LA-MRSA: On the definition of emergence

Some friends, called patriotic hackers, had secured a very interesting surveillance database and I tried to write a manuscript, when I was scooped (in CID) by friends that apparently had access to the same database. That made my work redundant. So, for your eyes only, my take on that data.

The database holds all S. aureus BSI (SAB) from Denmark from 2010 till 2016, with isolates categorized as MSSA, Live-stock Associated-MRSA (LA-MRSA) and other MRSA. There were 17 episodes of LA-MRSA BSI among 5.6 million Danes in 5 years (1 per 2 million Danish life years). Although the rates per year were not equally distributed (2011-15: n=1, 0, 4, 7, 3) I failed to see a “worrying” trend. In the most resourceful year, 2014, the proportion of LA-MRSA among all SAB was 0.36% (7 of 1,964). For the Danish population, in that year, the risk of SAB increased from 1,957 to 1,964 per 5.6M due to LA-MRSA. How to spin this?

What if LA-MRSA expressed as a proportion of all MRSA BSI. Hmmm; 16% in 2014 and that could increase even if the incidence of LA-MRSA BSI would go down (if other MRSA BSI incidence goes down more!). Not sure if that’s the best way to present data.

The number of skin and soft tissue infections (SSTI) also peaked in 2014 (n=211), but it was not apparent whether that reflected serious skin infections (as seen with USA300), or prosthetic joint infections or screening results of patients at risk for carriage. Necessary for risk assessment.

The genotyping data yielded evidence of nosocomial spread of LA-MRSA in 2 clusters of 2 patients each. Confirmation that LA-MRSA can be transmitted, just like any other bacterium, including Coagulase-Negative staplylococci. Yet, the data did not allow quantification nor comparison of transmission potential with other types of MRSA. So, no new information to the existing studies suggesting that LA-MRSA is, under comparable conditions, less transmissible than other MRSA (1,2).

Mortality: 6 of 17 LA-MRSA infected patients (35%) had died at day-30, as did 31 of 145 with other MRSA types (21%). Yet, not enough detail to challenge findings that methicillin-resistance in itself is not associated with attributable mortality in patients with SAB (3,4, and more). With a maximum addition per year of 7 SAB events, LA-MRSA does not add much to the total infection burden. As SAB is – without doubt – associated with attributable mortality (compared to no infection), an increase of the total burden of SAB would have severe consequences, regardless of methicillin-resistance.

So, what is the status of LA-MRSA BSI? In order to qualify for the term “rare disease” the prevalence of LA-MRSA BSI should not exceed 2,800 subjects, which is a 900-fold increase (and should be life-threatening or chronically debilitating), according to EU definitions.

Because of all this I could not make up my mind on a catchy title. “Sustained mildly fluctuating extremely low-incidence of LA-MRSA BSI despite growing reservoir in animals” did not sound appealing, and I hesitated until scooped. They found a much better one: “Emergence of LA-MRSA BSI in Denmark”. According to the Webster Dictionary emergence means: “the act of becoming known or coming into view”. I humbly take a deep bow for so much creativity.

Sorry guys (only male authors…), I greatly admire the Danish surveillance system, but sometimes we better agree to disagree on what we see in the data.

 

1: Bootsma MC, et al. The nosocomial transmission rate of animal-associated ST398 meticillin-resistant Staphylococcus aureus. J R Soc Interface. 2011 Apr 6;8(57):578-84. doi: 10.1098/rsif.2010.0349. Epub 2010 Sep 22. PubMed PMID: 20861037; PubMed Central PMCID: PMC3061118.

2: Hetem DJ, et al. Transmissibility of livestock-associated methicillin-resistant Staphylococcus aureus. Emerg Infect Dis. 2013 Nov;19(11):1797-1802. doi: 0.3201/eid1911.121085. PubMed PMID: 24207050; PubMed Central PMCID: PMC3837675.

3: Ammerlaan H et al. Adequacy of antimicrobial treatment and outcome of Staphylococcus aureus bacteremia in 9 Western European countries. Clin Infect Dis. 2009 Oct 1;49(7):997-1005. doi: 10.1086/605555. PubMed PMID: 19719417

4: Stewardson AJ, et al. The health and economic burden of bloodstream infections caused by antimicrobial-susceptible and non-susceptible Enterobacteriaceae and Staphylococcus aureus in European hospitals, 2010 and 2011: a multicentre retrospective cohort study. Euro Surveill. 2016 Aug 18;21(33). doi:10.2807/1560-7917.ES.2016.21.33.30319. PubMed PMID: 27562950; PubMed Central PMCID: PMC4998424.

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