Canadian researchers find drug-resistant Staph in pigs and farmers

US - Canadian researchers have found two major strains of the superbug MRSA on pigs - and pig farmers - in southwestern Ontario, the first time the pathogen has been reported in food animals in North America.
calendar icon 20 November 2007
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One of the strains, they believe, passed from people to pigs. But the other, first seen in pigs in the Netherlands in 2003, seems to have originated in animals and moved into people.

The senior author of the work said the findings don't cast in doubt the safety of meat produced on Ontario pig farms. But they do suggest pig farms could serve as sources of methicillin-resistant Staphylococcus aureus infections for people who work on them or live on them.

"The big public health concern in my mind is whether we might end up in the same situation as they have in Europe with this starting to become an important community pathogen," said senior author Dr. Scott Weese, a veterinarian at the Ontario Veterinary College in Guelph who specializes in the antibiotic resistant bugs that pass back and forth between people and animals.

"The concern is there's this reservoir in pigs that's being spread into people that work with pigs and now it's being spread into the general population."

People cooking or eating pork probably aren't at greater risk of acquiring a MRSA infection by doing so, said Weese and other experts not involved in his research.

"The likelihood of food being a source (of infection) would probably be pretty remote," said Dr. Jeffrey Bender, program director for veterinary population medicine at the University of Minnesota.

A Dutch microbiologist who was one of the first researchers to spot the emergence of MRSA in pigs went even further, saying he has no concern that pork - which he eats rare - could be a source of the potentially serious infections.

"I don't believe in it and there is no evidence that there might be any route of transmission of MRSA," said Dr. Andreas Voss, professor of clinical microbiology and infection control at Radboud University Medical Center in Nijmegen.

"It's really mainly the direct contact with living animals that is the main risk."

But whether MRSA in effluent from pig farms poses a human health risk is not yet known, Weese admitted. "It's too soon to tell. I really don't have any idea whether it's a potential problem. It's an area that needs to be looked at."

Voss and some colleagues first spotted the emergence of MRSA on pig farms in the Netherlands in 2003, when two infants and a veterinarian were all found to be carrying a new strain of the bacteria. MRSA rates in that country are so low that three cases rang alarm bells. The investigation traced the source to pigs.

Studies found that 25 per cent of Dutch pig farmers tested were carrying the strain, which has subsequently been found in Denmark, Germany, Austria, Italy, Singapore, Korea and now Canada.

"I assume it's all over the place," Voss said in an interview from Prague on Monday.

A followup three years later found that carriage rate in pig farmers had risen to 50 per cent. And a study Voss and his group are publishing in the December issue of Emerging Infectious Diseases showed that more than 20 per cent of all MRSA carriers or cases in the Netherlands have this particular strain.

(At any given time between 20 to 30 per cent of people will carry Staph in their nasal passages or on their skin without being ill. These people are said to be "colonized" by the bug. Some will never get ill, but can pass the bacteria to others. Others will can go on to develop infections that range from boils and skin abscesses to life-threatening pneumonias or bloodstream infections.)

Inspired by the Dutch data, Weese and his co-authors sampled pigs and pig farmers on 20 unidentified farms in southwestern Ontario. The findings of the research project were published in the journal Veterinary Microbiology.

Just under half - 45 per cent - of the farms were found to have pigs or farmers carrying MRSA. A quarter of 285 pigs swabbed in their snouts and rumps were found to be carrying the bug. And 20 per cent of farmers (five of 25) also tested positive for MRSA.

European strain
Nearly 60 per cent of the bacterial isolates from pigs and pig farmers were of the European strain, which Weese thinks may have come to Canada in a person. The most common human strain of MRSA in this country, called USA100, was also found in some of the pigs and some of the pig handlers.

"It does raise the question: How does a bug like this get spread?" Bender said of the findings.

"And there could be a number of ways. It could be animals. It could be antibiotic use. And clearly we don't know the answer to that."

While many questions remain to be answered, the findings should be a red flag to health-care institutions which struggle to keep MRSA infections out because of the possibility of spread among vulnerable patients.

Voss said in the Netherlands, where public health officials take aggressive measures to keep MRSA rates low, anyone who has contact with pig farms - the farmers, their families and veterinarians - is put in isolation and tested to see if they are carrying MRSA when they enter hospital.

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