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:
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.
All rights reserved.
Contact the author.
Ball DJ, Argentieri G, Krause R, Lipinski M, Robison RL, et al. (1991) "Evaluation of a microchip implant system used for animal identification in rats." Laboratory Animal Science, 1991 Apr; 41:185-86
Balkwill F and Mantovani A. (2001) "Inflammation and cancer: back to Virchow?" The Lancet, 2001 Feb 17; 357(9255):539-45.
Brand KG, Buoen LC, Brand I. (1975a) "Foreign-body tumorigenesis induced by glass and smooth and rough plastic. Comparative study of preneoplastic events." Journal of the National Cancer Institute, Aug 1975; 55(2):319-22.
Brand KG, Buoen LC, Johnson KH, Brand I. (1975b) "Etiological factors, stages,and the role of the foreign body in foreign body tumorigenesis: a review." Cancer Research, Feb 1975; 35(2):279-86.
Cooley DM, Beranek BC, Schlittler DL, Glickman NW, Glickman LT, and Waters DJ. (2002) "Endogenous gonadal hormone exposure and bone sarcoma risk." Cancer epidemiology, biomarkers & prevention, 2002 Nov; 11(11):1434-40.
Full text available:
Cousins, LM and Werb Z. (2002) "Inflammation and Cancer." Nature, 2002 Dec 19; 420, 860-867.
Gruys E et al. (1993) "Biocompatibility of glass-encapsulated electronic chips (transponders) used for the identification of pigs." Veterinary Record, 1993 Oct 16; 133(16):385-8.
Jansen JA et al. (1999) "Biological and migrational characteristics of transponders implanted into beagle dogs." Veterinary Record, 1999 Sep 18; 145(12):329-33.
Kass, PH et al (2003). "Multicenter case-control study of risk factors associated with development of vaccine-associated sarcomas in cats." Journal of the American Veterinary Medical Association, 2003 Nov 1; 223(9):1283-92.
Lambooij, E. (1995) "Electronic identification with injectable transponders in pig production: results of a field trail on commercial farms and slaughterhouses concerning injectability and retrievability." Veterinary Quarterly, 1995 Dec; 17(4):118-23.
Mader, C.H. et al. (2002) "Implantation of transponders at the bottom of the ear in equines" Berliner und Münchener Tierärztliche Wochenschrift. 2002 May-Jun; 115(5-6):161-6.
McCarthy PE et al. (1996) "Liposarcoma associated with a glass foreign body in a dog." Journal of the American Veterinary Medical Association, 1996 Aug 1; 209(3):612-4.
Moizhess TG and Vasiliev JM. "Early and late stages of foreign-body carcinogenesis can be induced by implants of different shapes." International Journal of Cancer, 1989 Sep 15; 44(3):449-53.
Munday JS, Stedman NL, and Richey LJ. (2003) "Histology and immunohistochemistry of seven ferret vaccination-site fibrosarcomas." Veterinary Pathology, 2003 May; 40(3):288-93.
Full text available:
Goldschmidt MH and Thrall De. (1985) "Malignant Bone Tumors in the Dog" in Newton, CD and Nunamaker DM. Textbook of Small Animal Orthopaedics., Lippincott Williams & Wilkins, 1985.
Ru G, Terracini B, and Glickman LT. (1997) "Host related risk factors for canine osteosarcoma." Veterinary journal, 1998 Jul; 156(1):31-9.
Troyk, Philip R. (1999) "Injectable Electronic Identification, Monitoring, and Stimulation Systems." Annual Review of Biomedical Engineering, 1999; 1:177-209.
Vascellari M et al. (2003) "Fibrosarcomas at presumed sites of injection in dogs: characteristics and comparison with non-vaccination site fibrosarcomas and feline post-vaccinal fibrosarcomas." Journal of Veterinary Medicine. A, Physiology, Pathology, Clinical Medicine, 2003 Aug; 50(6):286-91.
Articles Linking Microchips (Transponders) AND Tumors
[Where possible, I've quoted an excerpt from the abstracts or articles. Follow the links for the full abstracts or full-text.]
Vascellari M, Melchiotti E, Mutinelli F.
Fibrosarcoma with typical features of postinjection sarcoma at site of microchip implant in a dog: histologic and immunohistochemical study.
Veterinary Pathology. 2006 Jul; 43(4):545-8.
Le Calvez S, Perron-Lepage MF, Burnett R.
Subcutaneous microchip-associated tumours in B6C3F1 mice: a retrospective study to attempt to determine their histogenesis.
Experimental and Toxicologic Pathology. 2006 Mar; 57(4):255-65. Epub 2006 Jan 19.
Vascellari M, Mutinelli F, Cossettini R, Altinier E.
Liposarcoma at the site of an implanted microchip in a dog.
Veterinary Journal. 2004 Sep; 168(2):188-90.
Link to conference presentation on this subject by the authors:
European Medicines Agency, CHMP Safety Working Party. CHMP SWP Conclusions and Recommendations on the Use of Genetically Modified Animals Models for Carcinogenicity Assessment. 2004 June 23.
Floyd E, Mann P, Long G, Ochoa R.
The Trp53 hemizygous mouse in pharmaceutical development: points to consider for pathologists.
Toxicologic Pathology. 2002 Jan-Feb; 30(1):147-56.
Full text available:
Elcock LE, Stuart BP, Wahle BS, Hoss HE, Crabb K, Millard DM, Mueller RE, Hastings TF, Lake SG.
Tumors in long-term rat studies associated with microchip animal identification devices.
Experimental and Toxicologic Pathology. 2001 Feb; 52(6):483-91.
Cohen SM, Robinson D, MacDonald J.
Alternative Models for Carcinogenicity Testing
Toxicological Sciences. 2001; 64:14-19
Full text available:
European Society of Toxicologic Pathology. (2000) "MICROCHIP-ASSOCIATED TUMOUR IN A C57/BL MOUSE"
GTP [Gesellschaft für Toxikologische Pathologie] Meeting 2000: Case No 15.
Blanchard KT, Barthel C, French JE, Holden HE, Moretz R, Pack FD, Tennant RW, Stoll RE. Transponder-induced sarcoma in the heterozygous p53+/- mouse.
Toxicologic Pathology. 1999 Sep-Oct; 27(5):519-27.
Full text available:
Tillmann T, Kamino K, Dasenbrock C, Ernst H, Kohler M, Morawietz G, Campo E, Cardesa A, Tomatis L, Mohr U.
Subcutaneous soft tissue tumours at the site of implanted microchips in mice.
Experimental and Toxicologic Pathology. 1997 Aug; 49(3-4):197-200.
Copyright 2006, S. Pober. All rights reserved.
Contact the author.
The above article originally appeared on the VETMED discussion list in December 2006.
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