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Introduction
Methicillin-resistant Staphylococcus aureus (MRSA) is an emerging
community pathogen. It was first reported in the early 1990s among closed
communities of Aborigines in Western Australia [1]. Outbreaks of community-acquired
MRSA (CA-MRSA) infections in healthy children and adults have been described
worldwide [2]. CA-MRSA infections tend to occur in younger persons than
do hospital-acquired MRSA (HA-MRSA) infections. They often cause sporadic
cases of skin and soft tissue infections but cases of necrotising pneumonia
have also been reported [3]. CA-MRSA has been associated with virulent
strains producing Panton-Valentine leukocidin (PVL) and a variety of
other exotoxins [4]. It shows resistance to methicillin, which is encoded
by the mecA gene, mostly found on the type IV staphylococcal cassette
chromosome (SCC) [2]. The spread of these strains does not seem to be
limited to the community and may also concern the hospital setting [5],
although in Geneva, low prevalence of CA-MRSA on admission to the main
hospital has been reported [6].
Voluntary laboratory-based CA-MRSA surveillance was set up in Geneva
in 2003, to ensure adequate case investigation and contact tracing, estimate
incidence and transmission patterns, and develop prevention strategies.
We report the first results of this surveillance system.
Methods
Four laboratories (the Geneva University Hospital (Hôpitaux Universitaires
de Genève, HUG), clinical microbiology laboratory and three private
ones) participated on a voluntary basis in the surveillance system. Physicians
from public hospitals and private clinics also provided information on
cases. The population of the Canton of Geneva is estimated to be around
427 400 persons (2004).
For surveillance purposes, a laboratory reportable CA-MRSA was defined
as any MRSA isolate with an antibiotic resistance profile different from
the endemic strain prevailing in the Geneva healthcare setting, diagnosed
in a patient without history of hospitalisation in the previous 12 months.
All CA-MRSA cases reported by the laboratories since 2002 and fitting
this case definition were included in our database.
Presence of PVL or other exotoxins was determined with PCR-based assays
[2,8].
For each laboratory reported case, a questionnaire was sent to the clinician
in charge requesting demographic, clinical and epidemiological data information.
Data on the type of infection and other specific clinical features were
collected (questionnaire in French available on request).
For each infected CA-MRSA case, active contact tracing was done within
one week of identification of the index case. The case’s family
members or close contacts were offered screening and treatment (or decolonisation)
if required. This active search increased the number of infected or colonised
cases included in the surveillance database and allowed the identification
of several clusters.
Results
From January 2002 to December 2004, 58 CA-MRSA cases were reported; 41
cases (71%) had a clinical infection and 17 (29%) were colonised. Thirty
three patients (57%) were male. Median age was 32 years (inter-quartile
range: 11-49) and 22% of cases were younger than 10 years. Symptomatic
skin lesions such as furunculosis, impetigo or abscess were present in
40 (97%) of the 41 infected cases. Abscesses and furunculosis were the
most common clinical presentation. Sixteen cases (28%) had a close contact
person with similar skin lesions.
Thirty one cases (75%) were in patients who presented with their first
episode and 10 (25%) with a relapsing infection. The majority of infected
cases (34/41, 83%) had no comorbidity. No deaths or severe infection
were reported. Seven cases were in temporary residents who lived abroad
and 38 had travelled abroad in the preceding 12 months (Africa, 6; Europe,
11; Asia, 4 and North America, 2; not known, 15). The epidemic curve
is shown in the figure.
Forty of 58 isolates carried the PVL toxin. Twenty seven (66%) of these
isolates were recovered from infected cases and 13 (76%) from colonised
cases.
A total of 26 cases could be grouped in 13 distinct transmission clusters.
Of them, 9 clusters were family-related (size: 2-7 persons); 3 were heterosexual
couples and 1 occurred within an ambulatory health setting (private practice)
(size: 3). In two of the familial clusters, colonisation persisted over
a 12-month period despite several treatment attempts administered simultaneously
to all family members.
Discussion
It is essential to differentiate healthcare-associated MRSA infections
occurring in the community among patients at risk of HA-MRSA (such as
a previous history of hospital admission) from true CA-MRSA infections
due to strains which are present in the community only [3,6]. The possibility
that strains first identified in the community will disseminate further
within the hospital population is of great concern [7]. It should be
noted that, to our knowledge, Geneva is currently the only canton in
Switzerland where such specific surveillance (a systematic, patient-based
surveillance of CA-MRSA infections detected in both outpatients and inpatients)
exists.
In Geneva, PVL-producing CA-MRSA was first reported in 2002 [4]. Within
weeks of the alert, the Direction Générale de la Santé (DGS),
with the assistance of a CA-MRSA epidemiology working group (see appendix),
set up a voluntary CA-MRSA surveillance system. The medical community
and microbiology laboratories were informed and as a result, participate
actively and voluntary in the surveillance system.
The case definition of our surveillance system was based on microbiological
criteria (antibiotic resistance profile) together with epidemiological
criteria (no hospital contact within the previous 12 months). However,
more specific microbiological characteristics such as the SCC mec type
or the presence of PVL were also investigated in all cases. Because case
definitions may vary between surveillance systems, caution should be
applied when comparing CA-MRSA prevalence in different settings.
Skin and soft tissue infections caused by CA-MRSA may be an emerging
problem in Geneva and, probably, other parts of Switzerland. Continued
and expansion of surveillance is critical to asses the spread of this
new pathogen.
The majority of isolates (69%) carried the PVL toxin. Further studies
should be conducted to determine the role of PVL as a marker of community
acquisition and its importance to distinguish from healthcare-associated
acquisition.
The existence of more than a dozen clusters demonstrates the importance
of local transmission. More data are needed to clarify the risk profile
for infection and the relative contribution of imported cases versus
local transmission. Our population-based surveillance network will help
to better understand the extent of the spread of CA-MRSA not only in
the community but also to the healthcare setting. Specific epidemiological
studies are planned to better understand these potential risk factors
and transmission patterns.
Prevention and control of CA-MRSA infections represents a challenge for
the future, requiring better surveillance, contact tracing, education
and treatment of infected cases and colonised contacts.
Acknowledgements and thanks to Geneva physicians
and laboratories for their contribution, and to Pr. Jerome Etienne,
Centre National de Référence des Staphylocoques,
Faculté de Médecine, Lyon, France for encouragement,
support and contribution to laboratory testing.
Appendix
The Geneva CA-MRSA epidemiology working group is composed of Dr P Sudre,
M Girard, Dr C Aramburu (EPIET)/ DGS Geneva; Dr S Harbarth, Dr S Hugonnet,
Pr Didier Pittet/ SPCI, HUG Geneva ; Pr J Schrenzel, G Renzi/ DMI, HUG
Geneva ; Dr A Gervaix, Paediatric department/ HUG Geneva, Dr Na Liassine,
Bioanalytique-Riotton UNILABS, Geneva ; Dr L Gauthey, Dr M Pechère,
Association of Physicians in Geneva ; Dr CA Wyler/ Youth Health Service,
Geneva
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