on pasture, perhaps through contact with other animals or the environment, and took it back to the farm.

Sharing of agricultural machines was a risk factor for being positive in the fall sampling so moving vehicles between farms could be an issue. Being positive in both spring and fall, sampling was associated with farm size, with larger ones more likely to be positive, and combining milk and meat production.

Individual samples from calves on 12 dairy farms with STEC O157:H7, established by environmental sampling, were collected. Indicators of animal welfare and behavior to study individual differences were used to explore differences between colonized and non-colonized calves.

Only one farm had positive environmental samples from young and weaned calves, young stock and dairy cows. On all farms, the pathogen was found among calves between 2 and 6 months old. On six farms, groups including animals up to 12 months old were also positive in environmental sampling.

Mitigation measures
Results suggest social and active animals are more likely to be colonized by the pathogen while it was less likely for animals showing signs of poor health and welfare. Variables associated with carrying the bacteria were rubbing as well as self-licking and licking other calves. These findings were published in the journal Scientific Reports.

Colonized animals shedding high levels of the bacteria were important for transmission but environmental exposure also increased risk of transmission within pens.

Frequent transmission of virulent strains between nearby farms occur in cattle dense areas with regular contacts. Many neighboring farms increase the risk of infection on a farm and routes of transmission are related to human and animal contacts between farms.

On infected farms, STEC O157:H7 is most often found among calves and young stock and only occasionally in dairy cows. Transmission dynamics within farms is influenced by direct contacts between animals, presence of super-shedders as well as pen hygiene.

Farmers can prevent sporadic human cases caused by direct contact with animals by informing visitors, especially children, to wash their hands after this activity. Other actions are avoiding mixing of groups and preventing the pathogen from moving between pens by dirty boots, flies and other potential vectors.

Measures to help the farm clear infection faster include making sure that bedding is dry and trying to reduce stocking density in animal groups. Biosecurity measures reducing human and animal contacts between other farms will decrease risk. These could include providing protective clothing for visitors, especially those travelling between farms, and avoiding pastures where animals can have contact with those from other farms.

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