A new study shows canine fertility has decreased in recent years, and it may be the result of exposure to contaminants—contaminants that could also be affecting human fertility.
The study in question focused on dog semen. Looking at testes from different dog species between the years 1988 and 2014, researchers found that the quality of the sperm had declined significantly in that time period, with a smaller percentage of dog sperm being motile—or able to swim—than before. Dr. Richard Lea, who led the research at the University of Nottingham, tells Broadly that because this change happened in a relatively short period of time, it was unlikely caused by genetic changes. Instead, it's possible the decline could be the result of environmental contaminants.
Researchers found chemicals in the testes of dogs that, when tested, showed to have an effect on sperm function. Some of the materials were found in the dogs' commercial food brands. Though these findings don't provide hard evidence that the contaminants are the cause of the decreased fertility, the association shows it could be a viable hypothesis.
Though the findings are exclusively about canine balls, they are noteworthy because, Dr. Lea explains, dogs could be a "sentinel" for humans, "since dogs live in our homes, are exposed to same type of contaminants, and even get the same kinds of infections." Dr. Lea also says that the decline in canine sperm quality parallels an apparent decline in human sperm quality.
Yet it's difficult to prove scientifically that chemical contaminants are affecting fertility, especially among humans, says. Dr. Lea.
"The difficulty of this work is that we can detect individual chemicals and show they have effects, but reality is that humans exposed to a cocktail of chemicals that interact with one another. The effects of one chemical may not be same as it will be in the presence of lots of other chemicals. It's extraordinarily complicated to tease out the mixtures' effects."
Also, because humans live diverse lives, it's hard to study how environmental factors play a role in health, says Dr. Lea. "If you take a population of humans, we all have differing backgrounds, living in different place, eat different foods. That makes it very difficult to pin down environmental factors."
However, Dr. Lea points out that data across species has linked reproductive problems with exposure to contaminants, including fish swimming in estrogen-tainted rivers and orcas affected by PCB in the oceans. Meanwhile other studies have shown chemical exposure may be associated with health effects, including one that seemed to show chemicals as a potential cause for the average age of menopause going down.
Dr. Lea says more research needs to be done to discover with certainty what is causing what he calls a "cluster" of reproductive problems currently occurring among humans. "Cryptorchidism has been reported in humans and believed to have environmental cause, testicular cancer is on the increase, there's a condition called hypospadias… In addition, we have reports of semen quality declining — all these are linked together and it's believed they have a similar cause."