Numerous clinical studies indicate that sperm counts in men have declined over a period of more than 50 years. Many laboratories have investigated this worrying trend and the weight of evidence from meta-analyses, where data generated from multiple laboratories over time are analysed together, indicate that that this decline is real. The most recent and most extensive meta-analysis indicated that this problem was not limited to North America, Europe and Australia but was also evident from data generated from South/Central America, Asia and Africa: a truly global issue! In fact, the rate of decline per year appears to have doubled after the year 2000 (Levine et al., 2023). It would appear therefore, that many men today have half the sperm numbers their grandfathers had. These alarming reports have led to growing public concern alongside considerable academic debate around the underlying causes.
Other markers of male reproductive health also appear to have changed over time and these include a rising incidence of testicular cancer in younger men and increasing reports of malformations in baby boys, particularly a lack of descent of the testes into the scrotum (cryptorchidism): a process that normally occurs before or shortly after birth (Main et al., 2009). This raises the question on what may be causing these problems?
With regard to the decline in sperm quality, lifestyle is undoubtedly a factor e.g., poor diet, stress, age, sedentary lifestyle, smoking, alcohol and many more personal choices that characterise our daily lives. However, another worrying factor is that there is increasing evidence that declining reproductive health is linked to exposure to manmade environmental pollutants. These include chemicals that leach from plastics, flame retardants, non-stick coatings, pesticides, industrial solvents/surfactants and fertilisers to name but a few. There is also concern over chemicals we intentionally expose ourselves to, such as those found in medicines and contraceptives (Sharpe 2024).
Many non-contraceptive chemicals behave as hormones or block hormone action and a particularly sensitive period of exposure is before birth (via the mother) when the testes of the developing male baby are undergoing development in preparation for post-natal life and puberty. Even in the adult, there is evidence that exposure to chemicals known to be present in the testis reduce sperm motility (Sumner et al., 2019). The challenge around this important research area is that in reality, we are exposed to mixtures of chemicals from a variety of sources and therefore demonstrating cause and effect is difficult.
Given the reported chemical effects on the human, we wondered if there was any evidence of similar effects in animals. To test this idea, we investigated fertility in healthy stud dogs used in a breeding programme to generate support dogs for partially sighted or blind people. These dogs live in normal homes and have a regular veterinary clinical check-up to ensure they have good health including the collection of a sperm sample. In this population of healthy dogs, we found that sperm quality had declined over a period of 26 years: a very similar trend to that reported in the human (Lea et al., 2016). Since all of the data came from one clinical laboratory, there were fewer confounders than in the human studies e.g., alcohol consumption, lifestyle, age etc. This suggested that declining sperm quality also affects ‘man’s best friend’. We suggested therefore that since dogs live in our homes, they will be exposed to the same mixture of chemicals that we are and therefore chemical effects on our dogs reproductive health is likely to be the same as chemical effects on our own health. In a sense, our pet dogs are sentinels for our own exposure to chemical pollutants!
This raises questions around other species. Given how wide-spread the human reports are, is declining fertility and reproductive health a cross-species problem? Worryingly, this does seem to be the case. Rachel Carson’s first described possible adverse effects of environmental chemicals on bird and wildlife populations in her book Silent Spring published in 1962. Since then there have been numerous reports of chemical effects on wildlife. One example is the killer whale where exposure to polychlorinated biphenyls (PCBs) has been linked to a dramatic population decline. The problem is that PCBs and other pollutants accumulate in small fish and then “biomagnify” as they are consumed by larger fish. Consequently, since the killer whale is an apex predator, it absorbs the pollutants taken in by the different prey at each level of the food chain e.g., fish, seals etc. Another problem is that such chemicals sit in fat tissue such as blubber and are released into the milk when the mother suckles the young. In this way, the young calf is exposed to his mother’s lifetime exposure (Desforges et al., 2018).
The global impact of chemical pollutants is further exacerbated by their transport over long distances to the arctic where they have been detected in many species including narwhales and polar bears (Pederson et al., 2024). In a reproductive study of polar bears, changes in testis size, and intriguingly, a reduced size of the penile bone known as a baculum, were linked to exposure to concentrations of contaminants in these animals (Sprondly-Nees et al., 2019). Although further work is needed in this area, this inevitably raises concerns over effects on reproductive health, not only on polar bears and whales, but on the Inuit populations that consume them (Long et al., 2023).
In this short article, I have highlighted a worrying trend in male reproductive health not only in humans, but also in the domestic dog and in diverse species across the planet. I haven’t considered chemical impacts on female reproductive health but suffice to say that this is another area of significant concern. The question I am often asked is, what can we do about these alarming trends in reproductive health? There is of course no quick fix but we can look at our lifestyles and change what is under our control: diet (as a source of exposure), exercise etc. For those factors beyond our control such as environmental pollutant exposures, raising awareness is of key importance. We can’t avoid exposures to such chemicals, but we can collectively send a message to industry to take steps to reduce further input of chemicals into our environment or, at the very least, to use ‘safer’ alternatives. Ultimately, we should all strive to look after ourselves since good health and sensible life choices will inevitably impact on our fertility.
Professor Richard G Lea
School of Veterinary Medicine & Science, University of Nottingham, UK
Research
Levine H, Jørgensen N, Martino-Andrade A, Mendiola J, Weksler-Derri D, Jolles M, Pinotti R, Swan SH. Temporal trends in sperm count: a systematic review and meta-regression analysis of samples collected globally in the 20th and 21st centuries. Hum Reprod Update. 2023 29:157-176.
Main KM, Skakkebaek NE, Toppari J. Cryptorchidism as part of the testicular dysgenesis syndrome: the environmental connection. Endocr Dev. 2009 14:167-73.
Sharpe RM, Endocrine disruption and male reproductive disorders: unanswered questions, Human Reproduction, 2024;doi.org/10/1093/deae143
Sumner RN, Tomlinson M, Craigon J, England GCW, Lea RG. Independent and combined effects of diethylhexyl phthalate and polychlorinated biphenyl 153 on sperm quality in the human and dog. Sci Rep. 2019 ;9:3409.
Lea RG, Byers AS, Sumner RN, Rhind SM, Zhang Z, Freeman SL, Moxon R, Richardson HM, Green M, Craigon J, England GC. Environmental chemicals impact dog semen quality in vitro and may be associated with a temporal decline in sperm motility and increased cryptorchidism. Sci Rep. 2016 6:31281.
Carson R. Silent Spring. Houghton Mifflin Company, 1962
Desforges JP, Hall A, McConnell B, Rosing-Asvid A, Barber JL, Brownlow A, De Guise S, Eulaers I, Jepson PD, Letcher RJ, Levin M, Ross PS, Samarra F, Víkingson G, Sonne C, Dietz R. Predicting global killer whale population collapse from PCB pollution. Science. 2018 361:1373-1376.
Pedersen AF, Bayen S, Liu L, Dietz R, Sonne C, Rosing-Asvid A, Ferguson SH, McKinney MA. Nontarget and suspect screening reveals the presence of multiple plastic-related compounds in polar bear, killer whale, narwhal and long-finned pilot whale blubber from East Greenland. Environ Pollut. 2024 357:124417.
Spörndly-Nees E, Holm L, van Beest FM, Fakhrzadeh A, Ekstedt E, Letcher R, Magnusson U, Desforges JP, Dietz R, Sonne C. Age and seasonal variation in testis and baculum morphology in East Greenland polar bears (Ursus maritimus) in relation to high concentrations of persistent organic pollutants. Environ Res. 2019 173:246-254.
Long M, Sonne C, Dietz R, Bossi R, Jørgensen N, Olsen TI, Bonefeld-Jørgensen EC. Diet, lifestyle and contaminants in three east Greenland Inuit municipalities. Chemosphere. 2023 344:140368.