854 infection control & hospital epidemiology july 2018, vol. 39, no. 7
below-knee amputation, while the other was diagnosed with a psoas abscess. One patient had groin cellulitis requiring intravenous antibiotics, and the fourth patient required inci- sion and drainage of a finger pulp abscess. All patients recov- ered. The characteristics of all the affected wards and infection control interventions are described in the Supplementary Methods. Dermatological review of the patients in the affected wards
showed that most patients suffered from xerosis. Dry skin is caused by abnormalities in the integrity of the barrier function of the stratum corneum, possibly due to overall reduction in the lipid production in the skin of elderly patients, to the constant use of harsh soaps for baths, and to the lack of adequate moisturizing. The dry skin leads to pruritus, and constant scratching of the itchy dry skin results in breaks and cracks of the skin, allowing for bacterial infection. These patients may also have poorer hygiene; hence, they are at higher risk for scabies or fungal infection. Once infected, the itch results in increased scratching, damaging the skin and allowing for bacterial infections.
Microbiological Results
All S. pyogenes isolates from the affected institution were susceptible to penicillin, erythromycin, and clindamycin and were resistant to tetracycline. Staphylococcus aureus was also isolated with S. pyogenes from many of the swabs taken from skin lesions. These S. aureus isolates displayed many different antibiograms and were not typed further as part of this investigation (see Supplementary Results).
Epidemiologic and Genomic Investigation of the Outbreak
We performedWGS on 67 S. pyogenes isolates (22 from ward A, 5 from ward B, 2 from ward C, 4 from ward D, 3 each from wards D and F, 1 each from wards G and H, 2 from staff members, and 24 randomly selected community isolates) (see Supplementary Results). Sequencing identified 16 emm types that matched the emm types determined by Sanger sequencing (see Supplementary Results). In total, 16 different MLST types were identified, including 7 novel sequence types that were submitted to the PubMLST site (
http://pubmlst.org). The new MLST types were ST547, ST909, and ST915–ST919 (Figure 1). Manual examination of strain genetic relatedness identified 3 genomic clusters (C1–C3) (Figure 1). Based on pairwise SNP differences, cluster C1 contained the most closely related strains and was the candidate cluster most likely associated with recent person-to-person transmission of S. pyogenes. Cluster C1 included isolates from 16 patients from ward A, 2
patients from ward C, 3 patients from ward D, and 1 com- munity isolate (GAS008). Ward A was the site of the initial temporal cluster of 4 infections that prompted the investiga- tion in June 2016. Infections from this cluster occurred inter- mittently from June to the middle of October 2016 (Figure 2).
Wards A and C are male residential wards for those with
chronic mental heath issues. On investigation, most ward A patients (>80%) suffered with xerosis with a history of recurring superficial skin infections (Supplemental Material section 4). Most residents had frequent physical contact during their daily activities. Ward C was adjacent to ward A, and patients from these 2 wards shared communal living and dining areas, where they spent most of their time taking part in planned social activities. Most patients in ward C had no chronic skin lesions. Patients in ward D were mainly under- going psychiatric rehabilitation, which included interacting with patients in other wards. These patients were independent in their activities of daily living, with no history of skin infections or chronic skin lesions. We could not establish an epidemiological link between patients from wards A and C, and those from ward D by assessing patient movement, staff cross-coverage, and shared activities. All patients and staff on these wards were negative for throat carriage of S. pyogenes. No link could be established between the community isolate GAS008 and the affected patients in the institution. The results of WGS showed that the S. pyogenes isolates
from June (wards A and C) and July, August, October, and November (wards A, C, and D) clustered tightly on the phylogenetic tree (cluster C1) and were genetically distinct from most of the background community isolates (GAS001– GAS024) (Figure 1). Cluster C1 isolates had pairwise SNP differences of 0–5 and were emm type 4 and ST915. The low pairwise SNP differences were consistent with recent trans- mission of S. pyogenes among patients involved in this cluster. Patients involved in this cluster received treatment for skin infections, eradication of throat carriage (if present), and 5 days decolonization with chlorhexidine body wipes. These measures were coupled with terminal cleaning of ward A. Cluster C2 contained isolates from 5 patients from ward B, 1
patient each from wards G and H, and a community isolate, GAS022 (Figure 1). All ward B isolates were collected in July following a case of invasive infection, while isolates from wards G and H were obtained in September (Figure 2). Patients on these wards had xerosis but not recurrent skin infections or chronic skin lesions, which differentiated them from patients from outbreak ward A. Oropharyngeal carriage of S. pyogenes was identified in 1 staff member (STAFF 1) who worked on ward B. This discovery triggered a separate epidemiological investigation (see Supplementary Material). No epidemiolo- gical link could be established between ward B and wards G and H or the community isolate (GAS022). No further cases were related to cluster C2 after September 2016. The S. pyogenes strains in cluster C2 were all emm type 11 and ST 547. However, they were genetically more divergent than those in cluster C1 with pairwise SNP differences of 21–45, which is suggestive of a more distant common ancestor for this popu- lation and, therefore, independently introduced infections rather than recent transmissions in the wards. Additionally, the STAFF 1 isolate was genetically distinct from cluster C2 patient isolates.
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