Page 49 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

16 SECTION | I General

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FIGURE 1.5 Interrelationships for radioiodine (I 131 ): Intake by cows, cows thyroid and milk and human thyroid. Adapted from Bustad, L.K.,
McClellan, R.O., Garner R.J., 1965. The significance of radionuclide contamination in ruminants. In: Physiology of Digestion in the Ruminant.
Butterworth Inc., Washington, DC, pp. 131 146.

illustrates the results of a controlled study conducted at populations from ingestion of milk following the
the Hanford Laboratories, Richland, Washington, in the Chernobyl accident in 1986 and the Japanese population
early 1960s with I 131 administered to dairy cows (Bustad following the Fukushima, Japan accident in 2011.
et al., 1965). The “old” radiation units of microcuries for
quantifying the amount of radioactive material are used in Toxicokinetics
the scale on the left and the “new” International units of
Becquerls are used in the scale on the right. Rather than The simple schematic rendering shown in Fig. 1.1 can be
contaminate a large quantity of feed, instead the cows used to illustrate several important concepts. First, it is
were given two feed pellets each day, the feed pellets all important to recognize that contrary to common usage,
contained 5 μCi/L (185 Bq) of I 131 on the first day of the exposure and dose are not the same. The exposure envi-
study. The I 131 decayed with a physical half-life of 8.06 ronment is characterized by the concentration of the toxi-
days so the cows ingested less I 131 each day (the straight cant in the media, be it water, air or feed, the quantities
line in the figure). The cows’ thyroids and milk were taken in and the time course of the intake. Dose is the
monitored for I 131 content. Aliquots of the I 131 contami- concentration, over time, of the toxicant and/or its meta-
nated milk were ingested by five volunteers, and their bolites in the various tissues of the subject, whether it be
131
thyroids were monitored for I . As an aside, the calcu- a cow, a human, or a laboratory rat.
lated radiation dose to the thyroids of the volunteers was For example, in Fig. 1.5 the exposure of the cows
131 131
less than what would have been received from a I thy- would be the quantity of I ingested. An example of
roid uptake study conducted for diagnostic purposes. Let dose would be the measured quantity of I 131 in the thyr-
me hasten to note that the control limits for radionuclides oids of the cows and human volunteers that were ingest-
in milk intended for human consumption would be suffi- ing aliquots of the contaminated milk. A more precise
ciently stringent that the radiation dose to the cows would estimate of dose would be the calculated radiation dose in
be much lower than required to produce toxicity in the rem (old radiation unit) or Sievert (new radiation unit).
cows. These results have been used to estimate allowable The information presented in Fig. 1.5 may be viewed
intakes from I 131 contaminated milk and to develop guid- as being a limited toxicokinetic study. The key data
ance for monitoring pasture and hay to control milk con- acquired were the intake of I 131 and the changing concen-
tamination. Data such as presented in Fig. 1.5 were useful tration of I 131 in the cow’s thyroid and milk. The informa-
input to the control of radiation exposure of European tion on the relationship between I 131 intake and milk