DDT remains the prime suspect in declining male fertility

North American and especially European sperm
counts are way off the mark. Theories abound on why

By Owen Dyer

Widespread suspicions that male fertility is falling appear to have been confirmed by news from Britain, where a British Fertility Society meeting in Liverpool heard this month that men's average sperm count has fallen by nearly a third since 1989. Researchers say their findings, based on 16,000 semen samples taken from 7,500 men who attended the Aberdeen Fertility Centre, "cause concern." They say that the average sperm count has fallen from 87 million sperm per millilitre in 1989 to 62 million in 2002 -- a 29% drop. According to the lead researcher, Dr Siladitya Bhattacharya, "There has been an increase in men seeking treatment for male infertility... but whether this is due to a significant increase in this condition or because men are more aware of new techniques we cannot say." His group plans further studies to test the quality as well as the quantity of sperm in these samples.

The rate of decline is similar to reports from France and other European countries, although sperm counts among British men are even below the European average. It's generally agreed that sperm counts are falling twice as fast in Europe as in North America.

The problem first attracted serious attention in 1992 when Danish research in the British Medical Journal suggested a worldwide decline of about 1% a year. In 1995 a study of 1,300 fertile semen donors in Paris, published in the New England Journal of Medicine, found that sperm counts had fallen by an average of 2% each year in the previous two decades. The incidence of testicular cancer has been rising steadily. There is also strong evidence of an increase in hypospadias (when a male child is born with an abnormal penis) and cryptorchidism (when the testes fail to descend into their normal position).

The past year has produced a number of possible explanations for these phenomena. The January 1 issue of the American Journal of Epidemiology carried a Scandinavian study that found lower sperm counts and smaller testes among men whose mothers smoked during pregnancy. The American Society for Reproductive Medicine's 2003 annual meeting was told of studies linking marijuana to lower sperm count and lead pollution to poorer sperm motility. Another team, from Berkeley, found that sperm motility declines faster than previously thought as men age. But none of these theories could account for the scale of the decline. The same meeting heard that obesity correlates with low sperm count. But if obesity were the root cause of the problem, the US would have seen a faster decline than Europe.

One of the most interesting hypotheses appeared in the Journal of Epidemiology in 2002. Authors proposed an evolutionary mechanism for falling sperm counts. In previous centuries, they said, men with good reproductive health tended to have more children than those with lower sperm counts. Nowadays in the West, most men stop at two or three children for socioeconomic reasons, irrespective of sperm count. Natural selection, they argued, has therefore relaxed its vigilance against poor reproductive health in developed countries.

The theory is elegant but apparently wrong. When French researchers tested it against actual birth data, they found that changes in fecundity could account for only a small part of the problem. Some other mechanism is at work.

The prime suspect is the insecticide DDT (dicophane). At the end of WWII it was hailed as a wonder weapon against a range of diseases, especially malaria. A chemical in the insecticide called p,p'-DDE has strong estrogenic and anti-androgenic properties. Wherever it is found in high concentrations, there is evidence of demasculinisation. Florida's Lake Apopka, with extremely high levels of DDT pollution, is swimming with androgynous alligators.

Other problematic chemicals are alkyl phenol ethoxylates and nonylphenol ethoxylates widely used in industrial detergents, paints and pesticides as surfactants. Agricultural pesticides are clearly implicated and go a long way to explain the geographical variation of male fertility. Within Europe it is isolated Finland, with its minimal agriculture, that has the highest sperm counts.

Although DDT has been banned or restricted for two decades in the developed world, its persistence means that it can still be traced in all humans. It is still in widespread use in many malarial zones, from where it can be exported via food or the atmosphere. Little or no action has been taken to restrict other estrogenic compounds in the environment. The problem of falling sperm counts is bound to get worse before it gets better.