Although mercury in fish and shellfish is a widespread problem, that doesn’t mean you should completely eliminate it from your cat’s diet. Here’s what you need to know about mercury poisoning and how to prevent it.
Mercury poisoning is a big problem, especially when it comes to fish and shellfish. In humans, symptoms of methylmercury poisoning can include impaired peripheral vision, sensory disturbances (“pins and needles”), lack of coordination, muscle weakness, and problems with speech, hearing and walking. Cats do not show such signs, but that does not mean that they are immune to the toxic effects of mercury. A human being would receive some sort of signal that they were getting sick after being exposed to this heavy metal, but a cat would not show any signs of illness unless the exposure was much, much higher.
Chronic exposure to mercury causes two major problems:
- Serious neurological damage in humans.
- Kidney damage in cats and humans. In fact, the kidneys are the main organ in which inorganic mercury is absorbed and accumulated.
Why does fish contain mercury?
Mercury from human pollution ends up in the air, but eventually gets deposited in water or on land where it can be washed out in water. Once there, certain microorganisms transform this heavy metal into methylmercury, a highly toxic form that accumulates in fish and shellfish – and in the animals that eat them. Predatory fish at the top of the food chain can contain very high levels of heavy metals (including mercury), as well as PCBs, pesticides and other toxins. Tilefish (listed on many commercial cat food labels as “lake whitefish”) is among the worst, along with tuna, king mackerel, shark and swordfish. In fact, almost all fish and shellfish contain traces of methylmercury.
Pet food laws are lax
Fish and shellfish found in many cat foods are the main source of mercury exposure in felines. Unfortunately, the health laws for meat and fish in pet foods are much more lenient than they are for human foods. The FDA has not established toxic levels of heavy metals in pet foods. In fact, many pet foods have been found to contain concentrations of toxic metals that far exceed human limit values calculated at animal-sized doses. A study from a few years ago found that mercury levels in cats were five times higher than those typically found in humans!
Kidney disease and mercury
In my 30 years of practice, as mercury levels in fish have increased exponentially, I’ve noticed issues like hyperthyroidism, high blood pressure, and kidney disease in cats have also skyrocketed. .
Compromised kidney function in cats over 10 or 12 years old is epidemic. It might sound a bit complicated, but let’s look at how it all works in your cat’s organs. Mercury irreversibly inhibits selenium-dependent enzymes. Among their many functions, selenoenzymes prevent and reverse oxidative damage in organs. Since the kidney is the primary target organ in which mercury accumulates, I can easily extrapolate that the high fish content in many cat foods contributes to the large number of cases of kidney disease we are seeing in felines today.
The National Center for Health Research warns people with kidney failure that they can be harmed by exposure to mercury when eating fish. Yet many of my clients who have cats with kidney failure entice them to eat by adding tuna to their meals.
What should you do?
The EPA and FDA issue “fish advisories” (see box) for people with mercury-related health concerns, indicating that this health risk is very real. Both organizations advise limiting fish consumption and choosing fish species that are low in mercury. It makes sense to also limit the amount of fish your cat eats.
Many commercial cat foods contain minimal levels of fish in their recipes, either for flavoring or because people like to feed their cats food that contains fish, thinking it’s tastier for them. I recommend that you review cat food labels for the type and amount of fish used.
Although mercury in fish and shellfish is a widespread problem, that doesn’t mean you should completely eliminate it from your cat’s diet. By learning which fish to avoid, which are the safest to feed it, and how often you can treat it, you’ll minimize its exposure to this ubiquitous heavy metal.
