SOURCES

Mercury is a metal that exists in several physical and chemical forms. It occurs naturally in the earth’s crust as inorganic mercury. Through a natural process of degassing from the earth’s surface, it is released into the atmosphere as mercury vapor.¹ Mercury also enters the environment through volcanic activity.²

Human activity has generated sources of environmental mercury since Roman times, when the mining of mercury ore began.¹ Today, coal burning power stations and municipal waste incinerators liberate mercury vapor when household and commercial products that contain mercury are disposed of by this method. Elemental mercury can be found in fluorescent light bulbs, batteries, thermometers, barometers, lubrication oils, dental amalgams, and pressure-sensing devices. Inorganic mercury can be found in topical blistering agents, stool fixatives, latex paints, skin-lightening creams and soaps, laxatives, and old disinfectant and pesticide formulations.

From whatever source, mercury vapors emitted into the atmosphere can reside there for many months and be distributed over vast geographical areas.¹ In fact, mercury contamination is a worldwide problem.

Once in the atmosphere, mercury vapor is slowly transformed to a water-soluble, inorganic form.¹ Water-soluble inorganic mercury is then precipitated back to the surface and accumulates in fresh and marine water bodies by direct contamination, and through runoff into rivers and streams. Inorganic mercury is methylated to organic mercury by methanogenic bacteria, which are present in both fresh and marine waters.¹

The methylated, or organic, form of mercury could potentially pose a threat to both humans and companion animals, such as dogs and cats. Methylated mercury readily enters the aquatic food chain by first diffusing into forms of zooplankton. Bottom-feeding fish consume zooplankton. Bottom-feeding fish are eaten by other fish, and mercury continues up the aquatic food chain reaching highest concentrations in the largest, long-lived carnivorous fish in both fresh and marine waters.¹ In this manner, mercury is biomagnified from environmental contamination of water approximately 1 million-fold by the time it reaches the top of the aquatic food chain.¹

Fish-based diets therefore represent a potential source of mercury exposure for dogs and cats. There are many commercial and prescription veterinary diets that contain fish. Fish oil supplements are also frequently used in therapeutic regimens for a variety of conditions for dogs and cats. However, recent work has shown that commercial human grade fish oil supplements contain very low concentrations of mercury, ranging from less than 6 μg/L to 12 μg/L.³

Another potential dietary mercury source could occur when food animals are inadvertently fed seed grain that has been treated with a methylmercury fungicide. These types of fungicides have been used on seed destined for planting, but not for seed used as feed. When the meat from these exposed animals is fed to pets, severe mercury poisoning could occur. This type of scenario has been reported in a family that ate the meat from a hog that was accidentally fed treated seed grain. The family members who ate the meat over a 3-month period developed long-term neurological effects.⁴

Another potential source of mercury may be from ingestion of folk medicines. Nontraditional remedies may sometimes be used by animal owners on their pets and should be considered when taking a history in cases with unusual presentations. Some folk medicines contain inorganic mercury compounds, and some Latin American and Caribbean communities have used elemental mercury in religious and cultural rituals.⁵ Animals may be exposed intentionally or inadvertently.

Inhalation of elemental mercury vapors could conceivably happen in pets where mercury has contaminated a household through broken thermometers, barometers, or fluorescent light bulbs. Poisonings could also potentially occur in pets ingesting products containing the inorganic mercury forms, such as mercuric chloride (e.g., stool fixatives, latex paint, laxatives, old disinfectants and pesticides, and blistering agents).

TOXIC DOSE

When determining exposure doses in pet animals, it is important to consider whether the exposures have been acute or chronic. This information can be correlated with data from the primary literature, as both acute and chronic dosing studies have been performed in cats. Cats given oral doses of methylmercuric hydroxide at 1.29 and 0.86 mg/kg/day began exhibiting clinical signs after 15 days of daily exposure.⁶ Chronic doses of methylmercury at 0.25 mg/kg/day produced clinical signs between 76 and 100 days of daily dosing.⁷ In this chronic study, a total dose of between 19 and 25 mg/kg was required before clinical signs were observed.⁷ In a long-term diet study using fish contaminated with methylmercury, cats did not develop clinical signs or damage to the central nervous system (CNS) when doses were below 46 μg/kg/day, even after 1175 to 1227 days.⁸ Clinical signs referable to the nervous system and notable pathological findings in the CNS were found in cats dosed with 176 μg/kg/day for 92 to 119 days, and in cats dosed with 74 μg/kg/day after 276 to 526 days.^8,9

It is possible for the veterinary clinician to estimate the mercury exposure from fish diets. Edible tissues from swordfish and shark can contain 1 μg/g of mercury, whereas tuna, trout, pike, and bass can contain about 0.1 to 0.5 μg/g. Invertebrates, such as shellfish, have even lower concentrations.³ Unfortunately the specific fish and concentrations included in commercial pet foods are usually not known.

Because there are several possible sources of mercury exposure in companion animals, it is difficult to determine the toxic dose from chronic, possibly lifelong, exposure. Background exposure levels to pet animals do tend to mimic human exposure rates in certain geographical regions.⁶ So if this information is known from regional public health data, it may be possible to estimate potential exposure levels.