Over the past nearly three decades, researchers in England have studied the fertility of five types of purebred male dogs and found that fertility has dropped rapidly and significantly in all five breeds.

In the new study, published in the journal Scientific Reports, researchers found declining sperm counts and other infertility issues in those dogs that could be linked to environmental causes. They found certain chemicals in the dogs' sperm and testes — as well as in certain brands of commercial pet foods — that had a negative effect on fertility. The chemicals included polychlorinated biphenyls (PCBs) and diethylhexyl phthalate (DEHP). The dog food brands were not named in the study, but the researchers said they are sold worldwide.

The scientists were unable to determine how the chemicals made it into the pet food supply. But they speculated to the New York Times that they could be in the food packaging, as well as in water than may have come into contact with some of the ingredients.

“This is the first time that such a decline in male fertility has been reported in the dog, and we believe this is due to environmental contaminants, some of which we have detected in dog food and in the sperm and testes of the animals themselves," lead researcher Dr. Richard Lea of the University of Nottingham’s School of Veterinary Medicine and Science said in a statement.

“While further research is needed to conclusively demonstrate a link, the dog may indeed be a sentinel for humans — it shares the same environment, exhibits the same range of diseases, many with the same frequency and responds in a similar way to therapies.”

About the dogs

golden retriever puppies eating Several brands of commercial dog food, including puppy food, were found to have traces of chemicals. (Photo: Kotruro2/Shutterstock)

For the study, the researchers studied between 42 and 97 dogs each year from 1988 to 2014. They worked with a total of 232 different dogs from five different breeds: Labrador retrievers, golden retrievers, curly coat retrievers, border collies and German shepherds. The dogs were being bred as service animals for a center for the disabled in England.

Because the dogs were purebred and purebred dogs can be predisposed to genetic conditions, the researchers considered that the problems were due to inherited versus environmental factors.

“We looked at other factors which may also play a part, for example, some genetic conditions do have an impact on fertility," Lea said. "However, we discounted that because 26 years is simply too rapid a decline to be associated with a genetic problem.”

Over the 26 years of the study, researchers found a noteworthy drop in sperm motility — or sperm's ability to move forward. Between 1988 and 1998, sperm motility decreased by 2.5 percent each year. When dogs with the poorest sperm were removed from the study group, sperm motility from 2002 to 2014 continued to decline at a rate of 1.2 percent per year. The overall drop during the entire study was 30 percent.

What it means to humans

This study may support other findings about human semen and environmental factors, say the researchers.

The authors point out that over the past 70 years, studies have shown a decline in human sperm quality, as well as an increase of testicular cancer and other problems.

The issue of declining human semen quality, however, is a controversial issue. As the New York Times points out:

But there has been a debate in the scientific community about whether this trend is directly associated with industrial chemicals discharged in the environment. Critics have noted that lab conditions and standards for the many studies have varied widely. Establishing a cause-effect link over time, they say, is not reliable.

So will the canine fertility study help us take a broader look at human fertility?

Lea says that's a possibility: “The Nottingham study presents a unique set of reliable data from a controlled population which is free from these factors. This raises the tantalising prospect that the decline in canine semen quality has an environmental cause and begs the question whether a similar effect could also be observed in human male fertility.”

Mary Jo DiLonardo Mary Jo writes about everything from health to parenting — and anything that helps explain why her dog does what he does.