Combining
Several Pesticide Ingredients Increases Neurological Damage
Military
personnel operating in the Persian Gulf region between August
1990 and April 1991 have complained of numerous neurological
complications after returning home. Several theories have been
expressed regarding the occurrence of these problems. One
definite fact of exposure to potentially neurotoxic compounds
is that soldiers used large amounts of the chemicals DEET (a
personal insect repellent) and the pesticide permethrin.
Soldiers were also given the drug pyridostigmine bromide (PB)
to protect against possible nerve gas attack.
Because
of this concern, scientists at the Neurotoxicology Division at
the Environmental Protection Agency (EPA) and three other
Universities conducted a long term study to determine the
effects of combining these chemicals and then observing their
effects upon the nervous system of test animals.
The
study was conducted by exposing four groups of five hens each
to either one or several of these chemicals and at different
doses. Researchers continued the exposure daily to the animals
for approximately 60 days. During this period investigators
looked for various signs of health problems including
neurotoxicity.
Results
showed animals treated with only PB revealed no difference
between controls upon neuropathological examinations
(investigations of tissue samples under a microscope). Some
animals treated with permethrin or DEET exhibited minor
neuropathological changes that consisted of a small increase
in the frequency of slightly enlarge axons. (Axons are the
main connections between brain cells which send information).
Regarding
the effects of combining the chemicals, the researchers stated
that the animals treated with the DEET/permethrin combination
developed "hyperexcitability" between 1 and 4 weeks
of dosing. In one animal, a mild gait disturbance was
detectable at 27 days that progressed to a stumbling and
unsteady gait accompanied by moderate fine body tremors.
Microscopic examination of spinal cord and sciatic nerve found
mild neuropathological alterations in two of the animals
treated with permethrin/DEET which included a significant
increase in both the frequency and degree of enlargement of
the axons.
One
point of neurotoxic interest - even though the dosages of
permethrin were well below the amount needed to kill the
animals, it was found that when permethrin was combined with
DEET it created brain damage within the test animals that is
"similar to those observed following near lethal doses of
permethrin."
In
conclusion the scientists stated
"This
study demonstrates that concurrent administration of any two
compounds of PB, DEET, and permethrin results in
neurotoxicity that is markedly greater than that resulting
from treatment with any individual compound. ....Both DEET
and permethrin have been shown to produce tremors and
hyperexcitability in experimental animals (Ambrose et al.,
1959; Schoening et al., 1993).... In addition, these
findings suggest the need for additional studies into
potential health risks associated with coexposure of humans
to these agents at dosages likely to have been used by the
Gulf War veterans.
Generating
further concern for the pesticide permethrin, is that
research has found the chemical undergoes a biological
transformation within the human body by what is called
esterase and oxidase inhibitors (for the benefit of our
scientifically minded), thereby creating a new chemical
which researchers say,
"may
create unanticipated hazards by enhancing pyrethroid
toxicity to mammals.(4)"
In
other words, they are saying that this chemical is most
likely more dangerous than tests with animals show since the
human body can change the chemical into an entirely new and
even more toxic chemical."
DEET
is an aromatic amide used as a personal insect repellent
against mosquitoes, biting flies, and ticks, among other
insects. It has been used since 1946 by the U.S. Army and
since 1957 by the general population.
Approximately 30% of
the U.S. population uses DEET as a lotion, stick, or spray
at concentrations between 10 and 100% active ingredient.
Extensive and repeated topical applications of DEET
resulted in human poisoning including two deaths.
Symptoms
of poisoning are characterized by tremor, restlessness,
slurred speech, seizures, impaired cognitive functions,
and coma (McConnell et al., 1986).
DEET has been found to
be efficiently absorbed through the skin (Windheuser et
al., 1982; Spencer et al., 1979). Also, regarding the use
of permethrin, this chemical is currently used in schools
and homes for general insect or termite treatments.
Erroneously, pesticide applicators state that this
chemical is the same as pyrethrin (which is a natural
pesticide made from the chrysanthemum flower). This is
very inaccurate. Although the molecules are somewhat
similar, they are still very different and pyrethroid
pesticides such as permethrin are showing increased
evidence in the medical research of a variety of
neurological and immune system damaging effects (3).
Karl
F. Jensen
Neurotoxicology Division
U.S. Environmental Protection Agency, Research Triangle
Park, NC
Journal of Toxicology and Environmental Health, 48:35-56,
1996
COMMENTS:
Although pesticides are tested for many health effects such
as cancer, skin irritation, fatality risk, and major birth
defects, they are currently not required to be tested for
subtle neurological effects such as with memory, learning
problems or effects upon behavior.
