Detecting diphtheria is not enough: the missing toxigenicity link in outbreak surveillance

Abstract

Diphtheria surveillance in resource-limited settings may stop at clinical diagnosis, organism identification, or molecular detection of the diphtheria toxin gene, although these findings do not provide the same information. Identification of Corynebacterium diphtheriae establishes the organism, tox-gene detection indicates genetic potential for toxin production, and phenotypic testing demonstrates toxin expression. We propose that existing WHO case classifications be retained, while laboratory reports record the level of diagnostic resolution achieved. A tiered system linking peripheral specimen collection, regional culture and molecular testing, and reference-level phenotypic confirmation of selected isolates could strengthen outbreak characterization in Somalia without delaying treatment based on clinical suspicion.

Dear Editor,

The re-emergence of diphtheria in Somalia illustrates a diagnostic challenge that extends beyond recognition of cases. Ahmed et al. described 51 children treated for clinically diagnosed diphtheria in Mogadishu; eight died, and myocarditis and neuritis, complications compatible with toxin-mediated disease, were associated with severe outcomes. However, culture, polymerase chain reaction (PCR), and toxigenicity testing were unavailable in the treating hospital [1]. As of epidemiological week 33 of 2025, the national outbreak report recorded 1,811 suspected cases and 89 deaths; 17 cases were described as laboratory confirmed through molecular testing at the National Public Health Reference Laboratory, while no in-country Elek toxigenicity testing was available [2]. These observations highlight the need to distinguish the different questions answered by organism identification, toxin-gene detection, and demonstration of toxin production.

Three laboratory levels should be distinguished. First, culture and species identification establish the presence of Corynebacterium diphtheriae but do not establish toxin production. Second, PCR detection of the tox gene indicates genetic potential for toxin production; however, non-toxigenic tox-gene-bearing isolates can carry the gene without active toxin expression. Third, phenotypic demonstration of toxin production confirms expression. WHO’s 2024 interim guidance describes the modified Elek immunoprecipitation test as the current gold standard for confirming toxin production [3]. WHO surveillance standards center laboratory confirmation on isolation of the organism together with demonstration of toxin production, while the 2024 outbreak guidance also recognizes that, in a large established outbreak, where circulating strains have already been phenotypically confirmed as toxin producing, tox-gene PCR may be used pragmatically for subsequent confirmation [3, 4]. We, therefore, do not propose replacing WHO case definitions. Rather, existing suspected, epidemiologically linked/probable, and laboratory-confirmed classifications should be retained, while surveillance records separately document the laboratory resolution achieved: species identified, tox gene detected, and phenotypic toxin production demonstrated.

This distinction is epidemiologically and clinically important. A recent Somali report described a 3-year-old child whose immunization card documented three age-appropriate diphtheria-containing vaccine doses and from whom C. diphtheriae was cultured. Because toxigenicity testing was unavailable, the report explicitly noted that the case did not meet WHO criteria for laboratory-confirmed toxigenic diphtheria [5]. It is, therefore, more precise to describe this as isolation of C. diphtheriae in a vaccinated child rather than as confirmed vaccine-breakthrough toxigenic diphtheria. More broadly, laboratory results should be interpreted together with clinical presentation, specimen quality, antimicrobial exposure, vaccination history, and epidemiological context.

Importantly, greater laboratory resolution must not delay treatment. WHO surveillance guidance recommends that suspected cases be treated promptly without waiting for laboratory confirmation [4]. Diphtheria antitoxin and appropriate antimicrobial therapy should, therefore, be initiated on the basis of clinical suspicion when indicated, alongside isolation and contact management. The proposed laboratory hierarchy is intended primarily to improve epidemiological characterization, surveillance, and public-health interpretation rather than to serve as a prerequisite for clinical management.

Universal implementation of Elek testing at the hospital level is unlikely to be feasible or necessary in resource-limited settings. A more operational tiered model is preferable. Peripheral facilities should prioritize early recognition and treatment and collect appropriate pharyngeal and nasal specimens, ideally before antimicrobial treatment when feasible. WHO guidance supports the use of suitable transport media, such as Amies medium and prompt, regulated transport to testing laboratories [3, 6]. Regional laboratories should provide culture, species identification, and validated tox-gene PCR, with predefined referral routes and turnaround targets. The national reference laboratory should coordinate confirmatory testing, quality assurance, data integration, and referral of selected isolates to designated regional or international reference laboratories when phenotypic testing is not available in-country.

Phenotypic confirmation need not be performed on every tox-positive isolate in every phase of a large outbreak. WHO’s 2024 guidance recommends confirming all isolates when there is no outbreak or only a small number of cases, confirming at least 5–10 isolates during early outbreak characterization, using PCR to triage tox-positive isolates when Elek reagents are limited, and, during ongoing monitoring, testing 3–5 isolates per cluster per month with phenotypic confirmation of at least 30% where possible [3]. In addition, referral should be prioritized for unusual clusters, suspected vaccine-breakthrough events, changes in epidemiological patterns, and discordant molecular and clinical findings.

Phenotypic toxigenicity testing requires appropriate expertise, validated procedures, quality-controlled reagents, positive and negative reference strains, and internal quality-control processes; participation in external quality assessment is also desirable where available [6, 7]. Even with phenotypic testing, uncertainty is not eliminated: prior antimicrobial treatment, poor specimen quality, transport delays, culture results, vaccination status, and the clinical and epidemiological picture remain important to final interpretation [4, 6].

Somalia, therefore, needs not only more diphtheria testing, but greater diagnostic resolution. Surveillance that distinguishes organism identification, tox-gene detection, and demonstrated toxin production—while preserving established WHO case definitions and the urgency of clinical treatment—would improve outbreak characterization and interpretation of apparently vaccine-associated cases. This tiered approach may also provide a practical model for other fragile and outbreak-prone health systems in the Horn of Africa.

Abbreviations

DAT

Diphtheria antitoxin

PCR

Polymerase chain reaction

WHO

World Health Organization

Author contributions

KHU conceived the idea and developed the initial concept for the manuscript. KHU and AAB drafted the manuscript. IAK, MCA, and AHA contributed to the literature review, interpretation of the evidence, and critical revision of the manuscript. All authors contributed to the intellectual content of the manuscript, reviewed and approved the final version, and agreed to be accountable for the work.

Funding

No funding was received for the preparation or publication of this manuscript.

Data availability

No datasets were generated or analyzed during the current study.

Declarations

Ethics approval and consent to participate

Not applicable. This correspondence does not report original research involving human participants, human data, or human tissue.

Consent for publication

Not applicable.

Competing interests

The authors declare that they have no competing interests.