Chemistry Reference
In-Depth Information
Considering population effects of neurotoxic pollutants more generally, persis-
tence is clearly an important factor. With pollutants of short biological half-life,
effects will tend to be transitory, whereas persistent pollutants are likely to produce
longer-lasting behavioral disturbances. Thus, the environmental risks presented by
recalcitrant OCs such as dieldrin and DDT would appear to be greater than those
presented by readily biodegradable OPs, carbamates, or pyrethroids, from the point
of view of neurotoxic and behavioral effects. The use of persistent OCs has now been
largely discontinued, their global sales being estimated at only 2.1% of all insecti-
cides in 2003 (Nauen 2006). Thus, interest in them is mainly retrospective. However,
the use of nonpersistent neurotoxic pesticides is still widespread. OPs, carbamates,
pyrethroids, and neonicotinoids accounted for 24.7%, 10.5%, 19.5%, and 15.7% of
global sales of insecticides, respectively, in 2003 (Nauen 2006). Taken collectively,
this represents some 70% of all insecticide sales during that year. So, questions
remain because little is known about the importance or otherwise of sublethal neuro-
toxic and behavioral effects or consequent population effects that these compounds
may be having in the natural environment.
16.8 concLudIng remarkS
There is much evidence that neurotoxic pollutants, mainly pesticides, have had both
lethal and sublethal effects upon free-living vertebrates in the natural environment.
Lethal effects have, for obvious reasons, been much easier to recognize than sub-
lethal ones. At the same time, the mere fact that neurotoxic compounds have caused
mortality is, in itself, clear evidence that there must have been sublethal effects as
well, although the latter were seldom recognized at the time. As has been shown in
many well-designed studies, there are a variety of readily measurable neurotoxic
and behavioral effects in the early stages of poisoning by OCs, OPs, carbamates,
pyrethroids, and neonicotinoids before the onset of symptoms of severe poisoning
and death. Animals dying from poisoning in the field would have shown these symp-
toms in the early stages of intoxication, as in the case of birds and mammals show-
ing convulsions before succumbing to dieldrin poisoning in field incidents during
the late 1950s and early 1960s. By contrast, there would have been many cases of
individuals experiencing lower exposures and showing early symptoms of poisoning
but not receiving high enough doses to kill them outright. Such individuals may have
recovered completely and gone on to lead “normal” lives, or the sublethal effects
may have had harmful consequences in the shorter or longer term by reducing ability
to feed, breed, or avoid predation.
With recent advances in biochemical toxicology, incorporating new techniques of
molecular biology, it is now possible to develop better mechanistic biomarker assays
that will facilitate the identification and quantification of the different changes in the
sequence of events that underlie neurotoxicity. In this respect, medical toxicology is
much further advanced than ecotoxicology. However, techniques developed for the
former should be applicable to the latter. Microarray assays to monitor changes at
the level of the gene can run alongside assays to show changes at the cellular level
(e.g., interaction with sites of action, electrophysiological responses). Appropriate
combinations of assays can give an in-depth picture of the operation of this causal
Search WWH ::
Custom Search