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Community Acquired MRSA Part Deux.

By Chris Rangel MD ⋅ December 7, 2005 ⋅  Email this post ⋅  Print this post ⋅ Post a comment

Is community acquired MRSA actually more dangerous?

In response to my post on the media hype over community acquired methicillin resistant staphylococcus aureus (CA-MRSA) Dr. RW challenges my assertion that CA-MRSA is generally no more serious than Staph that is sensitive to antibiotics (MSSA).

In particular he sites a very good review of CA-MRSA in the Mayo Clinic Proceedings and brings up the fact that CA-MRSA is frequently positive for the Panton-Valentine leukocidin (PVL) gene, while hospital acquired MRSA and MSSA rarely have this gene. The PVL gene codes for a toxin that (in vitro) has been found to be lethal to neutrophils (infection fighting white cells) and is more likely to be seen in CA-MRSA associated severe skin infections and necrotizing pneumonia.

Yet, while epidemiologically linked to severe infections the PVL toxin has not been directly observed to be produced in vivo during infections or to contribute to the disease process. In his review for the New England Journal Dr. Chambers admits that the evidence linking the PVL gene to severe CA-MRSA and causing more virulent activity among CA-MRSA strains is “largely circumstantial, but compelling.”

Is CA-MRSA with the PVL gene more likely to cause severe necrotizing fasciitis (so called flesh eating disease) than MSSA? It’s very difficult to say because the data so far is very limited. A New England Journal study from this year looked at 14 cases of CA-MRSA associated necrotizing fasciitis presenting over a period of 16 months. None of the patients died though all had significant morbidities.

Yet nationwide there are 500-1500 cases of necrotizing fasciitis (usually caused by multiple different bugs like Streptococcus and anaerobic bacteria) which carries a mortality rate of 20-40%. This study didn’t look at the number of non-MRSA associated necrotizing fasciitis cases during this same time period. The decreased mortality of the CA-MRSA cases suggests a decrease in virulence compared to other causes of necrotizing fasciitis! The same lack of data can be said about the study of CA-MRSA necrotizing pneumonia of which only 19 cases were looked at during the 2003/4 flu season.

Is CA-MRSA more virulent in its transmissibility than MSSA? The Mayo Clinic review listed a study from 2004 that found that MRSA was 12 times more transmissible than MSSA in soldiers who were initially found to have MRSA colonization of the nasal mucosa. This is the most compelling evidence for the increased virulence of CA-MRSA however, although CA-MRSA was more likely to result in a skin infection, none of the CA-MRSA infections appeared to be any worse than the MSSA skin infections (though no outcome data was provided). In addition, like this study, most of the evidence for increased transmissibility of MRSA is from populations (day care centers, soldiers, prisoners, homeless persons, intravenous drug users) who share common risk factors for disease transmission in general such as crowding and/or decreased hygiene.

In general, I do agree with Dr. RW but I would characterize my position similar to the way that Dr. Chambers does. The evidence that CA-MRSA is more virulent than MSSA is largely circumstantial and far from definitive but some of the data is compelling. But that is a long way from the media calling CA-MRSA “killer Staph” just to sell a few more papers or get more readers/viewers.