Heat Stress Impacts Animal and Human Health

Master’s research compared effects of three-day heat waves

PhD student Katherine Bishop-Williams

Katherine Bishop-Williams

As summer temperatures rise, heat stress can pose a threat to livestock as well as humans. Katherine Bishop-Williams, a 2014 master’s graduate in population medicine, looked at the effects of heat stress on dairy cows and people.

“These health impacts are already happening, both at an animal and at a human level,” she says. As president of the Ecohealth Club on campus, she took an ecohealth approach to her research, looking at the interconnections between animal and human health and the environment.

Holsteins account for 94 per cent of Ontario’s dairy cow population. North American Holsteins can suffer heat stroke at temperatures as low as 23 C, which is only a few degrees warmer than average room temperature. Despite dairy cows’ sensitivity to heat stress, Bishop-Williams says they can withstand temperatures as low as -25 C.

“Dairy cows in Canada are very well adapted to extreme cold.”

Humans and livestock often exhibit similar symptoms of heat stress. “There are major similarities in the physiology of all large mammals in their response to heat stress,” says Bishop-Williams. Symptoms of heat stress in dairy cows include increased respiration, lack of energy and even death. Calves suffering from heat stress face a higher risk of mortality within 30 days of birth.

People are also susceptible to heat stress, particularly young children and the elderly. Symptoms include dizziness, nausea and vomiting. In severe cases, heat stress can also be fatal to humans.

Environment Canada defines a heat wave as three consecutive days of 32 C or higher. Prior to 2003, no heat waves had been recorded in Ontario for 50 years, says Bishop-Williams. In 2012, two heat waves were recorded in Ontario, although none were recorded this past summer.

“Those periods of high risk are becoming more frequent, and we can say that’s definitely a trend that’s already happening,” she says.

Bishop-Williams looked at on-farm cow deaths and rural hospital admissions during heat waves in southwestern Ontario, as well as three weeks before and after each heat wave to establish a control group. “We were comparing what was in excess rather than what was directly linked to heat stress,” she explains.

Farms and communities located in areas where heat waves occurred experienced more heat-related deaths. She found that 20 per cent more cows died during heat waves than during other times of the year. More than one cow died on each of the farms during heat waves, resulting in 27 additional deaths. When the cows died, she says, the temperature was 10 C higher than what they can tolerate. She noted that it could cost up to $2,500 to replace each cow.

dairy cows SIZED

Rural hospitals treated two extra patients per day during a three-day heat wave, resulting in 300 extra patients if extrapolated to 50 rural hospitals. “We’re dramatically underestimating the effects of heat stress on rural communities,” says Bishop-Williams.

Dairy cows face the additional stress of milk production, which stresses them physically and increases their body temperature. That can have dramatic impacts when combined with heat stress. Dairy cows also produce less milk under heat stress, which can result in $250 to $350 in lost milk production per farm, per three-day heat wave.

What can farmers do to help protect their livestock? Heat abatement strategies include wetting cows and installing fans in their barns, says Bishop-Williams, but more research needs to be done to find out which strategies are the most effective to ensure the benefits are worth the additional cost. Since cows enjoy lounging in the shade on hot days, planting trees could also help them keep cool.

Increased awareness of heat stress among farmers is encouraging, says Bishop-Williams. “Farmers are recognizing that heat stress is occurring in their herds.”

Although her research focused on dairy cows, she says heat stress can affect any type of livestock as well as pets. Dog owners are often warned not to leave their pets in vehicles during the summer because temperatures inside vehicles can rise quickly, causing animals to overheat and possibly die.

Supervised by professors Olaf Berke, David Pearl and David Kelotn in population medicine, Bishop-Williams received research funding from the Ontario Veterinary College. She is continuing her research to pursue a PhD, now looking at the effects of climate change and water quality on Inuit populations in Labrador.