MRSA (methicillin-resistant Staphylococcus aureus) has caused one of the major debates of antimicrobial resistance in humans and pigs – how much are pigs to blame?
MRSA strains have been a major problem in man especially associated with hospital infections but also increasingly in the community. The MRSA clone CC398, which has been found in many EU countries in pigs, is now is being found in man. It is mainly in countries which have high pig farm MRSA prevalence e.g. Netherlands, Germany, Spain, Italy and more recently reported, Denmark. Pig farmers in Germany can be highly infected (83%) but generally the infection has remained in farmers, their family members (4.3%) and it has not spread to any major extent into the general public in contact, such as schools that were monitored (Cuny et al, 2009). Swine veterinarians (36%), their laboratory staff and abattoir workers (14%) were also shown to have a higher incidence of infection than the national average (1-2%) in Germany (Blaha et al, 2009) or 1-2,000/100,000 population of non-pig associated isolates. In Denmark, there has been an increase in cases of MRSA in man. Cases here refer to colonisation and clinical infections (see Figure 1) and an increasing number are associated with the livestock-associated (LA) clone CC398.
Of the 1292 new human cases of MRSA (0.02% of population), 164 (12.7%) were associated with CC398. Approximately, 7.2% of 681 human MRSA clinical infections (0.012% population) were associated with CC398 in 2011(Danmap 2011, 2012). The reported incidence in pig farms was only 16% but at slaughter, the number of pigs infected had risen to 44% from 13% in 2009. Recently, reports suggest it has gone as high as 88% infection rate in 2012.
Overall, MRSA CC398 played a 12.7% role in a rapidly growing problem in Denmark, where MRSA new cases (both infected and only colonised) in humans increased from approximately 100 in 2001 to 1292 in 2011 (13-fold increase). This has led to a voluntary suspension of use of 3rd and 4th generation cephalosporins in veterinary medicine in 2010 and the implementation of a number of other controls to reduce antimicrobial use. However when the risk is examined on a population basis (5.5 million in Denmark), MRSA cases were only 0.023% in Denmark and LA MRSA cases accounted for 0.003% or 3 people/100,000 human population. However, over 90% of cases were associated with pig contact, so therefore only 0.3 people/100,000 population were not associated directly with pig contact but did live in rural areas. Either way the transmission rate into the general public can be considered low, especially, in comparison with other Member States carriage rate.
In comparison, in spite of the UK being free of LA MRSA in pigs, its Health Protection Agency (HPA) reported that it has gone through a major MRSA epidemic but this was linked primarily to the medical arena and poor hospital hygiene practices (see Figure 2).
The introduction of hospital hygiene initiatives has resulted in a dramatic fall (-85%) in the incidence of MRSA bacteraemia cases in the UK from 0.012% of population down to 0.0018%. This highlights the major nosocomial (hospital-associated) impact of clinical MRSA infections in man, in comparison with LA MRSA associated transmission from pigs. Interestingly, it also demonstrates the decline in vancomycin resistant enterococci (VRE), where vancomycin is commonly used to treat MRSA in humans.
The debate will go on, I am sure about who or what causes antimicrobial resistance in humans and animals but even using Denmark as a model, which has an unusually large pig production/capita (10 times) more than the rest of the EU, the risk of transmission from pigs to farmers etc is very high but fortunately the transmission into the general public is still amazingly low. However, we need to look more closely on how we can solve the problem in pigs? A bit of EC funding for something positive like this might be more effective than all the negative controls that might be introduced in the near future.