Original article

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

Philipp Justus Bless, Claudia Schmutz, Kathrin Sartori, Daniel Mäusezahl

DOI: https://doi.org/10.4414/smw.2017.14569
Publication Date: 27.12.2017
Swiss Med Wkly. 2017;147:w14569

BACKGROUND

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Keywords: Campylobacter, Salmonella, disease surveillance, denominator data, Switzerland, foodborne disease, seasonality, positivity, epidemiological trends, notification rate

Introduction

Human campylobacteriosis and salmonellosis are the most frequently reported foodborne bacterial infections in Europe. In 2014, notification rates in the European Union (EU) were 71.0 cases per 100 000 population (corresponding to approximately 236 900 cases) for campylobacteriosis and 23.4 cases per 100 000 population (approximately 88 700 cases) for salmonellosis [1]. In the same year, in Switzerland, the notification rate for Campylobacter infections was 92.9 cases per 100 000 population (approximately 7600 cases) and 15.2 cases per 100 000 population (approximately 1200 cases) for Salmonella infections [1]. During the mid-1990s, the annual number of notified human Campylobacter infections surpassed that of Salmonella infections in Switzerland [2]. This was owing to a reduction of human salmonellosis following the introduction of control measures in the egg and poultry industry, such as mandatory screening of layer hens, in the early 1990s [2]. So far, similar control measures for Campylobacter are lacking and campylobacteriosis is currently the most frequently notified foodborne disease in Switzerland [2].

In Switzerland, notifiable diseases are monitored by the Federal Office of Public Health (FOPH) through the National Notification System for Infectious Diseases (NNSID) [3, 4]. Laboratory-based surveillance of Campylobacter and Salmonella infections, as defined by the Epidemics Act of 1970 and its related ordinances, captured only those cases that tested positive [57]. Since the implementation of the new Epidemics Act at the beginning of 2016, the total number of tests conducted for these two pathogens, including the number of positive results, must be reported annually as aggregated numbers, stratified by month and test method [4, 8]. Hence, denominator data to help draw inferences from surveillance data about the epidemiological situation in the community have not been collected so far. The number of stool tests performed depends on the healthcare-seeking behaviour of patients with diarrhoea and the stool sampling rate of treating physicians [911]. As not all individuals affected by acute gastroenteritis seek medical care or have a stool sample examined for enteric pathogens, there are likely to be many undetected (at community level) and unreported (at healthcare level) campylobacteriosis and salmonellosis cases [12, 13]. Hence, changes in notification rates do not necessarily reflect an epidemiological trend, but could be attributable to changes in healthcare-seeking behaviour or stool sampling rates. A more informed interpretation of surveillance data is made possible by calculating positivity rates (proportion of positive tests among all tests performed). Because positivity rate calculations also consider denominator data, they adjust for the number of tests [14, 15]. We analysed laboratory data for stool tests performed for Campylobacter spp. and Salmonella spp. by Swiss diagnostic laboratories over a 10-year period to better interpret the trends of campylobacteriosis and salmonellosis case notifications seen in the NNSID.

Materials and methods

Selection of diagnostic laboratories

The study aimed to include private diagnostic laboratories from all geographical and linguistic regions of Switzerland to reach an optimal representation of the campylobacteriosis cases reported to the NNSID between 2003 and 2012. Eleven private diagnostic laboratories, each reporting more than 1000 campylobacteriosis cases during that decade, were contacted and invited to provide data for the study. The case-based laboratory data requested comprised patients’ demographic characteristics (sex, age, canton of residence, personal identification code assigned by laboratory) and test characteristics (pathogen tested, test result, date of test, test method) on all Campylobacter and Salmonella tests performed between 2003 and 2012.

Analysis of positivity rates

Datasets from individual laboratories were transformed uniformly, merged and analysed with STATA™ Version 13.1 (Stata Corporation; College Station, TX, USA). Firstly, double entries, repeated tests and tests for patients without Swiss residency were excluded. The following rules – based on disease durations and durations of organism excretion [16] – were applied to identify and exclude repeated tests: (i) control or follow-up tests, irrespective of result, following a positive result within 42 days for both, Campylobacter and Salmonella; (ii) negative tests following a negative result within 10 days (Campylobacter) or 21 days (Salmonella); and (iii) negative tests followed by a positive result within 10 days (Campylobacter) or 21 days (Salmonella). The patient population was characterised by sex, age, diagnostic laboratory, test year and residence by greater region (corresponding to the Nomenclature of Units for Territorial Statistics (NUTS) 2 level [17]). Age groups for statistical analyses were predefined. Residence by greater region was based on the patients’ canton of residence (NUTS 3 level). Descriptive analysis of positivity rates – defined as positive tests divided by total tests performed – and exploratory logistic regression analyses of predictors for and seasonality of positive tests were performed. Characteristics of laboratory-confirmed cases of campylobacteriosis and salmonellosis were additionally compared with national surveillance data. Time trends of annual positivity rates were investigated using stratification and direct standardisation for age groups and sex. Thus, the population of individuals tested from 2003 to 2012 was used as the reference population. The seasonality of monthly and weekly positivity rates was assessed by calculating positivity rates from laboratory data from the whole observation period pooled by month or calendar week.

Univariable and multivariable regression models

In a first step, univariable logistic regression analyses were performed to estimate the effect of sex, age group, laboratory, residence by greater region, test week, test month and test year on the test result. Afterwards, a multivariable logistic regression model estimated the unconfounded effects of sex, age groups, laboratories, residence by greater region and test year on the test result. The effect of seasonal within-year variations on test outcome were investigated with a second multivariable logistic regression model including test month and adjustments for sex, age groups, laboratories, residence by greater region and test year. For this model, the test month with a positivity rate closest to the mean positivity rate of all test months was used as a baseline and test year was introduced as a random effect. The significance of variables in the multivariable models was assessed by likelihood ratio tests and the category of each variable with the most observations (except for test month) was used as a baseline to make the model more robust. Patients with missing information on the canton of residence were assigned the greater region of their corresponding laboratory.

Ethics statement

The study was approved by the local ethical committee “Ethikkommission Nordwest- und Zentralschweiz” [Ethical committee of Northwestern and Central Switzerland] (No.: EKNZ:2014-164).

Results

Exclusion of test results and representativeness

Eight laboratories agreed to participate in the study and five of them provided complete data for Campylobacter and Salmonella tests performed as requested. The eight laboratories conducted a total of 196 307 Campylobacter tests (17 694 positive) and 199 062 Salmonella tests (4163 positive) between 2003 and 2012. Excluding data from the three laboratories with incomplete data led to the exclusion of 43 530 (3345 positive) Campylobacter tests and 45 114 (640 positive) Salmonella tests. Among the remaining laboratories (A to E), removal of double entries, repeated tests and tests of non-Swiss residents led to the exclusion of a further 17 211 (1245 positive) Campylobacter tests and 16 499 (689 positive) Salmonella tests. Additionally, we excluded 444 (9 positive) Campylobacter tests and 452 (2 positive) Salmonella tests because of missing information on sex and/or age. In the detailed analysis, 135 122 (13 095 positive) Campylobacter tests and 136 997 (2832 positive) Salmonella tests were included. Culture-based test methods accounted for 98.7% of all Campylobacter and Salmonella tests conducted, and polymerase chain reaction (PCR) tests accounted for 1.3%. Positive tests included in the analysis corresponded to 20.4% and 17.2% of campylobacteriosis and salmonellosis cases, respectively, registered in the NNSID between 2003 and 2012 (tables 1 and 2).

Table 1

Comparison of campylobacteriosis cases from laboratory data with cases registered in the National Notification System for Infectious Diseases by test year, Switzerland, 2003–2012.

 2003200420052006200720082009201020112012
Proportion of NNSID cases reported by study laboratories in %16.716.717.620.421.821.121.421.021.123.0
Proportion of NNSID cases by greater region reported by study laboratories in %          
    Lake Geneva1.52.52.03.53.03.04.04.03.54.0
    Espace Mittelland17.017.018.526.027.027.028.529.028.028.5
    Northwestern Switzerland26.022.523.025.027.526.027.025.027.033.0
    Zurich24.027.028.029.029.529.023.028.025.026.5
    Eastern Switzerland15.018.018.016.518.520.522.019.523.024.0
    Central Switzerland8.07.58.56.57.06.56.07.06.58.0
    Ticino45.547.044.063.069.554.560.052.557.058.0
Proportion of males in %          
    Laboratories56.655.453.557.755.253.755.853.054.654.9
    NNSID55.454.854.855.053.553.553.653.853.754.0
Median age in years          
    Laboratories34343435353637373936
    NNSID32333434353535373636

Table 2

Comparison of salmonellosis cases from laboratory data with cases registered in the National Notification System for Infectious Diseases by test year, Switzerland, 2003–2012.

 2003200420052006200720082009201020112012
Proportion of NNSID cases reported by study laboratories in %15.615.915.716.717.716.417.419.021.019.8
Proportion of NNSID cases by greater region reported by study laboratories in %          
    Lake Geneva0.52.52.03.04.54.02.01.03.01.0
    Espace Mittelland14.518.014.019.019.519.018.520.528.521.0
    Northwestern Switzerland20.021.021.024.022.517.523.023.019.029.5
    Zurich19.017.518.022.023.521.523.025.014.024.5
    Eastern Switzerland9.511.016.512.514.514.017.023.019.518.5
    Central Switzerland6.06.53.57.53.54.54.512.03.010.5
    Ticino49.551.043.548.546.049.545.047.072.548.5
Proportion of males in %          
    Laboratories53.253.456.058.857.155.057.952.349.652.2
    NNSID52.149.653.156.254.351.453.052.451.252.5
Median age in years          
    Laboratories18232325302829252425
    NNSID25252625272827272826

Characteristics of the patient population and overview of tests performed

The annual number of tests performed increased by 51.1% from 2003 to 2012 (11 674 to 17 641 tests) for Campylobacter and by 50.7% (11 842 to 17 842 tests) for Salmonella (fig. 1). For both diseases, annual test numbers decreased by at least 6% for the age groups <5 years and 5–9 years, and increased by at least 31% in the older age groups. The median age of patients tested for Campylobacter was 42 years (range <1–108 years) and 41 years (range: <1–108 years) for Salmonella. Patients’ age differed significantly between laboratories and test years for both pathogens (Kruskal-Wallis test: p <0.01 for all four tests). Slightly more tests were conducted among females than among males for Campylobacter (54.8%) and for Salmonella (54.3%). The sex ratio differed between laboratories and test years for both pathogens (chi-square test: p <0.01 for all four tests). The patients’ residence by greater region was associated with the geographical location of the laboratory that performed the test.

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Figure 1
Number of stool tests for Campylobacter (a) and Salmonella (b) by sex in five diagnostic laboratories, Switzerland, 2003–2012.

Annual positivity rates overall and by laboratory

Annual Campylobacter positivity rates standardised for age and sex increased by 46.1% from 2003 (7.6%) to 2012 (11.1%) (fig. 2). Annual standardised Salmonella positivity rates showed an inverse trend and decreased by 44.4% from 2003 (2.7%) to 2012 (1.5%). Campylobacter positivity rates stratified by laboratory (and standardised for age and sex) showed similar annual trends (supplementary fig. S1 in appendix 1). The annual positivity rates of laboratory C were remarkably lower throughout the investigated period compared with other laboratories. Laboratory-specific Campylobacter positivity rates ranged from 3.8 to 9.4% in 2003 and continuously increased to 7.0–13.2% in 2012. For Salmonella, annual positivity rates by laboratory differed only slightly between laboratories; the highest rates were observed for laboratory C, with two distinct peaks in 2007 and 2011. Overall, a decreasing trend was observed; positivity rates dropped from 2.1–3.8% in 2003 to 1.2–2.7% in 2012.

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Figure 2
National Notification System for Infectious Diseases (NNSID) notification rates and positivity rates (standardised for age and sex) of Campylobacter and Salmonella, Switzerland, 2003-2012.
NNSID data provided by the Federal Office of Public Health, Bern, Switzerland

Annual positivity rates by sex and age groups

The annual Campylobacter positivity rates for males and females increased by 43.6% (from 9.4 to 13.5%) and by 45.2% (from 6.2 to 9.0%), respectively, from 2003 to 2012. In the same decade, annual Campylobacter positivity rates by age group increased for all age groups. The largest increase was observed for the age group ≥85 years (193.7%) followed by the 5–9-year-olds (131.9%). Compared with 2003, annual Campylobacter positivity rates of sex-specific age groups were higher in 2012, except for females in the age group 10–14 years (fig. 3a). Annual Campylobacter positivity rates were generally higher for males than for females over the entire observation period. For males and females in the age groups <5 years, 5–9 years and ≥85 years, similar annual Campylobacter positivity rates were observed at the beginning of the decade but rates were later slightly higher for males in the age group ≥85 years and for females in the age groups <5 years and 5–9 years.

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Figure 3
Annual positivity rates of Campylobacter (a) and Salmonella (b) by age group and sex, Switzerland, 2003-2012.

Annual Salmonella positivity rates decreased from 3.3% to 1.6% (−51.5%) for males and from 2.5% to 1.2% (−52.0%) for females between 2003 and 2012. Annual positivity rates decreased for all age groups between 2003 and 2012 except for the age group 20–24 years, for which the rate remained rather stable. The largest relative decrease of positivity rates was observed for the age groups 10–14 years and ≥85 years, where rates decreased from 10.5 to 3.7% (−64.8%) and from 0.8 to 0.2% (−75.0%), respectively. Sex-specific Salmonella positivity rates were similar or slightly higher for males compared to females in all age groups although for some age groups, positivity rates varied strongly between years (fig. 3b).

Seasonal trends in stool sampling and positivity rates

The number of tests performed for Campylobacter and Salmonella started to increase in spring (fig. 4 panels a and c, fig. 5 panels a and c). Test numbers peaked in late August (calendar week 34) after a brief and strong temporary decline at the beginning of the month (calendar week 31). Afterwards, the number of tests decreased until the end of the year. Monthly test numbers were lowest in February for Campylobacter and Salmonella, even though calendar week 1 was the week with the fewest tests performed.

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Figure 4
Seasonality of Campylobacter tests and positivity rates (pooled over study period) per month and calendar week, Switzerland, 2003–2012.
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Figure 5
Seasonality of Salmonella tests and positivity rates (pooled over study period) per month and calendar week, Switzerland, 2003–2012.

After a continuous increase during spring, monthly Campylobacter positivity rates peaked during summer months, with the highest monthly rate occurring in July (13.8%) (fig. 4 panels b and d). Likewise, monthly Salmonella positivity rates started increasing during the spring. They peaked twice, first in late summer (August) and then in the autumn, with the highest rate occurring in October (3.1%) (fig. 5 panels b and d). The highest weekly positivity rate for Campylobacter (17.3%) was in calendar week 1 (January), whereas the peak of weekly Salmonella positivity rates (3.5%) was in calendar week 43 (October). The lowest monthly positivity rates for Campylobacter and Salmonella were in February (5.3%) and March (1.1%), respectively. The seasonal trends of Campylobacter and Salmonella positivity rates were also observable for sex- and age-specific positivity rates although less pronounced in certain groups.

Regression analyses

In the univariable regression analyses, sex, age, laboratory, residence by greater region, test week, test month and test year all had a significant effect on the test result for both diseases. The multivariable regression analysis of predictors for a positive Campylobacter test showed higher odds of a positive test for males than for females (odds ratio [OR] 1.53, 95% confidence interval [CI] 1.47–1.59) (supplementary table S1, appendix 1). Patients in the age groups 15–19 years and 20–24 years had higher odds for a positive test outcome compared with the age group 25–44 years, whereas patients of other age groups had reduced odds. The patients’ place of residence by greater region had similar odds for a positive test, except for patients from the Ticino region (OR 0.44, 95% CI 0.38–0.52). From 2003 to 2008, the odds increased continuously and decreased slightly between 2009 and 2011 compared with 2012.

The regression model for seasonal within-year variations showed that the odds for a positive Campylobacter test was highest in July (OR 1.52, 95% CI 1.40–1.65) and lowest in February (OR 0.55, 95% CI 0.49–0.61) compared with May, which had a positivity rate closest to the monthly average (supplementary table S2). Significantly higher odds were also observed for June (OR 1.38, 95% CI 1.26–1.50) and August (OR 1.24, 95% CI 1.14–1.35) compared with May.

In the multivariable regression model for Salmonella, males had higher odds (OR 1.30, 95% CI 1.21–1.40) of a positive test than females (supplementary table S3). The odds of a positive test outcome increased threefold for the age groups <5 years, 5–9 years and 10–14 years compared with the age group 25–44 years. Greater region was no longer significantly associated with the outcome in the multivariable regression model. The odds of a positive test outcome steadily decreased during the study period compared with 2012. In the second multivariable model for seasonality, the highest odds of a positive Salmonella test were observed in October (OR 1.61, 95% CI 1.36–1.90) and August (OR 1.44, 95% CI 1.23–1.70) compared with November (supplementary table S4). The lowest odds (compared with November) were observed in March (OR 0.55, 95% C: 0.44–0.68) and February (OR 0.57, 95% CI 0.46–0.72).

Discussion

Annual Campylobacter positivity rates standardised for age and sex increased from 2003 to 2012, whereas standardised Salmonella positivity rates decreased. During the same time period, campylobacteriosis notification rates increased from 72.7 to 105.5 notifications per 100 000 population, whereas salmonellosis notification rates decreased from 29.8 to 15.4 per 100 000 population. Campylobacter positivity rates were generally higher for males than females in all age groups. Monthly and weekly Campylobacter positivity rates showed a distinct seasonality, with a peak during the summer months and again at the beginning of the year, which was independent of sex and age group. Salmonella positivity rates showed a similar seasonality, but peaked in autumn. Annual Salmonella positivity rates were similar or slightly higher for males than for females, with the highest rates observed in the younger age groups, <5, 5–9 and 10–14 years. The observed seasonality and annual trends of positivity rates for both pathogens are congruent with reports from other countries [14, 18].

Annual positivity rates in relation to NNSID notification rates

Annual positivity rates of Campylobacter and Salmonella standardised for age and sex and annual NNSID notification rates showed similar trends. Multiple testing, data duplication or simultaneous testing of several pathogens could potentially affect both numerator and denominator data in different ways. However, similar trends were observed for the standardised annual positivity rates presented here and for the crude, non-standardised positivity rates calculated from raw data from all eight laboratories included in the study (supplementary fig. S2, appendix 1).

The stool test data analysed for this study originated mainly from culture-based test methods, which used to be the standard diagnostic method for detecting Campylobacter and Salmonella. Campylobacter, Salmonella and Shigella are often tested simultaneously [19]. In terms of relative frequency, more positive Salmonella tests (18.9%) than positive Campylobacter tests (8.4%) were excluded, whereas the proportion of excluded duplicate and repeated tests was similar for negative Campylobacter and Salmonella tests (11.2 vs 10.2%). The proportion of negative Salmonella tests excluded dropped only slightly from 10.2 to 9.4% when the same time span used for excluding negative Campylobacter tests was applied. Hence, only laboratory-confirmed campylobacteriosis and salmonellosis patients differ with regard to repeated testing. In summary, reducing the number of tests per patient and disease episode to one test result is crucial for an accurate calculation of positivity rates whereas the temporal trend of positivity rates is not considerably affected.

The relative increase in standardised annual Campylobacter positivity rates (+46.1%) and the relative decrease in standardised annual Salmonella positivity rates between 2003 and 2012 (−44.4%) are close to the increase in notification rates of Campylobacter (+45.0%) and the decrease in notification rates of Salmonella (−48.4%). During the same time period, the number of tests performed for Campylobacter and Salmonella increased by around 51%. The proportion of cases diagnosed by participating laboratories among NNSID case notifications increased by 37.7% for campylobacteriosis and by 26.9% for salmonellosis over the study period.

The observed increase of test numbers in our study was partially due to a single laboratory (laboratory A), where the number of tests increased 3.5 times for Campylobacter and 3.8 times for Salmonella between 2003 and 2012. This laboratory was founded a few years before the study period. For the remaining laboratories (B, C, D, E), a smaller increase of 32.0% for Campylobacter tests and of 29.0% for Salmonella tests was observed. An increase in testing frequency has also been observed in other European countries [14, 20], except in the Netherlands, where testing frequency remained rather stable [21]. Testing frequencies are largely influenced by physicians’ stool sampling behaviour and patients’ healthcare-seeking behaviour [14, 2225]. It is also possible that laboratories in the study increased their market shares.

The increase of Campylobacter notification rates is probably due to a combination of increasing test numbers and an upward epidemiological trend in the population, as suggested by the increase in positivity rates. The decrease of Salmonella notification rates presumably reflects an epidemiological trend in the population, as the notification rate decreased at the same time that testing frequency increased. The increase of campylobacteriosis cases in the population, together with the co-testing of Salmonella and Campylobacter, is probably responsible for the increase of Salmonella test numbers.

In summary, notification rates are influenced by both epidemiological trends in the population and test numbers. More infections in the population will lead to higher notification rates and fewer infections will lead to lower notification rates. On the other hand, increasing test numbers can lead to the detection of more cases in the population, i.e., higher notification rates without necessarily reflecting an increase in disease frequency. Consequently, an observed increase in notification rates does not necessarily represent an actual increase of disease frequency in the population. A change in test numbers can be due to a number of factors such as changes in the prevalence of risk factors leading to testing, altered healthcare-seeking behaviour, and changes in physicians’ testing practices, human susceptibility, and pathogenicity. Assessing the interplay of notification rates and test numbers by positivity rates provides more insights into the epidemiological situation in the population than one of these measures alone. Nevertheless, understanding underlying reasons for changes in one of these measures requires further investigation.

Positivity rates in relation to age and sex

A remarkable increase in Campylobacter positivity rates was observed for the age groups 5–9 years and ≥85 years (+131.9% and +193.7%). Test numbers for the age group 5–9 years decreased during the observation period (−6.1%), and they more than doubled for the age group ≥85 years (+131.5%). During the same time period, notification rates for the ≥85 years age group increased by 94.9% (47.2–92.0 per 100 000 population) and for the 5–9 years age group by 30.7% (55.3–72.3 per 100 000 population) [2]. It was found that adults and the elderly suffered increasingly more frequently from campylobacteriosis; this could be related to the frequent use of proton pump inhibitors and comorbidities in these age groups [2, 26, 27]. Others have also observed increasing test numbers among the elderly and related it to changes (increases) in healthcare-seeking and physicians’ testing behaviour [14, 18, 24]. Additionally, the Swiss population aged ≥85 years increased by 29% from 2003 to 2012, which probably also contributed to the observed increase in test numbers [28].

Salmonella notification rates and annual sex-specific positivity rates showed similar decreasing trends. The strongest decreases in age-specific annual Salmonella positivity rates were observed for the age groups 10–14 years and ≥85 years (−64.8% and −75.0%, respectively). At the same time, notification rates dropped by 55.4% (39.2–17.5 per 100 000 population) for the 10–14 year age group and by 55.6% (23.9–10.6 per 100 000 population) for the ≥85 years group. It appears, therefore, that these decreases are true epidemiological trends. Age-specific Salmonella positivity rates tended to be slightly higher for males but did not remarkably differ between sexes. Similar observations have been made for corresponding NNSID data [2].

The increasing trend in Campylobacter positivity rates was similar for males and females. Also, male and female notification rates to the NNSID likewise increased during this time [2]. Both positivity rates and notification rates for Campylobacter were higher among males than among females in nearly all age groups. Higher positivity rates for males have also been observed by others [18]. Higher stool sampling rates have been reported for male patients in Canada [18] and for female patients in Wales [14]. Sex-specific differences in healthcare seeking or in risk exposures could account for this observation.

Seasonality of positivity rates and notification rates

Monthly and weekly Campylobacter and Salmonella positivity rates showed seasonal trends corresponding to the NNSID notification rates, which peaked during the summer months and, for Campylobacter, also at the beginning of the year [2]. Summer peaks of Campylobacter and Salmonella positivity rates have also been described previously [18]. Monthly and weekly test numbers also peak in summer. The seasonal variation of test numbers could indicate seasonality of acute gastroenteritis, a temporal variation in the medical care-seeking behaviour of affected individuals and in the proportion of patients being tested. For instance, returning travellers are more likely to undergo stool diagnostics [24, 25, 29], leading to increased test numbers during the public school holiday season in the summer. The combination of high test numbers and high positivity rates in summer and autumn generates the observed peak in case numbers in the NNSID [2].

Peaks of Campylobacter and Salmonella notification rates during summer months are observed in most European countries [1, 2, 27, 30, 31]. The prevalence of Campylobacter in broiler flocks and the contamination of chicken meat with Campylobacter at retail are higher during summer months than during the rest of the year [3134]. This probably explains the observed seasonality as poultry meat from broilers is the main source of Campylobacter infections in Switzerland [3537]. However, it seems that the summer peak is not caused by a single common source of infection and is more likely driven by multiple sources of animal and environmental exposures and climatic conditions [27, 31, 38, 39]. An additional reason for the summer peak in Switzerland and parts of the EU could be related to the culture of barbequing during summer, which provides multiple occasions for disease transmission through undercooking of and cross-contamination by poultry and red meat [4043]. Travel abroad is a known risk factor for contracting campylobacteriosis [4245] – also in Switzerland [46, 47] – and a large proportion of notified Salmonella infections in Switzerland is travel-related [48]. Hence, travelling probably contributes to the observed seasonality of campylobacteriosis and salmonellosis test numbers and case notifications in Switzerland.

The highest weekly positivity rate for Campylobacter was found in calendar week 1 when test numbers were lowest. Notification rates of campylobacteriosis in Switzerland show a strong annual increase over Christmas and New Year (“winter peak”). A similar peak in notification data at the beginning of January has also been observed in Germany [30] and in the Campylobacter surveillance data of The European Surveillance System [1]. In Switzerland, the major driver for the winter peak is frequent consumption of meat fondue at festive occasions around this time, especially if it includes chicken meat [47]. The low test numbers over the festive season in December and January are probably related to a different healthcare-seeking behaviour and restricted access to healthcare services during the holiday period. Therefore, the winter peak in Campylobacter notification rates is probably attenuated and does not reveal the full magnitude of the problem.

Strengths and limitations

In Switzerland, private diagnostic laboratories operate on a regional or national level and predominantly serve the practices of general practitioners and medical specialists. The study did not consider hospital-based laboratories as their patient profile generally differs from the patient profile in private practices at the primary care level. Hospitalised patients are likely to be more severely affected by acute gastroenteritis and to undergo more extensive diagnostic testing. Hence, their pre-test probability for a positive Campylobacter or Salmonella test result is different from that of patients consulting at primary care practices [18]. The catchment population of the participating laboratories is not known. Therefore, it was not possible to describe the catchment population, adjust for potential changes therein or to estimate any population-based indicators like stool sampling rates. Similarly, we could not assess how well the data of the five participating laboratories represent the whole tested population in Switzerland, given the latter is not known. We could only assess the representativeness of the patient population by comparing “our” positively tested patients with all notified cases (and hence, supposedly, all positively tested patients in Switzerland; table 1 and table 2). From this comparison we conclude that estimated positivity rates are likely to represent accurately the epidemiological trends and situation in Switzerland as median age and the sex-ratio of cases identified in participating laboratories and in cases from the NNSID were comparable.

Conclusions

The study results support the assertion that the increase in notification rates of campylobacteriosis and the decrease in notification rates of salmonellosis are epidemiological trends in the population. These trends cannot be solely explained by changing test numbers. Still, we believe it is important to continuously assess test numbers or positivity rates to note changes in stool testing frequency that could lead to changes in case numbers seen in the notification system. This becomes especially important in the light of the increasing use of multiplex PCR panels where multiple pathogens are tested simultaneously and, hence, test numbers can change substantially [49]. The annual collection of test numbers of selected notifiable diseases as stipulated under the newly enforced Swiss Epidemics Act will allow for continuous assessment of positivity rates in the future.

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Appendix 1

Supplementary data

Table S1

Predictors for a positive Campylobacter stool test, Switzerland, 2003–2012.

 N*Adj. OR95% CIp-value§
Sex135 122  <0.01
      Female74 0061  
      Male61 1161.531.47–1.59 
Age group135 122  <0.01
      <5 years10 1960.530.49–0.58 
      5–9 years51370.710.64–0.79 
      10–14 years38430.890.80–0.99 
      15–19 years63621.321.23–1.43 
      20–24 years98101.261.18–1.35 
      25–44 years37 6931  
      45–64 years32 1470.850.81–0.89 
      65–84 years24 7120.520.49–0.55 
      ≥85 years52220.310.26–0.36 
Laboratory135 122  <0.01
      A18 8360.930.88–0.99 
      B17 6870.760.71–0.81 
      C19 8600.920.79–1.07 
      D33 7510.950.89–1.02 
      E44 9881  
Greater region135 122  <0.01
      Lake Geneva region36440.840.75–0.95 
      Espace Mittelland32 9121  
      Northwestern Switzerland28 7110.830.78–0.88 
      Zurich29 4720.730.68–0.79 
      Eastern Switzerland14 9310.930.86–1.00 
      Central Switzerland45040.900.81–1.00 
      Ticino20 9480.440.38–0.52 
Test year135 122  <0.01
      200311 6740.680.62–0.73 
      200411 2090.690.64–0.76 
      200511 0770.720.66–0.78 
      200611 6920.770.71–0.84 
      200712 4810.850.79–0.92 
      200814 2941.000.93–1.07 
      200914 6850.980.91–1.05 
      201014 6430.850.79–0.92 
      201115 7260.970.91–1.04 
      201217 6411  

* Number of records
† Adjusted odds ratio: adjusted for sex, age group, laboratory, greater region and year of test
‡ 95% confidence interval
§ p-value from likelihood ratio test

Table S2

Seasonality of a positive Campylobacter stool test by test month, Switzerland, 2003–2012.

 N*Adj. OR95% CIp-value§
Sex135 122  <0.01
      Female74 0061  
      Male61 1161.531.47–1.59 
Age group135 122  <0.01
      <5 years10 1960.540.50–0.59 
      5–9 years51370.720.65–0.80 
      10–14 years38430.900.81–1.00 
      15–19 years63621.311.22–1.42 
      20–24 years98101.251.17–1.34 
      25–44 years37 6931  
      45–64 years32 1470.850.81–0.89 
      65–84 years24 7120.530.49–0.56 
      ≥85 years52220.320.27–0.37 
Laboratory135 122  <0.01
      A18 8360.920.87–0.98 
      B17 6870.760.71–0.81 
      C19 8600.900.77–1.04 
      D33 7510.940.89–1.01 
      E44 9881  
Greater region135 122  <0.01
      Lake Geneva region36440.840.75–0.95 
      Espace Mittelland32 9121  
      Northwestern Switzerland28 7110.840.79–0.89 
      Zurich29 4720.740.68–0.80 
      Eastern Switzerland14 9310.930.87–1.00 
      Central Switzerland45040.900.81–1.00 
      Ticino20 9480.460.39–0.53 
Test month135 122  <0.01
      January10 5020.980.89–1.08 
      February93110.550.49–0.61 
      March10 6540.640.57–0.70 
      April96590.740.67–0.82 
      May10 6911  
      June11 6071.381.26–1.50 
      July11 7301.521.40–1.65 
      August13 9911.231.13–1.34 
      September13 1260.980.90–1.07 
      October11 7661.030.95–1.13 
      November11 5520.930.85–1.02 
      December10 5331.020.93–1.12 
Test year (random effect)135 122  <0.01

* Number of records
† Adjusted odds ratio: adjusted for sex, age group, laboratory, greater region and year of test
‡ 95% confidence interval
§ p-value from likelihood ratio test

Table S3

Predictors for a positive Salmonella stool test, Switzerland, 2003–2012.

 N*Adj. OR95% CIp-value§
Sex136 997  <0.01
      Female74 3741  
      Male62 6231.301.21–1.40 
Age group136 997  <0.01
      <5 years10 2873.242.88–3.66 
      5-9 years51793.192.75–3.70 
      10–14 years38453.172.68–3.74 
      15–19 years64641.921.62–2.26 
      20–24 years10 1591.301.11–1.53 
      25–44 years38 9471  
      45–64 years32 6091.040.93–1.17 
      65–84 years24 3600.670.58–0.78 
      ≥85 years51470.380.27–0.55 
Laboratory136 997  <0.01
      A20 4521.301.14–1.47 
      B17 6581.291.10–1.51 
      C19 5571.851.43–2.40 
      D34 3331.090.94–1.26 
      E44 9971  
Greater region136 997  0.13
      Lake Geneva region36431.120.87–1.46 
      Espace Mittelland32 8511  
      Northwestern Switzerland28 8890.870.75–1.00 
      Zurich29 6510.900.75–1.08 
      Eastern Switzerland16 6580.970.82–1.14 
      Central Switzerland45950.780.61–1.00 
      Ticino20 7100.860.65–1.13 
Test year136 997  <0.01
      200311 8421.851.56–2.19 
      200411 3421.701.43–2.02 
      200511 1161.721.45–2.05 
      200611 7411.691.42–2.01 
      200712 7501.721.45–2.04 
      200814 4851.611.36–1.90 
      200914 9631.060.88–1.27 
      201014 9101.060.88–1.28 
      201116 0061.251.05–1.49 
      201217 8421  

* Number of records
† Adjusted odds ratio: adjusted for sex, age group, laboratory, greater region and year of test
‡ 95% confidence interval
§ p-value from likelihood ratio test

Table S4

Seasonality of a positive Salmonella stool test by test month, Switzerland, 2003–2012.

 N*Adj. OR95% CIp-value§
Sex136 997  <0.01
      Female74 3741  
      Male62 6231.301.20–1.40 
Age group136 997  <0.01
      <5 years10 2873.383.00–3.81 
      5–9 years51793.322.86–3.85 
      10–14 years38453.212.71–3.79 
      15–19 years64641.901.61–2.25 
      20–24 years10 1591.291.10–1.52 
      25–44 years38 9471  
      45–64 years32 6091.050.94–1.18 
      65–84 years24 3600.690.59–0.80 
      ≥85 years51470.400.28–0.57 
Laboratory136 997  <0.01
      A20 4521.251.10–1.42 
      B17 6581.291.10–1.51 
      C19 5571.831.41–2.36 
      D34 3331.070.93–1.24 
      E44 9971  
Greater region136 997  0.18
      Lake Geneva region36431.110.86–1.44 
      Espace Mittelland32 8511  
      Northwestern Switzerland28 8890.870.76–1.01 
      Zurich29 6510.910.76–1.09 
      Eastern Switzerland16 6580.970.82–1.14 
      Central Switzerland45950.780.61–1.00 
      Ticino20 7100.910.70–1.20 
Test month136 997  <0.01
      January10 5220.720.59–0.89 
      February93580.570.46–0.72 
      March10 7000.550.44–0.68 
      April96690.730.59–0.90 
      May10 6780.890.73–1.08 
      June11 6961.130.94–1.36 
      July11 8311.331.11–1.58 
      August14 5251.441.23–1.70 
      September13 5511.291.09–1.53 
      October12 0421.611.36–1.90 
      November11 6121  
      December10 8130.740.60–0.91 
Test year (random effect)136 997  <0.01

* Number of records
† Adjusted odds ratio: adjusted for sex, age group, laboratory, greater region and year of test
‡ 95% confidence interval
§ p-value from likelihood ratio test

fullscreen
Figure S1
Campylobacter and Salmonella positivity rates stratified by laboratory (and standardised for age and sex).
fullscreen
Figure S2
Annual Campylobacter and Salmonella positivity rates standardised for age and sex in relation to the crude, non-standardised positivity rates calculated from raw data from all eight laboratories included in the study, 2003–2012, Switzerland.
fullscreen
Figure 1
Number of stool tests for Campylobacter (a) and Salmonella (b) by sex in five diagnostic laboratories, Switzerland, 2003–2012.
fullscreen
Figure 2
National Notification System for Infectious Diseases (NNSID) notification rates and positivity rates (standardised for age and sex) of Campylobacter and Salmonella, Switzerland, 2003-2012.
NNSID data provided by the Federal Office of Public Health, Bern, Switzerland
fullscreen
Figure 3
Annual positivity rates of Campylobacter (a) and Salmonella (b) by age group and sex, Switzerland, 2003-2012.
fullscreen
Figure 4
Seasonality of Campylobacter tests and positivity rates (pooled over study period) per month and calendar week, Switzerland, 2003–2012.
fullscreen
Figure 5
Seasonality of Salmonella tests and positivity rates (pooled over study period) per month and calendar week, Switzerland, 2003–2012.
fullscreen
Figure S1
Campylobacter and Salmonella positivity rates stratified by laboratory (and standardised for age and sex).
fullscreen
Figure S2
Annual Campylobacter and Salmonella positivity rates standardised for age and sex in relation to the crude, non-standardised positivity rates calculated from raw data from all eight laboratories included in the study, 2003–2012, Switzerland.

Table 1

Comparison of campylobacteriosis cases from laboratory data with cases registered in the National Notification System for Infectious Diseases by test year, Switzerland, 2003–2012.

 2003200420052006200720082009201020112012
Proportion of NNSID cases reported by study laboratories in %16.716.717.620.421.821.121.421.021.123.0
Proportion of NNSID cases by greater region reported by study laboratories in %          
    Lake Geneva1.52.52.03.53.03.04.04.03.54.0
    Espace Mittelland17.017.018.526.027.027.028.529.028.028.5
    Northwestern Switzerland26.022.523.025.027.526.027.025.027.033.0
    Zurich24.027.028.029.029.529.023.028.025.026.5
    Eastern Switzerland15.018.018.016.518.520.522.019.523.024.0
    Central Switzerland8.07.58.56.57.06.56.07.06.58.0
    Ticino45.547.044.063.069.554.560.052.557.058.0
Proportion of males in %          
    Laboratories56.655.453.557.755.253.755.853.054.654.9
    NNSID55.454.854.855.053.553.553.653.853.754.0
Median age in years          
    Laboratories34343435353637373936
    NNSID32333434353535373636

Table 2

Comparison of salmonellosis cases from laboratory data with cases registered in the National Notification System for Infectious Diseases by test year, Switzerland, 2003–2012.

 2003200420052006200720082009201020112012
Proportion of NNSID cases reported by study laboratories in %15.615.915.716.717.716.417.419.021.019.8
Proportion of NNSID cases by greater region reported by study laboratories in %          
    Lake Geneva0.52.52.03.04.54.02.01.03.01.0
    Espace Mittelland14.518.014.019.019.519.018.520.528.521.0
    Northwestern Switzerland20.021.021.024.022.517.523.023.019.029.5
    Zurich19.017.518.022.023.521.523.025.014.024.5
    Eastern Switzerland9.511.016.512.514.514.017.023.019.518.5
    Central Switzerland6.06.53.57.53.54.54.512.03.010.5
    Ticino49.551.043.548.546.049.545.047.072.548.5
Proportion of males in %          
    Laboratories53.253.456.058.857.155.057.952.349.652.2
    NNSID52.149.653.156.254.351.453.052.451.252.5
Median age in years          
    Laboratories18232325302829252425
    NNSID25252625272827272826

Table S1

Predictors for a positive Campylobacter stool test, Switzerland, 2003–2012.

 N*Adj. OR95% CIp-value§
Sex135 122  <0.01
      Female74 0061  
      Male61 1161.531.47–1.59 
Age group135 122  <0.01
      <5 years10 1960.530.49–0.58 
      5–9 years51370.710.64–0.79 
      10–14 years38430.890.80–0.99 
      15–19 years63621.321.23–1.43 
      20–24 years98101.261.18–1.35 
      25–44 years37 6931  
      45–64 years32 1470.850.81–0.89 
      65–84 years24 7120.520.49–0.55 
      ≥85 years52220.310.26–0.36 
Laboratory135 122  <0.01
      A18 8360.930.88–0.99 
      B17 6870.760.71–0.81 
      C19 8600.920.79–1.07 
      D33 7510.950.89–1.02 
      E44 9881  
Greater region135 122  <0.01
      Lake Geneva region36440.840.75–0.95 
      Espace Mittelland32 9121  
      Northwestern Switzerland28 7110.830.78–0.88 
      Zurich29 4720.730.68–0.79 
      Eastern Switzerland14 9310.930.86–1.00 
      Central Switzerland45040.900.81–1.00 
      Ticino20 9480.440.38–0.52 
Test year135 122  <0.01
      200311 6740.680.62–0.73 
      200411 2090.690.64–0.76 
      200511 0770.720.66–0.78 
      200611 6920.770.71–0.84 
      200712 4810.850.79–0.92 
      200814 2941.000.93–1.07 
      200914 6850.980.91–1.05 
      201014 6430.850.79–0.92 
      201115 7260.970.91–1.04 
      201217 6411  

* Number of records
† Adjusted odds ratio: adjusted for sex, age group, laboratory, greater region and year of test
‡ 95% confidence interval
§ p-value from likelihood ratio test

Table S2

Seasonality of a positive Campylobacter stool test by test month, Switzerland, 2003–2012.

 N*Adj. OR95% CIp-value§
Sex135 122  <0.01
      Female74 0061  
      Male61 1161.531.47–1.59 
Age group135 122  <0.01
      <5 years10 1960.540.50–0.59 
      5–9 years51370.720.65–0.80 
      10–14 years38430.900.81–1.00 
      15–19 years63621.311.22–1.42 
      20–24 years98101.251.17–1.34 
      25–44 years37 6931  
      45–64 years32 1470.850.81–0.89 
      65–84 years24 7120.530.49–0.56 
      ≥85 years52220.320.27–0.37 
Laboratory135 122  <0.01
      A18 8360.920.87–0.98 
      B17 6870.760.71–0.81 
      C19 8600.900.77–1.04 
      D33 7510.940.89–1.01 
      E44 9881  
Greater region135 122  <0.01
      Lake Geneva region36440.840.75–0.95 
      Espace Mittelland32 9121  
      Northwestern Switzerland28 7110.840.79–0.89 
      Zurich29 4720.740.68–0.80 
      Eastern Switzerland14 9310.930.87–1.00 
      Central Switzerland45040.900.81–1.00 
      Ticino20 9480.460.39–0.53 
Test month135 122  <0.01
      January10 5020.980.89–1.08 
      February93110.550.49–0.61 
      March10 6540.640.57–0.70 
      April96590.740.67–0.82 
      May10 6911  
      June11 6071.381.26–1.50 
      July11 7301.521.40–1.65 
      August13 9911.231.13–1.34 
      September13 1260.980.90–1.07 
      October11 7661.030.95–1.13 
      November11 5520.930.85–1.02 
      December10 5331.020.93–1.12 
Test year (random effect)135 122  <0.01

* Number of records
† Adjusted odds ratio: adjusted for sex, age group, laboratory, greater region and year of test
‡ 95% confidence interval
§ p-value from likelihood ratio test

Table S3

Predictors for a positive Salmonella stool test, Switzerland, 2003–2012.

 N*Adj. OR95% CIp-value§
Sex136 997  <0.01
      Female74 3741  
      Male62 6231.301.21–1.40 
Age group136 997  <0.01
      <5 years10 2873.242.88–3.66 
      5-9 years51793.192.75–3.70 
      10–14 years38453.172.68–3.74 
      15–19 years64641.921.62–2.26 
      20–24 years10 1591.301.11–1.53 
      25–44 years38 9471  
      45–64 years32 6091.040.93–1.17 
      65–84 years24 3600.670.58–0.78 
      ≥85 years51470.380.27–0.55 
Laboratory136 997  <0.01
      A20 4521.301.14–1.47 
      B17 6581.291.10–1.51 
      C19 5571.851.43–2.40 
      D34 3331.090.94–1.26 
      E44 9971  
Greater region136 997  0.13
      Lake Geneva region36431.120.87–1.46 
      Espace Mittelland32 8511  
      Northwestern Switzerland28 8890.870.75–1.00 
      Zurich29 6510.900.75–1.08 
      Eastern Switzerland16 6580.970.82–1.14 
      Central Switzerland45950.780.61–1.00 
      Ticino20 7100.860.65–1.13 
Test year136 997  <0.01
      200311 8421.851.56–2.19 
      200411 3421.701.43–2.02 
      200511 1161.721.45–2.05 
      200611 7411.691.42–2.01 
      200712 7501.721.45–2.04 
      200814 4851.611.36–1.90 
      200914 9631.060.88–1.27 
      201014 9101.060.88–1.28 
      201116 0061.251.05–1.49 
      201217 8421  

* Number of records
† Adjusted odds ratio: adjusted for sex, age group, laboratory, greater region and year of test
‡ 95% confidence interval
§ p-value from likelihood ratio test

Table S4

Seasonality of a positive Salmonella stool test by test month, Switzerland, 2003–2012.

 N*Adj. OR95% CIp-value§
Sex136 997  <0.01
      Female74 3741  
      Male62 6231.301.20–1.40 
Age group136 997  <0.01
      <5 years10 2873.383.00–3.81 
      5–9 years51793.322.86–3.85 
      10–14 years38453.212.71–3.79 
      15–19 years64641.901.61–2.25 
      20–24 years10 1591.291.10–1.52 
      25–44 years38 9471  
      45–64 years32 6091.050.94–1.18 
      65–84 years24 3600.690.59–0.80 
      ≥85 years51470.400.28–0.57 
Laboratory136 997  <0.01
      A20 4521.251.10–1.42 
      B17 6581.291.10–1.51 
      C19 5571.831.41–2.36 
      D34 3331.070.93–1.24 
      E44 9971  
Greater region136 997  0.18
      Lake Geneva region36431.110.86–1.44 
      Espace Mittelland32 8511  
      Northwestern Switzerland28 8890.870.76–1.01 
      Zurich29 6510.910.76–1.09 
      Eastern Switzerland16 6580.970.82–1.14 
      Central Switzerland45950.780.61–1.00 
      Ticino20 7100.910.70–1.20 
Test month136 997  <0.01
      January10 5220.720.59–0.89 
      February93580.570.46–0.72 
      March10 7000.550.44–0.68 
      April96690.730.59–0.90 
      May10 6780.890.73–1.08 
      June11 6961.130.94–1.36 
      July11 8311.331.11–1.58 
      August14 5251.441.23–1.70 
      September13 5511.291.09–1.53 
      October12 0421.611.36–1.90 
      November11 6121  
      December10 8130.740.60–0.91 
Test year (random effect)136 997  <0.01

* Number of records
† Adjusted odds ratio: adjusted for sex, age group, laboratory, greater region and year of test
‡ 95% confidence interval
§ p-value from likelihood ratio test

Bless Philipp Justusab, Schmutz Claudiaab, Sartori Kathrinab, Mäusezahl Danielab

a Swiss Tropical and Public Health Institute, Basel, Switzerland

b University of Basel, Switzerland

We are grateful to the diagnostic laboratories for providing data for the study, among others ADMED Microbiologie, La Chaux-de-Fonds; Medizinisches Labor Bioanalytica, Luzern and Viollier AG, Basel. The support of the Federal Office of Public Health, Division of Communicable Diseases, Bern, Switzerland during data acquisition is appreciated. The authors thank Jan Hattendorf from the Swiss Tropical and Public Health Institute (Swiss TPH), Basel, Switzerland for statistical support during data analysis and Amena Briët (Swiss TPH) for the critical language editing.

Financial support to conduct the study from the following institutions is gratefully acknowledged: the Federal Office of Public Health, Division of Communicable Diseases, Bern, Switzerland and the Rudolf Geigy Foundation, Basel, Switzerland

The authors declare that they have no conflicts of interest.

Header image: © Teo Lay Peng | Dreamstime

Daniel Mäusezahl, PhD, MPH, Socinstrasse 57, P.O. Box, CH-4002 Basel, daniel.maeusezahl[at]swisstph.ch

Campylobacter, Salmonella, disease surveillance, denominator data, Switzerland, foodborne disease, seasonality, positivity, epidemiological trends, notification rate