26-year study of canine sperm shows an overall decline in quality, and may also shed light on fertility changes seen in male humans

Britain’s dogs are becoming less fertile. Researchers who have systematically examined canine sperm over a span of 26 years say that overall sperm quality has been in decline.

Environmental chemicals are implicated. And the study may throw light on the fertility changes in male humans.

Richard Lea, of Nottingham University’s school of veterinary medicine and science, and colleagues collected samples of semen from a carefully monitored population of labradors, border collies, German shepherds and golden retrievers used as stud to breed dogs intended to help the disabled. They tested 1,925 samples of ejaculate from a total of 232 different dogs at the rate of between 42 and 97 dogs every year.

And they found a drop in sperm motility – the ability to swim in a straight line - of 2.4% per year from 1988 to 1998. Even once some dogs were excluded from the study because their fertility was in some way in question, from 2002 to 2014 the scientists still measured a decline of 1.2% per year.

They also confirmed the presence of environmental chemicals known as PCBs and phthalates in the canine semen, and in testicles of dogs castrated by veterinary surgeons in the course of routine neutering treatment. These chemicals are ubiquitous, and have been linked to both fertility issues and birth defects.

At the heart of the research is not the dog, but the question of male human fertility. Repeated tests over more than 70 years have shown a downward trend in male fertility, but there has always been argument about the consistency and accuracy of the findings.

“Why the dog?” said Dr Lea. “Apart from the fact that it is a great population of animals to work with, dogs live in our homes, they sometimes eat the same food, they are exposed to the same environmental contaminants that we are, so the underlying hypothesis is that the dog is really a type of sentinel for human exposure.”

One analysis of human sperm counts examined 60 separate studies over a 50-year span. But different research teams used different techniques, and in any case the advances in laboratory equipment over many decades meant that it would always be difficult to compare like with like.

There has been much less doubt about measured increases over the decades in rates of testicular cancer, and a condition known to affect a proportion of boy babies at birth: cryptorchidism, in which the testicles do not descend normally to the scrotum.

The Nottingham canine study group resolved a number of problems of consistency. All the dogs were healthy and well cared for. The semen sampling was supervised by Professor Gary England, Nottingham’s foundation dean of veterinary science, who launched the project, and all the samples were handled by just three technicians in the 26-year study. The researchers also looked at the dog’s food, and the chemicals in the food.

The decline in canine sperm quality does not, for the moment, augur the end of the dog as a species. “It’s very unlikely” Dr Lea said. “It’s very difficult to say at what point this becomes a problem.”

The researchers saw increases in cryptorchidism in the study dogs’ pups over the years. They also saw a clear connection between environmental chemicals and declining fertility. How this might work, however, is not so clear.

“If you think about it, we are exposed to a cocktail. Who knows how many chemicals are out there and what they are doing? It gets even more complicated when you start to look at the effects of mixtures of chemicals,” Dr Lea said.

“What we have been able to do here is just to pull out ones that we know are present, and we have tested those in terms of their effects and it does suggest there is an impact. The next stage – and it is a big next stage - is trying to tease out what else is there and how those chemicals are interacting.”