An outbreak in Australia of neurological disorders in cats fed irradiated diets is a first among pets but not unprecedented.
At least three instances of irradiated food causing severe nervous system disease have occurred around the world since 1998. The cases happened in Ireland and the United States in laboratory cats and in Australia among pet cats.
The Australian government in late May ordered an immediate stop to sterilizing cat food via irradiation after reviewing scientific studies pointing to food irradiation as the culprit behind illnesses characterized by ataxia or paralysis of the limbs, especially the hind limbs. How irradiation may be causing food to become toxic to cats is unknown.
The problem affected 90 cats in Australia, of which 30 died, mostly by euthanasia after they became paraplegic or tetraplegic, according to Dr. Georgina Child, a neurologist at the Small Animal Specialist Hospital and University of Sydney Teaching Hospital.
Child told The VIN News Service by e-mail that all the cats had eaten an imported dry diet, sold under the brand name Orijen and made in Canada by Champion Petfoods. The food was subject to gamma irradiation upon entry to Australia at levels greater than or equal to 50 kilo-Grays (kGy).
The U.S. Food and Drug Administration (FDA) also allows animal feed, including pet food and treats, to be irradiated up to 50 kGy. By comparison, most human foods in the United States allowed to be irradiated are limited to levels of 30 kGys and below. Packaged meats for astronauts are an exception; those may be treated up to 44 kGy.
The FDA requires all irradiated foods to be labeled as such; Australia does not require labels on irradiated pet food.
Symptoms in the Australian cats first appeared three to six months after they were exposed to the dry food, Child said. Some had eaten the food for as few as three weeks; others for more than six months. Most of the cats were fed other foods, as well. The affected cats ranged in age from less than one year to 15 years.
Early signs of illness included a wobbly gait, unwillingness to jump onto sofas or beds, and loss of balance exhibited, for example, by falling off tables, according to a Q&A for consumers prepared by Champion Petfoods.
Child said about half of the affected cats remain paraparetic or tetraparetic; some remain paraplegic or tetraplegic. “Many cats have been at their worst for two or more months before showing any improvement,” she said, noting that fewer than one-fifth have recovered fully.
Child said histopathology showed “diffuse, symmetric, severe white matter degeneration of predominantly the spinal cord but also (the) brain stem and cerebrum, with demyelination the predominant feature.”
“No specific treatment has resulted in an improvement in the outcome of affected cats,” she said. “A change in diet, nursing care, physiotherapy and time seem to be the only factors common in recovered cats.”
But even patients who are paralyzed and lose vision as a result of eating irradiated food can fully recover, given sufficient time, nursing care and food that hasn’t been irradiated, said Dr. Ian Duncan, professor of neurology at the University of Wisconsin-Madison School of Veterinary Medicine.
That’s what Duncan saw with cats housed in a private laboratory in Wisconsin that were fed irradiated diets. The irradiation was meant to help keep the cats — known as “specific pathogen-free” or SPF cats — from unwittingly acquiring infections.
In that episode, which was reported this spring in the Proceedings of the National Academy of Sciences (“Extensive remyelination of the CNS leads to functional recovery,” PNAS, March 30, 2009, Duncan I.D. et al.), the food was irradiated at levels of 25 to 50 kGy.
The symptoms took about four months to appear, and they abated after another two to four months once the cats were given a non-irradiated diet. Altogether, about 30 cats were affected, and most made impressive recoveries, Duncan said.
Oddly, only cats that became pregnant while on the irradiated diet developed neurological symptoms, he noted. Male cats and offspring exposed to the same diet did not become ill.
Male cats have been affected elsewhere, however.
In the Irish case, 190 domestic short-hair cats housed together developed hind limb ataxia and proprioceptive defects during a period of four years, from 1998 to 2001 (“Leukoencephalomyelopathy in Specific Pathogen-free Cats,” J.P. Cassidy et al., Vet Pathol 2007; 44:912-916). As with the Wisconsin colony, those animals were SPF cats.
Serendipitously, researchers had a control group in the form of kittens that had been transferred from the SPF colony to a conventional disease status colony on the same site.
Both groups of cats were fed on the same commercial formula ration, except that the food bound for SFP cats was irradiated to levels between 36.3 and 47.3 kGy.
The transferred cats did not develop any neurological abnormalities. More than one-third of the cats fed irradiated food did.
“Following supplementation of the irradiated diet with pasteurized proprietary tinned cat food in the winter of 2001, and ultimately, the replacement of the irradiated diet with an equivalent pasteurized diet, no further cases occurred,” the researchers reported.
Child said the same researchers since have induced neurological problems in cats at radiation levels as low as 26 kGy. A study with those results is due to be published shortly, she said.
Exactly how irradiation renders food toxic to cats is unclear. It was initially thought to be a vitamin A deficiency; the researchers in Ireland, analyzing the nutritional content of cat food subject to irradiation levels ranging from 28.9 kGy to 48.7 kGy, found that vitamin A content diminished to levels below that recommended for growth and reproduction.
By comparison, pasteurization reduced the vitamin A content in the cat food, as well, but not below recommended levels.
Curiously, the researchers found that irradiation did not alter vitamin A in dog food.
However, radiation treatment did increase peroxide content in dog food, cat food and rodent food, up to 25-fold at the highest dose.
“Gamma irradiation can have profound, selective effects on the vitamin A and peroxide contents of dry diets, and caution is advised when feeding such diets long-term and exclusively to SPF animals, particularly cats,” the researchers concluded (J Am Assoc Lab Anim Sci. 2008 Nov; 47 (6): 61-6, Caulfield CD et. al).
But subsequent studies found that affected animals were not deficient in vitamin A after all, Child said.
“Possible causes still include a micronutrient deficiency — although this seems less likely — or toxicity induced by irradiation-induced changes in the dry food (more likely),” she said.
Champion Petfoods, maker of the Orijen brand, noted in a Q&A dated November 2008 that its own testing of irradiated cat food found the treatment depleted vitamin A and formed free radicals in the food. Their statement reads in part:
“Irradiation does not affect all foods equally. Orijen is a nutritionally dense food with much higher levels of long-chain fatty acids (DHA, EPA) than conventional pet foods. These fatty acids are susceptible to oxidation following irradiation. Byproducts from fatty acid oxidation, mainly free radicals, are released into the body with the potential to cause tissue damage.
“Combined with the loss of antioxidant vitamins [such as vitamin A], free radicals are thought to be a major contributing factor.”
Although the irradiation ban applies only to cat food, Champion Petfoods has pulled its dog food from the Australian market as well, Sales and Marketing Manager Peter Muhlenfeld told the VIN News Service.
“Cats frequently eat from dog dishes and vice versa,” he said in an e-mail. “We do not believe the market to be safe unless both cat and dog foods are allowed entry without irradiation.”
According to the company Q&A:
* Only Australia requires irradiation of Orijen cat foods, out of 50 countries in which the food is sold.
* Orijen was sold in Australia only for a nine-month period — February through October 2008.
* In that period, only six pallets of Orijen cat food actually entered retail distribution, enough to feed 200 cats each day during the nine months.
* During the same period, shipments of Orijen cat food were made to 49 other countries, enough to feed 85,000 cats. In no other country has neurological illness been reported.
Why Australia requires irradiation of imported pet foods is unclear. Officials at the Australian Quarantine and Inspection Service (AQIS) did not respond to an e-mailed request for information.
Patty Lovera, assistant director of Food and Water Watch, a non-profit consumer organization based in Washington, D.C., that monitors food irradiation around the world, said Australia’s high vigilance on biosecurity may stem from its being an island nation.
“My impression is that they’re very tuned to things coming in that might be some kind of threat to their agriculture,” Lovera said.
A notice to industry posted by the AQIS before the government banned cat-food irradiation suggests that is the case. It states: “Heat or irradiation treatments are necessary to address animal quarantine pest and disease risks that may be associated with imported pet foods.”
A separate notice states that, as an alternative to radiation, importers may subject pet food to a moist heat treatment — through which the product must achieve a certain core temperature for a certain duration.
According to the Champion Petfoods Q&A, “The overwhelming majority of cat foods in Australia do not require irradiation.” Orijen was required to undergo irradiation because the food includes fresh meats that were cooked at low temperatures.
The company takes pride in the distinctions between its products and those of conventional pet-food makers. Its gourmet foods are marketed as “biologically appropriate,” and made with “fresh, regional ingredients.”
“We’re different. We sell only what we make ourselves,” it says on its Web site in an allusion to the use of co-packing facilities by some of its competitors. Co-packers are contracted to produce pet foods for a variety of companies sold under a variety of brand names.
The widespread use of co-packing contributed in 2007 to the largest recall of pet food in North American history, due to contamination by the industrial chemical melamine, which affected 180 brands of pet food.
Muhlenfeld said Champion Petfoods has a program to offset medical bills of owners of affected pets up to $1,000. It also donated $10,000 to organizations in Sydney that work with cat diseases.
The Australian ban on irradiation marks the first time in the world that authorities rescinded an approved use of irradiation because it is suspected to cause illness.
George Pauli, a retired chemist who worked for 29 years in the U.S. FDA’s Center for Food Safety and Applied Nutrition, said the FDA in one instance reversed an approval on irradiation, but not because anyone became ill.
In that case from the 1960s, the U.S. Department of Defense asked to sterilize bacon through irradiation to create a shelf-stable product that didn’t need to be refrigerated. “The FDA, when they reviewed it, thought everything looked good, but later on ... they concluded that the data were not nearly as solid and convincing as first thought,” Pauli said.
The Defense Department never came back with more data and a request to reconsider, Pauli said.
He speculated that the cat cases in Australia are an example of species sensitivity. “More testing has been done on rodents and dogs,” he said. “Normally, testing (for human safety) is not done on cats that much.”
VIN News Service commentaries are opinion pieces presenting insights, personal experiences and/or perspectives on topical issues by members of the veterinary community. To submit a commentary for consideration, email firstname.lastname@example.org.