Originally aired on March 9, 2019.

Researchers in the U.K. have found a link between the fertility declines in domestic male dogs and similar problems that have been widely reported in human males.

They traced the source back to two common chemicals that are persistent in our domestic environments and even our food.

Just a routine checkup

Richard Lea, from the School of Veterinary Medicine at the University of Nottingham, works closely with stud dogs that are used in an assistance breeding program.

Part of his work involves routine checks on their reproductive health. Two years ago he led a research team that discovered a noticeable decline in sperm quality in dogs over the last couple of decades.

This mirrored problems that have been reported in humans. Researchers in human reproductive health have been increasingly concerned about a decline in the quality of human sperm over roughly the last half-century.

One of the stud dogs routinely monitored for fertility at the University of Nottingham's veterinary clinic (Richard Lea)

"There was a decline in motility by 30 per cent [in dogs], and what's interesting is it parallels similar observations carried out on human sperm," said Lea.

This led Lea to suspect that both of these these problems were due to a common environmental exposure.

"There are so many modern-day chemicals being produced that affects our general health," said Lea. "We thought, the dog shares our home environment so they'd be exposed to the same chemicals, and whatever effect might be reflected in humans as well."

Putting theory to the test

Lea and his team analyzed the sperm samples collected from the stud dogs and detected several chemical contaminants.

Two of the chemicals he identified were DEHP, a phthalate plasticizer used in many common household products, and PCB 153, a persistent pollutant and carcinogen.

In previous work Lea found that both of these pollutants could be detected in many common dog foods.

Sperm DNA fragmentation, linked to reduced fertility, is observed in both men and dogs in the study. The normal sperm show a “halo” around the head whereas sperm with fragmented DNA have no “halo” (black arrows). (Rebecca Sumner)

Lea and his group exposed new sperm samples collected from donor men and stud dogs living in the same region in the U.K. to DEHP and PCB 153 at similar levels to those detected in the home environment.

The two chemicals were found to cause a significant decrease in sperm motility — the ability for sperm to swim — and an increase in DNA damage, which has been linked to reduced fertility.

"Our findings suggest that man-made chemicals that have been widely used in the home and work environment may be responsible for the fall in sperm quality reported in both man and dog that share the same environment," said Lea.

The chemical suspects

DEHP comes from a chemical group called phthalates that are used to make plastics flexible and harder to break. They can be found in carpets, plastic toys for children and dog toys.

Humans can also be exposed by eating microwaved food stored in plastic containers. Dogs can get exposed by sucking or chewing on soft plastic or vinyl products, and of course, in their food.

Studies have shown that phthalates can disrupt the operation of male hormones like testosterone. They've also been classified as an endocrine disruptor.

Dogs can get exposed to the chemical DEHP by sucking or chewing on soft plastic or vinyl toys. (Pierre-Philippe Marcou/AFP/Getty Images)

PCBs, or polychlorinated biphenyls, were previously used in coolants, electrical devices and in the manufacturing of paints, plastics, and rubber products. PCBs were banned in most jurisdictions around the world starting in the 1970s, when it was discovered that the chemicals were leaching into the environment and causing a variety of health problems.

But decades after the ban, PCBs still persist in the environment and is detected everywhere today.

Future implications

"The levels of the chemicals weren't particularly high in sperm samples in humans and dog," said Lea. "But it turns out that they still produce a marked effect on sperm quality. So I would challenge some of the safety limits of the chemicals."

"[The effects] haven't quite reached critical levels yet. But we have to limit further exposure to prevent those chemicals from damaging us even more," said Lea.

"We need to review the way those chemicals are regulated in the environment, and look into potential alternatives that don't have detrimental effects."