Page 444 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

P. 444

VetBooks.ir Chapter 23

Arsenic

Tam Garland

INTRODUCTION metal, the particle size, the purity, the solubility, the spe-
cies affected, and the physical condition of the animal
Arsenic is a ubiquitous element with several different
exposed. Susceptibility to inorganic arsenicals varies
forms. The toxicity of arsenic is determined by its form
among species and is highest in humans, followed by
(Vahter, 1983). The prevalent valences are the 13 and the
dogs, rats, and mice (Harrison et al., 1958; Hays, 1982).
15 form. Arsenic is found in both an organic form and
Clearly there are many variables affecting the absorption
an inorganic form with valence numbers ranging from and toxicity of this metal, which increases the difficulty
13
13to 15. As , or arsenite, is more toxic than arsenate, of making accurate predictions of lethal amounts.
15
or As .
Pentavalent organic arsenicals are more easily
It is found as different ores and rocks, which are
absorbed than are the trivalent arsenicals, especially
mined, then smelted resulting in elemental arsenic and
through the GI tract. Small amounts of either form may
arsenic trioxide. In the environment, arsenic usually exists
be absorbed via the intact skin, but it usually remains
in the pentavalent form and soil microorganisms may
locally within the skin. However, absorption is limited by
methylate it. Since it is ubiquitous in many forms, com-
the size of the arsenical particle size. If the particle size is
plete avoidance is nearly impossible.
too large, it is not absorbed. Hence, a more toxic arsenical
Arsenic’s sources and uses have a long and varied his-
that is not absorbed because of large particle size may
tory. A partial list is available in Table 23.1.
effectively be less toxic.
Once arsenicals are absorbed, the distribution is through
BACKGROUND the blood to all the organs of the body. Arsenic accumu-
lates in the liver and is slowly distributed to the other tis-
The ubiquitous element arsenic (As) is a nonmetal or met- sues. The spleen, kidneys, and lungs are able to accumulate
alloid in group V of the period chart. Frequently it is large amounts of arsenic. Arsenic has been shown to cross
referred to as arsenic metal and is classified for many tox- into the placental barrier, particularly in monkeys, ham-
icological purposes as a metal. It exists in several forms sters, and gerbils (Flora and Agrawal, 2017). Chronic doses
with a long history of various uses. It has been used in are stored in the bone, the skin, and other keratinized tis-
preparations from insecticides, wood preservatives, herbi- sues, such as skin, hair, hooves, and nails (Agency for
cides, and medicinal uses. It is responsible for many poi- Toxic Substances and Disease Registry, 2007).
sonings in people and animals, both large and small. The biotransformation of the arsenicals is poorly
understood. There is some conversion from the 15 state
to the 13 state, but the redox equilibrium favors the 13.
PHARMACOKINETICS/TOXICOKINETICS
Methylation occurs by microorganism in the soil, but
Different toxic disease syndromes are caused by the dif- inorganic arsenicals are also methylated in vivo. The
ferent forms of arsenic. Inorganic arsenicals and trivalent in vivo process may aid in the detoxification process. The
organics cause disease syndromes characterized by an kidneys may reduce a small amount of pentavalent arsenic
effect on the gastrointestinal (GI) tract and the capillaries. to the more toxic trivalent form.
In extremely low doses, it is possible the body will Arsenicals are excreted through many processes. In
develop a tolerance to the arsenic. Pentavalent organic most species, between 40% and 70% of the absorbed
arsenicals produce a neurological syndrome. amount of pentavalent arsenicals are excreted through the
There are many factors influencing the absorption of urine within 48 h (Vahter, 1983), while smaller quantities
arsenicals. Among those variables are the form of the may be excreted through the sweat. Trivalent forms of

439 440 441 442 443 444 445 446 447 448 449