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Humans may introduce MRSA CC398 into closed pig populations in Norway
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Outbreaks of livestock-associated MRSA infection are most likely to be introduced by humans, according to a study that examined all cases in humans and pigs in Norway from 2008 to 2014.
“To our knowledge, the present study is the first to describe the importance of the human introduction of MRSA CC398 to livestock populations,” Carl Andreas Grøntvedt, DVM, Dipl. ECPHM, researcher and swine health expert at The Norwegian Veterinary Institute, and colleagues wrote in Clinical Infectious Diseases. “Since there is virtually no import of live pigs to Norway, human transmission of [livestock-associated] MRSA should be regarded as the most important route of introduction into the Norwegian pig population.”
Starting in the mid-2000s, strains of livestock-associated strains of MRSA have emerged and represent a reservoir for transmission to humans. Grøntvedt and colleagues sought to describe the introduction and transmission of the CC398 variant of MRSA in humans and pigs in Norway to increase understanding of how to control this threat to human health.
The researchers analyzed data from all MRSA CC398 cases in humans and pigs that occurred there from 2008 to 2014. These data had been collected during an outbreak investigation that included contact tracing, stringent surveillance and a “search and destroy” policy at pig farms, they said.
The analysis included 74 pig farms and five slaughterhouses that were sampled among all 986 Norwegian sow farms. Further, researchers evaluated epidemiological data from 272 persons who had either been occupationally exposed to pigs or were household members who had been patients in a health care setting, worked in health care professions or were contacts of farm workers who tested positive for MRSA.
According to the researchers, the first human case of MRSA CC398 was identified in March 2009. By the end of 2014, 84 human cases were reported. MRSA CC398 was identified in three outbreak clusters, which included 26 pig farms, two slaughterhouses and 36 humans. Contact tracing identified two primary case sow farms as suppliers for the index case farms in outbreaks 1 and 2. In outbreak 3, the index case farm was considered the primary case farm. Although none of the farms investigated had imported pigs from abroad, all the primary case farms had farm workers and/or consultants from other European countries.
“Phylogenetic analysis revealed that the introduced MRSA strains were closely related to strains isolated in other European countries,” the researchers wrote. “The isolates from the primary case farms in outbreaks 1 and 2 showed close genetic relatedness to MRSA CC398 isolates from Denmark, and persons linked to the two farms had known contact with pig farms in Denmark. Further, the primary case farm in outbreak 3 involved farm workers from abroad, although without confirmed livestock contact outside Norway.”
Secondary transmission to other pig farms occurred mainly through animal trade and via humans or livestock trucks. The researchers noted, however, that 48 of the 74 farms sampled during the investigations were identified as MRSA-negative despite contact with MRSA-positive farms, suggesting that stringent hygiene practices may prevent the spread of MRSA.
“In these cases, both the epidemiological and the [whole-genome sequencing] data indicate that these introductions were the result of human-to-animal transmission,” the researchers concluded. “In addition, further transmission likely occurred via humans and livestock transport vehicles to farms not connected to MRSA CC398-positive farms through the trade of live animals. These findings have important implications for risk management to prevent the dissemination of MRSA CC398 among farms.” – by Sarah Kennedy
Disclosure: Grontvedt reports no relevant financial disclosures. Please see the full study for a list of all other authors’ relevant financial disclosures.
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