London, Aug 10 (IANS): While scientists continue to debate reported decline in human semen quality, a new study has now found that the fertility of "man's best friend" may have suffered a sharp decline over the past three decades primarily due to environmental contaminants.

"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," said lead researcher Richard Lea from the School of Veterinary Medicine and Science, University of Nottingham in Britain.

"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," Lea noted.

The research, published in the academic journal Scientific Reports, found that sperm quality in a population of stud dogs studied over a 26-year period had fallen significantly.

The work centred on five specific breeds of dogs - Labrador retriever, golden retriever, curly coat retriever, border collie and German shepherd -- with between 42 and 97 dogs studied every year.

Semen was collected from the dogs and analysed to assess the percentage of sperm that showed a normal forward progressive pattern of motility and that appeared normal under a microscope (morphology).

Over the 26 years of the study, they found a striking decrease in the percentage of normal motile sperm.

Between 1988 and 1998, sperm motility declined by 2.5 per cent per year and following a short period when stud dogs of compromised fertility were retired from the study, sperm motility from 2002 to 2014 continued to decline at a rate of 1.2 per cent per year.

Sperm collected from the same breeding population of dogs, and testes recovered from dogs undergoing routine castration, were found to contain environmental contaminants at concentrations able to disrupt sperm motility and viability when tested.

The same chemicals that disrupted sperm quality, were also discovered in a range of commercially available dog foods - including brands specifically marketed for puppies.

Over the past 70 years, studies have suggested a significant decline in human semen quality.

However, declining human semen quality remains a controversial issue which scientists continue to debate.