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It is over a decade since the revelation that livestock can harbour methicillin resistant Staphylococcus aureus (MRSA) which can be transmitted to humans. The subject of livestock-associated MRSA (LA-MRSA), particularly in pigs, has since been at the heart of debate about the human health impacts of antibiotic use in livestock. Although S. aureus is a ‘normal resident’ of the nose and skin of humans, only some 20% of people are permanent carriers. However, S. aureus is a premier ‘opportunistic’ pathogen of people, causing clinical infections that range from trivial to fatal, and risk is higher in carriers. Human infections with antibiotic resistant strains, most notably MRSA, incur higher medical costs and poorer outcomes. It is crucial to understand the impact of the livestock MRSA reservoir, and we are getting closer to answering some key questions.
S. aureus is likewise a normal inhabitant of many healthy mammals and birds. It is unsurprising, and long established, that microbes can be transmitted between livestock and people who interact with them. To date, the bulk of research has been done on the ST398 MRSA lineage that is widespread in pigs in much of Europe, although other variants (including ST9 in Asia; ST5 in North America) are also adapted to pigs and can acquire methicillin resistance and be transmitted to people. Opinions abound regarding the drivers of the emergence of LA-MRSA, but the occurrence of LA-MRSA among pig herds in time and space remains essentially unexplained. LA-MRSA appear to be relatively uncommon in pigs in the USA, although methicillin-sensitive variants of ST398, ST9 and ST5 are widespread in the industry (Sun et al, 2015). In contrast, LA-MRSA is highly prevalent in herds in some countries (e.g. Denmark and the Netherlands) that have greatly reduced antibiotic use in pigs, and factors other than antibiotics (e.g., high dietary zinc and disinfectants) may select for resistant strains (Slifierz et al, 2015).
There is no doubt that LA-MRSA cause clinical infections in people, and occasionally severe and even fatal cases. However, all MRSA are not created equal, and livestock strains carry fewer virulence genes, are less transmissible among humans, and typically cause less severe disease than human strains (Becker et al, 2017). The incidence of human ST398 MRSA clinical infections (from mild through severe) was recently estimated to be <3/100,000 inhabitants per year in Denmark and Germany (Goerge et al, 2017). For comparison, the annual incidence of all invasive and fatal MRSA cases in the USA was estimated to be 32 and 6 per 100,000 people respectively in the USA (Klevens et al, 2007) and a study in the pig dense state of Iowa found <0.5% of all MRSA cases were potentially of livestock origin (Nair et al, 2016).
One anomaly of LA-MRSA is that while exposure risk is overwhelmingly concentrated in people working with animals, approximately a third of reported clinical cases have occurred in people without livestock contact. A recent review cited 24% to 86% of nasal swabs of pig farmers were positive for ST398 MRSA across studies, but found only 12 reports of clinical infections, mostly mild, in people with livestock contact. The 3 fatal cases documented were all in medically compromised swine workers, a feature common to most of the severe human infections reported to date. Regarding the occurrence of LA-MRSA cases in people not exposed to animals, human to human transmission is the most likely explanation. It is increasingly evident that LA-MRSA carriage can persist for over a year in some swine workers who also carry relatively high numbers of organisms, and this may lead to transmission to their families and local social networks (Walter et al 2017; Sun et al submitted). To date there is little evidence that foodborne or environmental sources contribute significantly to human exposure risk.
Overall, after more than a decade there is little to suggest that LA-MRSA poses an imminent threat to public health, even in countries where prevalence is high in pigs. However, exposure risk is high for swine workers on positive farms, and farms are places where minor injuries are frequent. It is important to ensure that commonsense procedures are in place to mitigate potential risks. Workers should be encouraged to use personal protective equipment and practice good personal hygiene with regular hand washing with soap and water. It is important to pay appropriate attention to existing and new skin wounds (clean and cover with bandages until healed), and seek medical attention if infections develop.
