Although mercury toxicosis in animals has declined, it is important to recognize that it still occurs and to differentiate between acute and chronic forms of toxicosis. The toxicology of mercury is complicated because various forms can affect animals across species. Mercury is toxic by inhalation, ingestion, and topical exposures; oral exposures can also occur through feed because of bioaccumulation in certain feedstuffs. Because mercury is associated with certain feed ingredients, different risk factors are associated with species and production class in the case of livestock. Treatment of both acute and chronic toxicosis is challenging. Therefore, efforts should be focused on prevention and cessation of exposures once they occur. Clinical signs can affect multiple body and organ systems, depending on the form of mercury as well as the level and frequency of exposure.
Historically, mercury toxicosis was a common occurrence in both humans and animals. The replacement of mercury products with alternatives for medicinal, agricultural, and industrial purposes has resulted in a decline in acute and chronic toxicosis cases.
Although human and animal exposures to mercury vary, case reports of human exposures can provide insight into other ways in which pet animals, like dogs and cats, may also be exposed:
In 2017, a child was exposed to harmful concentrations of mercury at a home day care center where an older sphygmomanometer used to measure blood pressure broke and released mercury into the day care environment (1).
In a separate care report in 2014, an Iowa man developed acute mercury toxicosis while attempting to extract gold and silver from computer components in an unventilated, do-it-yourself smelting operation in his home (2).
In 2018, a 17-month-old child was exposed to an unlabeled artisanal skin-lightening cream containing inorganic mercury (3).
And in 2020, three siblings presented with inhalational elemental mercury exposure after mercury was spilled in the home and subsequently vacuumed, leading to inhalational toxicosis in all three children (4).
In each of these case reports, animals could have been similarly exposed in these environments. Therefore, clinicians should consider the possibility of these types of exposures in dogs and cats when other sources of exposure have been ruled out.
Despite an overall decline in cases of mercury toxicosis, many wildlife species remain at risk from environmental exposures. Predator species considered to be near the top of the food chain (eg, swordfish, seals, polar bears, and various bird species) bioaccumulate considerable quantities of mercury from dietary sources. In fact, mercury can bioaccumulate a million-fold from the lowest single-cell organisms that are exposed to elemental mercury in the silt of bodies of water to the highest predators (eg, sharks, king mackerel, and tilefish). Commercial fish-based dietary products have been associated with chronic toxicosis in cats, and long-term chronic dietary exposure limits have been well described.
Mercury exists in a variety of chemical forms:
elemental mercury (eg, older thermometers, fluorescent light bulbs, barometers)
inorganic mercurial (mercuric or mercurous) salts (eg, batteries, older latex paints)
organic mercury (aryl, methyl, or ethyl)
In the US, button cell batteries contain mercury. Button cell batteries are in the shape of a coin or button and are commonly used in watches, cameras, digital thermometers, calculators, and toys. While these do not pose a significant risk of mercury exposure while in use, dogs are known to bite into or ingest these batteries when they are left in their environment, and consequently, these batteries could pose a mercury exposure risk (5).
Fossil fuels represent an important environmental source of mercury. In the environment, inorganic forms of mercury are converted to methylmercury under anaerobic conditions in the sediment of most bodies of water. Similar conversions can also occur in the body.
Although industrial sources of environmental mercury contamination occur, mercury also exists naturally. Elemental mercury is degassed from minerals in the earth's surface, where it circulates globally in the outer atmosphere. After a period of 1 to 2 years, this source of elemental mercury is deposited back to the surface of the planet through weather events, eventually reaching bodies of water and settling into the sediment. There it is methylated and ingested by microscopic organisms, eventually traveling up the food chain to the highest predators. Therefore, regardless of the impact of industrial contamination, global contamination occurs at a baseline level.
Pathogenesis of Mercury Toxicosis
The physical, chemical, and kinetic properties of the various forms of mercury play an important role, influencing the clinical manifestations, the extent and nature of lesions, and the tissue distribution of mercury. Organic forms of mercury, primarily methylmercury, are lipid soluble and well absorbed orally. Consequently, bioaccumulation is extensive in tissues such as the brain, kidney, and fetus. Methylmercury interferes with metabolic activity, resulting in degeneration and necrosis in many tissues; however, the brain and fetus are more susceptible.
In the brain, histologically, neuronal degeneration and perivascular cuffing are evident in the cerebrocortical gray matter, and cerebellar atrophy or hypoplasia and Purkinje cell degeneration are present. Encephalomalacia, myelin loss, and axon necrosis might also be evident.
Methylmercury is mutagenic, carcinogenic, embryotoxic, and highly teratogenic. The inorganic forms of mercury, including elemental mercury, are poorly absorbed after skin exposure. Elemental mercury vapors are inhaled and rapidly absorbed. This highly toxic form of mercury produces corrosive bronchitis and interstitial pneumonia. All forms of mercury cross the placenta. Inorganic forms of mercury bind to sulfhydryl groups in enzymes and other thiol-containing molecules such as cysteine and glutathione. Tissues rich in these components, such as the renal cortex, accumulate notable concentrations of mercury. Inorganic forms of mercury are cytotoxic and highly corrosive. Consequently, these forms of mercury cause severe inflammation, ulcers, and tissue necrosis in the GI tract. Pale, swollen kidneys manifested histologically as tubular necrosis and interstitial nephritis are consistent findings.
Clinical Findings of Mercury Toxicosis
Toxic mercury exposures can occur through inhalation, ingestion, and topical application; however, ingestion of mercury is perhaps the most important route of exposure for animals. Differentiating between high-dose acute exposures and low-dose chronic exposures is important when anticipating the range of clinical effects that might be observed.
Acute inhalation of corrosive elemental mercury vapors at high concentrations produces severe dyspnea and compromised respiratory function that is usually fatal. Neurological manifestations can eventually develop at lower levels of exposure, especially if exposure continues at a low level.
Due to its corrosive nature, inorganic mercury produces primarily GI signs (including anorexia, stomatitis, pharyngitis, vomiting, diarrhea, and pain), as well as shock, dyspnea, and dehydration. Death often occurs within hours at high levels of exposure. Animals that survive can exhibit eczema, skin keratinization, anuria, polydipsia, hematuria, or melena. Neurological manifestations, including CNS depression and excitation similar to those that occur in cases of organic mercury toxicosis, can develop after chronic exposure.
Depending on the level of exposure to organic mercury compounds such as methylmercury, clinical manifestations can take days to develop. Because these compounds are not corrosive, GI signs do not occur. Common neurological manifestations of exposure to organic mercury compounds include blindness, ataxia, incoordination, tremors, abnormal behavior, hypermetria, nystagmus (cats), and tonic-clonic seizures. Advanced cases may be characterized by depression, anorexia, proprioceptive defects, total blindness, and paralysis, with high mortality. The nervous system of young, developing animals is particularly susceptible to organic mercury exposure, and this is frequently manifested as cerebellar ataxia associated with cerebellar hypoplasia and death.
Diagnosis of Mercury Toxicosis
Clinical signs
Evidence of exposure
Confirmation of mercury exposure includes analysis of source materials, whether exposure was through feed or from the environment. Both acute and chronic dietary concentrations have been described in various species, including cats, cattle, pigs, and poultry (6).
For cats, daily dietary exposures less than 0.3 ppm methylmercury are considered subtoxic (7). Various chronic exposure concentrations have been studied in cats, with a cumulative methylmercury dose of 20 mg/kg body weight associated with chronic toxic lesions, regardless of the time frame of exposure (7). Exposures at this concentration can take months to years to occur, particularly through the diet. Acute exposures over 1 mg/kg methylmercury result in acute toxicity in cats (8). Chronic toxicity in cats generally results in neurological and renal effects.
The considerable variation associated with clinical signs related to the various forms of mercury and the duration of exposure underscores the need for repeated tissue analyses. Because inorganic forms of mercury are excreted in the urine, urinary mercury concentrations are the most reliable indicator of exposure, especially during acute and ongoing exposures. In contrast, organic mercury compounds, which bioaccumulate in soft tissues, are most appropriately assessed in liver, kidney, or brain tissues.
In most species, blood, kidney, brain, and feed concentrations of mercury < 0.1 mg/kg (wet weight) are considered normal. When toxicosis is suspected, concentrations > 6,000 mcg/L (whole blood), 10,000 mcg/L (kidney), 500 mcg/L (brain), and 4 mg/kg (feed, dry weight) are consistent with a diagnosis of mercury toxicosis (9).Generally, mercury toxicity can be confirmed by testing mercury levels in postmortem biological tissue samples, ideally kidney and/or liver tissue. Concentrations in all tissues can be substantially higher after chronic exposure.
Pearls & Pitfalls
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Marine mammals and fish often contain substantially increased concentrations of mercury that might not be associated with clinical disease, which contrasts with toxic syndromes observed in mammals. This makes it difficult to eliminate certain fish from the diet on the basis of physical observation alone.
A minimum diagnostic database that includes urinalysis, BUN, creatinine, and hemoglobin/hematocrit is important for detecting proteinuria, azotemia, or nonregenerative anemia and can provide useful evidence to support a diagnosis of mercury toxicosis. When elemental mercury has been ingested, abdominal radiographs may confirm presence within the GI tract. The definitive diagnosis may be made on the basis of analysis of tissue specimens with appropriate histological and clinical testing, history, and clinical signs.
Differential diagnoses include conditions that produce GI distress, renal disease, or neurological dysfunction manifested by tremors, ataxia, or seizures. Metals such as lead, arsenic, thallium, or cadmium; insecticides, including organophosphate, carbamate, and organochlorine compounds; oxalates; vitamin D; mycotoxins such as T-2 toxin and ochratoxin; and thiamine deficiency should be considered. Infectious diseases, including hog cholera, erysipelas, and feline panleukopenia, can resemble mercury toxicosis.
Because mercury exposure can be due to environmental exposures via inhalation, ingestion, or cutaneous contact, animals have been used as sentinels for exposure to mercury and other heavy metals, such as lead. This type of sentinel exposure was evident during the Chisso Corporation dumping of mercury-containing waste into local waterways in and around Minamata, Japan, during the 1950s and 1960s. In this chronic exposure over many years, cats developed clinical signs related to mercury toxicity before humans who were exposed similarly in the same environment (10).
While environmental mercury exposures are rare today, they still occur, particularly near mining operations, and companion animals might serve as sentinels of exposure well before clinical disease is seen in humans.
Treatment and Control of Mercury Toxicosis
Not recommended in food-producing animals
Exposure reduction
Adsorption, chelation, antioxidant therapies
Because the neurological and renal damage resulting from mercury toxicosis is irreversible, treatment can be ineffective. Consequently, the prognosis for a complete recovery is very poor. In food-producing animals, considerable mercury accumulation in tissues intended as food and profound effects on reproduction limit treatment options. Euthanasia and appropriate disposal, in consultation with regulatory officials, is recommended.
In veterinary patients in which treatment is indicated, oral administration of activated charcoal (1–3 g/kg, PO, every 4–8 hours as needed) may bind mercury and limit absorption after acute intoxication (11). Vitamin E and selenium, which are antioxidants, may limit oxidative damage. Chelation therapy can be useful if treatment is started soon after exposure, before nephrotoxic effects become severe. The lipid-soluble chelator dimercaprol (2.5–5 mg/kg, IM, every 4 hours for 2 days, then every 12 hours until recovery) may be beneficial (12). For organic mercury toxicosis, 2.3-dimercaptosuccinic acid (10 mg/kg, PO, every 8 hours for 5–10 days) has been useful in dogs (13, 14) and cats (15).
If GI tract decontamination has been successful, administration of penicillamine (110 mg/kg, PO, every 24 hours for 1–3 weeks) might decrease clinical signs in cattle (16). Limiting the consumption of mercury-contaminated food (such as fish products) or water will lessen exposure.
Comprehensive guidelines reflecting the serious nature of mercury toxicosis have been established for humans and animals by the WHO and many individual countries. The water guidelines in most countries for mercury are 0.001 mg/L and 0.003 mg/L for humans and animals, respectively (17).
Key Points
Diagnosis of mercury toxicosis should be confirmed through analytical chemistry. Ideally, chemical confirmation should be performed on biological tissues (ie, kidney, liver when available), GI tract contents, and environmental or feed source materials.
The poor prognosis and lack of universally effective treatment options remain concerns globally.
Because of the potential for contamination of the food supply, attempted treatment in food-producing animals is not recommended.
Companion animals may serve as sentinels for environmental mercury exposure before toxic effects are evident in people. Therefore, veterinarians should alert public health officials upon making a diagnosis of environmental mercury toxicosis in animals.
For More Information
Beck AC, Lash EM, Hack JB. Environmental toxic exposures using companion animals as an indicator of human toxicity: a case report and discussion. J Emerg Med. 2020;59(1):e1-e7.
Mercury. EPA.
Mercury in Food. FDA.
Mercury. Agency for Toxic Substances and Disease Registry.
Mercury: Properties and Health Effects. Occupational Safety and Health Administration. US Department of Labor.
Also see pet owner content regarding mercury poisoning in animals.
References
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Koirala S, Leinenkugel K. Home gold and silver smelting—Iowa, 2014. MMWR Morb Mortal Wkly Rep. 2015;64(49);1365-1366. doi:10.15585/mmwr.mm6449a4
Ori MR, Larsen JB, Shirazi F. Mercury poisoning in a toddler from home contamination due to skin lightening cream. J Pediatr. 2018;196:314-317.e1. doi:10.1016/j.jpeds.2017.12.023
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Environmental Protection Agency. Mercury in batteries. Updated April 8, 2026. Accessed May 27, 2026.
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Puls R. Mineral Levels in Animal Health: Diagnostic Data. 2nd ed. Sherpa International; 1994.
Grandjean P, Satoh H, Murata K, Eto K. Adverse effects of methylmercury: environmental health research implications. Environ Health Perspect. 2010;118(8):1137-1145. https://pmc.ncbi.nlm.nih.gov/articles/PMC2920086/
DeClementi C. Prevention and treatment of poisoning. In: Gupta RC, ed. Veterinary Toxicology: Basic and Clinical Principles. 4th ed. Academic Press; 2025:1199-1215.
Peterson ME, Talcott PA, eds. Small Animal Toxicology. 3rd ed. Saunders; 2013.
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