Abstract and Introduction

Abstract

We investigated invasive group A Streptococcus epidemiology in Idaho, USA, during 2008–2019 using surveillance data, medical record review, and emm (M protein gene) typing results. Incidence increased from 1.04 to 4.76 cases/100,000 persons during 2008–2019. emm 1, 12, 28, 11, and 4 were the most common types, and 2 outbreaks were identified. We examined changes in distribution of clinical syndrome, patient demographics, and risk factors by comparing 2008–2013 baseline with 2014–2019 data. Incidence was higher among all age groups during 2014–2019. Streptococcal toxic shock syndrome increased from 0% to 6.4% of cases (p = 0.02). We identified no differences in distribution of demographic or risk factors between periods. Results indicated that invasive group A Streptococcus is increasing among the general population of Idaho. Ongoing surveillance of state-level invasive group A Streptococcus cases could help identify outbreaks, track regional trends in incidence, and monitor circulating emm types.

Introduction

Invasive group A Streptococcus (iGAS) disease is a severe bacterial infection caused by Streptococcus pyogenes, sometimes called strep A; ≈25,000 cases occurred in the United States in 2019^[1]. iGAS disease is defined as illness associated with detection of group A Streptococcus in a normally sterile site, such as blood, pleural fluid, joint fluid, or deep tissue. iGAS has different clinical syndromes, including bacteremia without focus, pneumonia, and cellulitis, and severe manifestations, include necrotizing fasciitis and streptococcal toxic shock syndrome (STSS). The highly variable M protein, encoded by emm (M protein gene), is essential for virulence and serves as the primary target for epidemiologic typing of GAS^[2]. Most patients with iGAS are hospitalized and the estimated case-fatality rate in the United States is 12%^[3]. Numerous risk factors are associated with iGAS, including older age, diabetes, HIV infection, heart disease, cancer, obesity, injection drug use, long-term care facility (LTCF) residence, homelessness, preceding or concurrent influenza infection, and exposure to children with sore throats^[4–9]. No licensed vaccines for GAS exist despite its substantial global disease burden: an estimated 1.78 million new cases of severe GAS (iGAS, acute rheumatic fever, rheumatic heart disease, and poststreptococcal glomerulonephritis) and 517,000 deaths occurring each year^[10]. In 2019, the World Health Organization recognized GAS as a leading cause of infectious disease burden and proposed better characterization of GAS epidemiology as a priority activity for vaccine development^[11].

During the past decade, iGAS incidence has increased in the United States, Canada, Ireland, Australia, and New Zealand^[1,12–16]. Reasons for these increases are unclear but could include changing demographics (e.g., an aging population at higher risk for iGAS), rising prevalence of other risk factors associated with iGAS infection, or changes in circulating emm types. In some US states, iGAS outbreaks among specific populations (e.g., persons experiencing homelessness or who inject drugs) or the emergence of rarely occurring emm types were linked to increases in iGAS incidence^[17–21]. In Idaho, a rural, western US state with a 2020 population of ≈1.8 million persons, iGAS is a reportable disease. We conducted a retrospective analytical study using surveillance data, emm typing results, and medical record review to describe the epidemiology of iGAS in Idaho during 2008–2019. In addition, we compared cases reported during 2014–2019 with cases from a lower-incidence 2008–2013 baseline period to determine whether clinical syndromes, patient demographics, and risk factors were associated with increased iGAS incidence.