Organophosphorus and Carbamate Insecticides - Organic Pollutants: An Ecotoxicological Perspective - page 209

Although inhibition of brain AChE is a valuable biomarker assay for identifying
lethal and other toxic effects of OPs in vertebrates, it needs to used with discretion.
The degree of inhibition of brain AChE associated with lethality varies between
species and compounds. Sometimes cholinergic effects appear to be more important
at extra-cerebral sites, including the peripheral nervous system, than they are in the
brain itself. Also, postmortem changes can occur in the field to confound analy-
sis; loss of enzyme activity and reactivation of the inhibited can occur postmortem.
Identifying OPs as the cause of lethal intoxication in the field is made easier if typical
symptoms of ChE poisoning are observed prior to death.
The emphasis has thus far been on lethal effects of OPs in the field. These have
been much easier to recognize than sublethal ones. The latter are much harder to
detect; but the very fact that animals die from poisoning is a good indication that
they will have experienced sublethal intoxication beforehand. After individuals have
taken up lethal doses, they inevitably pass through a stage when effects are sublethal
before they enter the final stage of lethal intoxication (Figure 10.5). Also, in the field
there will be a range of exposures, from high doses that will eventually prove lethal
to lower doses that will have some effects upon the animals, from which they later
recover. In one study of woodpigeons ( Columba palumbus ) that had been exposed
to grain dressed with chlorfenvinphos in eastern England (Cooke 1988), birds were
found that behaved abnormally, were uncoordinated, and were reluctant or unable to
fly. On examination of some of these birds, substantial residues were found of chlor-
fenvinphos in their crops and gizzards (50-170 mg/kg). Some of the birds that were
affected in this way recovered and eventually flew away. Examination of brains of
other birds that had displayed severe symptoms of poisoning revealed 83-88% inhi-
bition of AChE. This and other studies bear witness to the occurrence of transitory
sublethal effects when birds and mammals have been exposed to OP compounds in
the field.
Inevitably, terrestrial invertebrates are susceptible to the toxicity of OPs used in
the field. The honeybee is one species of particular importance, and the use of OPs
and other insecticides on agricultural land has been restricted to minimize toxicity
to this species. One practice has been to avoid application of hazardous chemicals
to crops when there are foraging bees. The use of some compounds, for example,
triazophos, has been restricted because of very high toxicity to honeybees.
As noted earlier, OPs are known to be highly toxic to aquatic invertebrates and
to fish. This has been demonstrated in field studies. For example, malathion applied
to watercress beds caused lethal intoxication of the freshwater shrimp Gammarus
pulex located downstream (Crane et al. 1995). Kills of marine invertebrates have
been reported following the application of OPs. Accidental release of OPs into riv-
ers, lakes, and bays has sometimes caused large-scale fish kills (see Environmental
Health Criteria 63).
10.2.5.2 Population dynamics
Some OPs are prime examples of pollutants that are highly toxic but of low persis-
tence, and serve as useful models for other compounds of that ilk that have been less
well investigated. Because of their limited persistence, toxic effects are expected to
be localized and of limited duration. As the compounds degrade quickly in tissues,