Mortality related to cold and heat. What do we learn from dairy cattle?

Extreme temperatures are associated with increased mortality among humans. Because similar epidemiologic studies in animals may add to the existing evidence, we investigated the association between ambient temperature and the risk of mortality among dairy cattle. We used data on 87,108 dairy cow deaths in Belgium from 2006 to 2009, and we combined a case-crossover design with distributed lag non-linear models. Province-specific results were combined in a multivariate meta-analysis. Relative to the estimated minimum mortality temperature of 15.4°C (75th percentile), the pooled cumulative relative risks over lag 0-25 days were 1.26 (95% CI: 1.11, 1.42) for extreme cold (1st percentile, -3.5°C), 1.35 (95% CI: 1.19, 1.54) for moderate cold (5th percentile, -0.3°C), 1.09 (95% CI: 1.02, 1.17) for moderate heat (95th percentile, 19.7°C), and 1.26 (95% CI: 1.08; 1.48) for extreme heat (99th percentile, 22.6°C). The temporal pattern of the temperature-mortality association was similar to that observed in humans, i.e. acute effects of heat and delayed and prolonged effects of cold. Seasonal analyses suggested that most of the temperature-related mortality, including cold effects, occurred in the warm season. Our study reinforces the evidence on the plausibility of causal effects in humans.