English Bulldogs are a quintessentially British breed said to symbolize the country's tenacious spirit. But the dogs are so inbred that new blood may be the only way to return them to health, scientists warned on Friday.
Veterinary scientists from the University of California studied the DNA of 102 registered English bulldogs from across North America and Europe. They found 78 of the dogs were so inbred they were more closely related than "offspring of full-sibling parents from a random breeding village dog population."
English bulldogs can suffer from a range of serious health problems, including breathing difficulties, skin problems, cleft palates and difficulties conceiving and giving birth naturally. They remain highly popular in the U.K. and U.S., though, because of their childlike demeanor and appearance.
"Bulldogs that reproduce without assistance, move freely, walk or run for long distances, and breathe normally even at rest are the exception," the University of California's Niels Pedersen, Ashley Pooch and Hongwei Liu said in a report in the Canine Genetics and Epidemiology journal.
In the U.K. and U.S., these dogs can sell for well over $1,000. An English Bulldog is also the official mascot of the U.S.'s University of Georgia.
"American breeders of English bulldogs have avoided the issue and even deny that the breed is unhealthy, pointing to its popularity as proof. However, the bulldog has been banned from plane travel in the cargo hold by many domestic and international airlines due to a high incidence of deaths," the report's authors said.
The scientists said the dogs —"one of the most popular and unhealthiest of dog breeds" — were so lacking in genetic diversity that efforts to improve the breed from existing genetic stock might be fruitless.
Instead, European rules regarding animal welfare might ultimately force breeders on the continent to crossbreed the dogs. After a constitutional amendment in Switzerland in 2013, Swiss breeders proactively began crossing English bulldogs with a separate breed from the U.S. known as the "Olde English Bulldogge."
"The feelings of individual English bulldog breeders about the health of their breed and what if anything should be done about it may ultimately be taken out of their hands," the University of California scientists said.
Other research suggests the rate of inbreeding for bulldogs is declining, according to the U.K. Kennel Club, which registers pedigree dogs.
"The Kennel Club continues to look at ways to ensure the sustainability of a breed if it is at risk and works with breed clubs and researchers in order to do this," Aimée Llewellyn-Zaidi, head of health and research at the organization, said in a statement emailed to CNBC on Friday.