Dose-response relationship between antimicrobial drugs and livestock-associated MRSA in pig farming

The farming community can be a vehicle for introduction of livestock-associated methicillin-resistant Staphylococcus aureus (LA-MRSA) in hospitals. During 2011–2013, an 18-month longitudinal study aimed at reducing the prevalence of LA-MRSA was conducted on 36 pig farms in the Netherlands. Evaluations every 6 months showed a slight decrease in MRSA prevalence in animals and a stable prevalence in farmers and family members. Antimicrobial use, expressed as defined daily dosages per animal per year, decreased 44% during the study period and was associated with declining MRSA prevalence in pigs. MRSA carriage in animals was substantially higher at farms using cephalosporins. Antimicrobial use remained strongly associated with LA-MRSA in humans regardless of the level of animal contact. A risk factor analysis outlined potential future interventions for LA-MRSA control. These results should encourage animal and public health authorities to maintain their efforts in reducing antimicrobial use in livestock and ask for future controlled intervention studies.

In 2005, sequence type (ST) 398 of methicillin-resistant Staphylococcus aureus (MRSA) emerged in Europe with proven transmission between pigs and humans. Since then, pigs, veal calves, and (to a lesser extent) poultry were increasingly found to harbor livestock-associated MRSA (LA-MRSA).

ST398 is widely spread across Europe, and ≈70% of pig farms in the Netherlands test positive. After transfer to humans, it can be introduced into hospitals and the community. In 2011, ST398 accounted for 39% of all new MRSA detected through screening of patients in the Netherlands.

To our knowledge, no intervention studies have been undertaken to assess the efficacy of MRSA-reducing measures on farms. Trade of animals is a major risk factor for introducing MRSA into a negative herd. Larger herds have been associated with higher antimicrobial use . Antimicrobial use could not be identified as a clear determinant for MRSA. Transmission dynamics within herds vary by animals’ ages and phase of production, potentially leading to endemicity.

In 2006, the European Union banned the use of antimicrobial drugs as growth promoters. In the Netherlands the most noticeable change started in 2010, when the government set objectives for a 50% reduction in antimicrobial use by 2013 and 70% by 2015, compared with 2009. This policy was combined with benchmarking of farms, and later veterinarians, to identify persistently high users of antimicrobial drugs. As part of this national program, farm treatment and health plans have to be drafted and reviewed annually, which has resulted in an almost 60% reduction for the major livestock industry sectors. Against the background of nationwide reduction of antimicrobial use, during 2011–2013, we evaluated MRSA carriage changes in pigs and humans and study the effect of introduction of an additional range of preventive measures on MRSA carriage in animals, and humans living and/or working on the farms. Read more:  Emerging Infectious  Diseases  Original web page at Emerging Infectious Diseases