Have you ever heard that microchips in pets can cause cancer? Yes, there’s one reported case … out of hundreds of thousands of microchipped pets.

Just one. But it's also true that mice, apparently, show an impressive susceptibility to cancer after microchip implantation. This, according to research the microchip industry allegedly buried for fear its chips wouldn’t get skin-time in humans and in pets.

So what's the vet's point of view? I've never much worried about the safety of these implants after the first few years they were available. Seeing as I’d never heard of a reaction — not even a simple infection — I figured we'd put this issue to rest for good. That is, until someone emailed me the story of Leon, a little French Bulldog, which urged me to consider the possibility that Leon’s might be the index case for microchip-related cancer in pets.

I had no doubt when I read the related article, Fibrosarcoma with Typical Features of Post injection Sarcoma at Site of Microchip Implant in a Dog: Histologic and Immunohistochemical Study, a 2006 study published in Veterinary Pathology (a respected, peer-reviewed journal), that this was a real-live case of fibrosarcoma induced by a foreign body — in this incident, apparently the microchip. Leon didn’t make it, but his story lives on in Leon's memorial Web site, a cautionary tale for those who think anything implanted or injected by doctors is 100 percent safe.

Fibrosarcomas are a common tumor implicated in vaccine reactions in pets (read my post on the new research into this issue in felines). Now it’s become a human and pet issue, too, with the breaking news of two studies in mice that show cancer can result at the microchip injection site. Humans are often microchipped when they have serious dementia (and risk getting lost). Perhaps that’s why this issue received so much press once it was revealed that the microchip companies were apparently hiding this data. Pets aren’t always big news, but pets and humans too? Now that’s a story!

Leon’s case is a perfect example of how animal bodies can mysteriously respond to foreign objects in aberrant ways. Nothing is completely safe — not herbs, not the gold beads used in acupuncture (also found to cause serious reactions in some cases), not vaccines, and not microchips.

Every action has a potential reaction. Leon’s case is the only example of cancer in pets that occurred as a potential result of a microchip, that we know of. He was vaccinated the same day in a nearby area, so it’s not 100 percent clear that the microchip caused it (though the cancer seemed to emanate from the microchip's specific spot). His case should give us some pause, and some solace too, that with all the microchips implanted we've only seen one so far (that we know of).

Though I’m 100 percent sympathetic to Leon’s tragedy, I wouldn't want to fan the flames of microchip-naysayers everywhere. It's still an excellent tool for getting pets back home where they belong. But knowing what we now do about Leon, we should approach it with a greater modicum of caution than we did before. Because as with every medical implant, there's always a risk. And the risk-benefit ratio must be weighed accordingly, with all the information at our disposal.

Dr. Patty Khuly