Abstract
Background: Classical toxin-mediated respiratory diphtheria has become less common because of widespread effective vaccination globally but invasive disease as a result of non-toxigenic strains of Corynebacterium diphtheriae is not prevented by vaccination and may result in severe disease, including infective endocarditis (IE).
Objectives: To describe the outbreak and subsequent investigation of a cluster of five cases of non-toxigenic C. diphtheriae endocarditis.
Method: A retrospective observational case series of five cases of non-toxigenic C. diphtheriae endocarditis identified in the rural West Coast district of the Western Cape province of South Africa between May 2021 and June 2021.
Results: Non-toxigenic C. diphtheriae IE had an aggressive clinical course with high mortality in this cohort. Only one of five patients survived to hospital discharge. The surviving patient received a prompt diagnosis with early surgical intervention but still had a complicated clinical course. Notably, only one case had a pre-existing risk factor for IE, namely a prosthetic valve. Whole genome sequencing of clinical isolates confirmed that all isolates were of the same novel sequence type of non-toxigenic C. diphtheriae but despite a thorough investigation no epidemiological link was ever found between the cases.
Conclusion: Non-toxigenic strains of C. diphtheriae are less well known but may be highly virulent and cause severe invasive disease.
Contribution: This is the largest cluster of non-toxigenic C. diphtheriae IE ever described in South Africa and expands the body of literature on this unusual but possibly emerging infection.
Keywords: infective endocarditis; non-toxigenic Corynebacterium diphtheriae; outbreak.
Introduction
Classical respiratory diphtheria is caused by toxin-producing Corynebacterium diphtheriae (or rarely by other toxin-producing Corynebacterium species) and has become less prevalent globally because of widespread vaccination in childhood. The diphtheria vaccine is an inactivated toxoid preparation and provides protection from toxin-mediated disease but does not prevent colonisation or infection with non-toxigenic C. diphtheriae. Infection with non-toxigenic C. diphtheriae most commonly manifests as mild respiratory disease or cutaneous lesions but has emerged as an important cause of more invasive disease in susceptible individuals.1
Infective endocarditis (IE) because of C. diphtheriae was first described by Howard in the Johns Hopkins Hospital Bulletin in 18932 with a non-toxigenic organism. Invasive infection with non-toxigenic C. diphtheriae was described infrequently3,4 following this case but has become more prolific in the literature since the 1980s. Infective endocarditis is the most commonly reported presentation, usually appearing as isolated sporadic cases.5,6,7,8,9,10,11,12,13 Splenic abscesses,14 septic arthritis15 and septicemia6,9,16,17 because of non-toxigenic C. diphtheriae have also been described.
This report describes a cluster of five cases of endocarditis caused by a non-toxigenic strain of C. diphtheriae. The cases occurred over a period of 6 weeks between May 2021 and June 2021 in the West Coast district of the Western Cape province of South Africa.
Methods
The study was a retrospective observational case series describing five cases of non-toxigenic C. diphtheriae endocarditis identified in the West Coast district of the Western Cape province of South Africa between May 2021 and June 2021.
Data were collected from the existing clinical and laboratory records for each patient and from documentation generated as part of the outbreak investigation including case investigation forms and line lists compiled by the Western Cape district and provincial public health outbreak response teams.
Results
Case presentations
Clinical findings for four individuals with confirmed IE because of non-toxigenic C. diphtheriae and a fifth individual with non-toxigenic C. diphtheriae bacteraemia and possible IE are summarised in Table 1. Three individuals had documented features of endocarditis on imaging, and one had endocarditis confirmed at autopsy. The remaining individual who presented with fever and cardiac failure was included retrospectively, based on blood culture results. The patient died before cardiac imaging could be performed, and a post-mortem examination was not pursued. In all five cases, the organism was isolated from blood culture and identified using routine laboratory methods at the tertiary care National Health Laboratory Service (NHLS) diagnostic laboratory.
| TABLE 1: Clinical characteristics of five individuals with non-toxigenic Corynebacterium diphtheriae endocarditis. |
The blood culture bottles of these cases were incubated in an automated microbial detection system. Once identified as positive for growth, a Gram stain was performed directly from the blood culture broth. Small Gram-positive bacilli were noticed, and inoculation and subsequent incubation of the broth were carried out on routine growth media (tryptose blood agar, MacConkey agar and Mueller Hinton agar with 5% sheep blood.) Typically, standard biochemical tests are used to identify the cultured isolates to genus level (e.g., Corynebacterium species); however because these cases were either known or suspected to have IE, automated biochemical profiling was performed. As invasive C. diphtheriae isolates are uncommon, the first two cases were additionally confirmed by matrix assisted laser desorption ionisation-time of flight (MALDI-TOF) spectrometry. Susceptibility testing for penicillin, ceftriaxone and vancomycin was performed on all isolates. Minimum inhibitory concentrations (MICs) were determined using the gradient diffusion method and were interpreted according to the latest breakpoint criteria published by the Clinical and Laboratory Standards Institute (CLSI).18 All isolates had intermediate susceptibility to penicillin and full susceptibility to vancomycin, whereas ceftriaxone varied between susceptible and intermediate breakpoints. The MIC for each antibiotic differed by only one double-dilution between the isolates.
All C. diphtheriae isolates were submitted to the national reference laboratory at the National Institute for Communicable Diseases (NICD) for confirmation and further characterisation. All isolates were confirmed as non-toxigenic on Elek and polymerase chain reaction (PCR) testing.19 Whole-genome sequencing was performed as previously described by Du Plessis et al.20 and the five isolates were the same strain, novel sequence type (ST) 885.
Four of the five individuals were residents of a small rural community approximately 180 km north of Cape Town. These four patients presented to the same local district hospital. One patient died shortly after presentation and the other three patients were referred to the next level of care at the general specialist care regional hospital before being transferred to the academic tertiary care institution in Cape Town. The fifth patient resided in a semi-urban setting in the same town as the regional hospital, where she presented before being transferred to the tertiary care facility. We were unable to find an epidemiological link or history of contact with the community or geographical area where the remaining four cases had presented.
Case 1 was a 13-year-old male who presented to his local district hospital with fever and cardiac failure in mid-May 2021. He had been previously well. His admission diagnosis was documented as acute rheumatic fever. He was transferred to the regional hospital for further care but died shortly after arrival. Laboratory investigations were notable for elevated white blood cell count and C-reactive protein, thrombocytopenia and renal failure. Blood cultures taken on admission yielded non-toxigenic C. diphtheriae. This case was identified retrospectively as belonging to the outbreak so more detailed clinical information was not available. No cardiac imaging was performed, and a diagnosis of IE could not be conclusively proven but was considered plausible.
Case 2 was a 25-year-old male who presented to his local district hospital 15 days after case 1 with fever, fatigue, myalgias and headache. An initial assessment of tick bite fever was made, and the patient was discharged on oral doxycycline. He returned 2 days later with ongoing fever, worsening headache and new-onset confusion and was transferred to the regional hospital for further investigation. Clinical examination revealed a loud pansystolic murmur in keeping with mitral regurgitation. Transthoracic echocardiography confirmed moderate mitral regurgitation and a large vegetation on the anterior leaflet of the mitral valve. Blood cultures were drawn prior to initiating intravenous penicillin and gentamicin and the patient was transferred to the tertiary care cardiology department. Blood cultures yielded C. diphtheriae. There were no clinical features of pharyngeal or cutaneous diphtheria.
Following transfer, the patient’s level of consciousness declined, and computed tomography (CT) imaging of the brain confirmed right frontal and cerebellar septic emboli and infarction with haemorrhagic transformation and raised intracranial pressure. An external ventricular drain was inserted. Decompression of the posterior fossa and drainage of the right frontal haemorrhage were performed. The patient was admitted to the neurosurgical intensive care unit postoperatively and significant improvement in neurological status was documented. Intravenous penicillin G 6 million units every 6 h with intravenous gentamicin 240 mg daily was continued. Vancomycin was added because of non-susceptibility to penicillin on the cultured isolate. Vancomycin was dosed according to therapeutic drug monitoring.
Four days after surgery, the patient’s level of consciousness suddenly deteriorated and he became haemodynamically unstable culminating in cardiac arrest. Resuscitation was unsuccessful and he died 16 days after his initial presentation.
Case 3 was a 24-year-old male who presented to the local district hospital in June 2021 with fever, back pain and jaundice. He gave a history of poly-substance abuse but denied any intravenous injection of drugs. A diagnosis of IE was suspected and empiric treatment with intravenous penicillin G, gentamicin and cloxacillin was commenced. The patient was referred first to the regional hospital and shortly thereafter transferred to tertiary care. Blood cultures yielded C. diphtheriae. During his admission, he developed a left hemiplegia and CT imaging confirmed a right middle cerebral artery infarct. He continued to deteriorate, developed multiorgan failure and died 5 days after admission. A postmortem examination confirmed endocarditis of the aortic and mitral valves with multiple obstructing vegetations, the largest of which measured 12 mm. Valve tissue PCR was positive for C. diphtheriae. There were no features of pharyngeal or cutaneous diphtheria on antemortem examination or at autopsy.
Case 4 was a 38-year-old woman who presented to the district referral hospital in mid-June 2021 with a 2-week history of fever, myalgias, lower abdominal pain and diminished effort tolerance. She had undergone mitral valve replacement for rheumatic heart disease 23 years prior. Her postoperative course had been uncomplicated with diligent follow-up for anticoagulation monitoring. On admission to the referral hospital, she was acutely ill and tachycardic with features of biventricular cardiac failure. Biochemistry was remarkable for elevated inflammatory markers, acute kidney injury and prolonged international normalised ratio (on warfarin). Initial transthoracic echocardiography did not show features of prosthetic valve endocarditis. Blood cultures were sent prior to initiating antibiotics (ceftriaxone) and subsequently yielded C. diphtheriae. The patient was urgently transferred to the tertiary level hospital for transoesophageal echocardiography and further care.
The patient was admitted to the medical high care unit at the tertiary level hospital. Intravenous penicillin G and vancomycin were initiated; gentamicin was administered for 72 h then withheld because of renal failure. Antibiotics were subsequently changed to penicillin G and ceftriaxone because of worsening renal failure. Transesophageal echocardiography confirmed multiple vegetations on the mitral valve prosthesis. Her clinical course was complicated by worsening renal failure, disseminated intravascular coagulation and pulmonary oedema. She died 11 days after admission and 15 days after her initial presentation to healthcare.
Case 5 was a 14-year-old female who presented to the local district hospital in June 2021 with a 2-week history of fever, malaise and sore throat. She had been previously well. Although she complained of a sore throat there was no significant pharyngitis or cervical lymphadenopathy. A murmur was detected on clinical examination at the district hospital and blood cultures were sent prior to initiating antibiotics. Blood cultures flagged positive within 72 h with Gram-positive bacilli and the patient was immediately referred to the regional hospital as a suspected C. diphtheriae IE. Transthoracic echocardiography confirmed vegetations on the anterior and posterior mitral valve leaflets with moderate to severe mitral regurgitation. The patient was transferred for urgent cardiothoracic surgery and underwent a mitral valve repair procedure within 24 h. She received intravenous penicillin G and vancomycin with therapeutic drug monitoring and completed a total of 42 days of antibiotics. Gentamicin was also initially administered but discontinued after three doses.
The patient developed significant mitral regurgitation postoperatively and required a second mitral valve repair procedure. Her admission was also complicated by acute kidney injury and nosocomial sepsis, but she was discharged home 2 months after initial presentation and continues to follow up with cardiology services.
Outbreak investigation
Following the identification of the second case of confirmed C. diphtheriae endocarditis from the same community, an outbreak investigation was launched by district and provincial public health services. The definitions applied for suspected, probable and confirmed cases and contacts are outlined in Table 221. A timeline of the five cases is presented in Figure 1.
| TABLE 2: Case definitions applied in the investigation of an outbreak of non-toxigenic Corynebacterium diphtheriae endocarditis. |
 |
FIGURE 1: Cases of non-toxigenic Corynebacterium diphtheria endocarditis. |
|
Detailed line lists of contacts of each case were generated, and each contact was followed up, tested for C. diphtheriae carriage or infection by nasopharyngeal swab and provided with azithromycin prophylaxis after the specimen had been taken. Two cases were school-going children (attending two different schools) and screening of the entire grade of each case was performed. The outbreak occurred during the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic while lockdown measures were in place throughout South Africa and learners were attending school only every second or third day making transmission via the school less likely. A potential additional case may have occurred prior to case 1: a child at the same school, although in a different grade, had died after a sudden illness in the month before case 1 presented. No further medical history surrounding this case could be obtained. No positive contacts were identified through screening of the school contacts.
Detailed line lists were also generated for the adult cases and each contact was tested and provided with antibiotic prophylaxis. A total of 119 nasopharyngeal swabs were submitted for C. diphtheriae screening. Swabs were inoculated on tellurite agar and incubated for a total of 72 h. Agar plates were halved to accommodate two swabs per plate. Growth of typical black colonies, suggesting Corynebacterium species, was identified by automated biochemical profiling. Gram stains and/or catalase tests were performed prior to biochemical identification if colonies were of doubtful significance (e.g., grey instead of black phenotype). Only one swab was positive for C. diphtheriae, confirmed by MALDI-TOF spectrometry. This positive screen was referred to the NICD where testing confirmed the same novel sequence type (ST-885) of C. diphtheriae that had been identified in the clinical cases. This specimen belonged to an asymptomatic contact of case 2. This contact was the life partner of case 2 and they had shared a residence.
Careful interviews of contacts and the community failed to identify any clear epidemiological links between the cases regarding residence or places of employment. As a result of lockdown measures, most public recreational areas (restaurants, community centres, places of worship, sports clubs, etc.) were not operating, which limited possible common sources of infection or routes of transmission.
Transmission of C. diphtheriae is usually by respiratory droplet and the outbreak investigation considered drugs of abuse to be potentially implicated in the transmission of non-toxigenic C. dipht | 
Hide
Show all
Email this article (Login required)
