Puppy problem points to reason for fewer baby humans

Scientists at Nottingham University recently published a study in the journal Scientific Reports that links environmental chemicals found in dog food with a 26-year decline in canine fertility rates. The researchers believe the same environmental chemicals may also affect fertility and sperm quality in humans.

Many studies have shown the quality of human sperm has significantly decreased over the past 70 years. Because male reproductive problems tend to cluster in geographical areas, scientists believe they may have a common cause and suspect exposure to certain environmental chemicals, particularly those known to disrupt the endocrine system, may be to blame.

Scientific studies with humans have not only detected a reduction in sperm count but also an increased incidence in cryptorchidism, a condition in which one or both of the testes fail to descend into the scrotum. The researchers hypothesized that if environmental chemicals are to blame, they should see a similar pattern in man’s canine companions, who share the same habitat and are therefore exposed to the same chemicals.

Each year between 1988 and 2014, the researchers studied 42 to 97 dogs from five different breeds. A statistical analysis revealed that during those 26 years, the dogs’ sperm quality and proportion of male pups decreased, while the incidence of cryptorchidism and female pup mortality increased. Sperm motility declined by 2.5 percent per year between 1988 and 1998, and then at a rate of 1.2 percent per year from 2002 to 2014.

In conjunction, the researchers found 12 environmental chemicals detected in the adult dog testes were also prevalent in 15 commercial dog foods. The researchers then narrowed their study to two of the chemicals: Diethylhexyl-phthalate (DEHP) and a polychlorinated biphenyl (PCB).

These two chemicals were in high abundance in the dogs’ testes and in some of the dog foods. During in vitro laboratory studies, researchers have determined DEHP and PCB affect sperm viability, motility, and DNA integrity. Other studies have found both chemicals in the semen of farm animals and humans in the same concentrations the current study found in its canine subjects. And some studies show these chemicals alter semen quality in other animals. The Nottingham researchers also found the increased incidence of cryptorchidism in the dogs paralleled that of humans. Cryptorchidism affects 2-9 percent of human newborn males and appears to be correlated with geographic location.

DEHP, an additive used to make plastics more flexible, is widely used in household and medical products. PCBs are a family of highly toxic and potentially carcinogenic chemicals to which humans may be exposed by eating contaminated fish.

The researchers concluded substantial evidence shows an environmental trigger for decreased rates of fertility in dogs and that the decline in canine semen quality “parallels that reported in the human.”

“This raises the tantalizing 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,” Richard Lea, the study’s lead researcher, told New Atlas.