DDT remains the prime suspect
in declining male fertility
North American and especially
counts are way off the mark. Theories abound on why
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
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
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.