Temporal association of the appearance of mucoid strains of Streptococcus pyogenes with a continuing high incidence of rheumatic fever in Utah

Objective. Our objective was to confirm an observed temporal association of increased numbers of rheumatic fever cases concomitant with the appearance of an increased prevalence of mucoid strains of Streptococcus pyogenes. During the resurgence of rheumatic fever (RF) that has occurred in the Intermountain area surrounding Salt Lake City, Utah, since 1985, the largest number of cases occurred in 1985 and 1986 and 12 years later in 1997 and 1998. During the initial outbreak and the later exacerbation of the resurgence, an increased number of mucoid strains of S pyogenes were present in the community. Methods. The referred cases of RF that fulfilled Jones criteria have been systematically reviewed by the medical staff at Primary Children's Medical Center yearly since 1985. Before the resurgence of RF, a program was initiated by the microbiology laboratory at Primary Children's Medical Center to store frozen isolates of S pyogenes. All frozen specimens were randomly selected and entered into a log; the coded entry allowed for comments regarding the origin of the isolate and whether the isolate had a mucoid appearance on the blood agar culture plate. This log was reviewed in October 2002 to determine whether the percentage of frozen mucoid isolates stored during the resurgence of RF would support the clinical and epidemiologic suspicion that mucoid isolates seemed to be present with a higher frequency during the 2 periods of high incidence of RF. The percentage of mucoid isolates was compared with the yearly number of cases of RF. A Pearson r correlation analysis was completed to determine whether there was a significant association between the percentage of mucoid isolates and the number of cases of RF. Results. The highest number of cases of RF was temporally associated with the highest percentage of mucoid isolates. There was statistically significant correlation between percentage of mucoid strains and the number of RF cases. Conclusions. The Utah experience with the resurgence of RF in a civilian population during the last decade and a half of the 20th century confirmed the temporal association of mucoid strains of S pyogenes, primarily M-type 18, with a high incidence of RF.