Pesticides Crash Frogs Immune Systems  
UPdate Fall 2002

Frogs exposed to trace amounts of the pesticides DDT, dieldrin and malathion suffered immune system collapse as severe as if they had been treated with the powerful immune suppression drug, cyclophosphamide.

The experiment found that frogs injected with minute amounts of DDT, malathion and cyclophosphamide had only 1-2 % of normal antibody production two weeks after exposure and were unable to fight off simple infections from viruses and bacteria.Frogs injected with dieldrin had 30% of normal antibodies.  It took 20 weeks of living in a pesticide-free environment before the frogs' immune systems returned to normal.

While DDT and dieldrin are now banned in Canada, malathion is widely used on crops and for mosquito control.  Spraying or fogging to kill West Nile Virus carrying mosquitoes is commonly done with malathion.

Brian Dixon, an immunologist at the University of Waterloo who worked on the project, was shocked that such negligible amounts of pesticides (one part per million, or 75 parts per billion) were so biologically active.  "That was the whole take home message," Dixon said. "The pesticides act exactly the same way as a medical drug they will give to suppress your immune system."

Frogs and mammals have the same type of immune system. In human terms, this
could mean that people with impaired immune systems could die of colds or other infections that otherwise they could easily fight off. The researchers say their results could help explain the rise in human autoimmune diseases including allergies and asthma.

Although DDT and dieldrin are now banned, they resist decay and continue to be found in the environment and in human tissues decades after they were widely used.