Prevalence of diphtheria and antimicrobial-resistant wound infections among asylum seekers in Heidelberg, Germany, August–October 2024

Figures Abstract Purpose In the second half of 2022 and 2023, an increase of diphtheria cases among asylum seekers was observed in Europe, with 120 cases reported in the state of Baden-Wuerttemberg, Germany. We aimed to determine prevalence of infection or colonization with toxigenic Corynebacterium among asylum seekers arriving between August and October 2024 in Heidelberg, Germany. In addition, we assessed antimicrobial-resistant wound infections and diphtheria immunity levels in this population. Methods Results Of the participants, 75% were male (n = 804), and ages ranged from 15 to 60 years (median = 27 years). The most common nationalities were Syrian (n = 281), Turkish (n = 189) and Afghan (n = 117). No toxigenic Corynebacterium was identified. Thirty-seven participants carried inflamed wounds, of whom 18 (49%) had wounds infected with methicillin-resistant Staphylococcus aureus (MRSA), corresponding to 1.7% (95% CI 1.0–2.6%) of the entire study population. Overall, 38.0% (95% CI 33.3–44.8%) showed non-protective diphtheria antibody titers, while acceptance of the on-site diphtheria vaccination was high at 93% (n = 998). Citation: Walter B, Lück K, Adam M, Winter B, Krauze P, Rau J, et al. (2026) Prevalence of diphtheria and antimicrobial-resistant wound infections among asylum seekers in Heidelberg, Germany, August–October 2024. PLoS One 21(6): e0350513. https://doi.org/10.1371/journal.pone.0350513 Editor: Igor Mokrousov, St Petersburg Pasteur Institute, RUSSIAN FEDERATION Received: December 22, 2025; Accepted: May 14, 2026; Published: June 9, 2026 Copyright: © 2026 Walter et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Data Availability: All relevant data are within the paper and its Supporting Information files. Sequencing data of MRSA isolates were submitted to the European Nucleotide Archive (http://www.ebi.ac.uk/ena) under the accession number PRJEB98196. Funding: The authors received no specific funding for this work. Competing interests: The authors have declared that no competing interests exist. The author B.W. is a fellow of the German Postgraduate Training in Applied Epidemiology (PAE) associated with the ECDC Fellowship Programme. The views and opinions expressed in the manuscript do not state or reflect those of ECDC. ECDC is not responsible for the data and information collation and analysis and cannot be held liable for conclusions or opinions drawn. Introduction Diphtheria is a vaccine-preventable infectious disease caused by toxigenic strains of Corynebacterium species. In recent years before 2022, diphtheria had become a rare disease in both Europe and Germany. Between 2011 and 2021, annual reported case numbers ranged from 20 to 66 across Europe and from 4 to 26 in Germany [1–3]. In Germany, cases occurred predominantly sporadically and were mainly associated with cutaneous infections caused by C. ulcerans [4]. Reports of local transmission were limited to isolated occasions [5]. In 2022–2023, diphtheria reemerged in several European countries [6]. Germany reported 310 cases, including 126 in the federal state of Baden-Wuerttemberg. The majority of cases were caused by C. diphtheriae and occurred among asylum seekers [7]. Most cases presented with cutaneous infections, though some individuals suffered from respiratory or both manifestations [8]. In case of cutaneous diphtheria, co-infections with Staphylococcus aureus, including strains with methicillin resistance (MRSA), and Streptococcus pyogenes were reported [9]. Previously, wound infections with extended-spectrum β-lactamase (ESBL)-producing Enterobacterales have also been reported among asylum seekers [10]. In addition, higher carriage rates of MRSA and other antimicrobial-resistant bacteria have been described among asylum seeker and refugee patients in comparison to the general patient population [10,11]. Migration history was found to be associated with presence of Panton–Valentine leukocidin (PVL), which is a virulence factor produced by certain strains of S. aureus that is associated with recurrent skin and soft tissue infections as well as severe necrotizing disease [12,13]. PVL formation is independent of the methicillin resistance of S. aureus. At the same time, vaccination records among asylum seekers are frequently unavailable and thus information on immunity against diphtheria in this population is rare [9]. If vaccination status is uncertain, the German Standing Committee on Vaccination (STIKO) recommends catch-up immunization against diphtheria at all ages for both the general population and asylum seekers, alongside other age- and risk-indicated vaccinations [14]. Recent observations suggested diphtheria transmission beyond the asylum-seeking population in Germany. In 2023, several cases of diphtheria occurred also among persons experiencing homelessness in Frankfurt, Germany. Identification of sequence type 574 (ST-574) in case-isolates suggested a possible epidemiological link to an earlier diphtheria cluster detected among asylum seekers in 2022 [15]. A second sub-cluster of ST-574 was identified in Germany since 2024, associated with cases of cutaneous and respiratory diphtheria, including three fatalities [16]. Besides symptomatic cases, studies have also reported asymptomatic respiratory carriage of C. diphtheriae among asylum seekers [17,18]. There is evidence that asymptomatic respiratory carriage can lead to disease transmission [19]. This highlights the potential for silent transmission. However, previous investigations have focused on symptomatic individuals or asymptomatic close contacts, which may result in an underestimation of absolute case numbers and risk. To address this, we conducted a cross-sectional study between August – October 2024 in a cohort of newly arrived asylum seekers in Heidelberg, Germany. Throat swabs were analyzed for toxigenic Corynebacteria. Wounds were screened for toxigenic Corynebacteria and antibiotic-resistant bacteria targeted by our screening approach, with a focus on pathogens associated with limited treatment options due to antimicrobial resistance. A random sub-sample of participants was serologically tested for IgG antibodies against diphtheria toxoid. This study aimed to provide evidence on the prevalence of diphtheria, its immunity levels and antibiotic-resistant wound infections in newly arrived asylum seekers. The findings could inform targeted vaccination and infection control strategies in reception facilities and support the clinical management in routine health care. Methods Study design Collected data and samples During the study examination demographic data (age, sex, nationality) was collected. The throat was examined for inflammation and a throat swab was taken of every participant. Head and limbs were examined for wounds, which were swabbed. Inflammation of wounds was assessed based on clinical signs such as redness, swelling, discharge or pain. The subsequent detection of pathogens in wounds clinically assessed as inflamed was classified as infection. Information was collected whether the participant received a diphtheria vaccination offered on site. All study participants were also eligible for serological investigations. Blood samples were taken from a simple random sub-sample of 263 participants. 27 serum samples resulted from a piloting week of the study between July 22nd and 26th 2024. Laboratory swab investigations The throat and wound swabs were spread on tellurite and blood agar plates (with 50 µg fosfomycin disks). Colonies suspected of being Corynebacterium were examined for catalase activity, Gram-stained and identified by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS, Bruker Corporation, Billerica, USA) with an extended database [20]. Potential toxin-producing Corynebacterium isolates (C. diphtheria and C. ulcerans) were forwarded to the consultant laboratory for diphtheria at the Bavarian Health and Food Safety Authority (LGL) in Oberschleißheim, Germany, where toxin production testing, toxin gene detection and antimicrobial resistance testing were performed according to the WHO laboratory manual for the diagnosis of diphtheria and other related infections [21]. Wound swabs were in addition spread on MacConkey agar plates and selective agar plates for drug resistant pathogens, including MRSA (CHROMID MRSA agar bioMérieux, Marcy-l’Étoile, France), vancomycin-resistant enterococci (VRE) (CHROMID VRE agar bioMérieux, Marcy-l’Étoile, France), extended-spectrum β-lactamase (ESBL)- or carbapenemase-producing organisms (CHROMID CARBA smart agar bioMérieux, Marcy-l’Étoile, France). These media can detect resistant pathogens identified in skin and soft tissue infections, including MRSA, Enterococcus faecium and Enterococcus faecalis (VRE), as well as multidrug-resistant Gram-negative organisms such as Escherichia coli, Enterobacter spp. (e.g., Enterobacter cloacae) and other Enterobacterales (e.g., Proteus mirabilis). Suspect colonies were biochemically identified using VITEK 2 Compact (bioMérieux) and an antimicrobial susceptibility testing (AST) was performed and assessed according to EUCAST 2024 guidelines. Colonies suspected to be MRSA were further analyzed by polymerase chain reaction (PCR) for the mecA/mecC gene (diarellaMRSA 3.0 TM, gerbion, Kornwestheim, Germany). PVL genes (lukS and lukF) were detected using a commercial assay according to the manufacturer’s protocol (GenoType MRSA, Hain Lifescience, Nehren, Germany). Sequencing of MRSA isolates Genomic DNA of S. aureus isolates was isolated as described [22]. Next, the sequencing library was prepared using the NEBNext® Ultra™ II FS DNA Library Prep Kit for Illumina, following the manufacturer’s instructions. Subsequently, the isolates were sequenced on a MiSeq system in a paired-end mode with a read length of 2 times 250 bp. For the processing of raw genomic data and further analysis, MBioSEQ Ridom Typer (Ridom GmbH, Germany) was used. Following quality control with FASTQC and processing with Trimmomatic [23] to remove Illumina adapters and quality trim the reads, the genomes were assembled using SKESA 2.3.0 [24]. Assemblies were analyzed and compared using core genome multilocus sequence typing (cgMLST) [25]. AMRFinder Plus (version 4.0.15) [26] was used to screen the genomes for genes encoding antimicrobial resistances. Sequencing data of MRSA isolates were submitted to the European Nucleotide Archive (http://www.ebi.ac.uk/ena) under the accession number PRJEB98196. Quantification of IgG antibodies against diphtheria toxoid Levels of IgG antibodies against diphtheria toxoid were measured using an enzyme-linked immunosorbent assay (ELISA) (SERION ELISA classic Diphtheria IgG), following the manufacturer’s instructions. Results were categorized according to the World Health Organization as follows [27]: <0.1 IU/ml: insufficient immunity 0.1 - ≤ 1 IU/ml: sufficient immunity >1 IU/ml: long-term immunity Statistical analysis Data were analyzed using R software (version 4.4.0). Confidence intervals (CI) for prevalence represent exact Clopper-Pearson intervals. Observations with “unknown” or “other” category for sex were excluded prior to regression analysis. Age was included as a continuous variable in regression models to preserve statistical power and avoid sparse data issues. A p-value <0.05 was considered to be statistically significant. To assess the association between independent variables and the outcome of MRSA infected wounds, a multivariable analysis was performed. To account for the low number of MRSA cases and potential separation, penalized logistic regression using Firth logistic regression was performed [28]. Analyses were conducted using the logistf package in R. Age and sex were included as independent variables in the multivariable model. For serological analyses, antibody titers were dichotomized by combining values ≥0.1 IU/ml into a single category. The association between age and sex with low antibody titers <0.1 IU/ml was assessed using univariable logistic regression models fitted with the glm function from the R stats package. Geometric mean concentrations (GMCs) of antibody levels were calculated by log-transforming the data, calculating means and 95% CIs on the log scale, and back-transforming to the original scale. Results Participant characteristics and microbiological findings The median age of the participants was 27 years (IQR: 22–36). The majority of participants were male (75%). Most common nationalities were Syrian, Turkish and Afghan (Table 1). In total, 68 wounds were identified on 48 participants. Among these, 54 wounds from 37 individuals were assessed as inflamed (Fig 1). This corresponds to an inflamed wound prevalence of 3.4% [95% CI: 2.4–4.7%]. The detection of pathogens in wounds clinically assessed as inflamed was classified as infection. In this study asylum seekers arriving in Heidelberg, Germany, August – October 2024 were systematically screened for wounds. Inflammatory status of wounds was assessed based on clinical signs of inflammation. Wound swabs were tested for antimicrobial-resistant pathogens. The presence of the Panton-Valentine leukocidin (PVL) gene was confirmed in methicillin-resistant and methicillin-susceptible Staphylococcus aureus samples. Some participants had multiple wounds and may appear in more than one category. One participant had both an inflamed and an uninflamed wound, with no antimicrobial-resistant bacteria identified in either wound. Furthermore, four participants presenting inflamed wounds had at least two wounds that were assigned to different diagnostic categories following laboratory analysis. AMR: Antimicrobial resistance. MSSA: Methicillin-susceptible Staphylococcus aureus. MRSA: Methicillin-resistant Staphylococcus aureus. One throat swab of an asymptomatic participant was identified with C. diphtheriae. The isolate was toxin gene negative during subsequent PCR. No other non-toxigenic or toxigenic Corynebacterium spp. was detected in any throat or wound swab. MRSA was detected in 25 inflamed wounds from 18 individuals, representing approximately half of all inflamed wounds. This corresponds to a prevalence of MRSA wound infections of 1.7% [95% CI: 1.0–2.6%] among the entire study population (Table 1). PVL was identified in MRSA infected wounds of three individuals. In multivariable analysis including sex and age as independent variables, female sex was associated with lower odds of a MRSA wound infection (odds ratio (OR) = 0.09, 95% CI 0.001–0.70, p = 0.014), while increasing age was associated with decreased odds (OR = 0.93 per year, 95% CI 0.86–0.99, p = 0.014). The overall model was statistically significant (likelihood ratio test p = 0.001). Methicillin-susceptible Staphylococcus aureus (MSSA) was detected in 12 inflamed wounds from 7 individuals. Additionally, 14 wounds from 12 individuals were assessed to be not inflamed. MRSA was detected in one of these wounds. MSSA was found in six non-inflamed wounds from five individuals, and in two of these, the PVL gene was present. These results, without clinical signs of inflammation, may reflect colonization rather than infection. No VRE or ESBL or carbapenemase-producing pathogens were identified in wound swabs. Antimicrobial resistances among MRSA isolates Antimicrobial susceptibility testing of MRSA isolates from 14 participants with inflamed wounds revealed high resistance rates to fusidic acid, clindamycin and tetracycline (Table 2). Two isolates showed phenotypic resistance to trimethoprim/sulfamethoxazole. Phylogenetic analysis using core genome multi locus sequence typing (cgMLST) demonstrated high genetic diversity, with pairwise allelic distances across the minimum spanning tree ranging from 88 to 1,655 (S2 Fig). All isolates harbored resistance genes against erythromycin, chloramphenicol, tetracycline and tigecycline (S3 Table). Diphtheria immunity levels Immunity levels against diphtheria were analyzed from serum samples of 290 participants (Table 3). Overall, 7% of individuals showed high antibody concentrations (>1.0 IU/mL), indicating long-term immunity. The majority of participants (54%) had antibody levels in the range of 0.1–1.0 IU/mL, which are considered protective. A total of 38% had low antibody levels (<0.1 IU/mL), indicating incomplete protection against diphtheria. Univariable logistic regression analysis showed increasing odds of low antibody levels (<0.1 IU/mL) with increasing age (OR 1.03 per year, 95% CI 1.01–1.06, p = 0.011). There was no evidence of an association between sex and low antibody levels (<0.1 IU/mL) (OR 1.03, 95% CI 0.59–1.80; p = 0.91). Due to small numbers in several nationality categories, nationality was not included in the regression analysis to avoid sparse data bias. The overall Geometric Mean Concentration (GMC) was 0.153 IU/mL (S4 Table). Of 1073 study participants, 996 (~93%) received a vaccination against diphtheria during the health examination in the reception center. Discussion A recent diphtheria outbreak among asylum seekers in Europe peaked in 2022–2023, with most confirmed cases reported by Germany [7]. Cases were mainly cutaneous and limited to reception centers. By 2024–2025, reported diphtheria cases in asylum seeker populations had declined sharply [29]. This is consistent with our findings: no symptomatic infection or asymptomatic colonization with toxigenic Corynebacteria was detected during our study. We identified one asymptomatic participant with non-toxigenic C. diphtheriae in a throat swab. Non-toxigenic C. diphtheriae has been previously isolated among asylum seekers in UK, Switzerland and Germany [17,30]. Infections caused by non-toxigenic strains do not result in classical toxin-mediated diphtheria. However, they can still lead to severe invasive disease, particularly among vulnerable populations [31]. A cross-sectional study conducted in Denmark 2016–2018, prior to the emergence of diphtheria among asylum seekers in Europe, did also not identify C. diphtheriae among 104 throat swaps of asymptomatic Syrian refugees [32]. Diphtheria prevalence among asylum seekers arriving in England 2022 was estimated to be 0.15% [17]. However, this estimate was derived from surveillance data, which primarily capture symptomatic disease and may therefore underestimate the true prevalence of infection and colonization during the corresponding period. The decline in cases by 2024 may relate to reduced migration: Frontex reported a 78% drop in irregular border crossings along the Western Balkan route and a 38% overall decline in illegal entries to the EU from 2023 to 2024 [33]. This substantial reduction of migration, particularly through Balkan countries, may have contributed to fewer transmission opportunities and a subsequent decline in diphtheria incidence. In addition, increased awareness following the rise in cases in 2022 and 2023, together with the potential implementation of recommendations issued by the European Centre for Disease Prevention and Control (ECDC), may have contributed to the prevention as well as earlier diagnosis and treatment of infections [7]. Notably, our serological results indicate persisting immunity gaps in the studied population, particularly among nationalities most affected in 2022 and 2023, suggesting that vaccination coverage remained incomplete. In contrast, autochthonous diphtheria cases have emerged in Germany since mid-2023 [16]. Afghanistan, one of the main countries of origin, reported 207 diphtheria cases in 2024, indicating a remaining risk for imported cases [34]. Serological testing showed that only 7% of participants had long-term immunity, while 39% showed non-protective titers. The overall GMC was 0.153 IU/mL. Antibody titers above 0.1 IU/mL are considered protective. However, it should be noted that ELISA assays primarily quantify binding antibodies and may not fully reflect functional neutralizing capacity. Previous studies have shown that ELISA measurements overestimate diphtheria antibody titers particularly below 0.1 IU/mL in comparison to in vitro toxin neutralization tests. Consequently, the actual GMC may be lower than suggested [21]. Immunity declined with age, which mirrors trends in displaced but also in European populations [35,36]. However, 93% of participants accepted diphtheria vaccination during their health screening, underlining the potential of low-threshold vaccination offers. Although no diphtheria case was detected, 1.7% of participants had MRSA-infected wounds, all being male. Other studies have highlighted increased rates of MRSA carriage and infection among migrants in Europe in comparison to the general population. The MRSA prevalence among forced migrants was estimated to be 11% [37]. At the same time, evidence from Germany suggests that although colonization with resistant organisms may be common among refugees, clinically manifest resistant skin infections remain relatively uncommon: in a pediatric refugee cohort, 22 of 325 hospitalized children were colonized with MRSA, but only three had MRSA-associated skin and soft tissue infections [38]. Literature on antimicrobial-resistant skin and tissue infections among asylum seekers in Europe remains scarce. In a Finnish study between 2010 and 2017, resistant wound infections were predominantly caused by MRSA: among 447 asylum seekers screened during hospital admission, 97 (21.3%) were colonized with MRSA and four presented with MRSA wound infections, while only two ESBL-associated wound infections caused by Enterobacter cloacae and Proteus mirabilis were identified. [10]. These findings are consistent with our results, in which MRSA represented the only resistant pathogen detected in wounds. In addition to C. diphtheriae and S. aureus, S. pyogenes has been reported in wound infections among asylum seekers [9,39]. However, because our microbiological screening was restricted to organisms with antimicrobial resistances relevant to therapeutic management, we are unable to draw conclusions regarding the prevalence of susceptible organisms such as S. pyogenes in this cohort. In our study, 5 of 44 S.aureus isolates (11%) were positive for PVL, which is markedly lower than the 40–51.1% reported in two German ambulatory care studies of skin and soft tissue infections [13,40]. This considerable difference might partly be related to the different study settings as our data is derived from wound screening rather than clinically presented infections. A Dutch study among asylum seekers found PVL in 42% of MRSA isolates [11]. However, these isolates were obtained not only from wound swabs but also from broader screening samples such as nasal swabs limiting direct comparability. According to the cgMLST scheme of S. aureus, a maximum allelic distance of 24 alleles is used to define a complex type [41]. The supporting study shows a highly probable common origin of different S. aureus isolates (up to 18 allelic differences) in a nosocomial setting [25], taking into account the mutation rate for MRSA of one nucleotide per six weeks [42]. Therefore, for a close genetic relationship, we would expect < 24 allelic differences among the isolates. However, the phylogenetic analysis revealed no close genetic relationship among the isolates, with the closest allelic distance being 88. This indicates probably independent origins. Isolates displayed multiple resistances against antibiotics and resistance rates were considerably higher than those reported among inpatients in Germany [43]. Our findings underscore the importance of timely diagnostic and treatment options for arriving asylum seekers, considering potential antibiotic resistances. Proactive vaccination offers can be an effective measure to close immunity gaps. Supporting information S1 Fig. Flowchart of participant recruitment and sample collection procedure. https://doi.org/10.1371/journal.pone.0350513.s001 (PDF) S2 Fig. Phylogenetic analysis of the 14 sequenced Staphylococcus aureus isolates. For cluster analysis, a minimum spanning tree was generated based on core genome multilocus sequence typing (cgMLST) comparing a total of 1,861 genes. The numbers on top of the circles represent the internal laboratory sample numbers. The number on each connecting line indicates the number of allelic differences between the respective isolates. https://doi.org/10.1371/journal.pone.0350513.s002 (PDF) S3 Table. Number and frequency of specific antibiotic resistances detected in MRSA isolates (n = 14) from skin wounds of asylum seekers arriving in Heidelberg, Germany, August – October 2024 based on resistance gene detection. https://doi.org/10.1371/journal.pone.0350513.s003 (PDF) S4 Table. Geometric mean concentrations (GMC) of anti-diphtheria toxoid antibodies stratified by age, sex and nationality. https://doi.org/10.1371/journal.pone.0350513.s004 (PDF) Acknowledgments The authors would like to thank staff members of the Local Health Authority Rhine-Neckar-District who were involved in organizing and conducting the study examinations. At the State Health Office Baden-Wuerttemberg, we would also like to acknowledge the laboratory staff who analyzed the samples and Kim Hemmer for her legal advice. Finally, we would like to thank Gamze Aktuna for her constructive feedback and Achim Dörre for his statistical advice (both Robert Koch Institute). References - 1. ECDC EC for DP and C. Annual epidemiological report 2016. Stockholm: ECDC. 2014. https://www.ecdc.europa.eu/sites/default/files/documents/Diphtheria%20AER_0.pdf - 2. ECDC. AER for 2018. European Centre for Disease Prevention and Control: ECDC. https://www.ecdc.europa.eu/sites/default/files/documents/diphtheria-annual-epidemiological-report-2018.pdf - 3. European Centre for Disease Prevention and Control. Annual Epidemiological Report - Diphtheria 2021. 2021. https://www.ecdc.europa.eu/sites/default/files/documents/AER-Diphtheria-2021.pdf - 4. Robert Koch-Institut. Infektionsepidemiologische Jahrbuch meldepflichtiger Krankheiten für 2021. 2025. https://doi.org/10.25646/10143.2 - 5. Berger A, Dangel A, Schober T, Schmidbauer B, Konrad R, Marosevic D, et al. Whole genome sequencing suggests transmission of Corynebacterium diphtheriae-caused cutaneous diphtheria in two siblings, Germany, 2018. Euro Surveill. 2019;24. pmid:30646974 - 6. European Centre for Disease Prevention and Control. Diphtheria - Annual Epidemiological Report for 2022. 2024. https://www.ecdc.europa.eu/en/publications-data/diphtheria-annual-epidemiological-report-2022 - 7. European Centre for Disease Prevention and Control. Epidemiological update: diphtheria cases in Europe. https://www.ecdc.europa.eu/en/news-events/epidemiological-update-diphtheria-cases-europe? 2023. Accessed 2025 June 17. - 8. Hoefer A, Seth-Smith H, Palma F, Schindler S, Freschi L, Dangel A, et al. Corynebacterium diphtheriae Outbreak in Migrant Populations in Europe. N Engl J Med. 2025;392(23):2334–45. pmid:40466062 - 9. Zink A, Hofer J, Schneider C, Kessler F, Klenze H, Klauwer D. Management and outcome of cutaneous diphtheria in adolescent refugees in Germany, June 2022 - October 2023. Infection. 2025;53:329–37. pmid:39190269 - 10. Aro T, Kantele A. High rates of meticillin-resistant Staphylococcus aureus among asylum seekers and refugees admitted to Helsinki University Hospital, 2010 to 2017. Euro Surveill. 2018;23(45):1700797. pmid:30424828 - 11. Ravensbergen SJ, Berends M, Stienstra Y, Ott A. High prevalence of MRSA and ESBL among asylum seekers in the Netherlands. PLoS One. 2017;12(4):e0176481. pmid:28441421 - 12. Hussain K, Bandyopadhyay A, Roberts N, Mughal N, Moore LSP, Fuller LC. Panton-Valentine leucocidin-producing Staphylococcus aureus: a clinical review. Clinical and Experimental Dermatology. 2022;47:2150–8. pmid:36040400 - 13. Klein S, Menz M-D, Zanger P, Heeg K, Nurjadi D. Increase in the prevalence of Panton-Valentine leukocidin and clonal shift in community-onset methicillin-resistant Staphylococcus aureus causing skin and soft-tissue infections in the Rhine-Neckar Region, Germany, 2012-2016. Int J Antimicrob Agents. 2019;53(3):261–7. pmid:30412736 - 14. Koch Institute R. Recommendations of the Standing Committee on Vaccination (STIKO) at the Robert Koch Institute – 2026. 2026. https://www.rki.de/EN/Topics/Infectious-diseases/Immunisation/STIKO/STIKO-recommendations/Downloads/04_26_english.pdf?__blob=publicationFile&v=4 - 15. Haller J, Berger A, Dangel A, Bengs K, Friedrichs I, Kleine C, et al. Diphtheria outbreak among persons experiencing homelessness, 2023, linked to 2022 diphtheria outbreak, Frankfurt am Main, Germany. Emerging Infectious Diseases. 2025;31:547–54. pmid:40023808 - 16. Robert Koch Institute. Signal für bundesweiten Diphtherie-Ausbruch mit Corynebacterium diphtheriae ST-574. Robert Koch Institute. 2025. - 17. O’Boyle S, Barton HE, D’Aeth JC, Cordery R, Fry NK, Litt D, et al. National public health response to an outbreak of toxigenic Corynebacterium diphtheriae among asylum seekers in England, 2022: a descriptive epidemiological study. Lancet Public Health. 2023;8(10):e766–75. pmid:37777286 - 18. Kofler J, Ramette A, Iseli P, Stauber L, Fichtner J, Droz S, et al. Ongoing toxin-positive diphtheria outbreaks in a federal asylum centre in Switzerland, analysis July to September 2022. Euro Surveill. 2022;27(44):2200811. pmid:36330823 - 19. Truelove SA, Keegan LT, Moss WJ, Chaisson LH, Macher E, Azman AS, et al. Clinical and Epidemiological Aspects of Diphtheria: A Systematic Review and Pooled Analysis. Clin Infect Dis. 2020;71(1):89–97. pmid:31425581 - 20. Rau J, Berger A, Dangel A, Dyk M, Eisenberg T, Hiller E, et al. Up-to-date MALDI-TOF MS based identification of the complete Corynebacterium diphtheriae species complex for improved diagnostics. bioRxiv. 2025. - 21. WHO laboratory manual for the diagnosis of diphtheria and other related infections. 1st ed. Geneva: World Health Organization. 2022. - 22. Köser CU, Fraser LJ, Ioannou A, Becq J, Ellington MJ, Holden MTG, et al. Rapid single-colony whole-genome sequencing of bacterial pathogens. J Antimicrob Chemother. 2014;69(5):1275–81. pmid:24370932 - 23. Bolger AM, Lohse M, Usadel B. Trimmomatic: a flexible trimmer for Illumina sequence data. Bioinformatics. 2014;30(15):2114–20. pmid:24695404 - 24. Souvorov A, Agarwala R, Lipman DJ. SKESA: strategic k-mer extension for scrupulous assemblies. Genome Biol. 2018;19(1):153. pmid:30286803 - 25. Leopold SR, Goering RV, Witten A, Harmsen D, Mellmann A. Bacterial whole-genome sequencing revisited: portable, scalable, and standardized analysis for typing and detection of virulence and antibiotic resistance genes. J Clin Microbiol. 2014;52(7):2365–70. pmid:24759713 - 26. Feldgarden M, Brover V, Haft DH, Prasad AB, Slotta DJ, Tolstoy I, et al. Validating the AMRFinder Tool and Resistance Gene Database by Using Antimicrobial Resistance Genotype-Phenotype Correlations in a Collection of Isolates. Antimicrob Agents Chemother. 2019;63(11):e00483–19. pmid:31427293 - 27. World Health Organization. Recommendations to assure the quality, safety and efficacy of diphtheria vaccines (adsorbed). https://cdn.who.int/media/docs/default-source/biologicals/vaccine-standardization/diphtheria/diphtheria-recommendations-trs-980-annex-4.pdf?sfvrsn=7b38b45d_2&download=true - 28. Firth D. Bias reduction of maximum likelihood estimates. Biometrika. 1993;80(1):27–38. - 29. European Centre for Disease Prevention and Control. Outbreak of Corynebacterium diphtheriae ST-574 among migrants, people experiencing homelessness, older adults and unvaccinated people – Germany – 2025. 2025. https://www.ecdc.europa.eu/sites/default/files/documents/communicable-disease-threats-report-week-20-2025.pdf - 30. Meinel DM, Kuehl R, Zbinden R, Boskova V, Garzoni C, Fadini D, et al. Outbreak investigation for toxigenic Corynebacterium diphtheriae wound infections in refugees from Northeast Africa and Syria in Switzerland and Germany by whole genome sequencing. Clin Microbiol Infect. 2016;22(12):1003.e1-1003.e8. pmid:27585943 - 31. Dangel A, Berger A, Konrad R, Bischoff H, Sing A. Geographically diverse clusters of nontoxigenic Corynebacterium diphtheriae infection, Germany, 2016-2017. Emerging Infectious Diseases. 2018;24:1239–45. pmid:29912709 - 32. Eiset AH, Stensvold CR, Fuursted K, Nielsen HV, Wejse C. High prevalence of methicillin-resistant Staphylococcus aureus, Giardia, and Blastocystis in asymptomatic Syrian asylum seekers in Denmark during 2016 through 2018. J Migr Health. 2020;1–2:100016. pmid:34405169 - 33. FRONTEX. Annual Brief 2024. FRONTEX. 2024. https://www.frontex.europa.eu/assets/Publications/General/Annual_Brief_2024.pdf - 34. Immunization Data. https://immunizationdata.who.int/global/wiise-detail-page/diphtheria-reported-cases-and-incidence?CODE=AFG&YEAR=. Accessed 2025 June 18. - 35. Jablonka A, Behrens GMN, Stange M, Dopfer C, Grote U, Hansen G, et al. Tetanus and diphtheria immunity in refugees in Europe in 2015. Infection. 2017;45(2):157–64. pmid:27541038 - 36. Wagner A, Jasinska J, Schmid D, Kundi M, Wiedermann U. Lack of seroprotection against diphtheria in the Austrian population, in light of reported diphtheria cases in Europe, 2022. Euro Surveill. 2023;28:2300206. pmid:37103786 - 37. Chukwudile B, Pan D, Silva L, Gogoi M, Al-Oraibi A, Bird P, et al. Antimicrobial resistance among migrants in Europe: a systematic review and meta-analysis - update from 2017 to 2023. EClinicalMedicine. 2024;75:102801. pmid:39296945 - 38. Tenenbaum T, Becker K-P, Lange B, Martin A, Schäfer P, Weichert S, et al. Prevalence of Multidrug-Resistant Organisms in Hospitalized Pediatric Refugees in an University Children's Hospital in Germany 2015-2016. Infection control and hospital epidemiology. 2016; 37:1310–4. pmid:27523304 - 39. Spielberger BD, Hansel A, Nazary A, Kleißle E-M, Lehr C-G, Utz M. Imported Toxigenic Corynebacterium Diphtheriae in Refugees with Polymicrobial Skin Infections, Germany, 2022. Emerging Infectious Diseases. 2023;29:2112–5. pmid:37690442 - 40. Friesen J, Neuber R, Fuhrmann J, Kietzmann H, Wenzel T, Schaumburg F, et al. Panton-Valentine leukocidin-positive Staphylococcus aureus in skin and soft tissue infections from primary care patients. Clin Microbiol Infect. 2020;26(10):1416.e1–1416.e4. pmid:32619735 - 41. Staphylococcus aureus cgMLST. https://www.cgmlst.org/ncs/schema/Saureus/ Accessed 2026 April 20. - 42. Harris SR, Feil EJ, Holden MTG, Quail MA, Nickerson EK, Chantratita N. Evolution of MRSA during hospital transmission and intercontinental spread. American Association for the Advancement of Science. 2010. - 43. Layer-Nicolaou F, Strommenger B, Cuny C, Werner G. Häufigkeit, Eigenschaften und Verbreitung von MRSA in Deutschland – zur Situation 2021/2022. Epidemiologisches Bulletin. 2023.