Brachycephalic Research Shows Body Condition Is Key to Thermoregulation

Among the Boxer’s distinguishing characteristics is a chiseled head in correct proportion to the body. The broad, blunt muzzle, which must be in proper form and balance with the skull, is a distinctive feature that contributes to the Boxer’s dignified appearance. The muzzle also gives the Boxer a “pushed-in” or “snub-nosed” face, making the breed brachycephalic and at risk for brachycephalic respiratory syndrome.

Most Boxer breeders and owners are aware of the signs of respiratory distress and the breed’s increased susceptibility to heat stress. Unlike dogs with conventional faces, Boxers and other brachycephalic breeds have shortened facial bones without the same proportionate shortening of the overlying soft tissues. The excess soft tissue makes them prone to upper airway obstruction that compromises their ability to take in air. Their inefficient panting and inability to cool down can result in inflamed, swollen airways that lead to a more severe obstruction and further overheating. 

Recent research by Michael Davis, D.V.M., professor of physiological sciences and director of the Com­par­ative Exercise Physiology Laboratory at Oklahoma State Univer­sity in Stillwater, showed that a dog’s body condition score has an even greater impact on thermoregulation, or the ability to maintain a steady body temperature, than does being a brachycephalic breed. The AKC Canine Health Foundation and several parent clubs helped to fund the research.

“Brachycephalic dogs are at greater risk for heat-related illness, presumably due to the structure of their respiratory tract,” Davis explains. “Dogs rely on the respiratory tract to dissipate metabolic heat, and this process is hampered in brachycephalic breeds due to their airway anatomy.”

The study confirmed that brachycephalic breeds are not as equipped to withstand endurance activities and heat as are nonbrachycephalic breeds but that other factors should be considered for individual dogs. “While brachycephaly had an important impact on our research results, body condition score seemed to have a larger impact,” Davis says. “In other words, being overweight is probably more risky than being brachycephalic and a lean brachycephalic dog may not have that much of a risk. The overweight brachycephalic dogs had two strikes against them.”  

Monitoring a dog’s body condition is important for all breeds but particularly brachycephalic breeds. Boxer owners are encouraged to conduct visual and hands-on evaluations to assess their dogs’ physique on a nine-point scale. A body condition score is an assessment of the proportion of fat and lean and reflects the balance between calorie intake and energy requirements. As dogs overeat, more fat is deposited and body condition scores increase.

While it is well-known that brachycephalic breeds are at risk of overheating during exercise and in warm climates, the parameters in which they are able to achieve homeostasis, or maintain physiological stability, have not been understood. Davis and his research team set out to identify those parameters.  

Their study included 200 dogs — 100 brachycephalic dogs and 100 size-matched nonbrachycephalic dogs. Among the brachycephalic breeds were Boxers, Boston Terriers, English and French Bulldogs, Pugs, Japanese Chins, and Shih Tzu. Ten to 20 dogs of each breed were examined.

Using a whole-body plethysmograph, a custom-built box similar in size to a dog crate with rapid, sensitive pressure, temperature and humidity sensors, the researchers were able to detect and measure the impact of temperature and humidity changes on dogs by assessing their breathing patterns. 

“In a nutshell, when compared to ‘normal’ dogs, the brachycephalic dogs had higher breathing rates and more shallow breaths,” Davis says. “We speculated that faster, shallower breathing is less sustainable than slower, deeper breathing.”

The heat stress levels used in the study were based on the maximum allowed by the U.S. Department of Agriculture (USDA) for dogs being transported by commercial airlines. The research showed that under mild heat stress conditions — 33 degrees Celcius (91.4 degrees Fahrenheit) and 62 percent relative humidity — the brachycephalic dogs were close to overheating. As the intensity of heat stress increased from a normal room temperature, defined as 22 degrees Celcius (71.6 degrees Fahrenheit) and 62 percent relative humidity, to mild heat stress conditions, the brachycephalic dogs overheated sooner than the nonbrachycephalic breeds.

“Only a few obese brachycepahlic dogs in our study could not handle the mild heat stress conditions, and they would be in jeopardy if they were transported under those conditions,” Davis says. “More dogs would be in jeopardy if conditions managed to exceed USDA guidelines.

“Our findings confirm what the airline industry has suggested for a number of years, except I hope the industry will begin to recognize that there is variation among individual dogs,” Davis says.

Some airlines require owners of brachycephalic breeds to sign a release accepting responsibility for mishaps. In fact, some airlines exclude Boxers and other brachycephalic breeds from their flights. Airlines’ commitments to ensuring dogs’ safety have resulted in temperature restrictions. Thus, some airlines embargo dogs from flying each year from mid-May to mid-September. Though temperature-related cargo compartments generally range from 55 to 70 degrees Fahrenheit, they still can become uncomfortable.

Airlines also generally set regulations regarding kennels or crates used to transport dogs. A crate must be large enough for a dog to stand up and turn around. Wire crates usually are not allowed, and those made of solid metal or fiberglass are preferred. Crates come with air ducts on the sides, but owners can add holes in the back to create more circulation.

Breeder Responsibilities

Linda Abel and her husband, Skip, of Elk River, Minn., have bred Boxers under the Storybook prefix since 1993. Their love for their breed motivates them to drive their dogs no matter the distance. “We travel only by car to deliver a bitch to be bred or a puppy to his or her new home,” Abel says. “We also travel by car when going to dog shows or taking a dog to a handler. We recognize that this is best for our dogs, and we simply don’t feel comfortable putting a Boxer into a plane’s cargo hold.

“When you have a brachycephalic breed, you have to accept limitations when traveling. You learn to adapt to their needs.”

They also practice caution at home. “When it’s hot, we turn on the air conditioning for the dogs,” she says. “I love to run but do not run with our dogs. It’s because of the detrimental effect the heat and humidity could have on them.” 

A stressful experience transporting a Boxer by air travel led to the Abels’ sensitivity about potential risks. In 1998, Abel accompanied their Boxer, CH Storybook’s Rip It Up (“Jake”), to Atlanta on a commercial airline flight, where a professional handler would pick up the dog for campaigning on the dog show circuit.

“It took hours for Jake to cool down and calm down,” Abel says. “He was panting steadily and excessively as soon as I got him out of the crate. He was obviously stressed. I don’t know how much of it was due to the heat in the plane’s cargo hold versus the new environment and people. It probably was a combination. Jake’s distress was a red flag to me about the potential risks involved.”

Educating people who buy Boxer puppies about the possibility of heat distress also is important. “I caution new puppy owners about these issues,” Abel says. “A lot of times they are young families or a young, single person who wants to go jogging with a Boxer. I question them about whether a Boxer is the right breed for them. In general, a Boxer is not a breed that spends a lot of time outdoors. These dogs simply can’t lie around in the sun without the heat becoming an issue.”

Despite the limited cooling capacity of Boxers, the muscular Working breed was among the first used as police canines in Germany more than 100 years ago and is capable of physical activity in sports. The Abels have participated in flyball with their dogs. A female they bred, Bratt’s Gretchen of Crystal, FD (“Gretchen”), was the first Boxer to earn a flyball title. 

“While Gretchen and I were competing in flyball, there were no other Boxers in the trials,” Abel says. “All the trials were held in temperature-controlled environments, so I was not concerned about heat stress. I probably would not have run her in trials if the environment had been hot and humid.”

Most Boxer enthusiasts, who are drawn to the breed’s handsome appearance, keen intelligence and even-tempered disposition, realize it is important to take precautions against heat and humid conditions. Importantly, thanks to the research of Davis and his team at Oklahoma State University, more is known today about the parameters that define the risk of heat stress. Keeping Boxers and other brachycephalic dogs in ideal body condition goes a long way to help prevent respiratory problems. 

Purina appreciates the support of the American Boxer Club and particularly Joyce Campbell, D.V.M., chair of the ABC Health and Research Committee and a trustee of the American Boxer Charitable Foundation, in helping to identify topics for the Purina Pro Club Boxer Update newsletter.

This article is used with permission from the Purina Pro Club Boxer Update newsletter, Nestlé Purina PetCare.