Refined Avian Risk Assessment for Chlorpyrifos in the United States - Reviews of Environmental Contamination and Toxicology - page 166

which is the chemical primarily responsible for toxicity (Solomon et al. 2014 ).
The oxon of CPY binds to the enzyme that hydrolyzes the neurotransmitter acetyl-
choline, i.e., acetylcholinesterase (AChE). The resulting accumulation of acetylcho-
line causes overstimulation of cholinergic synapses (Testai et al. 2010 ). Exposure to
CPY in birds can be detected biochemically as reduced activity of AChE in blood
plasma or brain (Parsons et al. 2000 ; Testai et al. 2010 ). Other symptoms of toxic-
ity include loss of mass, ruffl ed appearance, loss of coordination, reduced reaction
to sound and movement, wing droop, prostrate posture, weakness of lower limbs,
lethargy, gaping, salivation, muscle fasciculation, convulsions, and death
(Gallagher et al. 1996 ).
Acute dietary studies have been conducted to determine toxicity in birds.
However, birds tend to avoid treated food with high CPY concentrations (see SI
Appendix 3, Sect. 1.2), which limits the usefulness of studies in which CPY is fed
in the diet to derive dose-response relationships. Five-d GLP (Good Laboratory
Practice) dietary studies yielded LC 50 values of 2,772 mg ai kg −1 diet for northern
bobwhite ( Colinus virginianus ) (Beavers et al. 2007 ), and 1,083 mg ai kg −1 diet for
mallards ( Anas platyrhynchos ) (Long et al. 1991 ). Based on the results of acute, oral
gavage studies, LD 50 s ranged from 5.62 mg ai kg −1 bwt for adult common grackles
( Q. quiscula ) (Schafer and Brunton 1979 ) to 112 mg ai kg −1 bwt for mallard duck-
lings ( A. platyrhynchos ) (Hudson et al. 1984 ). In an 8-wk study, in which adult
mallards were fed CPY, a NOEC (no-observed effects concentration) of 100 mg ai kg −1
diet was observed (Fink 1977 ). Reduced consumption of food and production of
fewer eggs, and overt signs of toxicity, such as ataxia, ruffl ed appearance, weakness
of lower limbs, and lethargy, were observed at the lowest observed effect concentra-
tion (LOEC) of 215 mg ai kg −1 diet or greater concentrations. A reproductive study
in mallard of CPY effects, in which adults were exposed to treated diet for 9-wk
prior to egg laying and for 8-wk during egg laying, reported a NOEC of 25 mg ai
kg −1 diet and a LOEC of 125 mg ai kg −1 diet (Fink 1978a ). A similar study of CPY
effects on reproduction of the northern bobwhite reported a NOEC at the greatest
concentration tested, 125 mg ai kg −1 diet (Fink 1978b ).

2.6 Conceptual Model

A conceptual model provides a written and visual description of possible exposure
routes between ecological receptors and a stressor. The model includes hypotheses
for how a stressor might come into contact with and affect receptors. These hypoth-
eses are derived by use of professional judgment and information available on
sources of exposure, characteristics of the stressor (e.g., chemistry, fate, and trans-
port), ecosystems at risk, and anticipated effects to birds. The conceptual model for
evaluating potential risks to birds from the application of CPY as a fl owable ( Fig. 1 )
product illustrates that the most likely routes of exposure of birds are ingestion of
foliage, seeds, fruits, insects, and drinking water from pools or foliage in the treated
area. For granular CPY (Fig. 2 ), exposure is most likely to be the result of direct
consumption of granules mistaken for grit.