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.
