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Could Mercury in Treats Prove Harmful to My Cat?

Christie Long is a small animal veterinarian with 10 years experience in medicine, surgery, and acupuncture. As PetCoach's Chief Veterinarian, she directs a global team of veterinarians fielding inquiries regarding all aspects of pet car...

Humans worry about mercury levels in their seafood, but what about all the fish your cat eats?

Awareness that consumption of foods could lead to mercury poisoning began in the 1950s when people and animals in and near Minamata, Japan, began to exhibit unexplained neurological symptoms, such as tremors, staggering, muscle weakness and seizures. Eventually, the symptoms were traced back to the consumption of fish and seafood from the nearby bay, which served as a dumping ground for the wastewater from a local chemical company. The wastewater contained large amounts of mercury, and thousands of people and animals were sickened and died from the exposure.

Cats that ate the fish from the bay at Minamata were dubbed “dancing cats” because of the symptoms they exhibited. So can seafood and fish in today’s cat foods and treats be dangerous to your cat? Read on to get educated about mercury in cat food.

More: 25 Random Cat Behaviors Finally Explained

How did mercury get into my cat’s food?

Mercury originates from three sources. About 30 percent of mercury in our environment is there as a result of human activities, such as coal burning, iron mining and cement production. Natural sources of mercury account for 10 percent of the mercury in our environment, from volcanoes and geothermal vents. And finally, the majority of the mercury in our environment — a whopping 60 percent — comes from a phenomenon known as reemission, which means mercury that is released into the environment from soil during floods or from burning plant life in a forest fire.

How does mercury in the environment get into cat foods?

It occurs through a process known as bioaccumulation. Bioaccumulation happens when a substance accumulates in the body faster than it can be excreted. Mercury is a great bioaccumulator. Tiny organisms like krill and plankton take in ocean water, which contains mercury from the environment, and it builds up in their bodies. They are then consumed by bigger fish. This process continues on up the food chain, and the relative amount of mercury increases at each level. When cats consume these fish and shellfish, they’re consuming the accumulated mercury in the flesh.

More: 9 of Your Most Pressing Cat Health Questions Answered

Which fish have the most mercury?

Small fish and seafood have, on average, the smallest amount of mercury in their bodies. Large predator fish have the most. A shrimp has about 0.001 parts per million of mercury, salmon has 0.008 ppm and sharks come in close to 1 ppm. Bioaccumulation is responsible for this phenomenon.

So just how much mercury is dangerous?

Suspected mercury toxicity is rare to see in cats, but there are veterinary laboratories that can perform the test to measure mercury levels in the blood. Normal levels are considered to be 0.1 to 0.3 ppm, high are between 0.3 and 5.0 ppm, and toxic levels are between 6 and 20 ppm. It’s when the cat’s blood measures in the toxic range that we would expect to see the types of signs that were seen in the “dancing cats” in the 1950s in Japan.

Are there dangers to your cat having lower levels of mercury in its blood than those levels that are considered toxic? The truth is we don’t know. We’ve tried to pin a number of maladies on mercury exposure, most recently the development of autism in children after vaccination, but so far, no definitive links to any specific illnesses in cats has been established.

How much mercury is my cat getting in its food?

This is a tough question to answer. While we know the levels of mercury in the fish itself, no data exists on the amount of mercury in cat foods and treats. Obviously, it’s likely a lot less, since foods and treats aren’t made solely of fish, but we don’t have exact numbers.

If you know that a shrimp contains 0.001 ppm and the toxic level of mercury in a cat begins at 6 ppm, you can see that it’s highly unlikely that your cat will build up enough mercury in its body to get mercury poisoning from eating fish-flavored cat foods and treats.

More: How Much Does it Actually Cost to Own a Cat?

OK, but I’m still worried about mercury in my cat’s food

And you have every right to be. There are quite a few unknowns in this whole picture, and since we don’t routinely measure mercury levels in cats, we don’t have a good understanding of how much mercury the average cat is walking around with. It’s also possible that we’re actually seeing mercury poisoning in cats from time to time, but because it’s not on our radar screen, we don’t test for it and we miss the diagnosis.

If you want to keep your cat’s mercury exposure as low as possible, don’t feed any whole fish and limit or completely avoid feeding any fish-containing foods or treats. If you give your cat fish oil as a supplement (and it’s a great idea!), buy fish oil that is tested for mercury levels.

What about an occasional treat of tuna? It’s OK as long as you keep it small and occasional. Canned salmon is probably a better choice. And be sure to avoid albacore tuna, since as a bigger fish there are much higher mercury levels present in it.

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