Background
During the past decade, community acquired infections with methicillin
resistant Staphylococcus aureus (CA-MRSA) have been observed
with increasing frequency. Distinct genetic lineages associated with
CA-MRSA infections have been determined through typing and their geographic
dissemination evaluated [5]. In the United States (US), the predominant
CA-MRSA clone is the USA300, characterised by a particular pulsed field
gel electrophoresis (PFGE) pattern, staphylococcal protein A (spa)
type t008, multi locus sequence type 8 (ST8), Staphylococcal Cassette
Chromosome mec (SCCmec) type IVa and encoding Panton-Valentine
leukocidin (PVL) [2]. USA300 is the single most prevalent MRSA clone
obtained from skin and soft tissue infections (SSTI) infections in several
metropolitan areas across the US [6], and has for instance been transmitted
in relation to contact sports [7], and prison inmates [8]. However, an
increasing number of reports describe USA300 as a rapidly emerging cause
of hospital infections causing severe infections such as septicaemia,
and neonatal death [4].
Transatlantic spread of USA300 has recently been documented in a case report,
but the extent to which it is disseminated in Europe is still unknown [9].
Denmark has been a low prevalence country, with MRSA comprising <1%
of all blood isolates for three decades [10]. Recently, the number of new
MRSA cases (infected persons and/or carriers) in Denmark has increased
rapidly, from 100 in 2002 to 243, 549 and 864 cases in 2003, 2004 and 2005,
respectively [11] with a large proportion of infections being community
acquired.
In Denmark, the European CA-MRSA clone (ST80) has been the predominating
cause of CA-MRSA for a decade (1995-2004). However, a remarkable increase
in MRSA isolates belonging to clonal complex 8 (CC8) has been recognised
in different parts of the country and especially in the Copenhagen area
since 2003.
This article describes the presence of USA300 in Denmark between 2000 and
2005, and gives epidemiological characteristics of its dissemination.
Materials and methods
As a part of the national surveillance system, all MRSA isolated from
infections or from healthy carriers in Denmark since 1988 has prospectively
been referred to and stored at Statens Serum Institut (SSI) on a voluntary
basis. Since, 1997 the first isolate from each MRSA case has been subjected
to PFGE typing (n=1986) and isolates assigned to clonal complex based
on spa and MLST typing of representative isolates in each PFGE cluster.
The results have prospectively been registered in a database with each
patient reported only once. In the present report, all MRSA isolates
belonging to CC8 in the period 1997-2005 were investigated.
Clinical and epidemiological information
Since 1999, epidemiological and clinical data have been registered for
patients and healthy carriers by their primary MRSA isolate. The data
has been obtained from hospital discharge summaries and general practitioner
(GP) records. The following clinical data were recorded: reason for specimen
collection (infection or screening), infection onset, risk factor for
acquisition of MRSA, and infected body site (skin and soft tissue, blood,
respiratory tract, bone/joint, urinary tract or postoperative wound).
For classification of the infection onset, we used a recent definition
for MRSA infections [12] including five possible types of MRSA infections:
(i) hospital acquired (HA); (ii) imported (IMP); (iii) community onset
(CO-MRSA) infections with no identified risk fac-tors (CO-NR); (iv) CO-MRSA
infections with an identifiable community risk factor (CO-CR), for example
infected persons with other family members as known MRSA carri-ers/patients
and (v) CO-MRSA infections with identified healthcare risk factors (CO-HCA),
for example persons living in residential homes for elderly people or
with a history of hospitalisation in the previous 12 months. In the
present report the CO-CR and CO-NR will be grouped together as CA-MRSA.
Molecular characterisation
PFGE: SmaI macrorestriction profiles were performed according to the
HARMONY protocol [13] and analysed using Bionumerics 4.6 (Applied
Maths, Sint-Martens-Latem, Belgium). The Danish isolates were compared
to the US reference strain, kindly provided by Fred C. Tenover, CDC,
Atlanta, USA. Spa typing and MLST were performed as previously described
[14,15]. The spa type (t) and MLST sequence types (ST) were assigned
through the Ridom (http://www.ridom.de)
and MLST databases (http://www.mlst.net),
respectively.
SCCmec types I-V and mecA confirmation were determined by two multiplex
PCR strategies [16,17] and PVL was detected as previously described [18].
Results
Through the period 1999-2005, 516 out of 1986 MRSA cases belonged to
CC8. Based on PFGE and detection of PVL genes, 44 (8.5%) of these isolates
were found to be USA300.
By SCCmec and spa typing, all these isolates harboured SCCmec type IVa,
and spa type t008 or variants thereof (t068, t211, t304 and t622).
Discharge summaries were obtained for 42 of the patients, but there was
insufficient patient information for two of the cases. In 41 cases these
isolates caused infections, and one isolate was found as result of screening
a family member to a hospitalised person infected with USA300.
Discharge summaries suggested that 28/41 (68%) of the USA300 infections
had community onset (CO-MRSA), and import (IMP) and hospital acquired
(HA) infections were re-ported in two and 11 cases, respectively. Healthcare
(CO-HCA) or community (CO-CR) risk factors were recognised in four cases
each. In the remaining 20 cases no risk factors (CO-NR) were identified.
Thus, 24/41 (59%) were regarded as true CA-MRSA infections.
The first USA300 was isolated in Denmark in 2000 by a GP in a rural area
(Viborg), followed by two cases from different parts of the country in
2002, and another two cases in 2003. One of the cases in 2003 had been
working in Canada when he visited a GP in Denmark with abscesses on his
chest. In 2004, 11 new cases were encountered, six of them in the Copenhagen
area. In 2005, the number increased to 28 cases, of which 16 were found
in the Copenhagen area. Travel to the US was reported for two patients
in 2004 and five patients in 2005. In addition, two had traveled in the
east Asia and another two cases reported unspecified travel, which makes
import a suspected source for 11/42 (26%) of all cases. The annual distribution
of USA300 cases in Denmark is summarised in the figure.
SSTIs accounted for 90% (38/42) of the infections: two patients suffered
from respiratory tract infections and one had a deep seated wound infection.
Discussion
The finding of USA300 in Denmark illustrates the ease of MRSA spread
between countries and continents. Import of USA300 was very likely
on several occasions, and travel to the US was highly overrepresented
among patients where travel destination was noted by the physicians.
However, domestic spread seems to be the most prominent way of dis-semination
in Denmark, especially in 2004 and 2005. A single transatlantic event
of transmission has previously been reported [9]. In Belgium, three
cases of PVL positive isolates with spa type t008 were reported in
2005 and in the Netherlands an increase in PVL positive ST8 isolates
from 2002-2003 have been detected [19,20]. Some of these isolates may
likely be identical to USA300, but confirmation by PFGE has only been
re-ported once in a tertiary care center in the Netherlands [21]. The
USA300 clone may therefore already be disseminated in several European
countries.
In the US, the USA300 clone has proven successful in causing CA-MRSA
as well as healthcare acquired infections [3,4]. It is therefore of concern
that it now seems to have been established in the Danish community, causing
2%-5% of the annual MRSA infections in the period 2002-2005.
The general increase in MRSA cases in Denmark during the study period
could be due to increased surveillance activity. However, only one USA300
isolate was found by screening, while all the other isolates reported
caused infections, so the increase of reported USA300 isolates does not
seem to be a study artifact. Since 1995, ST80 has been the predominant
CA-MRSA in Denmark, but USA300 is now competing for the same niche, which
may cause either a general increase in CA-MRSA or a shift in the clonal
distribution. So far, the dissemination of USA300 has primarily occurred
in the Copenhagen area, which is also the most densely populated area,
thereby supporting the hypothesis of community spread. In Denmark, USA300
has also been found in a few hospitalised pa-tients, indicating that
PVL positive MRSA could also become a healthcare-associated problem in
Denmark as observed in the US [6]. In contrast, spread of ST80 isolates
into hospitals has been reported only occasionally [12]. At present,
the national recommenda-tions for infection control of MRSA in Denmark
primarily concern precautions and interventions in hospital settings,
while interventions against CA-MRSA infections are not in-cluded.
All persons who have been in contact with foreign hospitals outside the
Nordic countries and the Netherlands are screened for MRSA at admission
to Danish hospitals. However, domestic patients admitted to hospitals
are only screened when they have known risk fac-tors for carrying MRSA,
resulting in an unhindered access to hospital settings by patients carrying
CA-MRSA with no established risk factors for MRSA
It is therefore of concern if USA300 turns out to be a successful nosocomial
pathogen as indicated by the experience from the US [6]. In order to
diminish entry and transmission of CA-MRSA (including USA300) into hospitals,
it is important to increase awareness of MRSA as the cause of SSTI or
other typical staphylococcal infections. Increased use of diagnostic
sampling from SSTI should be considered, both in primary healthcare and
in hospitals. Furthermore, the use of proper hand hygiene routines is
known to be the most effective way to prevent transmission of MRSA.
In conclusion, the number of USA300 isolates has increased several-fold
in Denmark since 2003. With the US experience in mind, this is of great
concern, especially since this is observed in a country with a long reputation
for controlling MRSA.
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