Full typing also indicates the potential for the rising spread of CA-MRSA into health care institutions, according to a population-based study presented here at the 16^th European Congress of Clinical Microbiology and Infectious Diseases (ECCMID).

With concern in Norway about the increases in MRSA incidence over the past few years, Aina Fossum, PhD, research fellow with principal investigator Geir Bukholm, MD, PhD, professor of clinical epidemiology, Epi-Gen Institute, University of Oslo, Akershus University Hospital Trust, Lorenskog, Norway, designed a study to investigate these changing trends.

The researchers analyzed all MRSA genotypes found in the Eastern Health Region of Norway from 1991 to 2005. Dr. Fossum presented the results of this analysis on April 3^rd.

Norway's Eastern Health Region represents over 1.65 million inhabitants, about 35% of the country's population. The collected isolates were classified according to two major groupings: those from health care institutions (HCI), and those from non-health care institutions (non-HCI).

"In the health care institution group, isolates from institutions like nursing homes are included, making this group a mixture of hospital-acquired MRSA and CA-MRSA, while in the non-health care institute group, MRSA is believed to be almost exclusively community-onset," Dr. Fossum said.

All isolates were analyzed by multilocus sequence typing (MLST), Staphylococcus Cassette Chromosome mec (SSCmec) typing, and Panton-Valentine leukocidin (PVL) typing.

In the isolates obtained from 541 people, the incidence of MRSA infection/colonisation showed a gradual increase from 1997, with an increasing rate apparent since 2003. When analyzed according to HCI/ non-HCI status, although the incidence in the HCI setting has generally been higher than that for non-HCI, the patterns of the increases seen in both group are such that for 2005 this difference has disappeared, mainly due to the continuous steady increase in the non-HCI setting.

In the MLST molecular characterization, 37 known and 12 novel sequence types were identified. Within the known sequence types, ST8, ST80 and ST239 dominated, representing more than 40% of the isolates. The novel sequence types included two variants of ST80 and one variant of ST8. In the HCI isolates, ST8 dominated, while ST80 dominated in the non-HCI setting.

For the SCCmec types, Dr. Fossum said, "Type IV dominated in both groups, and it was represented in almost 70% of the isolates as a whole." She also indicated that in both of the isolate groups, the SCCmec-IV isolates have continued to increase over the last 10 years, with little or no changes in the non-SCCmec-IV isolates in more recent years.

The PVL gene locus is considered a marker of CA-MRSA, and it was found in 27% of the isolates. Overall, it was seen in 9 sequence types across the isolates, with ST80 dominating, but ST8 showed more recent increases.

Reflecting its potential as a CA-MRSA marker, only 9.5% of the PVL⁺ isolates fell into the HCI group, as opposed to the 48.0% incidence in non-HCI. The HCI group showed five PVL⁺ sequence types and the non-HCI showed 9. In both cases, ST80 was the major sequence type in these PVL⁺ isolates, representing 43% and 49% incidences, respectively.

"The PVL⁺ isolates found in health care institutions do not differ in sequence type or SCCmec type from those discovered outside health care institutions, most likely meaning that the PVL⁺ isolates are a result of MRSA infection acquired in the community but discovered inside institutions."

The incidence of SCCmec-IV showed close to a 2-fold increase from 2004 to 2005, and PVL typing showed only very small, but recently increasing, proportions of SCCmec-IV/PVL⁺ in the HCI setting. In contrast, some 50% of the non-HCI SCCmec-IV were PVL⁺; which supports an increasing transfer of CA-MRSA into the HCIs.

A rapid increase (about 4-fold) in ST8-IV/PVL⁺ isolates was also seen from 2004 to 2005, while ST80-IV/PVL⁺ continued its slower increase. This potentially indicates a shift in the lineages of the CA-MRSA, from being dominated by ST80 to being dominated by ST8, Dr. Fossum said.

Therefore, along with the worrying increases in MRSA incidence in Norway, this detailed MRSA type analysis also indicates both a shift in the lineages of CA-MRSA and a shift of CA-MRSA into the health care institution setting. The true levels and significance of these trends, however, remain to be seen, Dr. Fossum said.


[Study title: Molecular Characterisation of MRSA in the South-eastern Part of Norway 1991-2005. Abstract O312]

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