Refined Avian Risk Assessment for Chlorpyrifos in the United States - Reviews of Environmental Contamination and Toxicology - page 181

is determined by randomly sampling from a binomial distribution defi ned by N
(number of particles ingested that could be either granules or natural sand) and p .
This calculation is made separately for medium- and coarse-sized pesticide granules,
and for spatial zones of the fi eld that differ from one another in either the relative
availability of granules or relative use by birds (e.g., end rows, fi eld margin, fi eld
center). The number of particles the bird obtains from a given zone ( N ) is estimated
from the zone's relative size and use by birds.
The version of GranPARAM described herein estimated exposure to 20 birds on
each of 1,000 fi elds, as previously described for LiquidPARAM. The 1,000 fi elds
were intended to represent the range of soil characteristics for the crop and region
of interest. In GranPARAM, characteristics of birds such as grit counts in their
gizzard and proportion of time they forage in treated fi elds are chosen randomly
from distributions. Thus, individuals on fi elds differ from one another and the model
is designed to explicitly incorporate the variation observed in nature. Similarly,
characteristics of fi elds such as differences in size distribution and composition of
soil are randomly chosen from distributions in GranPARAM. This approach ensures
that the variability in fi eld soils observed in nature is refl ected in the model.
The outputs from the exposure portion of GranPARAM are estimated acute
doses for each of 20 birds on each of 1,000 fi elds. The effects and risk components
of GranPARAM and LiquidPARAM for acute exposure are the same. The risk
output from GranPARAM is a bar chart showing the percentages of fi elds with 0/20
dead birds, 1/20 dead birds, 2/20 dead birds, etc.
The components of the model and input variables for GranPARAM are described
in detail in SI Appendix 3. Simulations were run for each of the exposure scenarios
listed in Table 3 . All simulations were carried out using Latin Hypercube Sampling in
Oracle Crystal Ball (2009), Version 7.3.2 with 1,000 trials (i.e., fi elds) per simulation
and 20 birds per fi eld.

5 Effects Assessment

Upon ingestion, CPY is rapidly absorbed and undergoes oxidative metabolism to
the oxon form, which is the metabolite primarily responsible for toxicity (Testai
et al. 2010 ). Chlorpyrifos inhibits acetylcholinesterase activity causing acetylcho-
line to accumulate at nerve terminals and neuromuscular junctions, which leads to
cholinergic overstimulation (Testai et al. 2010 ). In birds, CPY poisoning can be
detected biochemically as reduced cholinesterase activity in plasma and brain
tissues (Cairns et al. 1991 ; Parsons et al. 2000 ; Timchalk 2010 ). A gavage study
with northern bobwhite ( C. virginianus ) found reduced brain cholinesterase activity
at concentrations of 47 mg ai kg −1 bwt of CPY and greater (Cairns et al. 1991 ).
Cholinesterase activity remained inhibited for at least 24-48 h thereafter.
The following sections present a review of the available acute and chronic effects
studies and the derivation of the acute and chronic effects metrics that were used to
characterize risks to birds from the use of CPY.