A paper appearing online in August 2016 by lead researcher Richard G. Lea and others in the prestigious science journal Nature opened the door for speculation on the causes of a decline in canine fertility and whether the problem might extend to humans.
Scientists collected sperm from 232 stud dogs including Labrador retrievers, golden retrievers, curly coat retrievers, border collies and German shepherds who participated in a British controlled breeding program from 1988 – 2014. This group of dogs was an optimal study population because they remained under similar conditions, and their fertility was regularly assessed by looking to sperm motility (the ability of sperm to swim in a straight line).
Even after eliminating the dogs with the lowest sperm counts, scientists observed that fertility declined by 1.2 percent every year for the 12-year period. Total fertility in these dogs declined 30 percent during the entire period.
Curiously, these findings also parallel an increase in cryptorchidism (undescended testicles) in dogs. Scientists also observed a decrease in the total number of males per litter, but this decrease disappeared when stillbirths and post-natal deaths were eliminated from the analysis. The mortality of female puppies also increased significantly during this period.
Reasons for the decline in male canine fertility remain unclear, although scientists suspect that PCBs (polychlorinated biphenyls) and DEHP (diethylhexyl phthalate) may have played a role. These chemicals are not dog food ingredients so their source remains unclear. Contaminated water, used in the processing of dog food, or perhaps packaging, are possible culprits. Scientists focused on these chemicals, which are associated with fertility problems because traces were found in biological samples taken from the dogs when the dogs were neutered.
Although PCBs were banned in the United States in 1979, they remain in the environment. Unlike PCBs, DEHP remains in production, which is estimated at two million tons per year. Among its many uses, DEHP is a plasticizer in making PCV piping and a fragrance carrier in personal and home care products. Currently. the FDA allows DEHP to be used in packaging for foods that are largely water. In Europe, it is commonly found in high-fat foods such as milk and cheese.
These canine results are particularly interesting because they parallel a drop in human fertility. Meta-analysis of 61 studies between 1938 and 1991 confirms a decrease in human male fertility. Markers for the fertility include declines in sperm concentration, declines in motility, and impaired sperm morphology. Even more telling, perhaps, is the recent increase in undescended testicles in human babies, as this too parallels the observation in canine populations.
Since humans and dogs tend to share the same environment, it is not surprising to see studies of canine well-being showing results similar to that of their owners. Closer analysis is needed to determine if PCBs or DEHP is the culprit or whether their presence merely correlates with these findings. Our dogs may indeed be trying tell us something.