Impetigo in epidemic and nonepidemic phases: an incidence study over 4½ years in a general population

Abstract

Little is known about incidence and natural variation of impetigo in general populations. To investigate the natural course of impetigo in a well-defined population, and to study the resistance pattern of the causal bacteria over time. This is a population-based incidence study in Austevoll, an island community of 4457 inhabitants in Norway, in the years 2001-2005. Incidence rates are given as events per person-year. Epidemic periods were identified by statistical process-control analyses. The incidence rate of impetigo for the whole study period was 0.017 events per person-year, corresponding to a total of 334 cases. The incidence rates were 0.009, 0.026, 0.019, 0.016 and 0.009 in the years 2001, 2002, 2003, 2004 and 2005, respectively. Three epidemics were identified, starting in August of 2002, 2003 and 2004, lasting for 11, 11 and 5 weeks, respectively. Incidence rates in these epidemic periods were 0.099, 0.045 and 0.074, respectively. In epidemic periods, Staphylococcus aureus was the causal bacterium in 89% (117/132) of cases, while this proportion was 68% (84/123) in nonepidemic periods (P < 0.01). Staphylococcus aureus was resistant to fusidic acid in 84% (98/117) and 64% (54/84) of impetigo cases in epidemic and nonepidemic periods, respectively (P < 0.01). When investigating all types of infections caused by S. aureus in the study period, the proportion of fusidic acid resistance in impetigo cases (152/201, 76%) differed significantly from fusidic acid resistance in other infections (18/116, 16%) (P < 0.01). Distinctive epidemic outbreaks occurred during the summer of three of the five follow-up years. In outbreaks, S. aureus was more frequently the causal agent and the sensitivity to fusidic acid decreased significantly.