title: Heat stress related dairy cow mortality during heat waves and control periods in rural Southern Ontario from 2010-2012.
abstract: BACKGROUND
  Heat stress is a physiological response to extreme environmental heat such as heat waves. Heat stress can result in mortality in dairy cows when extreme heat is both rapidly changing and has a long duration. As a result of climate change, heat waves, which are defined as 3 days of temperatures of 32 °C or above, are an increasingly frequent extreme weather phenomenon in Southern Ontario. Heat waves are increasing the risk for on-farm dairy cow mortality in Southern Ontario. Heat stress indices (HSIs) are generally based on temperature and humidity and provide a relative measure of discomfort which can be used to predict increased risk of on-farm dairy cow mortality. In what follows, the heat stress distribution was described over space and presented with maps. Similarly, on-farm mortality was described and mapped. The goal of this study was to demonstrate that heat waves and related HSI increases during 2010-2012 were associated with increased on-farm dairy cow mortality in Southern Ontario. Mortality records and farm locations for all farms registered in the CanWest Dairy Herd Improvement Program in Southern Ontario were retrieved for 3 heat waves and 6 three-day control periods from 2010 to 2012. A random sample of controls (2:1) was taken from the data set to create a risk-based hybrid design. On-farm heat stress was estimated using data from 37 weather stations and subsequently interpolated across Southern Ontario by geostatistical kriging. A Poisson regression model was applied to assess the on-farm mortality in relation to varying levels of the HSI.
  
  
  RESULTS
  For every one unit increase in HSI the on-farm mortality rate across Southern Ontario increases by 1.03 times (CI95% (IRR) = (1.025,1.035); p = ≤ 0.001). With a typical 8.6 unit increase in HSI from a control period to a heat wave, mortality rates are predicted to increase by 1.27 times.
  
  
  CONCLUSIONS
  Southern Ontario was affected by heat waves, as demonstrated by high levels of heat stress and increased on-farm mortality. Farmers should be aware of these risks, and informed of appropriate methods to mitigate such risks.
authors.affiliation: Department of Population Medicine, Ontario Veterinary College, University of Guelph, 50 Stone Rd East, Guelph, ON, N1G 2 W1, Canada. kbishop@uoguelph.ca.
authors.forename: Katherine E
authors.initials: KE
authors.lastname: Bishop-Williams
authors.affiliation: Department of Population Medicine, Ontario Veterinary College, University of Guelph, 50 Stone Rd East, Guelph, ON, N1G 2 W1, Canada. oberke@uoguelph.ca.|Department of Mathematics and Statistics, University of Guelph, Univof Guelph, 50 Stone Rd East, Guelph, ON, N1G 2 W1, Canada. oberke@uoguelph.ca.|Institute of Biometry, Epidemiology and Information Processing, University of Veterinary Medicine, Hanover, Germany. oberke@uoguelph.ca.
authors.forename: Olaf
authors.initials: O
authors.lastname: Berke
authors.affiliation: Department of Population Medicine, Ontario Veterinary College, University of Guelph, 50 Stone Rd East, Guelph, ON, N1G 2 W1, Canada. dpearl@uoguelph.ca.
authors.forename: David L
authors.initials: DL
authors.lastname: Pearl
authors.affiliation: Strategic Solutions Group, 142 Hume Rd, Puslinch, ON, Canada. karen.hand@strategicsolns.ca.
authors.forename: Karen
authors.initials: K
authors.lastname: Hand
authors.affiliation: Department of Population Medicine, Ontario Veterinary College, University of Guelph, 50 Stone Rd East, Guelph, ON, N1G 2 W1, Canada. dkelton@uoguelph.ca.|Strategic Solutions Group, 142 Hume Rd, Puslinch, ON, Canada. dkelton@uoguelph.ca.
authors.forename: David F
authors.initials: DF
authors.lastname: Kelton
mesh_terms: D000818:Animals; D002417:Cattle; D002418:Cattle Diseases / Q000453:epidemiology / Q000401:mortality*; D018882:Heat Stress Disorders / Q000453:epidemiology / Q000401:mortality / Q000662:veterinary*; D009864:Ontario / Q000453:epidemiology; D012189:Retrospective Studies