Photo by D. Sillman
Every time I write about cats my email responses go up. Many people love cats and also love to think about their evolution, behaviors, and ecology. I came across a very interesting article in the New York Times a few months ago (May 16, 2017) that talked about the hyperthyroid epidemic that is being seen in “senior” cats (cats over ten years old). The ideas and inferences from this article are really worth considering.
The facts: prior to the 1970’s feline hyperthyroidism was a very rare condition. Today, though, 10% of all senior cats are hyperthyroid. Symptoms of hyperthyroidism include weight loss (a wasting syndrome) with no apparent change in eating habits (often, in fact, appetite and food consumption increases). The cat is restless, has a rapid heart rate, and has poor coat quality. It is not a set of symptoms that a cat owner could ignore!
The question is, why has this change occurred in cats? What is causing this epidemic of hyperthyroidism? Many factors were explored as possible causes: there seemed to be a correlation between types of cat foods and the incidence of hyperthyroidism, and there were suggestions that chemicals in cat litters or in flea control medications might be having a stimulatory effect on the cats’ thyroid glands. The strongest correlation, though, the most consistent factor regularly seen in hyperthyroid cases was the amount of time the cat spent indoors. Feral cats and barn cats, for example, did not show this trend toward hyperthyroidism. What was inside of houses and apartments that might be causing this endocrine reaction, and what might this feline hyperthyroid trigger be doing to the rest of us?
Figure by Leyo and Rhododenrdon Busch, Public Domain
The answer turned out to be the presence of a chemical called polybrominated diphenyl ether (or PBDE, for short). PBDE’s were developed in the 1970’s as fire retardants, and they were quickly incorporated into many household items (rug pads, furniture cushion fabrics, electronic devices (like television sets, etc.) and many types of plastics). The addition of these PBDE’s made homes and apartments safer and less likely to flash into flames during a fire. Widespread use of these chemicals, though, soon led to some serious problems.
Photo by D. Sillman
One problem is that PBDE’s are incredibly resistant to degradation. They tend persist in any environment to which they are introduced or in any subsequent environment into which they travel. A second problem with PBDE’s is that they are fat soluble and, thus, will bio-accumulate in any organism exposed to low levels of the chemicals either through ingestion or inhalation. Certain fatty foods (like salmon, cheese, ground beef and butter) can have high levels of PBDE’s as can breast milk (leading to great concerns over PBDE exposure in infants)! The third problem with PBDE’s is that they act as endocrine stimulators which can disrupt control of body’s homeostasis. These disruptions can involve the reproductive system (reduced fertility), the adrenal glands (one of my old students, Kirk Dineley, in fact, is working on PBDE impacts on adrenals!), and the thyroid! Thyroid disruption, in particular, is of great concern in infants and children because of the role that thyroid hormone plays in the development of brain tissue. Experiments in mice have shown that early exposure to PBDE’s can permanently alter brain activity and behaviors and can even cause hyperactivity. Studies in humans have suggested that IQ scores may be reduced from 5 to 8 points after early life exposure to PBDE’s.
Photo by M. Hamilton
But how do the PBDE’s that are intermixed with the polymers and other complex chemicals in the household items get into the people (and cats) that inhabit those homes? It turns out that PBDE’s don’t stay attached to their surrounding molecules. They tend to break loose and because they are so resistant to degradation float out into the household ecosystem in which they reside and accumulate in the dust. Contact with the dust of a household in which PBDE infused products reside, then, leads to inhalation or ingestion of PBDE’s (the Times article estimated that children ingest 200 mg of household dust each day!).
Cats and small children seem to be particularly pre-adapted to dust exposure and PBDE bioaccumulation. They both crawl about on the floor, rub and lounge on cushions and furniture, and both put potentially dust laden objects and body parts into their mouths.
Testing hyperthyroid cats has shown that they tend to have 20 to 100 times higher levels of blood PBDE’s than controls. Certain cat foods (especially seafood flavored canned foods) have very high levels of the fat soluble PBDE’s. Also, examination of thyroid samples taken from 7000 autopsies of cats conducted over the past decades showed that prior to the late 1970’s thyroid tumors were exceedingly rare in cats, but since 1979 they have become increasingly common.
Photo by M. Hamilton
Almost every person that has been tested has had detectable levels of PBDE’s in their blood, and children, very disturbingly, regularly have higher PBDE levels that adults! These chemicals may be causing a variety of fairly subtle brain changes and also endocrine changes in our population. High levels of PBDE’s have also been correlated with increased incidence of thyroid cancer, and thyroid cancer cases in the United States have been rising at a rate of 3.8% per year over the past ten years according to the National Cancer Institute. The good news is that blood levels of PBDE’s in humans seems to have peaked back in the early 2000’s and have been declining after these chemicals were effectively banned from use. The persistence of PCBE’s in the environment, though, means that older products will continue to be a PBDE source until they are removed from our ecosystems!
Now, the changes in human IQ levels and behavioral syndromes (not to mention the later in life occurrence of thyroid cancer) may be hard on their own to obviously link to PBDE’s in the home. These human impacts, though, when linked to the very clear cause and effect observations of hyperthyroid cats, describe a very frightening (and preventable) story of self-pollution and self-poisoning! Our cats are acting as sentinels, as canaries in our metaphorical coal mine warning us of the impact of these persistent chemicals on our health and well being.
Photo by D. Sillman
Let’s take care of our cats (veterinarians are developing treatments for our senior hyperthyroid felines), and immediately dispose of all of those PBDE laden products in an environmentally responsible way. Remember, PBDE’s will persist and travel out through our environment! They will bio-accumulate in any species unlucky enough to encounter them (everything from peregrine falcons to beluga whales!) and cause extensive endocrine disruption and disease! We can’t just pitch them into a landfill and be done with them! We have to be smarter than that! Let’s also remember that there are hundreds of other biological active, human created molecules floating around in our homes (and blood streams!). We had better take a good look at them, too! We owe it to our cats!