May 17, 2023 Everyday Medicine

Chihuahuas to Great Dane: The Medical Impact of Dog Size

Chihuahua with a veterinarian and a great dane with a veterinary assistant

Chihuahuas to Great Dane: The Medical Impact of Dog Size

The dog is the first animal domesticated by humans, and we have done a brilliant job creating a variety of dog breeds to meet our needs. Selective breeding, which began roughly 9,000 years ago and expanded dramatically during the Victorian era, has resulted in dogs specialized for herding, hunting, retrieving, ratting and companionship. We also created a single species ranging in size from two to 200 pounds. While veterinarians love the variety of dogs we care for, the challenge for us is to understand all the variations that make up normal dogs in order to provide excellent veterinary care. Here are a few of the issues in medical care for species with a 100x range in size.

Dog Size and Longevity

I think it is fairly common knowledge that small dogs live longer than big dogs. This is supported by the data. In a study of breed-specific longevity, the miniature poodle was the longest-lived purebred dog studied between the years of 1980 and 1990. More recent data confirms this information.

As veterinarians, this information means we think about a ten-year-old poodle as being younger than a ten-year-old Great Dane. The 2019 American Animal Hospital Association Life Stage guidelines define a geriatric dog as one in the last 25% of their expected lifespan. A dog with an expected lifespan of 12 years is geriatric at 9 years of age, and a dog with a lifespan of 15 years is not geriatric until 11 years of age. The age at which dogs become geriatric influences the care veterinarians recommend as outlined in the AAHA guidelines. Expected lifespan can be estimated from charts such as the one below.

Chart shows life expectancy of dogs and cats over time according to size

Dog Size and Intestinal Function

Dogs of different sizes also have differences in their intestinal function, primarily colon function. The bacteria normally found in the colon varies based on dog size, and large dogs have more short chain fatty acid producing bacteria in their feces.

Feces diagram

Short chain fatty acids are responsible for flatulence. I couldn’t find any studies on what dogs have the most flatulence, but Modern Dog Magazine has a list, and it trends toward large breed dogs. One might hypothesize that differences in colon bacteria produce the differences in flatulence. However, excessive flatulence may also suggests diet intolerance or intestinal disease. If your dog is passing a lot of gas, be sure to tell your veterinarian.

Dog Size and Anatomy

A recent publication featuring AMC researchers demonstrates the necessity of understanding a wide range of “normal” to provide high quality veterinary care. In the tripartite panel below, you see three “normal” CT scans of the heart and lungs of a Havanese in panel A, a Cockapoo in panel B and a Great Pyrenees in panel C.

CT scan of the heart and lungs of a Havanese in panel A, a Cockapoo in panel B and a Great Pyrenees in panel C
A) Havanese B) Cockapoo C) Great Pyrenees

While the basic anatomy is the same in all three, the heart of the deep chested Pyrenees looks much smaller compared to the lungs than in the barrel chested Havanese, yet all three CT scans are considered normal breed variation.

I could go on and on about the variation of normal in our canine companions, but it’s this variation that makes them so lovable.

Tags: chihuahua, cockapoo, dog, dog breed, Dog Health, dog size, dogs, great dane, great pyrenees, havanese, pet health, veterinary,

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