Page 489 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

456 SECTION | V Metals and Micronutrients

VetBooks.ir associated with adverse effects. Foods other than fish that ride and mercuric chloride) are absorbed 10% 40% from
Inorganic mercury compounds (e.g., mercurous chlo-
may contain higher levels of mercury include wild ani-
the GI tract upon ingestion, distributed to different organs
mals, birds, and mammals (bears) that eat large amounts of
contaminated fish (ATSDR, 1999). Meat and/or fat from and mainly accumulate in the kidneys. In an experimental
fish, marine mammals, fish-eating wildlife and birds, and study, female Sprague-Dawley rats given a single dose of
mercury-based fungicide-treated grains have the highest mercuric chloride (7.4 or 9.2 mg Hg/kg, po) showed 12.6
mercury levels. Certain species of commercially available and 18.9 ppm mercury, respectively, in the kidneys when
saltwater fish, such as shark, swordfish, kingfish and tile- sacrificed 14 days postexposure (Lecavalier et al., 1994).
fish, can contain high levels of methylmercury. These are Trace amounts were also detected in the liver, brain, and
all potential sources of mercury poisoning. In horses, mer- serum. These compounds do not readily cross the BBB or
cury toxicity occurs from wound dressings (blisters) when placental barrier. Inorganic mercury excretes in the urine
dimethyl sulfoxide (DMSO) is applied simultaneously, and feces, and only detectable levels pass through the
because DMSO enhances the absorption of mercury milk.
(Schuh et al., 1988). Organic mercury, such as methylmercury, is readily
absorbed from the GI tract (about 90% 95%) and has a
long retention time (half-life of B70 days). After inges-
TOXICOKINETICS tion, the distribution to the blood compartment is com-
plete within 30 h, and the blood level accounts for about
Absorption of mercury from oral ingestion depends upon 7% of the ingested dose. Methylmercury distributes to all
the form of mercury. Metallic mercury is maximally vital organs (Fig. 31.1). Circulating methylmercury accu-
absorbed (about 80%) from the lungs, while very little is mulates predominantly in the red cells where it binds to
absorbed from the gastrointestinal (GI) tract. Once mer- cysteinyl residues ( SH) on the hemoglobin beta-chain,
cury enters the circulation, it is rapidly distributed to other and is then slowly distributed to other tissues, reaching
tissues, but more so in the kidneys, where it accumulates. equilibrium with other tissues at B4 days. The distribu-
Metallic mercury can stay in the body for weeks to tion of methylmercury is similar to that of metallic mer-
months. Due to its high lipophilicity, metallic mercury cury, i.e., a relatively large amount of mercury can
can readily cross the blood brain barrier (BBB) and pla- accumulate in the brain and fetus (compared to inorganic
cental barrier. When metallic mercury enters the brain, it mercury) because of its ability to penetrate the BBB and
is readily converted to an inorganic divalent mercury (oxi- placental barrier and its conversion in the brain and fetus
dized by the hydrogen peroxidase catalase pathway), and to the inorganic divalent cation mercury. Organic mercury
it gets trapped there for an extended period. The inorganic excretes in the form of inorganic mercury in the feces
divalent cation can, in turn, be reduced to metallic mer- over a period of several months. Some organic mercury
cury. Most of the absorbed metallic mercury excretes in also excretes in the urine and milk.
the urine and feces, some amount passes in the milk, and Depending upon the route of exposure, dose, and single
very little in the exhaled air. versus repeat exposure, toxicokinetics of mercury can

FIGURE 31.1 The distribution of MeHg in the human
body. MeHg is readily absorbed by the lung, skin, and
gastrointestinal tract and distributed to the CNS, kidneys,
and liver. MeHg can cross the placental blood barrier
and it accumulates in the fetus at higher concentrations
compared to the mother.