Notes from the Field: Increase in Nontoxigenic Corynebacterium diphtheriae— Washington, 2018–2023

Toxin-producing Corynebacterium diphtheriae, an aerobic Gram-positive coccobacillus, is the predominant causative agent of diphtheria and is responsible for substantial morbidity worldwide (1).Infection with nontoxigenic C. diphtheriae is also associated with disease, but little is known about the clinical spectrum of illness or the incidence of nontoxigenic C. diphtheriae infections in the United States (2).Toxin gene acquisition and expression by nontoxigenic C. diphtheriae is biologically plausible and could lead to reintroduction of diphtheria into the United States, where diphtheria is no longer endemic (3).Understanding diseases caused by nontoxigenic forms of C. diphtheriae is important because diphtheria toxoid-containing vaccines create immunity to the toxin itself but cannot protect against infection or illness caused by nontoxigenic strains.In the state of Washington, detection of C. diphtheriae in any clinical specimen is immediately notifiable (4).Beginning in 2000, Washington mandated submission of all C. diphtheriae isolates to Washington State Public Health Laboratories (WSPHL).The number of reported nontoxigenic C. diphtheriae isolates in Washington has increased approximately tenfold, from 17 during 2012-2017 to 179 during 2018-2023; most infections occurred among King County residents.In November 2023, Washington State Department of Health, Public Health -Seattle & King County, and CDC conducted a statewide investigation of nontoxigenic C. diphtheriae cases to determine factors contributing to this increase and to describe the epidemiology of nontoxigenic C. diphtheriae and clinical characteristics of patients with nontoxigenic C. diphtheriae infections in Washington. Investigation and OutcomesDuring January 1, 2018-September 30, 2023, C. diphtheriae isolates from 176 patients were identified in 14 (36%) of 39 Washington counties; all isolates were identified as C. diphtheriae at WSPHL and subsequently determined to be nontoxigenic by CDC.A public health team abstracted patient data § from medical charts.Descriptive statistics were * These authors contributed equally to this report.