Mercury Poisoning in Cats – Causes, Symptoms, And Treatment (Vet Answer)
By Dr. Sharon Butzke, DVM (Vet)
Mercury is a heavy metal found everywhere in the environment. It exists in several forms:
- Elemental mercury: found in some thermometers
- Inorganic mercury salts/compounds: used in industry and production of certain chemicals
- Organic mercury (e.g., methylmercury): known to bioaccumulate through food chains, especially in fish
Cats are very sensitive to the effects of methylmercury in particular.
Since exposure to elemental and inorganic mercury is not very common in cats, this article will focus on mercury poisoning caused by chronic exposure to methylmercury.
What Causes Mercury Poisoning in Cats?
Historically, methylmercury poisoning has been observed in cats who consumed large amounts of contaminated fish. You may have heard of the “dancing cats” of Minamata, Japan, whose symptoms resulted from large amounts of mercury waste being dumped into Minamata Bay by a petrochemical plant in the 1950s (thousands of people were also affected). There was another report of cats with Minamata Disease in Ontario, Canada in the 1970s.
Several recent studies (like this one) have raised concern about methylmercury levels in commercial pet foods. The practical application of this information is currently limited, however, because:
- It is not known how much of the mercury detected is bioavailable (i.e., able to be absorbed by the body)
- Veterinarians do not frequently test cats for methylmercury exposure, so we don’t know if elevated levels in cat food are causing elevated levels in cats
- There is no regulation of mercury levels in pet food in the United States, so there is no direct course of action to improve the safety of commercial diets
Chronic exposure to methylmercury in cat food certainly has the potential to result in toxicity. However, we do not currently have clinical evidence to show that this is actually happening.
What Are the Signs of Mercury Poisoning in Cats?
Symptoms of mercury poisoning in cats are primarily due to damage of the nervous system (including the brain), because this is where methylmercury tends to accumulate in the body. The kidneys are also frequently affected, as well as unborn kittens (mercury crosses the placenta).
Signs may include:
- Ataxia (general incoordination)
- Tremors or convulsions
- Unusual behavior
- Exaggerated gait (hypermetria)
- Vision loss
It can take several weeks for methylmercury to build up in the body to a level where signs of toxicity become apparent.
Can Mercury Poisoning Be Treated?
Unfortunately, there is no specific antidote for chronic methylmercury toxicity. Treatment involves providing supportive care and preventing further exposure. There is currently no evidence to show that chelation therapy (which has been used in cases of acute poisoning caused by inorganic mercury salts) is helpful in these cases.
The damage to organs caused by methylmercury is irreversible, and the prognosis for severely affected patients is poor. Cats who do survive may have permanent neurologic impairments and reduced kidney function.
Other than the specific cases mentioned in this article, there are very few reports of confirmed methylmercury toxicity in cats. Symptoms can be similar to other neurologic conditions, so veterinarians may not consider testing for methylmercury and mild cases may be going undiagnosed. Therefore, we have little information about the overall prevalence of methylmercury toxicity in cats and whether mildly affected patients are surviving.
How Can I Protect My Cat From Mercury Poisoning?
While the risk of mercury poisoning is probably very low for most cats, there are some steps you can take to help keep your cat as safe as possible. These may be particularly important for pregnant cats and kittens:
- Limit your cat’s intake of predator fish (e.g., tuna) that are known to contain higher levels of mercury (consult this chart for safer choices)
- Check local fish consumption advisories before sharing wild-caught fish with your cat
- Consider purchasing cat food from companies who voluntarily perform rigorous quality control of their products, including testing for heavy metals and other toxins
At this time, fish oil supplements do not appear to be a significant source of mercury.
Hopefully, future research will continue to investigate methylmercury levels in commercial cat food and determine whether they are a significant cause for concern.
Development of regulations dictating maximum allowable levels of mercury in pet food would be a good step towards ensuring our cats’ food is as safe as possible.
It may also be helpful to determine whether non-invasive methods of testing methylmercury levels in cats (such as fur sampling) could be used for widespread monitoring.
Featured Image Credit: BeataGFX, Shutterstock