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My previous blog reported on evidence for a widespread decline in human sperm counts between 1940 and 1990. In 1992, Danish reproductive biologist Elisabeth Carlsen and colleagues published a key review that can be seen as a turning-point. The Carlsen review evoked considerable criticism, including a riposte from obstetrician Ian Tummon and reproductive biologist David Mortimer. For me, their most telling counter-argument was that methods of semen analysis had changed over the period concerned and that the results reported by Carlsen and colleagues were artifactual.
The Decline is Real
Of course, the objection raised by Tummon and Mortimer is potentially devastating and demands serious consideration. But in fact veterinarian Brian Setchell neatly disposed of this possibility in a 1997 paper. Similar methods have been widely used to assess semen quality in farm animals, so their sperm counts should also show any decline due to changing techniques. Sperm counts dating back to the early 1930s are available for cattle, pigs and sheep. Setchell reviewed data from over 300 publications reporting sperm counts in these farm animals over the previous 60 years. There was no significant change in sperm counts over time for either bulls or boars. Moreover, with sheep there was actually a slight, but significant, increase. Setchell sagely concluded: “It would appear that, if the fall in human sperm counts is real, then it must be due to something which is not affecting farm animals.”
The fall in human sperm counts in many human populations is clearly real and evidence continues to mount. Recent reports have, for instance, provided convincing evidence of marked declines in sperm counts over the past two decades in countries as far apart as Israel and France. Entirely new information for Israel was published in a 2012 paper by a team led by Ronit Haimov-Kochman, an infertility researcher. It reported results of a retrospective analysis of over 2,000 weekly sperm samples collected from 58 young, paid donors over a 15-year period (1995-2009). After controlling for any effect of age, average total sperm counts were found to have fallen significantly by 37%, from over 300 million to around 200 million. Haimov-Kochman and her colleagues concluded: “The rapid deterioration of sperm quality among fertile semen donors is alarming and may lead to cessation of sperm donation programs." An even more recent paper identified a similar decline in semen concentration in France. This nationwide study, also retrospective, examined sperm counts for almost 27,000 men participating in assisted reproductive technology procedures because their partners were totally infertile, with blocked or absent oviducts. Over the 17-year period 1989-2005 there was a continuous decrease in semen concentration of almost 2% a year. Across the study period the overall decline was some 32%, falling from about an average of 220 million sperms per ejaculate in 1989 to under 150 million in 2005.
But is Fertility Affected?
The evidence is now overwhelming that human sperm counts are declining, albeit at different rates and times in different localities. But the key question is whether the lower levels now seen are likely to have a significant impact on male fertility. Successful fertilization definitely needs large numbers of sperms. Research by infertility clinics in the 1950s revealed that men are commonly infertile if total sperm counts in an ejaculate remain persistently below 70 million. Above a certain level, however, sperm counts and fertility are not tightly linked. A 1953 study by medical endocrinologist Edward Tyler, for example, showed that the rate of conception improved progressively as sperm counts increased from 70 million up to 200 million per ejaculate, but the conception rate then leveled off.
Recent studies have reinforced these findings. In 2002, a team led by public health researcher Rémy Slama examined the relationship between sperm counts and time to pregnancy in almost a thousand fertile couples living in four European cities. Couples achieved pregnancy more and more rapidly up to a total sperm count of around 200 million, but not beyond that level. Complementing these findings, in 2010 andrologist Trevor Cooper and colleagues examined semen samples from almost 5,000 men in 14 countries spread across four continents to provide reference values for the World Health Organization. For pregnancy to be achieved within a year, a lower limit of some 60 million sperms per ejaculate was identified. Overall, it seems that between 60 million and 200 million sperms are required for normal fertility.
Although we still do not know why this is so, vast numbers of sperms are evidently needed for male fertility. As sperm counts decline, more and more men are now drifting into the zone of 60 million or less, where fertility is surely affected. The striking decrease in semen quality and increasing abnormalities of the male sex organs detected over only fifty years are surely due to environmental factors. Several authors, including Carlsen and colleagues, have speculated that compounds with oestrogen-like activity or similar factors might be implicated. This steroid disruptor hypothesis seems quite plausible.
We badly need to know whether sperm counts are levelling off, or whether there is a serious danger of widespread male infertility in the future. 2012 marked the fiftieth anniversary of Rachel Carson’s widely influential book Silent Spring, warning us of the perils of environmental toxins. Do we really have to wait until somebody pens the sequel, Silent Cribs?
My book, How We Do It, will be released on June 11. You can currently purchase the hardcover and Kindle editions at a pre-publication discount.
References
Carlsen, E., Giwercman, A., Keiding, N. & Shakkebaek, N.E. (1992) Evidence for decreasing quality of semen during past 50 years. Brit. med. J. 305:609-613.
Cooper, T.G., Noonan, E., von Eckardstein, S., Auger, J., Baker, H.W.G., Behre, H.M., Haugen, T.B., Kruger, T., Wang, C., Mbizvo, M.T. & Vogelsong, K.M. (2010) World Health Organization reference values for human semen characteristics. Hum. Reprod. Update 16, 231-245.
Haimov-Kochman, R., Har-Nir, R., Ein-Mor, E., Ben-Shoshan, V., Greenfield, C., Eldar, I., Bdolah, Y. & Hurwitz, A. (2012) Is the quality of donated semen deteriorating? Findings from a 15 year longitudinal analysis of weekly sperm samples. 14, 372-377.
Rolland, M., Le Moal, J., Wagner, V., Royère, D. & De Mouzon, J. (2012) Decline in semen concentration and morphology in a sample of 26 609 men close to general population between 1989 and 2005 in France. Hum. Reprod. 28, 462-470.
Setchell, B.P. (1997) Sperm counts in semen of farm animals 1932-1995. Int. J. Androl. 20, 209-214.
Slama, R., Eustache, F., Ducot, B., Jensen, T.K., Jørgensen, N., Horte, A., Irvine, S., Suominen, J., Andersen, A.G., Auger, J., et al. (2002) Time to pregnancy and semen parameters: a cross-sectional study among fertile couples from four European cities. Hum. Reprod. 17,503-515.
Tummon, I.S. & Mortimer, D. (1992) Decreasing quality of semen Brit. Med. J. 305, 1228-1229.
Tyler, E.T. (1953) Physiological and clinical aspects of conception. J. Am. Med. Ass. 153, 1351-1356.
