Page 48 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

Concepts in Veterinary Toxicology Chapter | 1 15

VetBooks.ir use, require the development of new risk-based safety
standards, and provide increased public transparency for
chemical hazard/risk information. During the first year
after passage of the Lautenberg amendment, over 300
chemical reviews were completed by the USEPA, more
than in any comparable period in the past. It will be of
Percolation
interest to see if the Lautenberg amendments accelerate Deposition
the validation and use of in vitro (nonanimal systems) to on water
Deposition
evaluate the toxicity hazards and risks of new chemicals Runoff on ground
entering commerce.
Deposition
on food
and feed Eating
A FRAMEWORK FOR ACQUIRING vegetables
Irrigation
INFORMATION
Drinking
Linkages From Sources to Health Impacts milk Soil
Inhalation
ingestion
Eating
The purpose of this section is to provide a conceptual
fish Dermal
framework for using information to evaluate specific Drinking absorption
cases of actual or alleged toxicosis and to facilitate the water
acquisition of new knowledge that will have an impact in
Uptake by
understanding potential toxic effects. Earlier, in Fig. 1.1,
biota
a conceptual framework was provided for evaluating the
linkages extending from a source of a toxic material to
manifestation of an adverse health outcome in an individ-
ual or a population. The conceptual framework is equally FIGURE 1.4 Schematic rendering illustrating exposure pathways
applicable to humans or other animal species. extending from a source of toxicants to exposure of livestock and people.
The source to exposure linkage has been expanded in From Paustenbach (2008).
Fig. 1.4 (Paustenbach, 2001). In this example, an indus-
trial plant is illustrated as the source. The figure serves to ecosystem may be impacted as individuals. Moreover,
illustrate the complex nature of the exposure pathways natural populations may be impacted. In addition, these
that may be encountered including the role of livestock. pathways may ultimately result in the toxicant reaching
The focus in the figure is on the multiple pathways by people. An example is mercury in fish. In practice, veteri-
which a potential toxicant may reach a human population: narians may encounter situations where poisonous plants
inhalation, drinking water, dermal absorption, ingestion of in the pasture or in harvested forage may be the source of
soil, and ingestion of a variety of foodstuffs including the toxicant. Feed may be contaminated at a mill or pro-
milk and meat from domestic animals. All of these path- cessing plant and serve as the pathway by which a toxi-
ways might also serve to expose the cow in the figure to cant reaches the livestock. A recent case in Europe
the toxicant. Obviously, the quantities of the toxicant involves contamination of eggs by a pesticide used to
taken in by the single cow could also cause toxicity in a control lice in chicken houses. In other cases, the potential
herd of cows if the combination of exposure intensity and human toxicant may be a pharmaceutical agent purpose-
duration were sufficient. Equally as important is the role fully given to the cow with the pharmaceutical appearing
of the cow as a pathway for the toxicant to reach people. in the milk or in the tissues of the cow if it were
For example, the figure illustrates that a toxicant could be slaughtered and, thus, available for human consumption.
present in cow’s milk and the milk could be consumed by Likewise, the “contaminated” materials may be used to
people. The cow could also be slaughtered and the meat produce products intended for consumption by other ani-
ultimately consumed by people. Thus, it is important to mals such as pet food.
recognize that the cow, or any other food animal species, The utility of using the conceptual approach in
can both manifest toxic effects and serve as a pathway for Fig. 1.4 can be illustrated using the transfer of a specific
toxicants to reach people via the food supply. radionuclide, I 131 , from the diet of cows to the cow’s milk
It is readily apparent that the schematic rendering to intake by humans and thyroid uptake. This situation
shown in Fig. 1.4 can be expanded or contracted. In natu- was of concern when nuclear weapons were being tested
ral ecosystems, multiple species might be involved as a in the atmosphere resulting in world-wide fallout and for
toxicant moves from a source or multiple sources via releases from reactor accidents in England (Windscale),
various pathways. In some cases, various species in the Ukraine (Chernobyl), and Japan (Fukushima). Fig. 1.5