The search and destroy strategy prevents spread and long-term carriage of methicillin-resistant Staphylococcus aureus: results from the follow-up screening of a large ST22 (E-MRSA 15) outbreak in Denmark

In the aftermath of a methicillin-resistant Staphylococcus aureus (MRSA) ST22 hospital outbreak, we investigated the prevalence of long-term carriage, the efficacy of MRSA decolonization treatment (DT) and the spread of MRSA to households of patients and healthcare workers (HCWs). Furthermore, we evaluated the efficacy of repeated DT in long-term MRSA carriers. Of 250 index persons (58 HCWs and 192 patients), 102 persons (19 HCWs and 83 patients) and 67 household members agreed to participate. Samples from all 169 persons were taken from the nose, throat, wounds and devices/catheters, and urine samples were additionally taken from index persons. Samples from companion animals (n = 35) were taken from the nostrils and anus. Environmental sites (n = 490) screened were telephone, television remote control, toilet flush handle, favourite chair and skirting board beside the bed. Sixteen (19%) patients and two household members, but no HCWs, were ST22-positive. The throat was the most frequent site of colonization. In a multivariate analysis, chronic disease (p <0.001) and pharyngeal carriage (p <0.001) were associated with long-term MRSA carriage. MRSA was found in the environments of four long-term carriers. All animals tested were negative. MRSA-positive households were decolonized using nasal mupirocin TID and daily chlorhexidine body and hair wash for 5 days. Pharyngeal MRSA carriers also received fucidic acid (500 mg TID) combined with rifampicin (600 mg BID) or clindamycin (600 mg BID) for 7 days. The home environment was cleaned on days 2 and 5. At the end of follow-up, ten of 16 long-term carriers and the two household contacts were MRSA-negative. In conclusion, decolonization of MRSA carriers is possible, but should include treatment of household members and the environment.

Genetic diversity in CC398 methicillin-resistant Staphylococcus aureus isolates of different geographical origin

Staphylococcus aureus of sequence type 398 has emerged in Europe, North America and Asia, and has typically been associated with livestock and their human contacts. We analysed two Panton-Valentine leukocidin (PVL)-negative t034-ST398 isolates from humans in contact with pigs and two t034-ST398 PVL-positive isolates from two unrelated, adopted Chinese children, using multistrain microarrays to determine genomic variability between the two sets of isolates. The ST398 isolates clearly belong to the same lineage when compared to other clonal lineages. However, the four isolates cluster into two distinct groups corresponding to differences in epidemiology based on mobile genetic elements and resistance patterns, suggesting that the two groups are epidemiologically distinct.

Prevalence of methicillin-resistant Staphylococcus aureus among veterinarians: an international study

Pig farmers and veterinarians in contact with livestock in The Netherlands have a higher risk of methicillin-resistant Staphylococcus aureus (MRSA) carriage than the general population. The objective of this study was to investigate whether this is also true for other professionals in contact with pigs in an international setting. A convenience sample of 272 participants at an international conference on pig health in Denmark was screened for MRSA carriage using combined nose/throat swabs and were asked to complete a questionnaire concerning animal contacts, exposure to known MRSA risk-factors, and the protective measures taken when entering pig farms. In total, 34 (12.5%) participants from nine countries carried MRSA. Thirty-one of these isolates were non-typeable by pulsed-field gel electrophoresis following SmaI digestion of chromosomal DNA. All of the non-typeable isolates belonged to spa types (t011, t034, t108, t571, t567 and t899) that correspond to multilocus sequence type 398. All of the above-mentioned spa types, with the exception of t899, have been isolated previously from either Dutch pigs, pig farmers and/or veterinarians. Protective measures, e.g., masks, gowns and gloves, did not protect against MRSA acquisition. Transmission of MRSA from pigs to staff tending to these animals appears to be an international problem, creating a new reservoir for community-acquired MRSA (CA-MRSA) in humans in Europe, and possibly worldwide. The rise of a new zoonotic source of MRSA could have a severe impact on the epidemiology of CA-MRSA, and may have consequences for the control of MRSA, especially in those countries that maintain a low prevalence by means of search-and-destroy policies.

Emergence and dissemination of the methicillin resistant Staphylococcus aureus USA300 clone in Denmark (2000-2005)

The problem of methicillin resistant Staphylococcus aureus (MRSA) is increasing worldwide, and the spread of MRSA in the community challenges infection control since it is no longer restricted to hospital settings but involves private homes, places of work and kindergartens [1]. Furthermore, community acquired (CA)-MRSA may circumvent existing hospital infection control, since patients are rarely screened at admission. In the United States, the predominant CA-MRSA is defined by the Center for Disease Control (CDC) as the USA300 (ST8) clone. USA300 primarily causes skin and soft tissue infections (SSTI) in the community [2], but healthcare acquired infections with USA300 are rapidly emerging in the United States [3,4]. Comparison of the Danish collection of MRSA from 1997-2005 with the USA300 reference strain showed that USA300 has been introduced into Denmark on several occasions. Between 2000 and 2005, we identified 44 isolates which in addition to identical pulsed-field gel electrophoresis (PFGE) pattern shared other molecular characteristics with USA300: spa type t008 or closely related variants, Panton-Valentine leukocidin (PVL) positive and Staphylococcal Cassette Chromosome mec (SCCmec) type IVa. The isolates primarily caused SSTI, but cases of invasive infections were also encountered. The number of USA300 has increased several-folds in Denmark from 2003 to 2005 (2, 11 and 28 new cases, respectively) and with the experience from the US in mind, this is of great concern, especially as it is observed in a country with a long reputation for controlling MRSA.

Prevalence, persistence, and molecular characterization of glycopeptide-resistant enterococci in Norwegian poultry and poultry farmers 3 to 8 years after the ban on avoparcin

Environmental reservoirs of glycopeptide-resistant enterococci (GRE) in Norway have been linked to former growth promoting use of the glycopeptide avoparcin in poultry production. We have examined the prevalence of fecal GRE in poultry and poultry farmers 3 to 8 years after the Norwegian avoparcin ban in 1995 and performed molecular analyses of the GRE population. Fecal samples from poultry farmers and their flocks on 29 previously avoparcin-exposed farms were collected on five occasions during the study period (1998 to 2003). All flocks (100%) were GRE positive in 1998. Throughout the study period, 78.5% of the poultry samples were GRE positive. Glycopeptide-resistant Enterococcus faecium (GREF) was isolated from 27.6% of the farmer samples in 1998 and from 27.8% of the samples collected between 1998 and 2003. The prevalence of fecal GRE in poultry declined significantly during the study period, but prevalence in samples from the farmers did not decline. PCR analysis revealed a specific Tn1546-plasmid junction fragment in 93.9% of E. faecium isolates. A putative postsegregation killing (PSK) system linked to Tn1546 was detected in 97.1% of the isolates examined. Multilocus sequence typing of glycopeptide-susceptible (n = 10) and -resistant (n = 10) E. faecium isolates from humans (n = 10) and poultry (n = 10) on two farms displayed 17 different sequence types. The study confirms the continuing persistence of a widespread common plasmid-mediated vanA-pRE25-PSK element within a heterogeneous GRE population on Norwegian poultry farms 8 years after the avoparcin ban. Moreover, it suggests an important role of PSK systems in the maintenance of antimicrobial resistance determinants in reservoirs without apparent antimicrobial selection.

Persistence of animal and human glycopeptide-resistant enterococci on two Norwegian poultry farms formerly exposed to avoparcin is associated with a widespread plasmid-mediated vanA element within a polyclonal enterococcus faecium population

The evolutionary processes responsible for the long-term persistence of glycopeptide-resistant Enterococcus faecium (GREF) in nonselective environments were addressed by genetic analyses of E. faecium populations in animals and humans on two Norwegian poultry farms that were previously exposed to avoparcin. A total of 222 fecal GREF (n = 136) and glycopeptide-susceptible (n = 86) E. faecium (GSEF) isolates were obtained from farmers and poultry on three separate occasions in 1998 and 1999. Pulsed-field gel electrophoresis (PFGE) and plasmid DNA analyses discerned 22 GREF and 32 GSEF PFGE types within shifting polyclonal animal and human E. faecium populations and indicated the presence of transferable plasmid-mediated vanA resistance, respectively. Examples of dominant, persistent GREF PFGE types supported the notion that environmentally well-adapted GREF types may counteract the reversal of resistance. PFGE analyses, sequencing of the purK housekeeping gene, and partial typing of vanA-containing Tn1546 suggested a common animal and human reservoir of glycopeptide resistance. Inverse PCR amplification and sequence analyses targeting the right end of the Tn1546-plasmid junction fragment strongly indicated the presence of a common single Tn1546-plasmid-mediated element in 20 of 22 GREF PFGE types. This observation was further strengthened by vanY-vanZ hybridization analyses of plasmid DNAs as well as the finding of a physical linkage between Tn1546 and a putative postsegregation killing system for seven GREF PFGE types. In conclusion, our observations suggest that the molecular unit of persistence of glycopeptide resistance is a common mobile plasmid-mediated vanA-containing element within a polyclonal GREF population that changes over time. In addition, we propose that "plasmid addiction systems" may contribute to the persistence of GREF in nonselective environments.

VanA-type vancomycin-resistant enterococci (VRE) remain prevalent in poultry carcasses 3 years after avoparcin was banned

Avoparcin was used as a growth promoting feed additive in Norwegian broiler and turkey production from 1986 until it was banned in 1995, when an association between vancomycin-resistant enterococci (VRE) and avoparcin use became apparent. A recent study regarding faecal samples documented a continuing high prevalence of VRE among Norwegian poultry 3 years after avoparcin was banned. In the present study, carcasses of broilers and turkeys from farms where avoparcin had previously been in use and carcasses of layer chickens from farms where avoparcin had never been used were examined for the presence of VRE. One carcass from each of 150 different farms was included. By a direct plating method, VRE were isolated from 30 of 100 samples of broilers and turkeys, but not from any samples of layer chickens. When an enrichment step was included, VRE were isolated from a total of 81 of the 100 samples of broilers and turkeys and from nine of the 50 samples of layer chickens. All VRE isolated were highly resistant to vancomycin (MIC > or = 256 microg/ml) and possessed the vanA gene. These results correspond to the prevalence of VRE recently documented in faecal samples from Norwegian poultry. The present study reveals a high prevalence of VRE in broiler and turkey carcasses. Consequently, consumers are exposed to VRE when handling raw poultry meat, although the public health significance of such exposure is unclear.

Persistence of vancomycin-resistant enterococci (VRE) on Norwegian broiler farms

Five Norwegian broiler farms previously identified as housing broilers carrying vancomycin-resistant enterococci (VRE) were examined for the presence of VRE 4 years after avoparcin was banned. Environmental samples were obtained from empty, cleaned broiler houses. Faecal samples were collected weekly from the flock housed after the environmental sampling. The hatchery from where the chicks originated was also sampled. VRE were found to be present in the farm environment after depopulation and cleanup of the broiler houses. Within 3 weeks after introduction to the farm, all broiler flocks tested positive for VRE. VRE were not isolated from the hatchery.