2008-03-27 16:57:27 UTC
Ann Clin Microbiol Antimicrob. 2006; 5: 26.
Published online 2006 November 10. doi: 10.1186/1476-0711-5-26. PMCID:
Copyright © 2006 Huijsdens et al; licensee BioMed Central Ltd.
Community-acquired MRSA and pig-farming
Xander W Huijsdens,1 Beatrix J van Dijke,2 Emile Spalburg,1 Marga G
van Santen-Verheuvel,1 Max EOC Heck,1 Gerlinde N Pluister,1 Andreas
Voss,3,4 Wim JB Wannet,1 and Albert J de Neeling1
1National Institute for Public Health and the Environment (RIVM),
Diagnostic Laboratory for Infectious Diseases and Perinatal Screening,
P.O. Box 1, 3720 BA, Bilthoven, The Netherlands
2St. Jansgasthuis, Department of Medical Microbiology, P.O. Box 29,
6000 AA, Weert, The Netherlands
3Radboud University Medical Centre, Department of Medical
Microbiology, P.O. Box 9101, 6500 HB, Nijmegen, The Netherlands
4Canisius-Wilhelmina Hospital, Department of Medical Microbiology,
P.O. Box 9015, 6500 GS, Nijmegen, The Netherlands
Xander W Huijsdens: ***@rivm.nl; Beatrix J van Dijke:
***@stjansgasthuis.nl; Emile Spalburg:
***@rivm.nl; Marga G van Santen-Verheuvel:
***@rivm.nl; Max EOC Heck: ***@rivm.nl; Gerlinde N
Pluister: ***@rivm.nl; Andreas Voss: ***@cwz.nl; Wim
JB Wannet: ***@rivm.nl; Albert J de Neeling:
Received July 14, 2006; Accepted November 10, 2006.
This is an Open Access article distributed under the terms of the
Creative Commons Attribution License
(http://creativecommons.org/licenses/by/2.0), which permits
unrestricted use, distribution, and reproduction in any medium,
provided the original work is properly cited.
Sporadic cases of CA-MRSA in persons without risk-factors for MRSA
carriage are increasing.
We report a MRSA cluster among family members of a pig-farmer, his
co-workers and his pigs. Initially a young mother was seen with
mastitis due to MRSA. Six months later her baby daughter was admitted
to the hospital with pneumococcal otitis. After staying five days in
hospital, the baby was found to be MRSA positive. At that point it was
decided to look for a possible source, such as other family members
and house-hold animals, including pigs on the farm, since those were
reported as a possible source of MRSA earlier.
Swabs were taken from the throat and nares of family members and
co-workers. A veterinarian obtained swabs from the nares, throat and
perineum of 10 pigs. Swabs were cultured following a national protocol
to detect MRSA that included the use of an enrichment broth. Animal
and human strains were characterized by PFGE, spa-typing, MLST
analysis, SSCmec, AGR typing, and the detection for PVL, LukM, and
TSST toxin genes.
Three family members, three co-workers, and 8 of the 10 pigs were MRSA
positive. With the exception of the initial case (the mother) all
persons were solely colonized, with no signs of clinical infections.
After digestion with SmaI, none of the strains showed any bands using
PFGE. All isolates belonged to spa type t108 and ST398.
1. This report clearly shows clonal spread and transmission between
humans and pigs in the Netherlands. 2. MLST sequence type 398 might be
of international importance as pig-MRSA, since this type was shown
earlier to be present in epidemiologically unrelated French pigs and
pig-farmers. 3. Research is needed to evaluate whether this is a local
problem or a new source of MRSA, that puts the until now successful
Search and Destroy policy of the Netherlands at risk.
References BackgroundStaphylococcus aureus is a major pathogen causing
both nosocomial and community-acquired infections. MRSA strains have
emerged worldwide and became resistant to a variety of antibiotics.
The prevalence of MRSA varies widely between countries, from less than
1% in the Netherlands to more than 30% in several other European
countries . Bacterial strain typing is an important tool to
investigate MRSA outbreaks, to evaluate the transmission of MRSA
strains, and to study evolution. PFGE with SmaI is considered to be
the gold standard for molecular typing of MRSA . When no SmaI
digestion occurred, MRSA strains were classified as non-typeable by
PFGE. Recently, Voss and colleagues described a possible link between
non-typeable MRSA and pig farming . French farmers were shown to be
colonized by a small number of S. aureus strains which exhibited MLST
sequence types (ST) 9, 398, and 433. These STs were found in isolates
from pig farmers as well as from swine but were not present in
non-farmers suggesting a high rate of S. aureus strain exchange
between pig farmers and pigs had occurred . MRSA of animal origin
may be genetically related to MRSA recovered from humans . MRSA in
companion animals have also been described as source for infection for
animals and humans [6,7].
The aim of this study was to find the source of MRSA in a family of a
pig-farmer that had no known risk-factors for MRSA carriership, but
were found to be permanent carriers of PFGE non-typeable MRSA.
References The caseIn October 2004 a young mother with mastitis
suffering from high fevers (> 39°C), general malaise, and pleural
effusions, was admitted to our hospital. Cultures taken at her GP's
office unexpectedly revealed MRSA. The patient recovered quickly after
treatment with teicoplanin. When repeated attempts to eradicate her
MRSA carriership failed, her family was screened for MRSA. The father
and the baby daughter were found to be MRSA positive. Six month later,
the baby girl was admitted with an acute pneumococcal infection. Due
to the history of MRSA the baby was isolated and screened on
admission. While initial screening cultures were negative, follow-up
cultures during antibiotic treatment revealed MRSA. At this point all
family members were re-screened and the parents were found to still
carry MRSA. The source of MRSA remained unclear. As animals have been
described as a source of MRSA and the father was a pig-farmer, we
decided to screen his pigs. Furthermore, three co-workers on the farm
The farm consisted of 8000 pigs located in 4 different holdings. We
randomly picked 10 pigs from the holding closest to the living
quarters of the family. A veterinarian took cultures from the anterior
nares, throat and perineum of the animals. All cultures were processed
in the laboratory according to a national guideline for the detection
of MRSA in human samples. Swabs were put into an enrichment broth that
was incubated for 24 hours at 37°C and subcultured on blood agar. The
cefoxitin disc method was used to screen for methicillin-resistance in
colonies suspected to be S. aureus.
Identification of MRSA was confirmed by a multiplex PCR in which a S.
aureus specific DNA fragment  and the mecA gene for methicillin
resistance  is amplified. Oxacillin susceptibility was tested by
E-test (AB Biodisk) on Mueller-Hinton agar (BBL) containing 2% NaCl
with 24 h incubation at 35°C and results were interpreted according to
the criteria of the Clinical and Laboratory Standards Institute ).
In bovine mastitis, the leukocidin LukM is considered to be a
virulence factor . Since the mother of the pig-farming family
suffered from mastitis, all non-typeable MRSA isolates were tested for
the presence of the LukM gene . The presence of the tst gene,
encoding for the toxic shock syndrome toxin (TSST), was also
investigated . This gene was found significantly more often in
mastitis-associated S. aureus strains . All PFGE non-typeable MRSA
strains were characterized by staphylococcal protein A (spa) gene
typing , multi-locus sequence typing (MLST) , staphylococcal
chromosome cassette (SCC) mec typing , accessory gene regulator
(AGR) typing , and the detection of the Panton-Valentine
leukocidin (PVL) genes. PVL is a virulence factor thought to be
associated with community-acquired MRSA .
Using different typing methods all (animal as well as human) PFGE
non-typeable MRSA isolates were shown to be genetically identical.
They were characterized by spa type t108, ST398, SCCmec type V, AGR
type 1, and negative for the PVL, LukM and TSST toxins.
References DiscussionThe MLST results are in concordance with a study
reported by Armand-Lefevre and colleagues, who compared S. aureus
isolates from healthy pig farmers, human controls, and pigs . They
recovered methicillin-susceptible S. aureus exhibiting ST9, 398, and
433 from pig farmers and swine; only one ST 398 isolate of a pig
farmer was methicillin resistant. ST398 was first recognised by our
group, and reported to the international MLST database in 2004. At
that time no correlation between S. aureus with ST398 and pig farming
had been reported. In Hong Kong, two ST398 strains were described to
have been isolated from patients with bacteremia . No relation
with pig farming was reported. Typing results of the French ST398
strains (4 pig-related MSSA and 1 pig-related MRSA isolate) revealed
the same typing result as the Dutch ST398 strains. At our lab the
French strains were PFGE non-typeable, spa type t034 and t1250, and
were PVL negative. Spa type t108, t034, and t1250 are related to each
other, indicating to have a common ancestor.
Voss and colleagues reported for the first time the isolation of PFGE
non-typeable MRSA strains from pig care-takers . The strains were
closely related to each other as shown by spa typing. They screened a
total of 26 farmers of whom 6 were colonized with MRSA. The authors
identified three different MRSA strains by spa typing, type t108,
t567, and t943. Spa type t108 was also found in the present study,
indicating the relatedness of this spa type with pig-farming. Only one
pig was found to be MRSA positive, carrying the same strain type as
the farmer. In contrast, we found MRSA in 8 out of 10 randomly chosen
pigs. The difference in prevalence could perhaps be explained by
sampling differences, MRSA transmission among pigs or to differences
in risk factors between the farms.
All pig MRSA isolates were PFGE non-typeable by PFGE and had the same
typing characteristics as the human MRSA isolates. Furthermore, the
pig-related MRSA isolates were related to PFGE non-typeable MRSA
strains from the national MRSA database. It seems that the PFGE
non-typeable MRSA strains are not only transmitted between human and
pigs but also between humans. The human to human transmission was
elucidated by the fact that among the PFGE non-typeable MRSA isolates
from the national institute of public health (RIVM) MRSA database in
at least 3 cases a family member was colonized with an MRSA strain
with identical typing characteristics. Furthermore, the child of the
pig farmer's family had no contact with pigs and was colonized with
the same strain as the parents.
An earlier report of a significant association between pig farming and
resistant commensal bacteria was published by Aubry-Damon et al. .
The authors showed that levels of commensal bacteria with
antimicrobial resistance were higher among pig farmers than among
controls, including a higher isolation rate of S. aureus in pig
farmers. The cause of the higher S. aureus isolation rate in pig
farmers remained unclear.
More research on a larger scale is necessary to further address the
prevalence of MRSA among pigs, pig farmers and their contacts.
Furthermore, it would be interesting to what extent the PFGE
non-typeable MRSA isolates were associated with pig farming, which may
elucidate the importance of the clonal cluster.
References ConclusionThis report clearly shows the clonal spread and
transmission between man and pigs in the Netherlands. MRSA isolates
characterized by spa type t108 (or related spa types) and MLST ST 398
might be of international importance as pig-MRSA, since this type was
shown earlier to be present among epidemiological unrelated MRSA
isolates from French pigs and pig-farmers. Further research has to
evaluate whether pigs are a new source of MRSA, that warrants a change
in the Search & Destroy strategy, namely by adding pig-farmers pigs to
the group of possible MRSA carriers.
The prevalence of MRSA in farming animals, as well in the humans
working with them, (e.g. farmers, veterinarians) needs to be
Top Competing interestsThe author(s) declare that they have no
Top Authors' contributionsXWH designed the study, collected and
analyzed the data and drafted the manuscript. ES, MGS, MEOCH, GNP
performed experimental work. BJD and AV were involved in the pig-MRSA
related case. WJB and AJN participated in the design of the study and
drafting of the manuscript. All authors read and approved the final
We would like to thank Dr. Raymond Ruimy and Dr. Antoine Andremont for
providing the French ST398 strains.
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