Epidemiology of Campylobacter infections in Ireland 2004-2016: What has changed?

Campylobacter is the most common notifiable cause of bacterial gastroenteritis in humans in Ireland. However, epidemiological information is limited. We aimed to describe Campylobacter epidemiology in Ireland and trends over time, to inform future surveillance and research. We reviewed data completeness and described notified cases of campylobacteriosis (2004-2016) by age, sex, geographical area, season and trends over time. We used negative binomial regression to estimate incidence rate ratios (IRR) and adjusted IRR (aIRR) by age group, sex, geographical area and season. We undertook interrupted time-series analysis by age group and geographical area incorporating terms for trend and period (2004-2010 and 2011-2016). There were 27,034 cases of campylobacteriosis notified between 2004 and 2016. Data were >99% complete for notification date, geographical area, sex and date of birth. Crude annual incidence ranged from 36.2 to 54.4 per 100,000 population. The incidence was higher in, males (aIRR 1.15, 95% confidence intervals (CI) 1.12-1.19), those aged <5 years compared with the lowest incidence age group (45-64 years) (aIRR 4.65, 95% CI 4.43-4.88), other seasons compared with winter and all other areas compared with the north-east area (aIRR range 1.22-1.71, p-values <.001). In 2011, we observed a stepped increase in annual crude incidence overall, in both sexes, all age groups and most geographical areas. This pattern was mirrored on time-series analysis, with significant increases in trend-adjusted incidences of 30%-45% (p-values ≤.008) detected for all age groups and 30%-66% (p-values ≤.012) for seven out of eight geographical areas after 2011. Campylobacter remains the most commonly notified bacterial cause of gastroenteritis in Ireland. With available information, we could not fully explain a stepped increase in incidence observed in 2011. The transition of regional laboratories from culture-based to molecular-based Campylobacter diagnostic methods was a possible contributor. However, further investigation is required to fully explain the identified changes.