Community-acquired staph on rampage

Virulent MRSA strain rages unchecked in western Canada

By Jennifer Laliberté

Cases of the superbug MRSA in otherwise healthy individuals who haven't been in hospital are increasing at an alarming rate. What's more, many physicians are mistaking the diagnosis of the community-acquired strain, causing a delay in potentially life-saving treatments, according to infectious disease experts. "It's a growing concern in the Canadian perspective," warns Dr John Conly, director of the Centre for Antimicrobial Resistance at the University of Calgary. "The problem will continue to increase at a fairly significant pace."

The community-acquired strains of MRSA (CA-MRSA) are more virulent than the more familiar strains that lurk in hospital corridors. CA-MRSA tends to cause more severe skin and soft-tissue infections and has been associated with both necrotizing fasciitis — flesh-eating disease — and necrotizing pneumonia, which can be fatal, particularly in young children. "The [fatality] rates are currently very low, but as the prevalence of CA-MRSA increases in the community, the potential is there for the number of deaths to increase as well," says Dr Michael Mulvey, chief of antimicrobial resistance and nosocomial infections at the National Microbiology Laboratory in Winnipeg. Dr Conly has seen several cases of necrotizing pneumonia in Calgary. "These young individuals in their 20s or 30s are admitted with pus pockets in the linings of the lungs. We lose about one a month," he says.

TOO MUCH TOXICITY
The more nasty presentations associated with CA-MRSA are believed to be caused by the so-called Panton-Valentine leukocidin (PVL) virulence factor that characterizes the strain. PVL is a toxin that produces tissue necrosis and lowers numbers of circulating white blood cells. "This is a fully-loaded strain in terms of toxin genes," says Dr Conly.

The good news is that CA-MRSA is susceptible to more antibiotics than hospital-acquired (HA) strains. Community-acquired and hospital-acquired MRSA differ both in terms of biology and epidemiology, says Dr David Patrick, director of epidemiology at the BC Centre for Disease Control. "CA-MRSA infects people who haven't had contact with the hospital system, and though it's resistant to methicillin, you don't see as much multi-drug resistance."

So CA-MRSA, though more virulent, is a tad easier to treat than hospital-acquired strains — so long as you know what you're dealing with. "Physicians need to be made more aware of CA-MRSA," says Dr Patrick. "Although it's still methicillin-susceptible staph in your general practice population, there's more MRSA in the community. The only way a physician can be relatively sure is to send a culture to the lab."

"The biggest concern for the average physician is that if you put a patient on cloxacillin or cephalexin [standard treatment for run-of-the-mill staph infections] and they've actually got CA-MRSA, they could end up with bloodstream infection," adds Dr Conly. "That could have been avoided if you chose the appropriate treatment within the first few hours of infection." (see "Community-acquired MRSA Primer", top)

SPREADING LIKE WILDFIRE
Initial reports of community-acquired MRSA (CA-MRSA) were limited to certain populations in the Western US — namely injection drug users, the incarcerated, Native Americans and some athletes. But over the course of the last few years, Canadian physicians in BC, Alberta and Saskatchewan have reported staggering increases in the number of MRSA infections in young, healthy patients with no previously identified risk factors. Dr Conly says the Calgary health region has been the hardest hit, with 20-25 cases a month. The Canadian Hospital Epidemiology Committee Surveillance at Vancouver General Hospital, which has been tracking MRSA isolates since 1994, reported that isolates of CA-MRSA have risen from 1% in 2001 to between 25-28% in 2005. "So far it's been observed the most on the western side of the country, but nothing is limiting its ability to spread further east," says Dr Patrick.

As the Public Health Agency of Canada is gathering information to monitor the spread of CA-MRSA and establish treatment guidelines for physicians, specialists have been closely observing the situation as it develops across the US. And it's not looking good.

CASE IN POINT
A study presented at the Annual Meeting of the Society of Healthcare Epidemiology of America on March 20 showed that in certain patients, the annual incidence of CA-MRSA shot up by 250% in 2004 and 500% in 2005 compared to the 2003 rate. Lead study author Dr Seemi Andrabi, of the Washington Hospital centre, reported that incision and drainage and standard antimicrobials were ineffective for two thirds of patients, more than half of whom required hospitalization.

In a second study published in the Annals of Internal Medicine on March 7, researchers from Emory University School of Medicine looked at patients admitted to an Atlanta hospital. They found that 87% of the skin and soft tissue infections caused by MRSA in 2003 were community-acquired. They also found that 65% of patients with MRSA were treated with the wrong antibiotics because physicians didn't suspect MRSA as a possible cause of infection. "Improved recognition by physicians that CA-MRSA is the major cause of staphylococcal skin and soft-tissue infection is needed to ensure that appropriate therapy is initiated and to reduce the risk of horizontal transmission in healthcare settings," conclude the study authors.

Dr Conly agrees that recognizing MRSA is half the battle. "Have you inappropriately treated a patient and allowed it to progress with inadequate or inappropriate therapy?" he asks.