Page 147 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

114 SECTION | I General

VetBooks.ir pesticides are more commonly associated with foods of range of conditions expected to occur in practice, includ-
ing application methods, seasons, cultural practices, and
plant origin and it is the use of plants and plant products,
crop varieties. If residue levels in the processed commodity
such as grain, in the production of animal feed that is usu-
ally the source of residues. Organophosphates, pyre- exceed the residue levels in the raw agricultural commodity
throids, and carbamates are relatively quickly degraded by a margin sufficient to require an MRL higher than the
and therefore less likely to be found in high concentra- raw agricultural commodity MRL, it is necessary to esti-
tions if a long period of time has lapsed after application. mate a MRL for the processed commodity. The pesticide
In the EU, legislation that limits pesticide residues in feed residue dietary burden for livestock is derived from super-
is listed beside those for all undesirable substances. vised residue trials for feed commodities multiplied by
Persistent organic pollutants and, as such, residues can be standard animal diets based on OECD livestock feed tables.
found in the environment and can also be present in ani- Estimated maximum residue levels as well as HRs found
mal feed ingredients. Where they are used according to in the supervised trials and supervised trial median residues
GAP, residues of these pesticides should not exceed (STMRs) derived from external animal treatments are com-
MRLs, which are set on the basis of a toxicological RA pared with those derived from exposure through the feed.
and in consideration of what is achievable by best prac- The recommended MRLs, HRs, and STMRs are based on
tices (i.e., correct application rates and minimum harvest whichever values are higher from this comparison.
intervals). Estimates of chronic exposure are based on the STMRs
During drought conditions, the feeding of potentially from the supervised trials and food processing studies and
contaminated crop byproducts, such as stubbles and fod- long-term food consumption. For short-term exposure
der, and processed fractions, including grape marc, citrus assessment, estimates of high intake of pesticide residue on
pulp, fruit pomace, and cannery wastes, is likely to a single day are based on the HRs from the supervised
become more prevalent. In all cases, chemical residues trials (WHO, 2015). Substances such as abamectin, cyper-
may appear in the edible tissues, milk, or eggs derived methrin, alpha-cypermethrin, cyfluthrin, cyhalothrin,
from these animals. Some regulatory bodies also now rou- deltamethrin, diflubenzuron, emamectin benzoate, thiaben-
tinely consider the need to set an ARfD for all pesticides dazole, and treflubenzuron are used both as veterinary
it evaluates. The suggested numerical cut-off for setting drugs and as pesticides.
ARfDs for pesticides is 5 mg/kg bw; consequently, if cal-
culations indicate that an ARfD value would be greater
than this value, then it would not be necessary to set an
BIOCIDAL SUBSTANCES USED IN ANIMAL
ARfD (WHO, 2015). This ARfD is not necessary for HUSBANDRY
compounds with low acute toxicity value.
The approach adopted for establishing these MRLs is Biocidal substances used in animal husbandry are those
fundamentally different from that which applies to veteri- used for the purposes of caring for and rearing food-
nary drug residues. Animal transfer studies, which allow producing animals, and to which food-producing animals
for a determination of the relationship between the level are exposed during some stage of their life. However, bio-
of chemical in the animal diet and the concentration of cidal substances are used in many different situations and
residue found in edible tissues, milk, and eggs, are pivotal their residues may potentially enter the food chain as a
in determining MRL. MRL for animal tissues, milk, and result of a number of these uses (including exposure of
eggs are established at concentrations that cover the high- plants, exposure of food-producing animals, and contami-
est residues (HRs) expected to be found from the esti- nation of food commodities). For instance, a pesticide is
mated livestock dietary exposure. In animal production defined as a plant protection product (Regulation (EC)
systems, compliance with animal commodity MRL relies No. 1107/2009) (EC, 2009a), and biocidal products are
on adherence to a stipulated period to allow residues in defined as active substances and preparations containing
the crop to deplete prior to the commencement of animal one or more active substances put up in the form in which
feeding, a stipulation period to allow residues in the ani- they are supplied to the user, intended to destroy, deter,
mal to deplete prior to slaughter, or a combination of both. render harmless, prevent the action of, or otherwise exert
MRLs for pesticide residues and residues of veterinary a controlling effect on a harmful organism by chemical or
drugs are the maximum concentrations of residues to be biological means (Article 1(a) of Directive 98/8/EC (EC,
permitted in or on a food. Livestock and crop metabolism 1998a)). Biocidal products protect us against pests and
studies are the prime determinants of the residue defini- other harmful microorganism (e.g., insect repellents, dis-
tion in food and feed commodities. The recommended infectants). Biocidal product types include those intended
MRLs in various crops depend mainly on the data from to control vertebrates; however, the actual use of such
supervised residue trials conducted in line with maximum types might give rise to concern. Where a biocidal prod-
registered uses within GAP. These trials should cover the uct is identified as an insecticide, acaricide, rodenticide,