Time trends of positivity rates from foodborne pathogen testing in Switzerland, 2003 to 2012

Campylobacteriosis and salmonellosis are important foodborne diseases in Europe, including in Switzerland. In 2014, notification rates for Switzerland were 92.9 per 100 000 population for campylobacteriosis and 15.2 per 100 000 population for salmonellosis. These notification rates originate from laboratory-based surveillance whereby positive test results are reported to the National Notification System for Infectious Diseases. Consequently, notification rates do not directly correspond to the disease burden among the population as the number of positive tests depends on patients' healthcare-seeking behaviour, stool sampling rates and other factors.

We assessed laboratory positivity rates (proportion of positive tests among all tests performed) of diagnostic tests for Campylobacter and Salmonella from five private laboratories in Switzerland between 2003 and 2012. We analysed demographic characteristics, temporal and spatial distribution of test numbers and positivity rates. Predictors for a positive test and disease seasonality were assessed with logistic regression analyses.

A total of 135 122 (13 095 positive) Campylobacter tests and 136 997 (2832 positive) Salmonella tests were obtained with positive tests corresponding to 20.4% and 17.2% of notified campylobacteriosis and salmonellosis cases, respectively. The number of tests conducted annually increased for both pathogens by 51% from 2003 to 2012. Annual positivity rates of Campylobacter increased from 7.6 to 11.1% and rates of Salmonella decreased from 2.7 to 1.5%. The largest increases in annual Campylobacter positivity rates were observed for patients older than 85 years (+193.7%), followed by children aged 5-9 years (+131.9%). Positivity rates and test numbers for both diseases by month or calendar week showed a distinct seasonality, with peak rates for Salmonella occurring in autumn and for Campylobacter in summer and at the turn of the year. These findings were independent of patients' age and sex.

Both positivity rates and notification rates showed increasing trends for Campylobacter and decreasing trends for Salmonella, suggesting that these trends reflect changes in disease epidemiology at population level. The continuous assessment of positivity rates remains important to appropriately interpret changes observed in the notification system especially considering the increasing use of multiplex polymerase chain reaction test panels where multiple pathogens are tested simultaneously.