Babies and toddlers of California farmworkers exposed to the insecticide DDT have neurological effects that are severe enough in some cases to slow their mental and physical development, according to research by UC Berkeley scientists published today.
The federally funded research involving the children of women who recently emigrated from Mexico to the Salinas Valley is the first in the United States to indicate that the pesticide harms human brain development.
"This suggests that ... DDT has effects that no one even thought to test for back when it was in use," said Dr. Walter Rogan, an epidemiologist with the National Institute of Environmental Health Sciences. He was not involved in the new study, published in the journal Pediatrics.
Because DDT was banned more than 30 years ago in the United States and most developed countries, the findings have particular relevance for the ongoing, controversial use in Africa to combat malaria.
UC Berkeley scientists measured levels of various pesticides in 360 pregnant women, nearly all of whom were born in Mexico, and tested the mental and motor skills of their infants and toddlers, who were born in the Salinas Valley.
For every tenfold rise in DDT exposure, the children's scores on mental tests dropped 2 to 3 points. Their motor skills were also reduced. In the worst cases, the highest DDT doses were associated with a 7- to 10-point drop in the mental scores of 24-month-old children compared with those who were not exposed.
Those drops are significant, because the average score in the study was 86 at that age and anything below 85 indicates a developmental delay and potential learning disability. The tests measure the children's ability to learn and think, including memory and problem-solving skills.
"If you had a whole population with a downward shift like this, you'd be seeing more kids with developmental problems," said Brenda Eskenazi, a UC Berkeley professor of maternal and child health and epidemiology, who directed the project.
The Salinas Valley women had very high exposures, eight times higher than average levels in the U.S. population reported recently by the Centers for Disease Control and Prevention. They were probably exposed in Mexico, because most of them had lived in the United States for less than five years. Mexico allowed the use of DDT on farms until 1995 and for mosquito control until 2000. All uses in the United States ended in 1972.
"These women probably received very little exposure while here in the U.S.," said Asa Bradman, associate director of UC Berkeley's Center for Children's Environmental Health Research and coauthor of the study.
Virtually every human body on Earth still carries traces of DDE, a compound formed as DDT breaks down. But the effects in the Salinas study were mostly associated with DDT, rather than the DDE that is found in most animals and people.
Rogan said that means that the babies' brain development was mostly affected by relatively new spraying, not the residue that remains in the environment from spraying decades ago.
"This finding is mostly relevant to the current debate about new use of DDT, or any place that still uses DDT, and is less important to places with historical use," said Rogan, who studied DDE's effects on children in the 1980s.
"The take-home message," he said, "is that this is not an entirely benign compound even though the great advantages of its use when you're saving lives with effective malarial control are very important."
Under a United Nations pact, the Stockholm Convention, DDT is used only for killing mosquitoes that transmit malaria, which claims nearly 1 million children and pregnant women in sub-Saharan Africa annually. President Bush's year-old Malaria Initiative, the new chief of the World Health Organization's malaria program and some environmental groups support continued use of DDT as one of many strategies until safer options are found.
In Africa, small amounts are squirted on interior walls, unlike the broadcast spraying of the 1940s and 1950s that contaminated most of the world's food, soil and wildlife.
Eskenazi and her colleagues caution in their new report that "the benefit of using DDT to control malaria should be balanced carefully against the potential risk to children's neurodevelopment. Whenever possible, alternative antimalarial controls should be considered, especially in areas where pregnant women and children may be exposed."
Nobody knows if the effects found in the Salinas toddlers will persist. The UC Berkeley team plans to study the same children until they enter school.
"It remains to be seen whether it's a lasting effect or not," Rogan said.
Because DDT and DDE are so similar, scientists were surprised that DDT seemed to harm brain development while the other had little impact. That suggests that the mental abilities of U.S. children born between World War II and the early 1970s -- when DDT was routinely sprayed -- could have been affected, but not those born years later and exposed to old residue in the environment.
Nevertheless, the researchers reported that breastfeeding is beneficial to babies even when the milk contains large doses of DDT. The children's test scores increased with every month of nursing even for the most contaminated mothers.
Doctors know that breastfeeding boosts a baby's intelligence, yet they have long wondered if contaminants in the milk erase that benefit. The new study "provides additional evidence that breastfeeding may help to compensate for the subtle perinatal insult associated with DDT/DDE exposure," the authors wrote. The insecticide's damage probably occurs in the womb, not during breastfeeding.
The researchers tested the women for other pesticides, but only DDT was connected to neurological effects.
The study is part of a federally funded UC Berkeley project that assesses whether agricultural chemicals in the Salinas Valley, one of the world's most intensely farmed areas, are harming children.
Although animal tests have repeatedly shown that DDT causes neurological damage, the new study is the first in the United States to find such effects in humans.
In North Carolina and New York, large studies in the 1980s and 1990s detected no effect on babies' mental abilities, but they tested for DDE, not DDT. A smaller study in Spain did report some neurological effects.
The insecticide, which mimics estrogen, also affects reproduction. California women exposed in the womb are more likely to experience delays in getting pregnant decades later, according to a 2003 UC Berkeley study. Again, the effect was predominantly found with DDT, not its older residue.
Developed as an insecticide in 1939, DDT was popular because it killed insects but wasn't acutely poisonous to people or animals. But by the 1950s, it was accumulating in food chains, nearly wiping out eagles and other birds.
Canadian scientists recently reported that DDT still contaminates farm soils and will seep into the air for another generation.