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Can open-air pens reduce the prevalence of MRSA?

A recent study tracking the prevalence of MRSA on pig farms and hospitals in Sri Lanka has found remarkably low rates of infection in both humans and pigs despite poor biosecurity and antimicrobial stewardship on farm.

2 January 2020, at 10:15am

A new study published in Frontiers in Sustainable Food Systems has reported low MRSA levels on Sri Lanka’s pig farms despite endemic levels of community-associated MRSA in the rest of the country. The rate of MRSA infections was also remarkably low considering the poor biosecurity and antimicrobial stewardship practised on farm sites.

Sri Lanka’s pig industry and MRSA

Most of Sri Lanka’s pig farms are in the western coastal part of the country, known as the “pig belt”. Around 60 percent of the farms are small-scale (up to 50 animals), 25 percent are medium-scale (between 51-100 animals) and 15 percent are large-scale (over 100 animals). Researchers have reported poor biosecurity standards on these sites.

Globally, Sri Lanka has the highest reported rate of MRSA as a proportion of hospital-associated Staphylococcus aureus infections. MRSA is a considerable medical and financial burden for Sri Lanka’s health services. Community-associated MRSA is endemic in Sri Lanka and accounts for over 38 percent of MRSA infections.

Evidence suggests that intensive livestock farming can increase the prevalence of MRSA and leave both animals and humans susceptible to untreatable S. aureus infections. Pigs are the reservoir species for livestock-associated MRSA and contact between pigs and farm staff can increase the likelihood of zoonosis. Low biosecurity and poorly regulated use of antimicrobials in livestock production allow resistance genes to proliferate – leaving both humans and animals at risk.

The study

The researchers wanted to fully document the presence of MRSA in Sri Lanka’s pig farming industry. They evaluated pigs, farm staff, the surrounding environment and dust for the presence of the bacteria. They also compared the genotypes of MRSA in the trial to the types found in human clinical samples from a hospital to see if there was any cross-over.

In order to collect samples from farm sites, the researchers cold-called 100 pig farms along a vet’s mobile clinic. Vets and researchers collected data on farm management and biosecurity through a structured questionnaire and farmer interviews. They also gathered samples from pigs, pig pens, farm employees, farmers and family members. They avoided sampling young, sick or medicated animals.

Results

Most of the farms were small- or medium-scale and had open air pens for the pigs. The animals tended to be swill-fed. Researchers reported poor biosecurity scores. Only five percent of the sites provided access to pig pens through foot baths and only four percent of farms prevented unauthorised visitors from entering the pigs’ housing area.

Based on interviews and on-site observations, the researchers found poor antimicrobial stewardship. Most antimicrobials were used to treat piglet diarrhoea. These drugs were often administered without consulting a vet and producers did not indicate extensive knowledge of resistance challenges from antimicrobial use.

Lab results showed a low prevalence of MRSA in Sri Lanka’s pig farms – 90 percent of sites were MRSA-free. On sites with positive MRSA samples, the bacteria usually had a human origin and pigs remained uninfected.

The researchers tested the resistance profiles of the farm samples and found that all the MRSA isolates were susceptible to chloramphenicol and nitrofurantoin. Pig MRSA isolates were also susceptible to an additional five antimicrobials. Conversely, the hospital-isolated MRSA strains were resistant to 10 of the 12 antimicrobials.

Key conclusions

These results indicate that Sri Lanka has a lower pig-associated MRSA infection rate than the Netherlands or Canada. This suggests that MRSA infections are more closely associated with intensive farming than other factors. It also indicates that the presence of MRSA on site doesn’t immediately translate into infected pigs or humans.

The researchers had different theories as to why the prevalence of MRSA was so low in this study. When focusing on the human side of the infection, they noted that MRSA-positive humans usually only worked on one site. This means that the disease was geographically bound and couldn’t spread to other sites.

They also suggested that the small- and medium-scale of farms limited the spread of MRSA. Housing also played a critical role – since most pigs are reared in open houses with half-high walls, air circulation and sunlight could dispel more bacteria than closed production systems. Farmers also washed the pens twice a day, using enough water to clean the floors after swill-feeding.

Based on the results of this study, it appears that intensive rearing can compound the risks of MRSA, while smaller-scale and alternative rearing methods can dispel some of the risk factors of MRSA.

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