Robert Roos | News Editor | CIDRAP News | Sep 16, 2013

Researchers offered new insights on MRSA infections today, with good news on the overall incidence of healthcare-related cases and a separate finding that where a person lives may influence his or her risk of infection.

Amplifying previous findings, one team reported substantial declines in the incidence of serious healthcare-related methicillin-resistantStaphylococcus aureus (MRSA) infections from 2005 to 2011, with those contracted in hospitals down more than 50%. Community-associated cases, however, showed only a small decline.

At the same time, another group reported that living near farms where swine manure is used as fertilizer or near high-density livestock operations may increase a person’s risk of having a MRSA infection. The study is billed as the first to suggest such a link.

Both studies were published online today by JAMA Internal Medicine.

Healthcare-linked cases dropping

The good news on incidence came from officials with the US Centers for Disease Control and Prevention (CDC) and several state health departments. They analyzed data on invasive MRSA infections gathered in nine US urban areas from 2005 to 2011. The data were collected through the Emerging Infections Program—Active Bacterial Core Surveillance system (EIP-ABCs).

The authors divided MRSA cases into three classifications: (1) healthcare-associated community onset (HACO), meaning those identified within 3 days after hospital admission or in patients exposed to the healthcare system in the previous year via dialysis, hospitalization, surgery, long-term care residence, or in other ways; (2) hospital-onset infections, meaning those cultured more than 3 days after hospital admission; and (3) community-associated infections, those not fitting into either of the other two groups.

In estimating invasive MRSA cases, the authors refined a previous CDC estimate of the national MRSA burden in 2005, as that one didn’t adjust for dialysis as a risk factor or include patients with recurrent infections.

The team estimated the total number of invasive MRSA infections in 2011 at 80,461 (95% confidence interval [CI], 69,515 to 93,914), or 30,800 fewer than in 2005, for an overall 31% drop. Of the 80,461 cases, 48,353 (60.1%) were HACO infections, 14,156 (17.6%) were hospital-onset, and 16,560 (20.6%) were community-associated

The authors calculated that the annual incidence of HACO infections dropped by 27.7% and the incidence of hospital-onset cases by 54.2% from 2005 to 2011. The incidence of community-associated cases, however, decreased by only 5.0%.

The new estimate marks the first time that estimated community-associated infections outnumber hospital-onset cases, the report says.

The 54% decrease in hospital-onset MRSA cases “is highly encouraging and may be attributable to increased awareness and implementation of local and nationwide infection prevention measures in many healthcare settings, including those targeting intravascular catheter-related infections,” the article states.

It also says the findings indicate that the nation is on track to achieve the federal target of reducing healthcare-related invasive MRSA infections by 50% as of this year.

In an accompanying editorial, Franklin D. Lowy, MD, of the Columbia University College of Physicians and Surgeons writes that the findings are consistent with several earlier reports, including one from the same group.

“This impressive decline may well be due to the implementation of infection control efforts at the hospital level; however, it is also possible that the epidemic MRSA strains circulating in the health care setting have evolved, becoming less virulent,” Lowy writes.

He agrees with the researchers’ suggestion that the study points up the need for a better understanding of how epidemic MRSA strains spread and cause infections in the community.

Livestock-related risks?

In the other new study, investigators from the Johns Hopkins Bloomberg School of Public Health in Baltimore and a Pennsylvania healthcare system sought to determine if living near livestock affected people’s risk of MRSA infections.

The report notes a US Food and Drug Administration estimate that 80% of antibiotics in the United States are used in livestock and adds that about 75% of the drugs given to food animals end up in manure.

The researchers designed a case-control study focusing on patients from the Geisinger Health System, which operates in much of eastern and central Pennsylvania. They identified patients who had MRSA or skin and soft-tissue infections (SSTIs), as well as controls.

In addition, they identified high-density livestock operations and crop fields that were fertilized with manure exported from the livestock operations. Also, the researchers mapped the locations of patients, controls, livestock operations, and fields, and then calculated distances from the homes of patients and controls to the fields and livestock sites.

The authors identified 1,539 community-associated MRSA cases, 1,335 healthcare-associated MRSA cases, 2,895 SSTI cases, and 2,914 controls.

The main finding was that, compared with controls, MRSA and SSTI patients lived closer to fields most heavily fertilized with swine manure. The risk increased with the amount of swine manure applied to fields near patients’ homes.

The analysis showed a similar but weaker link between the risk of community-associated MRSA and SSTIs and living near high-density swine operations. Links to dairy and veal operations were weaker and less consistent.

The authors say their data suggest that about 11% of the community-associated MRSA and SSTI cases in the study population could be attributed to the field use of swine manure.

As part of the study, the team examined 200 MRSA isolates from patients and found that most of them were common community or healthcare strains. None matched up with clonal complex 398, the most common variant found in swine.

The authors say previous findings on the aerosolization of MRSA indicate that their findings have “biologic plausibility,” but that “more research is needed to establish sources, media, routes, and behaviors that may lead to infection. Other animals, slaughterhouses, and meat consumption may be involved.”

In the accompanying editorial, Lowy writes that the study is the first to indicate a link between MRSA cases and proximity to antibiotic-exposed manure or high-density livestock farms. To further examine the apparent association, he says, future studies should look at the types and amounts of antibiotics in manure, the prevalence of resistance-related genes in manure, and comparison of MRSA isolates from human cases and manure.

The study, he adds, provides “yet another reason” to be concerned about the use of antibiotics to enhance livestock growth and “argues for legislation that restricts the use of antibiotics in this setting.”

Dantes R, Mu Y, Bellflower R, et al. National burden of invasive methicillin-resistantStaphylococcus aureus infections, United States, 2011. JAMA Intern Med 2013 Sep 16 (Early online publication). [Abstract]

Casey JA, Curriero FC, Cosgrove SE, et al. High-density livestock operations, crop field application of manure, and risk of community-associated methicillin-resistant Staphylococcus aureus: infection in Pennsylvania. JAMA Intern Med 2013 Sep 16 (Early online publication). [Abstract]

Lowy FD. Methicillin-resistant Staphylococcus aureus: Where is it coming from and where is it going? (Editorial) JAMA Intern Med 2013 Sep 16 (Early online publication). [Abstract]