Study: Wind can spread resistant bacteria from pig farms

Danes living close to pig farms have an increased risk of being infected with the antibiotic-resistant bacteria livestock-MRSA – even without direct contact with animals.

A new nationwide study from Aarhus University shows for the first time that wind direction may play a role in disease transmission.

The researchers conducted the largest study to date of livestock-MRSA infection among Danes without livestock contact.

The study, which includes 518 infected individuals and nearly 5,000 control subjects, shows a clear correlation between how close one lives to pig farms and the risk of infection.

"Approximately half of all clinical infections with livestock-MRSA in Denmark occur among people with no known livestock contact. It is striking that people become infected with the bacteria apparently without being in direct contact with animals," says Martin Rune Hassan Hansen, PhD from the Department of Public Health, who is the lead author of the study.

A previous study from 2018 showed that people with livestock-MRSA lived closer to pig farms than others. However, that study did not account for wind direction, and it has been documented that the bacteria can be detected in air and soil around pig farms.

"The idea behind investigating wind direction's impact on infection risk was that if wind direction influenced the occurrence of livestock-MRSA among people without known livestock contact, it would suggest that part of the transmission occurs through environmental spread," explains Martin Rune Hassan Hansen.

Wind direction only important at medium distances

A key finding in the study is that wind direction's impact on infection risk apparently varies with distance to pig farms.

"The most surprising finding was that wind direction only appears to be important at medium distances. We had expected that wind direction would have a more uniform effect at all distances," says Martin Rune Hansen.

According to the researcher, this pattern can be explained by the fact that bacteria will spread regardless of wind direction if the pig farm is close to housing, because the concentration of bacteria is highest near the source.

At medium distances, transmission only occurs if the wind predominantly blows from the farm toward the address. And at large distances, the bacteria become so diluted that the risk disappears.

Low risk for individuals

The researchers emphasize that individual Danes should not be concerned about living close to a pig farm.

"The occurrence of clinical infections with MRSA CC398 among people without known livestock contact is low – there were 114 cases in 2024, despite the fact that most addresses in Denmark are located less than ten kilometers from a pig farm," explains Martin Rune Hansen.

The researchers generally emphasize that it is too early to draw too many conclusions based on the study.

It is thus too early to say anything definitive about how bacteria spread from pig farms to humans.

"We cannot say with certainty that the bacteria spread through dust in the air. It is also possible that the bacteria are carried by flies, whose movement patterns are affected by wind direction," says Martin Rune Hansen, who explains that the research group's results must first and foremost be confirmed based on new datasets from the period 2022 to 2025.