The BBC television documentary “Pedigree Dogs Exposed” caused an international uproar in 2008, with footage of a cavalier King Charles spaniel with a skull too small for its brain and basset hounds described as “deformed congenital dwarfs.” In 2011, under the headline “Can the Bulldog Be Saved?,” an article in the New York Times Magazine detailed serious health problems in that breed, including the untimely demise of two University of Georgia bulldog mascots.
With that kind of media exposure, is it any wonder that purebred pets have become synonymous with poor health?
But that’s an unfair rap, says Jerold Bell, a geneticist at the Cummings School of Veterinary Medicine and coauthor of the “Veterinary Medical Guide to Dog and Cat Breeds” (Teton NewMedia, 2012). Any dog or cat can have diseases and disorders linked to genetics, says Bell, “but what is lacking in the popular press about purebred dogs and pedigreed cats is that there are choices about how you acquire them and that health-conscious breeding can insure a healthier future.”
Because all individuals in a dog or cat breed are related to each other, you might assume that any health problems are the result of inbreeding. Not so, says Bell, who notes that experiments with laboratory animals show that repeated matings between full siblings over generations will cause many family lines to die out because of infertility and genetic defects—and others to thrive. The results depend entirely on whether a particular family line propagates or loses disease-causing genes in successive generations.
What does produce inherited disease in our pets is the unchecked propagation of defective genes.
Most breeders do the right thing as best they can, says Leslie Lyons, a professor of genetics at the UC Davis School of Veterinary Medicine. From chatty Siamese to sunny golden retrievers, she says breeders create “beautiful animals with wonderful personalities and traits,” including ones that are quite useful, such as bomb-sniffing dogs.
“But everybody—whether you’re a dog, cat or human being—carries different genetic mutations,” she says.
For example, the Afrikaner population in South Africa has an unusually high incidence of Huntington’s disease, the inherited neurodegenerative disorder, because most are descendents of a small group of Dutch settlers, one of whom carried that gene.
In livestock, genetic health is considered vital to quality control, notes Bell. “It’s only in dog and cat breeding that we have had a long history of pairing mates without any regard to their genetic health.” As a result, he says, “we see diseases in cats and dogs that should have been prevented over and over again.”
Inherited health problems don’t hound only purebred animals. Thirteen of the most common hereditary disorders in dogs—including degenerative hip disease, an eye condition that causes blindness, some cancers and slipping kneecaps—occur with equal frequency in mixed-breed and purebred animals, according to research done at UC Davis and just published in the “Journal of the American Veterinary Medical Association.”
It’s not unusual to see inherited diseases in mixed-breed animals—be it a randomly bred mongrel or a designer breed such as a Labradoodle (Labrador retriever crossed with a poodle)—because they are “ancestrally down line from where those original mutations occurred,” Bell says. “Frankly, if we said today that every animal that is a carrier or has a genetic disorder can’t be bred, we might as well just go ahead and say goodbye to domestic animals.”
Origins of Canine Disease
First domesticated more than 15,000 years ago, the dog, more than any other animal, has been defined by artificial selection, says Lyons, who will speak at the Tufts Canine and Feline Breeding and Genetics Conference Sept. 27–29 in Boston.
Humans tamed wolves and then almost immediately began selecting from within this small group of founders for ones that were good for protection or hunting. “We then further refined hunting dogs to be masters at specific types of hunting,” Lyons says, such as breeding to produce experts at pursuing quarry into holes.
Distinct dog types appeared 3,000 to 4,000 years ago. Modern breeds, however, trace their roots to much smaller groups of individual animals chosen over the last couple of centuries, when it became popular to raise dogs with specific physical attributes for showing.
“It took only a handful of dogs to establish each breed,” says Noriko Tonomura, a research assistant professor at the Cummings School who conducts research at the Broad Institute of MIT and Harvard, where scientists are studying the DNA of purebred dogs to gain insight into how cancers, diabetes, cardiovascular problems and other diseases develop. “To use a very crude analogy, each breed is like a population expanded from a few families of humans,” she says.Continued...