Janice A. Dye, DVM, Ph.D., at the U. S. Environmental Protection Agency in Research Triangle Park, N.C., and colleagues from there as well as Indiana University and the University of Georgia, report evidence linking the disease to exposure to environmental contaminants called polybrominated diphenyl ethers (PBDEs), which the researchers found to be elevated in blood samples of hyperthyroid cats.
Their findings were based on analysis of blood samples from 23 pet cats, 11 of which had the disease, termed feline hyperthyroidism (FH). PBDE levels in the hyperthyroid cats were three times as high as those in younger, non-hyperthyroid cats.
Concerns about the possible health effects of PDBEs arose in the late 1990s, and studies have reported that PDBEs cause liver and nerve toxicity in animals. FH is one of the most common and deadly diseases in older cats, and indoor pets are thought to be most at-risk. For starters, cats ingest large amounts of PBDE-laden house dust that the researchers believe comes from consumer household products.
Dye, a toxicologist, began by hypothesizing that prolonged contact with certain polyurethane foams and components of carpet padding, furniture and mattresses would pose the greatest hazard for developing FH. In addition, the researchers suspected that diet might be another risk factor for developing FH. To see if a link existed, they analyzed PBDE content in several cat food brands.
Their analysis found that PBDE content of canned fish/seafood flavours, such as salmon and whitefish, was higher than dry or non-seafood canned items. Based on the analysis, they estimate that diets based on canned food could have PBDE levels 12 times as high as dry-food diets. The researchers indicate that pet cats might be receiving as much as 100 times greater dietary PBDE exposure than American adults.
With their meticulous grooming behaviour, cats may ingest large amounts of dust that collect on their fur.
"Our results showed that cats are being consistently exposed to PBDEs,. Because they are endocrine-disrupting agents, cats may well be at increased risk for developing thyroid effects," Dye said.
The danger of contracting feline hyperthyroidism might be greater in America, where people have the highest reported PBDE levels worldwide, the study said. Also, by the late 1990s, North America accounted for almost half of the global demand for PBDEs from commercial materials like furniture or upholstery, the report added.
The epidemic of hyperthyroidism in cats began almost 30 years ago, at the same time when PBDEs were introduced into household materials as a fire-prevention measure. Although the disease was first discovered in the U.S., it has since been diagnosed in Canada, Australia, Japan and many parts of Europe.
Symptoms of the syndrome in cats include weight loss, an increase in appetite, hair loss and irritability. Cats and humans are the only mammals with high incidences of hyperthyroidism, Dye said. The study concludes that hyperthyroid cats could serve as modern-day versions of the canaries in the cage that alerted coal miners to poisonous gas.
"While the link between hyperthyroidism in cats and their elevated PBDE levels requires additional confirmation, it is clear that house cats may be able to serve as sentinels for indoor exposure to PBDEs for humans who share their houses," said Linda S. Birnbaum, Ph.D., a co-author of the study.
No link between human hyperthyrodism and PBDE exposure has been established, Birnbaum noted, adding that some ongoing studies do suggest such a connection. Although several states have banned use of certain PBDEs in commercial products, there are no regulations limiting PBDE content in foods, according to Birnbaum.
The study is published in the Aug. 15 online issue of Environmental Science and Technology.