Heavy Rainfall, Dairy Cattle, and Campylobacter in Aotearoa New Zealand 2015 to 2019: Space-Time Stratified Case-Crossover Study

Abstract Objective: It is well-established that contamination of water supplies by heavy rainfall can cause large outbreaks of enteric disease. Associations of rainfall with sporadic disease are less well understood. Here we investigate short-term associations between rainfall, dairy cattle density, drinking water characteristics, and notifications of sporadic campylobacteriosis. Methods: Daily notifications of Campylobacter were spatially linked to geographic boundaries of water distribution zones. We created 10 km circular buffer zones around source water abstraction points for each supply and calculated average rainfall and maximum temperature over 7-day lag periods within the source water abstraction zones. We fitted conditional Poisson regression models in a space-time stratified case-crossover study. Results: In a linear model, adjusted for temperature, there was a weak association between campylobacteriosis and rainfall in the 7 days prior to disease onset: incidence rate ratio (IRR) 1.01; 95% CI: 1.008–1.017. In the lowest quintile of dairy cattle density (zero cows/km2), there was an increased risk of Campylobacter in the 10th (highest) decile of 7-day average lagged rainfall: IRR 1.11 (95% CI: 1.02–1.22). In the highest quintile of dairy cattle density (∼150 cows/km2), there was an increased risk of Campylobacter in the sixth to 10th deciles of rainfall. Conclusions: The risk of sporadic campylobacteriosis was significantly associated with heavy rainfall in the 7 days prior to disease onset, especially in locations served by surface water supplies with a high density of dairy cattle. Implications for public health: Heavy rainfall may increase the risk of sporadic waterborne enteric disease, especially where there is a high environmental load of zoonotic pathogens.