Centuries of selective breeding has destroyed the English bulldog's genetic diversity and crossing it with another breed is the best way to ensure its survival, scientists say.
Due to selective breeding for physical traits, the Bulldog has become so inbred it cannot be returned to health without an infusion of new bloodlines, a genetic study suggests.
Among other problems, English bulldogs have difficulty breathing, moving and mating. These traits are a result of how the dogs were selectively bred to promote characteristics like its shortened muzzle and stature. Decades of heavy inbreeding have caused further problems, including autoimmune diseases and allergies, the New Scientist reported, quoting a study that appears in the journal Canine Genetics and Epidemiology.
Study co-author Niels Pedersen, from the University of California, Davis, said, "We tried not to be judgemental in our paper. 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 you want to re-build the breed, these are the building blocks you have, but they're very few. So if you're using the same old bricks, you're not going to be able to build a new house," Davis told the BBC.
The English Bulldog breed - also known as the British Bulldog - has a long-standing cultural association with the UK, but is also sought after worldwide because of its child- like appearance and gentle temperament.
Prof Pedersen and colleagues from the Center for Companion Animal Health at UC Davis examined the DNA of 102 registered English Bulldogs to know whether there was enough genetic diversity - a measure of relatedness among individual dogs - to breed out the harmful traits through programmes that use existing genetic stock.
But the analysis found they had very low levels of diversity resulting from a small initial pool of founding dogs, followed by so-called bottlenecks, caused by selective breeding for "desirable" traits like the short nose, which have further reduced variety in the Bulldog gene pool.
"The fastest way to get genetic diversity is to outcross to a breed that looks similar but is genetically distinct... Trying to manipulate diversity from within a breed if it doesn't have much anyway is really very difficult," Pedersen said.
Although some diversity still exists in the Bulldog gene pool, including in genes that affect the brachycephaly (distinctive short face and snout) trait, other genetic loci show very little variety.
Breeders differ widely on what should be done to tackle the illnesses. Some argue that any deviation from the breed's standards would no longer make it an English Bulldog.
Others argue that the English Bulldog has constantly evolved over the centuries and favour the introduction of new genetic material, known as outcrossing.
One candidate mentioned in the research paper is the Olde English Bulldogge, a 1970s attempt by an American breeder to recreate the healthier working bulldog that existed in England during the early 1800s.
