Medical scientists present the “strongest evidence” yet that antibiotic use in farm animals has driven the emergence of a new strain of methicillin-resistant Staphylococcus aureus (MRSA) in people. Writing this month in PLoS One they say the strain, known as ST398, is a potential threat to public health.

 

“The observation that MRSA carriage in humans is associated with MRSA prevalence among their calves is remarkable,” write Haitske Graveland and colleagues. “This might indicate that the prevalence in humans in close contact with animals follows the prevalence of MRSA among animals over time.”

 

ST398 MRSA first appeared in four pigs and a farmer in France in 2004, and since then it has been spotted in people living in several countries worldwide. Although scientists know that farmers are at highest risk of contracting the bug, the factors that put farmers at risk have not been evaluated until now.

 

To get a better handle on this, the researchers visited more than 100 veal farms in the Netherlands between October 2007 and March 2008. They took nose swabs from 390 farm workers and their families, and asked them about the nature of their contact with the animals.

 

They also collected swabs from the nostrils of 10–25 randomly selected veal calves on each farm, and asked the farmers about any antibiotics given to the animals, the type of farm and hygiene practices. All the swabs were then tested in the laboratory for MRSA. Any strains that the animals harboured were genotyped to assess the prevalence of the ST398 strain.

 

A third of the farmers carried MRSA, and in all but four cases the strain was identified as ST398, say the researchers. Statistical analysis revealed “strong associations” between MRSA carriage and the time workers spent in contact with the animals as well as specific farming tasks, such as feeding the calves and giving them veterinary care.

 

Overall, almost 16% of the study participants, who lived and worked on veal calf farms, carried MRSA. By comparison, the prevalence of MRSA in the Dutch community is estimated to be 1%.

 

The prevalence of MRSA among the calves was almost double that of the people studied, with 28% of the 2150 calves tested found to be carrying the bug. Almost all these bacteria were of the ST398 strain, and MRSA was more often found in calves that had been treated with antibiotics.

 

“This is the strongest evidence of a direct relationship between antibiotic use in animals and transfer of antimicrobial resistant organisms to humans at present,” write Graveland and colleagues.

 

Previous links have been indirect, they note, as they were made based on similarities in the genetic code or resistance patterns between MRSA bacteria found infecting animals and people. In this paper the authors demonstrate the “parallel occurrence” of MRSA ST398 in people and animals, and make a “direct observation” of the risk that the bug will get transferred between the two groups.

 

“The presence of livestock-associated MRSA in farmers forms a potential threat for public health,” they write.

 

In light of their results Graveland and colleagues say that antibiotics should be used “prudently” in livestock. Veal calf producers in the Netherlands and the Dutch Government recently agreed a covenant to reduce antibiotic resistance, they add.