The majority of dog breeds are highly inbred, contributing to a lifelong increase in disease and health care costs.

That’s according to a new collaborative study by researchers in California and Finland, who used a database of commercial DNA tests of nearly 50,000 dogs across 227 breeds to analyze the average genetic similarity of dogs within a breed, then compared their results to data from previous studies that used a smaller pool of breeds.

The researchers found that the average inbreeding was close to 25%, which is similar to the amount of genetic similarity found between two human siblings. Those levels of shared genetic material are well above what would be considered safe for either humans or wild animal populations.

Lead author Danika Bannasch, DVM, PhD, a veterinary geneticist at the University of California-Davis, said, “Data from other species, combined with strong breed predispositions to complex diseases like cancer and autoimmune diseases, highlight the relevance of high inbreeding in dogs to their health.” The authors point out that a higher risk of hereditary disorders like cancer start to show up in humans at around 3 to 6%.

Bannasch and her team also cross-referenced their findings with data from a pet insurance database, using insurance claims for nonroutine vet visits as a measure of canine health. They found that dog breeds with higher levels of inbreeding were more likely than other breeds to need additional veterinary care.

Factors like size play a key role.

Bannasch noted that while previous studies have shown that small dogs live longer than large dogs, none had reported on the presence of disease: “This study revealed that if dogs are of smaller size and not inbred, they are much healthier than larger dogs with high inbreeding.”

As to why some breeds are more inbred than others, Bannasch told NEWStat that it’s likely due to the individual population history of each breed: “This includes the number of founders for the breed, historical bottlenecks [the drastic reduction of the breed’s population at some point in the past], strong selection for traits, and dog-breeding practices [such as] close breeding and population sires.”

A well-known example of breeding by selection for traits that can lead to health problems are brachycephalic—or short-snouted—dogs like French and English bulldogs, who are bred to have relatively short muzzles because the look is considered desirable by some pet owners. These dogs can have trouble breathing, but Bannasch said that brachycephalic dogs were excluded from the findings because their health problems were due to their morphologies to such a great extent that they skewed the results. However, she added that “they have similarly high levels of inbreeding to other nonbrachycephalic breeds.”

Unfortunately, Bannasch says there’s not much veterinarians can do about the problem other than to educate pet owners before they get a dog that may face health problems due to inbreeding: “At this point, this is more about awareness of the issue. The solutions are really challenging.”

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