Link between microchips implanted in dogs and cancer reviewed

Microchips and cancer: a review
The VETMED email list had a discussion of the potential link between implanted identification microchips and cancer. I did some research to see what has been published on this topic.

A lot of our assessments of risk are based not on evidence, but on general impressions, as in "Everybody says [something commonly held to be true]" or "I've never seen [some rare adverse effect]." But sometimes, what "everybody says" is wrong. I prefer to look for evidence from scholarly studies when figuring out the risks of a specific drug or device.

When you look at scholarly research, you may not find the large-scale studies that would give you an accurate, quantified assessment of risk. Some years back, one of my dogs had his broken hock rebuilt by an orthopedic surgeon. I wanted to know whether we should remove the implanted bone nails and plates after he recovered, because some implantable devices, such as hip replacements in human patients, are associated with an increased rate of cancer.

When I discussed this with his surgeon and with other vets, it turned out that none of them could give me any evidence-based statistics on the incidence of bone cancer or other malignant neoplasms at the site of bone fixation devices. So, being the curious type, I looked at some of the research on that. I never did find any firm numbers, but what I found was the following things were all associated with an increased risk of cancer: Breaking a bone, implanting metal into the body, and implanting many other types of material into the body. Basically, it seems like anything that encourages more bone to grow increases the bone cancer risk (Goldschmidt and Thrall, 1985). (This is probably one reason that early-age spaying and neutering of dogs is associated with a significant increase in bone cancer risk. (Cooley et al, 2002 and Ru et al, 1998) Early desexing is known to produce taller animals. In intact dogs, the sex hormones of puberty help trigger the closure of the growth plates of the bones. If you spay or neuter a young puppy, the bones grow for a longer period and more bone growth increases the risk of bone cancer.

So, there is some risk of cancer from bone fixation devices, and the risk they pose is higher than the risk of simply breaking a bone - but I never did find research that quantified that specific risk. And I'm certain that the benefit of having a sound leg to walk and run on far outweighs the small risk of cancer from implanting bone fixation devices.

Going back to the issue of microchips, I did find multiple studies and some case reports that indicate that implanting a microchip raises the risk of cancer in animals. Here are some relevant principles that we know from veterinary research on related risks:
Malignant tumors in animals have been linked to implantation of foreign bodies (Brand, 1975b and Moizhess, 1989). Even foreign bodies consisting of relatively inert materials such as glass (McCarthy, 1996 and Brand, 1975a) have been found to cause malignant tumors in animals.


Vaccinations and injections have been found to lead to sarcomas in cats (Kass, 2003), dogs (Vascellari, 2003), and ferrets (Munday, 2003). In the cat study by Kass, the sarcomas are not linked to one brand or type of vaccine, as was previously thought. (In some older studies, specific brands of vaccines were thought to be implicated, but some researchers now feel that was simply a reflection of the popularity of those brands.)


Inflammation, usually transient, occurs at the implantation sites of microchips (Mader, 2002, and Lambooij, 1995).


Tissue inflammation has a role in the development of cancer (Cousins, 2002, and Balkwill, 2001).


A fibrous capsule is formed around implanted microchips (Ball, et al, 1991, Gruys, 1993, Troyk, 1999) even in the absence of a gross inflammatory reaction (Jansen, 1999). This indicates that there is enough inflammation to cause fibrous tissue growth. Fibrosarcoma, which is the most common sarcoma associated with vaccinations in animals, is also rich in fibrous tissue.


So, if you're wondering how microchips could be harmful, the answer is that they can cause inflammation, fibrous tissue growth, and are implanted via injections, a method that is already known to increase the sarcoma risk. Then, add the fact that implanted foreign bodies are known to increase the risk of cancer. It follows that we have good reason to be cautious about microchip implantation.

When people want to dismiss out of hand the idea that there may be a cancer risk in implanting microchips, they should think about the many years that vaccines were given to cats before the issue of injection-site sarcomas was recognized and understood to be a risk.

I am not saying that the risk of implanting a microchip necessarily outweighs the benefit. I think each pet owner needs to decide that for themself. Vaccines are linked to sarcomas, but I vaccinate all my pets for rabies, because I believe the protection from a fatal disease is worth the small risk. The situation with microchips is different, as there are other identification methods available.

It would be unfortunate if the government mandated microchipping of pets and took this decision out of the hands of pet owners. There have been a few municipalities that have passed laws requiring this. I believe that the decision of whether to microchip an animal should be left to the pet owner, particularly since the owner is the one who will foot the bill for veterinary treatment in the case of any adverse effect.

Looking at the studies and case reports that link implanted microchips (also known as "passive transponders") to the development of tumors in various species of animal, it's interesting to note that most of the studies were not done specifically to find problems with microchips. Rather, the researchers implanted microchips in the animals they were using for some other study, and they noticed that their research subjects were developing tumors at the microchip implantation sites.

Some of these articles refer to specific lines of laboratory animals that may be more prone to cancer than the species as a whole. That's not a reason to dismiss the research. Just as some humans carry certain genes predisposing them to some form of cancer, a similar phenomenon is found in some dogs and cats. With implanted microchip devices becoming common as an identification method for pets, you have to assume that some of the dogs and cats that get them will have health issues, such as a genetic susceptibility to cancer. When deciding if a device or drug is safe, you don't just look at the risk to healthy animals, you have to look at the risk to the most vulnerable animals. too.

If anyone wishes to repost or republish this, please email me and ask for permission. I usually say yes, but I like to be asked.

Copyright 2006, S. Pober.
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