Breeders of the English bulldog are facing concerns about the way "extreme" physical changes have affected the health of the breed. And the only way to reverse these unhealthy changes now would be to crossbreed the English bulldog and introduce new bloodlines, researchers suggest.

In the study presented in Canine Genetics and Epidemiology, the researchers examined whether the English bulldog as a breed exhibits sufficient "genetic diversity" in order to rectify phenotypic and genotypic defects.

Some of the physical and genetic attributes of the English bulldog have been linked to poor health. The breed is known to suffer from reproductive issues, such as the presence of a narrow pelvis in females, which makes birthing difficult.

Other physical traits, such as the dog's short snout, have also made breathing difficult for the canine, and this has been one of the animal's leading cause of death.

Genetic diversity may, however, help to rid the gene pool of mutations and bring about additional phenotypic changes to the breed, such as changes to the dog's body structure or coat.

Selective breeding over the centuries has led to inbreeding which, in turn, has resulted in "very low genetic diversity" among English bulldogs, the study authors note.

This low diversity, or limited genetic stock, is attributed to the breed's small founder population from which the breed evolved. Aside from a small initial population, "artificial genetic bottlenecks" have also occurred as breeders attempt to produce desirable traits in the canine.

The team proved the point on genetic diversity when it studied a sample of 102 English bulldogs registered for breeding purposes. Against this sample, the researchers examined 37 additional English bulldogs that were brought to the University of California Davis Veterinary Clinical Services because of health problems.

"We tried not to be judgemental in our paper," Niels Pedersen, a UC Davis professor who co-authored the study, tells the BBC News. "We just said there's a problem here, and if you are going to decide to do something about it, this is what you've got to work with."

If inbreeding has placed the English bulldog's health in danger, how then can breeders save the canine?

The short answer is to crossbreed with another canine that carries similar phenotypic traits but has a different genetic constitution.

"Trying to manipulate diversity from within a breed if it doesn't have much anyway is really very difficult," Pedersen says. "If all your dogs are highly related to one another, which ones are you going to pick?"