Globe and Mail
Wednesday, April 24, 2002
By MARTIN MITTELSTAEDT
Frogs given trace amounts of DDT and other pesticides
experience a near total collapse in their immune
systems, a finding that could help explain the
puzzling rise in human autoimmune diseases such as asthma and allergies,
according to Canadian researchers who made the discovery.
The scientific team also says the work could shed light on
the global decline in amphibians, animals that may no
longer have strong enough immune systems to survive
exposures to everyday viruses and parasites.
The pesticides had an effect on frogs identical to
cyclophosphamide, a drug used on human transplant
recipients to suppress their immune systems so they
don't reject their new organs.
Frogs and mammals essentially have the same type of immune
system, so the finding could have widespread
implications for humans, who also have elevated
"There is a lot of immune dysfunction going on that
could be due to environmental chemicals," said
Brian Dixon, an immunologist at the University of
Waterloo who worked on the project.
Mr. Dixon said he was "shocked" that negligible
amounts of pesticides were so biologically active and
could mimic one of the most powerful drugs in the
arsenal of modern medicine. "That was the whole take-home message," he
said. "The pesticides act exactly the same way as a
medical drug they will give to suppress your immune
In recent decades, frog species have inexplicably died out
in many habitats in which they were once plentiful,
while in other areas they have been suffering from
horrific physical defects, such as growing extra limbs.
The findings on immune-system impairment have received
peer review and will be published in a research
journal later this year.
In laboratory experiments, the team injected northern
leopard frogs - shiny, brown-green amphibians common
in Canada's swamps and forests - with tiny, sublethal
doses of DDT, dieldrin or malathion. For comparison purposes,
some other frogs were given the immune-suppressing drug cyclophosphamide.
DDT and dieldrin, two deadly insecticides on the United
Nations' list of the most toxic substances ever
produced on Earth, have been banned in Canada, but
they resist decay and continue to be found throughout wildlife in
Canada and in human tissues decades after they were in widespread use.
Malathion is still widely used on crops in Canada and for
mosquito control. It is often sprayed from planes in
large-scale efforts to control mosquitoes, such as
recent programs to control the West Nile virus in New York
City and to knock down populations of the insect in Winnipeg.
Health Canada's Pest Management Regulatory Agency is
reviewing its registration for malathion, while
regulators in the United States have recently given it
a clean bill of health for mosquito control.
The pesticide research project was funded by Health Canada
and Environment Canada.
The experiments found that frogs injected with DDT,
malathion or cyclophosphamide had only 1 per cent to 2
cent of normal antibody production, while dieldrin led
to 30 per cent of normal production, two weeks after
exposure. It took frogs 20 weeks of living in a pesticide-free environment
to have their immune systems return to normal.
In human terms, impaired immune systems could lead to
people dying of common colds or other infections that
a healthy person would be able to resist easily. Frogs
live in bacteria- and parasite-infested environments, and
consequently may not be able to shake illnesses because of their weakened
immune systems, according to the research.
In their experiments, the researchers also tested wild
leopard frogs from a number of locations in Ontario
and found major differences in their immune systems,
depending on their exposure to pesticides.
Specimens collected near Point Pelee National Park in
Southwestern Ontario had weaker immune systems,
compared with those from regions of the province, such
as around Collingwood, less polluted by agricultural chemicals.
Point Pelee is a DDT hot spot because a children's camp in
the park was once heavily sprayed to kill mosquitoes
and it lies near one of Canada's largest
concentrations of farms.
The research project was prompted partly to investigate
the mysterious disappearance of leopard frogs in Point
Pelee, to see if it was linked to DDT.
Many experiments use exceptionally high chemical doses -
hundreds of times normal environmental exposures - to
cause deleterious effects, but the frog tests were
conducted with doses of less than one part per million, similar
to the DDT levels found in many animals.
The dose of one ppm is minute - the equivalent of the
passage of only one minute of time over a two-year
After making their discoveries, the researchers started
giving frogs ever smaller doses of the chemicals to
find the level that does not have an adverse effect.
With DDT, even as little as 75 parts per billion, a small fraction
of what was used in their original experiments, caused immune-system
In the 1970s, as DDT use in North America was winding
down, concentrations of the chemical in human breast
milk ranged from 50 parts per billion to 200 ppb.