Veterinarians are rethinking the widely held belief that mixed breeds are hardier than purebred dogs in the wake of new research that shows both are equally susceptible to a variety of genetic disorders.
The study — one of the first of its kind in terms of breadth — shakes the notion that mixing breeds is a surefire way to help lessen the incidence of disease. Published in the June 1 Journal of the American Veterinary Medical Association
, the work comes from University of California, Davis (UC Davis) researchers who tapped some 90,000 medical records of dogs seen between 1995 and 2010 at the institution's School of Veterinary Medicine. Among the cases reviewed, 27,254 dogs presented with at least one of 24 genetic conditions, the researchers concluded.
The cases revealed that mixed-breed dogs are nearly as susceptible as purebred dogs to 13 heritable medical conditions such as hypertrophic cardiomyopathy, mitral valve dysplasia, lymphoma and hip dysplasia. The study also found some statistical basis for the perception that purebreds are more vulnerable to certain genetic diseases including cataracts, dilated cardiomyopathy and hypothyroidism.
Mixed-breed dogs are more likely to suffer from just one condition — ruptured cranial cruciate ligaments — than their purebred counterparts. The condition is the equivalent to anterior cruciate ligament (ACL) injury in humans.
Dr. Robert Hutchison, an expert in canine reproduction, says the study tests his confidence in hybrid vigor, which involves crossbreeding genetically different animals to increase their vigor or other superior genetic qualities.
"This article is so very interesting," opined Hutchison, director of the Center for Canine and Feline Reproduction and Infertility in North Ridgeville, Ohio. "My first thoughts were different than the third time I read it. I am a big believer in hybrid vigor. When I first read it, it looked like it was trying to prove everything in the world under one cap. The more I read it, the more it made sense."
Hutchison's takeaway is that mixed breeding alone will not eliminate the most common and frequently seen genetic disorders in dogs. "That's why (some genetic disorders) show up in mixed breeds," he said of the study's findings.
Anita Oberbauer, an animal physiologist and the study's lead author, agrees with that assessment.
"The public is under the impression that mixed breeds exhibit vigor and will not express genetic disorders," said Oberbauer, who chairs the animal science department at UC Davis and has bred Belgian Tervuren for more than 20 years. "This is simply not true."
The question of hybrid vigor has long intrigued Oberbauer, prompting the researcher and her colleagues to construct a study that goes beyond the scope of others like it. Previous research on the topic has been limited to examining genetic diseases in specific breeds or one condition in numerous types of breeds, Oberbauer said.
When asked to explain why some genetic disorders appear in multiple breeds or are equally present in mixed-breed dogs, Oberbauer said she believes that casual genetic mutations might have arisen multiple times or the affected dogs may have derived from a common distant ancestor carrying the defect.
Even so, selective breeding can minimize the potential for disseminating genetic mutations, Oberbauer reasoned. “Some dog lineages are more likely to express particular disorders, and for those lineages, breeders must maintain vigilance," she said, adding that the prevalence of genetic disorders among purebred and mixed-breed dogs depends on the specific condition.
For Hutchison, the research confirms what he says specialists have long believed: When recessive genetic traits are concentrated, as occurs in purebred lines, they appear more often. Purebred dogs are more likely to have genetic diseases due to the concentration of genetic traits, he said.
"... You would expect the frequency of a genetic trait to concentrate more as dogs are bred closer together because you have a greater chance of the gene doubling up," he said.
Dr. Jerold Bell, a private practitioner and clinical associate professor of genetics at Tufts Cummings School of Veterinary Medicine, likens some of the statistical aspects of the study to "comparing apples to oranges.”
For some diseases, Bell said, the study's results skew toward making purebred dogs more prone to disease simply because they are more likely to be diagnosed during pre-breeding screening rather after signs appear, as happens with mixed breeds.
"We diagnose some diseases with a bias towards purebreds,” Bell said, using elbow dysplasia as an example. "The majority of affected dogs have grade 1 elbow dysplasia. They don't get arthritis, and it does not affect the dog clinically."
While the study's findings reveal that purebreds have more elbow dysplasia than mixed-breed dogs, Bell points out that purebreds often are screened and diagnosed with grade 1 of the degenerative joint disease. "Mixed breeds are not screened so they aren't getting diagnosed for grade 1, but they are for clinical grades 2 and 3," he said. "If you want to compare them, you need to compare what's actually being diagnosed clinically and ignore the grade 1s."
Bell said the same is true of hypothyroidism, which according to the study is more prevalent in purebred dogs. He explained that many purebred dogs are diagnosed with the condition through pre-breeding profiles before there are any clinical signs of the disease. Conversely, mixed-breed dogs typically are diagnosed by a veterinarian after clinical signs are present.
Comparing the prevalence of a genetic condition in dogs that are either screened or tested for the disease creates a bias, Bell said.
“There are certainly genetic disorders that we see at higher frequency in purebred dogs,” Bell said. “However, for the most common genetic disorders seen in practice, they tend to be seen equally in mixed and pure breeds.”
Other criticisms of the study center on the medical conditions considered by researchers. Hutchison said that none of the 17 veterinarians at his practice consider Cushing's disease (hyperadrenocorticism) to be genetic disorder, although it was included in the study. The same goes for ruptured cranial cruciate ligaments, the only condition that the study found to occur more commonly in mixed breeds.
Oberbauer countered that cranial cruciate ligament rupture is reported in several studies
to have a genetic component. The conditions, she said, were chosen this way: "All the genetic disorders selected for the study represented those expected to be present in the dog population at a measurable prevalence and to be debilitating, and having confidence in the reliability of diagnosis. Additionally, disorders that affected a variety of anatomic locations and physiologic systems were chosen."
She explained that cases of senile cataracts, despite not being genetic conditions, were mixed in with cases of inherited juvenile cataracts
under the study category "cataracts."
The research isn't perfect, Hutchison said, but it's prompted him to expand his thinking about genetics and preventing disease in veterinary medicine.
"Let's say every golden carries two genes for hemangiosarcoma," said Hutchison, referring to the aggressive and invasive cancer of the blood vessels that occurs almost exclusively in dogs.
"Do you do a preventive splenectomy? We could prevent it rather than breed it out." he mused.
Oberbauer hopes the spread of information in the study can help lessen the incidence of some genetic disorders by reshaping canine breeding practice. If a condition is associated with the morphology determined by the breed standard, perhaps the breed standard needs revisiting, she said.
If a condition has a phenotypic or genetic test, implementing testing within that population can reduce the disease's prevalence, she added.