The reported incidence of human campylobacteriosis in Norway increased steadily, like in many other countries during the 90's and reached a maximum of 2890 cases in 2001 (64 reported cases per 100,000 inhabitants in 2001 [1, 2]). The proportion of domestically acquired cases has been stable at about 40–50%. A few outbreaks have been reported, however the majority of Campylobacter infections seem to be sporadic as has also been observed in other developed countries [3, 4].
Campylobacter jejuni ssp. jejuni (C. jejuni) is the most frequently isolated Campylobacter from humans with campylobacteriosis in Norway (personal communication, Kapperud, Georg the National Reference Laboratory, Norwegian Institute for Public Health) as observed in other developed countries [3]. However, the exact distribution of species in the reported human cases remains to be revealed and there are indications of that other species also can be important in some countries and in some age groups [5].
Case control studies and epidemiological surveys have identified risk factors for Campylobacter infections in humans, the most frequently revealed being untreated drinking water, consumption of broiler meat bought raw, raw milk, contact with cats and dogs and other animals and eating other types of meat at barbecue [6–12]. The risk connected to "consumption of broiler meat bought raw" might imply inadequate cooking of the meat or more likely; cross contamination in the kitchen to products that are not cooked before being eaten.
A rather high seasonal prevalence of Campylobacter in some surface water sources used as drinking water in Norway were found in the survey by Brennhovd et al. [13] and is also found elsewhere [14, 15]. Most of the water works in Norway apply sanitation before releasing the drinking water into the net work. In some geographical areas however, there are still a high number of private water sources often localised on farms and also in houses for recreation purposes, (an aspect of the lifestyle which is common in Norway).
Case-control studies, analysis of some outbreak and comparison of strains isolated from various sources with those causing infections in humans have provided evidence that poultry represent an important risk factor [3, 4, 6–11, 16, 17]. The official import of chicken in Norway is negligible, but an unknown volume is imported through travelling and illegal import may be substantial in some areas. Because of the increased sale and consumption of fresh chicken (61% increase from 1995 to 2000), transmission of Campylobacter from raw or insufficient heat-treated broiler meat has been the main theory explaining the increased incidence of domestically acquired infections since 1995. However, no substantial evidence supporting this theory has been provided so far [18].
Although many studies of risk factors for campylobacteriosis in humans have been conducted world wide, there are still uncertainties related to the relative importance of the probable causal factors. As most case control studies have been matched on geography, identifying risk-factors associated with geographical location has not been possible. In regard to geographical differences in the number of sporadic and outbreak related cases, the modelling of human exposure to Campylobacter taking geographical areas into account can supplement traditional case-control studies and help identify risk factors of geographical origin.
Climatic variables have very often a specific geographical pattern. The association between rainfall and the incidence of Campylobacter infections in humans in different geographical regions in Norway has so far not been examined. Temperature is another variable that could be included in such a model in order to account for the observed north south gradient difference in the reported incidences during the summer season [7, 19].
Ecological studies have featured prominently in environmental epidemiology because exposure has often been measured at the group level, or because limited resources for conducting the study prohibit collection of individual-level data [20].
The objectives of this study were to describe the reported incidence of domestically acquired campylobacteriosis from 1995 to 2001 in Norway, and to investigate whether the increase in incidence was statistically significant when accounting for variations between counties.
Moreover, we wanted to investigate the association between reported domestically acquired human Campylobacter infections in 2000 and 2001 and assumed biological risk factors such as consumption of chicken, proportion of people drinking treated water, density of dogs, cattle, sheep and goats, rainfall, temperature (accounting for the north south gradient in the reported incidence) and whether the population is urban/suburban or rural.