[back] Polio and DDT
Toronto Globe and Mail
24 April 2002
Study finds DDT may spur disease
By MARTIN MITTELSTAEDT ENVIRONMENT REPORTER
Wednesday, April 24, 2002 - Print Edition, Page A1
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 rise in human autoimmune diseases such as asthma and
allergies, Canadian researchers say.
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 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 implications for humans, who also have elevated
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
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
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
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
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
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
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
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