Page 488 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

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VetBooks.ir Chapter 31

Mercury

Ramesh C. Gupta, Dejan Milatovic, Rajiv Lall and Ajay Srivastava

INTRODUCTION BACKGROUND
Mercury (Hg) is a naturally occurring element that exists Mercury exists naturally in the environment (soil, water,
in several forms, such as elemental (metallic), inorganic and air), and as a result everyone is exposed to very low
and organic. About 80% of the mercury released into the levels. Aristotle named it “quicksilver,” because it is a
environment is metallic mercury; it comes from human silver-colored liquid. Animal poisoning by mercury is
activities, such as fossil fuel combustion, mining, smelt- rare because of strict federal, state, and local regulations.
ing and from solid waste incineration, as well as from vol- The most common natural forms of mercury found in the
canoes and forest fires. Human activities can lead to environment are metallic mercury, mercury sulfide, mer-
mercury levels in the soil as much as 200,000 times high- curic chloride, and methylmercury. Methylmercury is of
er than natural levels. Metallic mercury in a pure form particular concern because it is bioaccumulated and bio-
looks like a shiny-white liquid substance at room temper- magnified in certain edible freshwater and saltwater fish
ature. It is commonly used in thermometers, barometers, and marine mammals to levels that are many times
blood pressure measuring devices, button-cell batteries, greater than levels in the surrounding water. As a result,
electric switches, dental fillings (amalgams), etc. older and predatory fish living in contaminated water
Inorganic mercury compounds, or mercury salts, occur build up levels of mercury in their bodies (especially in
when Hg combines with other elements, such as chlorine, the liver, kidneys, brain, and muscle). Inorganic mercury
sulfur, and oxygen. Most of these compounds are white, does not bioaccumulate in the food chain to any extent.
except mercuric sulfide or cinnabar ore (i.e., red, which Cultivation of edible mushrooms, where waste as compost
turns black after exposure to light). Some mercury com- material with unsafe levels of mercury is used, can also
pounds are used as fungicides, while others are used for accumulate high levels of mercury (Bressa et al., 1988).
medicinal purposes, e.g., laxatives, deworming agents, The release of methylmercury into an ocean bay
antiseptics, and disinfectants. When mercury combines (Minamata) in Japan in the 1950s led to a massive health
with carbon, it is called organic mercury (organomercur- disaster, and the clinical syndrome was named Minamata
ials). Methylmercury, ethylmercury, and phenylmercury disease (Tsubaki and Krukuyama, 1977; Tsubaki and
are a few examples. In the environment and mammalian Takahashi, 1986). Thousands of people were poisoned,
systems, various forms of mercury are interchangeable. and hundreds developed severe brain damage.
For example, inorganic mercury can be methylated to Currently, Hg is ranked third by the US Agency for
methylmercury and methylmercury can change to inor- Toxic Substances and Disease Registry (ATSDR, 2011)
ganic or elemental mercury. Animals at the top of the Substance Priority List and is listed as one of the WHO’s
food chain tend to bioaccumulate methylmercury in their top 10 chemicals of major public health concern (Ray
bodies. Therefore, poisoning by mercury is due to con- et al., 2014). The Food and Drug Administration (FDA)
sumption of meat or grain contaminated with mercury. estimates that, on average, most people are exposed to
Poisoning can also result from excessive exposure to inor- about 50 ng mercury/kg body weight/day in the food they
ganic and organic mercury compounds from misuse of eat. This level is not enough to cause any harmful effects.
mercury-containing products. Much of the information A large part of this mercury is in the form of methylmer-
presented in this chapter is from experimental studies con- cury and the majority of that comes from eating fish. Fish
ducted in laboratory animals and poisoning incidents in for food consumption is not allowed to have more than 1
humans, animals and wildlife. ppm mercury. This level is below a level that can be

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