In 1979 reports began to emerge of a new disease in cats. Older cats were losing weight rapidly despite a good appetite. A veterinarian in New York figured out that these cats had developed benign tumours in their thyroid glands that caused the gland to produce excess thyroid hormone - a condition called hyperthyroidism. Soon hyperthyroid cats were being diagnosed all around the world. By the late 1980s, when I was going to veterinary school, it was estimated that 1 in 10 cats would develop it. Where did this disease come from? New diseases did occasionally arise, but they were always infectious diseases with clear origins, such as canine parvovirus, which was the mutation of the cat distemper virus, and heartworm, which was the northward migration of a tropical disease.
Some speculated that it was just that cats were living so much longer that we were now seeing more geriatric diseases, but this made no sense as the gains in life expectancy were gradual and the apparent emergence of hyperthyroidism was relatively sudden. Veterinarians, being neurotic as a group, also blamed themselves, assuming they had just missed it before. This also made no sense as the disease is dramatic and obvious in its advanced form. One researcher looked through 7000 old autopsy reports and found no evidence of hyperthyroidism. It really was a new disease.
So various other more reasonable, but still flawed, hypotheses were put forward through the 1990s and 2000s, but to speed the story along I'll take you straight to what appears to be the answer. In four letters it is PBDE. This is the acronym for polybrominated diphenyl ether, a common fire retardant found especially in furniture foam, carpet underlay, some clothing and bedding, and in the plastic housing for some electronics. PBDEs gradually, microscopically, shed into the home environment and become part of the dust. Cats, being close to the ground, are exposed to dust even in relatively clean houses. And crucially, PBDEs have been shown to be endocrine disruptors, meaning that they can interfere with hormonal functions. Thyroid is a hormone. Tellingly, for this story, PBDEs first became wide-spread during the 1970s. This is all circumstantial evidence, but the research evidence is mounting as well with a steady stream of ever more persuasive studies, the most recent just in 2016.
PBDEs were declared "toxic" by the Canadian government in 2004 and their manufacture and import was restricted. Unfortunately though they are still pervasive in the environment and industry has side-stepped the regulations by devising new fire retardant chemicals which may or may not have the same effects. Nobody knows yet. Government regulations are slow to play catch-up. Nonetheless, I think I am seeing fewer cases of hyperthyroidism than I did back in the 90s. What I am seeing far more of is pancreatitis. Canine pancreatitis is more or less unchanged, but feline pancreatitis is sky-rocketing from a very rare diagnosis twenty years ago, to a weekly one now. Did we just miss it before? The discussion is starting to sound familiar...
So back to the canary metaphor. The incidence of human thyroid cancer has increased more rapidly than most other cancers since the late 1970s. This is far from conclusive and studies are ongoing, but maybe our cats are telling us something. Maybe we should listen more carefully.