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Animal MRSA infections found in Canada

Signs of novel strain found increasingly often in humans, but questions remain over public health importance

Source: Flickr/c@rljones

A strain of methicillin-resistant Staphylococcus aureus (MRSA) primarily associated with pigs has been found to be infecting humans in Canada, say researchers this week in Emerging Infectious Diseases.¹ They are the first cases of infection with the bug to be reported in North America, the authors say, although the strain was known to be colonising livestock and farmers in both the USA and Canada.

“Recent emergence of infections resulting from this strain is of public health concern,” write the researchers, led by George Golding at the National Microbiology Laboratory in Winnipeg, Canada. “Additional surveillance efforts are required to monitor the emergence and clinical relevance of this MRSA strain in Canada.”

The strain called livestock-associated MRSA (LA-MRSA), also referred to as MRSA clonal complex (CC) 398, was first identified colonising pig farmers in the Netherlands in 2005, and has since been found colonising and infecting farmers and livestock across the globe. In the Netherlands, where there is a high density of pig farming, LA-MRSA now accounts for an increasingly high percentage of all newly detected MRSA strains in people — 42% of them at the end of 2008.

LA-MRSA is capable of causing disease in humans, from mild skin infections, wound infections and abcesses to severe invasive infections leading to organ failure. The strain can also spread between people, as shown by reports of healthcare workers carrying LA-MRSA after contact with infected patients. But how easily this can occur is unclear, as there are relatively few reports and no epidemiological studies of LA-MRSA in people who have not had direct contact with livestock.

To determine the prevalence of LA-MRSA in the general population in Canada, Golding and colleagues analysed 3687 MRSA isolates collected in Saskatchewan and Manitoba between 2007 and 2008. The sample represented about 66% of all MRSA samples collected in Saskatchewan and 17% of those collected in Manitoba during this period, both provinces with a low density of pig farming. The researchers found five isolates with genes characteristic of LA-MRSA, four of which were associated with cases of skin and soft-tissue infection. One of the isolates came from a post-operative infection in a patient who was unlikely to have had any contact with livestock, according to the authors.

Although the number of reports of LA-MRSA is increasing worldwide, it is difficult to say whether this represents genuine emergence or just increased surveillance for the strain, says Wannes Vanderhaegen, of the Veterinary and Agrochemical Research Centre in Uccle, Belgium. “It’s a very recent problem,” he points out. “There’s not enough evidence yet.”

The public health implications of the finding are also unclear, Vanderhaeghen says. Most of the LA-MRSA strains that have been found, including the isolates reported in the Canadian study, have lacked virulence factors such as Panton–Valentine leukocidin (PVL), a cytotoxin thought to play a role in some severe infections. Some research suggests that strains without these virulence factors may be less infectious, Vanderhaeghen says. But some LA-MRSA isolates with PVL have been identified in China, he points out, raising the possibility that more infectious clones of LA-MRSA could emerge in the future.

Furthermore, the prevalence of LA-MRSA in the general population in Canada appears to be low. In the Golding et al. study, the animal strain accounted for only 0.14% of the MRSA isolates. Even in the Netherlands, where the strain accounts for a high proportion of all isolates of MRSA in people, a study published last month in PLoS ONE² found that out of a representative sample of 534 people who had had no contact with livestock, only one harboured LA-MRSA. This figure is in line with the low overall MRSA rates reported in the Netherlands, and comparable to a 2009 study of 422 schoolchildren in Germany, where no LA-MRSA was detected, say the authors, led by Brigitte van Cleef at the National Institute for Public Health and the Environment in Bilthoven.

But the prevalence of LA-MRSA in farmers and veterinarians is very high, despite the relatively small number of infections detected so far. Studies in pig farmers have shown colonisation rates of up to 25% in Canada and 45% in the USA, Golding et al. report. In the Netherlands, Van Cleef and colleagues found the bug in 26% of people who had had contact with livestock — and other European studies have shown rates as high as 45% in pig farmers, they say. For this reason, hospitals in the Netherlands screen people in regular contact with pigs or veal calves for MRSA on admission.

The wide range of animal hosts for the strain, and what appears to be a high rate of transfer to humans based on the high rates of colonisation in livestock workers, means the strain should be watched closely, Vanderhaeghen says. “Staphylococcus is usually quite host-specific,” he points out. “It’s very strange that you find the same strain in pigs, veal calves, dairy cows, dogs, chickens, rats and goats.”

Reference and links

1.	Golding GR, Bryden L, Levett PN, McDonald RR, Wong A, Wylie J, et al. Livestock-associated methicillin-resistant Staphylococcus aureus sequence type 398 in humans, Canada. Emerg Infect Dis 2009. Article
2.	Van Cleef BA, Verkade EJM, Wulf MW, Buiting AG, Voss A, Huijsdens XW, et al. Prevalence of Livestock-Associated MRSA in Communities with High Pig-Densities in The Netherlands. PLoS ONE 2010. 5:e9385. doi:10.1371/journal.pone.0009385
US Centers for Disease Control and Prevention information about MRSA