Livestock density a factor in MRSA risk – study

agnified 20,000X, this colorized scanning electron micrograph (SEM) depicts a grouping of methicillin resistant Staphylococcus aureus (MRSA) bacteria. See PHIL 617 for a black and white view of this image. These S. aureus bacteria are methicillin-resistant, and are from one of the first isolates in the U.S. that showed increased resistance to vancomycin as well. Note the increase in cell wall material seen as clumps on the organisms’ surface.
Magnified 20,000 times, this colorized scanning electron micrograph (SEM) depicts a grouping of methicillin resistant Staphylococcus aureus (MRSA) bacteria. © Janice Carr

People who live near livestock or in livestock farming communities may be at greater risk of catching Methicillin-resistant Staphylococcus Aureus (MRSA), a new study suggests.

A comparison of livestock density, place of residence and existing information on risk factors found that regional density of livestock is an important risk factor for nasal carriage of livestock-associated (LA) MRSA for persons with or without direct contact with livestock.

The results of the study, carried out by researchers from the Johns Hopkins Bloomberg School of Public Health, the Dutch Institute for Public Health and the Environment (RIVM) and VU University Medical Center in Amsterdam, are featured in the November issue of the journal, Emerging Infectious Diseases.

Staphylococcus aureus is a pathogen that can cause a range of illnesses in humans, from minor to life-threatening skin, bloodstream, respiratory, urinary and surgical site infections.

MRSA is a type of staph bacteria resistant to penicillin and certain first-line antibiotics called beta-lactams.

MRSA infections are most commonly infections of the skin.

Though nasal carriage – the indicator under study in this research – does not indicate that someone is infected with MRSA, it is associated with increased risks of eventual infection.

Moreover, in this study it is a measure of exposure to MRSA.

“In the past, MRSA has been largely associated with hospitals and other health care facilities, but in the last decade the majority of infections have been acquired in the community outside of a health care setting,” said Dr Ellen Silbergeld,  co-author of the study and a professor with the Bloomberg School of Public Health’s Department of Environmental Health Sciences.

The study is the first to suggest the importance of indirect routes of transmission of livestock-associated MRSA.

Dr Jan Kluytmans,  co-author of the study and professor of Medical Microbiology and Infection Control at VU University Medical Center in Amsterdam, said: “In the Netherlands LA-MRSA was first found in 2003 and was initially almost exclusively found in persons with direct contact to livestock.

“In recent years LA-MRSA is found with increasing frequency in community-dwelling individuals with no known contact with livestock. It is important to determine the routes of transmission outside of the farms since this may have important consequences for public health.

“ … we found that as the density of veal calves, pigs, or cattle doubles in a specific area, the odds of carrying LA-MRSA increases between 24 per cent and 77 percent, depending on the animal.

Dr Beth Feingold, of the Johns Hopkins Zanvyl Krieger School of Arts and Sciences and lead author of the study, said the results challenge researchers to understand how such exposures could be occurring.

“This work has potential policy implications for MRSA surveillance in countries with a substantial percentage of total MRSA cases being livestock-associated MRSA,” she said.

“Controlling the spread of livestock-associated MRSA requires attention to community members in animal-dense regions who are otherwise unaffiliated with livestock farming.”

The team of US and Dutch researchers examined how regional-level factors played into an individual’s risk, after accounting for their personal risk factors such as their direct contact with animals, whether they lived in a rural or urban locale, and in what situation they likely acquired MRSA.

Using geographic information systems, the researchers geocoded livestock-associated MRSA cases in the Netherlands and other types of MRSA controls by postal codes.

They used the geographic information to examine spatial patterns of livestock-associated MRSA compared to other types of MRSA.

Researchers found that the odds of LA-MRSA as compared to other types of MRSA are highest in the southeast region of the Netherlands, a major livestock production area.

The authors conclude that the study may also have serious implications for individuals living in the US due to the sheer volume of pig farms in the country.

“Swine production is a significant industry in the Netherlands, but its density and scale are much less than in the United States. Future work should investigate the relationship between intensive livestock operations in the US and exposures to drug-resistant microbes including MRSA,” said  Silbergeld.


The study, entitled Livestock Density as Risk Factor for Livestock-associated Methicillin-Resistant Staphylococcus aureus, the Netherlands, was written by Beth Feingold, Ellen Silbergeld, Frank Curriero, Brigitte Van Cleef, Max Heck and Jan Kluytmans. The research was funded by the Johns Hopkins Center for a Livable Future and the Pew Charitable Trusts.



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