We included all cases of C. diphtheriae species complex infections reported to the regional health agency and recorded at Réunion Island University Hospital during 2015–2020. We analyzed medical records and extracted age, sex, country of residence, recent travel, contact with animals, socioeconomic status, and diphtheria vaccination status for each case. We performed antimicrobial susceptibility testing; identified co-infecting strains; and determined tox gene presence, diphtheria toxin production, and biovar and sequence type (ST). We sent each isolate to the National Reference Center for Corynebacteria of the diphtheriae Complex (Institut Pasteur, Paris, France) to confirm species identity through multiplex PCR and biotyping as previously described (8–10). We detected the tox gene by using conventional PCR or, since 2019, by using multiplex real-time PCR (10). We assessed toxin production by using a modified Elek test (11). We determined antimicrobial drug susceptibility by using disk diffusion or by determining MICs (E-test; bioMérieux, https://www.biomerieux.com), in accordance with CASFM/EUCAST2021 (https://www.sfm-microbiologie.org/2021/04/23/casfm-avril-2021-v1-0) recommendations for benzylpenicillin, amoxicillin, cefotaxime, clindamycin, rifampin, and ciprofloxacin. We genotyped each isolate by using multilocus sequence typing (MLST) (12).

Figure

Figure. Number of cases diagnosed per year in study of emerging Corynebacterium diphtheriaespecies complex infections, Réunion Island, France, 2015–2020. Number of cases were classified according to geographic origin (A)...

A total of 26 cases of C. diphtheriae species complex infections were recorded, from which 27 C. diphtheriae and 2 C. ulcerans isolates were cultured. Most (88.5%) infected patients were male; median age was 60 (interquartile range 32.5–67) years. Fourteen (50%) patients lived on Réunion Island, 3 (11.5%) in Mayotte, 4 (19.2%) in mainland France, 3 (11.5%) in Comoros, and 2 (7.8%) in Madagascar. Most (84.6%) patients had skin manifestations, and 16 patients were vaccinated (Table 1; Appendix Figure). Of 24 C. diphtheriae infections, 8 occurred in patients who had recently traveled to or originated from Madagascar, 4 who traveled to or originated from Mayotte, and 3 who traveled to or originated from Comoros. Since 2018, a total of 9 cases on Réunion Island have been considered locally acquired; all of those patients lived in poor socioeconomic conditions. C. ulcerans infections occurred in 2 patients living on Réunion Island who had not traveled recently but had contact with animals (Table 1; Figure). We performed a Spearman rank correlation to compare locally acquired strains isolated during 2015–2018 and 2019–2020; a 75% increase in locally acquired C. diphtheriae infections occurred in 2019–2020 (ρ = 0.8452; p = 0.0341).

Isolates were obtained from cutaneous lesion (n = 24), bone (n = 4), and respiratory (n = 1) samples. Eight of 27 C. diphtheriae isolates were toxigenic, yielding positive Elek test results. The 2 C. ulcerans isolates were nontoxigenic. C. diphtheriae isolates were characterized as biovars Mitis (n = 20) and Gravis (n = 7).

Patient isolates were co-infected most frequently with Staphylococcus aureus (n = 17) and Streptococcus pyogenes (n = 18). Benzylpenicillin resistance was observed in 80% of isolates according to CASFM/EUCAST2021 recommendations, but isolates were categorized as susceptible increased exposure according to EUCAST version 13.0 proposed breakpoints (https://www.eucast.org/clinical_breakpoints) (Appendix Table). One (3.5%) C. diphtheriae isolate was resistant to amoxicillin (CD8/FRC0402; MIC 1.5 mg/L), and 1 was resistant to rifampin. Both C. ulcerans isolates were resistant to clindamycin (100%, natural low susceptibility), whereas clindamycin resistance was observed for only 1 C. diphtheriae isolate.

We identified 21 STs by MLST analysis, including ST88 for C. diphtheriae isolates from 4 patients and ST339 for both C. ulcerans isolates (Table 2). All C. diphtheriae STs had 2–5 mismatches, except ST87 and ST237, which had 1 mismatch between them. ST339 (C. ulcerans) had 7 mismatches with all C. diphtheriae STs.

We report increased prevalence of cutaneous C. diphtheriae species complex infections on Réunion Island during 2015–2020. Introduction of mass spectrometry analysis in hospital laboratories and increased clinician awareness might have led to increased case reporting. Our study confirms that C. diphtheriae species complex members are circulating and are likely underestimated in the southwest Indian Ocean (7,13). Moreover, we observed emergence of locally acquired cutaneous C. diphtheriae infections on Réunion Island since 2019. The number of imported cases in 2020 was probably limited because of the COVID-19 pandemic, which reduced travel. Indeed, all C. diphtheriae cases identified during 2015–2018 occurred in patients who had traveled from other islands in the Indian Ocean. In addition, cutaneous diphtheria appeared to be associated with poor socioeconomic living conditions, in which alcoholism, drug dependence, and homelessness are factors that increase risk for human-to-human transmission and virulence (14).

A total of 8 (30%) C. diphtheriae isolates were toxigenic and caused cutaneous infections. Nontoxigenic C. diphtheriae isolates (70%, n = 19) were obtained from cutaneous lesions, respiratory samples, and bone samples. Clinicians should be aware that nontoxigenic C. diphtheriae can potentially cause severe disease (1,14,15). Moreover, all isolates were co-infected with pyogenic bacteria, suggesting diphtheria infection should be considered under polymicrobial conditions.

MLST analysis identified 21 different STs; most were unrelated (>2 mistmatches) reflecting marked genetic diversity of isolates. ST88 was found in 4 patients living on Réunion Island who had not traveled recently, indicating probable local acquisition. ST88 had previously been reported only in patients from Mayotte. Therefore, our results show that multiple C. diphtheriae species complex clones are circulating in the southwest Indian Ocean (8). Both C. ulcerans strains belonged to ST339. The National Reference Center reported that ST339 is the predominant C. ulcerans ST found in animals in France. Although considerable ST diversity was revealed, whole-genome sequencing will be required to further evaluate circulating C. diphtheriae clones in this region.

In conclusion, we describe emergence of locally acquired C. diphtheriae species complex infections on Réunion Island during 2019–2020. Local clinicians and microbiologists should remain aware of this neglected infection; improvements should be made in diagnostic methods and management of infected patients, such as maintaining availability of diphtheria antitoxin.