Dr Mickey Mayhew wants owners warned about feline injection-site sarcoma after his 19-year-old cat was put to sleep
A grieving cat owner is calling for England’s compulsory microchipping rules to be reviewed after his pet was put to sleep with an aggressive tumour in the area where her chip had been implanted.
Princess, who was around 19, was euthanised in October 2025 after the tumour on her back grew so large that she could no longer walk.
Her owner, Dr Mickey Mayhew, an author and historian from London, has now launched a UK Parliament petition calling for a review of the law and the information owners receive before their cats are microchipped.
“Princess was part of our family,” Dr Mayhew said. “We know microchipping has huge welfare benefits and we’re not trying to discourage people from doing it.
“We simply believe owners deserve honest, balanced information about both the benefits and the recognised risks before giving their consent.”
The petition asks the Government to review compulsory microchipping in England. It says this could include making the procedure optional or setting clearer requirements to ensure owners are informed about recognised but rare risks, including feline injection-site sarcoma.
Princess’ Law
Supporters hope any resulting change could become known as “Princess’ Law”.
Princess was microchipped in June 2024, when microchipping became compulsory for owned cats in England. Dr Mayhew said the sarcoma appeared approximately ten months later in the area where the chip had been implanted.
Pets Magazine has not independently reviewed Princess’s veterinary or histopathology records, and the presence of a tumour beside a microchip does not prove that the implant caused it.
For Dr Mayhew, however, the campaign is rooted in the loss of a cat he had known for approximately 15 years.
Princess originally belonged to his former partner, and he would look after her whenever they were away for work. In March 2020, shortly before the first Covid lockdown, she came to live with him permanently.
Dr Mayhew said she had been used for breeding and spent her first few years confined to a cage before being rehomed. Although the experience had left her timid, she quickly established herself as the commanding presence in her new household.
“That didn’t stop her becoming top dog when she came to ours,” he said.
She was inseparable from another cat, Humpty, whom Dr Mayhew affectionately describes as her “husband”.
“They didn’t have any kittens, but they were very much an old married couple,” he said.
Humpty has struggled since Princess died and frequently cries and searches for her. Dr Mayhew said his vet had prescribed gabapentin to help calm him.
‘The presence of a microchip does not establish causality’
Feline injection-site sarcoma, or FISS, is a rare but aggressive cancer that develops at a site of inflammation beneath a cat’s skin.
It is most commonly associated with vaccinations and other injected medicines. Sarcomas have also been reported beside implanted microchips, but the published evidence is limited to a very small number of case reports.
Shirley van Lelyveld, an EBVS European Veterinary Specialist in Small Animal Oncology at Dovecote Veterinary Hospital, said the first published studies on FISS appeared in 1991.
A 2013 UK study, based on a limited sample of veterinary practices, estimated an annual incidence of between one case in every 16,000 and 50,000 cats registered with those practices.
Those figures relate to FISS generally, rather than to microchipping. Sarcomas reported in association with microchips appear to be rarer still, and the risk cannot be reliably quantified.
One case report published in 2008 described a fibrosarcoma beside a cat’s microchip, although the cat had also been vaccinated in the same area, making it impossible to determine the cause.
A further case, published in 2011, involved a cat that had not received vaccinations or other injections at the tumour site, providing stronger evidence of a possible association.
However, Ms van Lelyveld cautioned: “The presence of a microchip in or near a tumour does not establish causality.”
She said vaccinations and other injections may also be given in the same general area as a microchip, meaning that any of these interventions could potentially be a contributing factor.
Dr Mayhew said Princess had not, to his knowledge, been vaccinated at the tumour site. He is considering requesting her clinical records from the practice that treated her.
The precise cause of FISS is not fully understood, although chronic inflammation and an individual cat’s genetic susceptibility are thought to play a part.
The cancer can be particularly difficult to treat because it grows into surrounding tissue rather than remaining contained. Surgery may require very wide margins, while radiotherapy or chemotherapy may also be considered. Even after extensive treatment, the cancer can return.
What must owners be told?
Since June 2024, owned cats in England must be microchipped and registered on an approved database before they reach 20 weeks of age. The requirement includes indoor cats but does not apply to free-living cats with little or no human interaction or dependency.
The Microchipping of Cats and Dogs (England) Regulations 2023 do not require implanters to give owners a specific warning about FISS.
However, the Royal College of Veterinary Surgeons told Pets Magazine that its Code of Professional Conduct requires veterinary surgeons and veterinary nurses to obtain informed consent before any treatment or procedure, including microchip implantation.
What should be included in that discussion is a matter for the veterinary professional’s judgement.
Defra said the requirement to discuss risks and obtain consent also applies to approved implanters who are not veterinary surgeons or veterinary nurses.
A Defra spokesperson said: “In the vast majority of cases microchipping is completely safe and any adverse reactions are extremely rare. However, implanters must discuss risk and obtain informed consent from pet owners.”
Approved implanters must undertake training which demonstrates that they can obtain consent and explain relevant information to owners, including potential risks, aftercare and when veterinary advice should be sought.
Defra also has the power to prevent an implanter from continuing to implant microchips, or require them to undertake further training, if the required standards are not met.
The question raised by Dr Mayhew’s petition is therefore whether those existing consent requirements are sufficiently clear, and whether an exceptionally rare but serious possible complication such as FISS should be expressly mentioned.
Ms van Lelyveld said: “As with any medical procedure, information regarding risk should be given to the owner, regardless of how uncommon the risk is.”
She added, however, that the extremely low risk posed by microchipping was outweighed by its benefits.
Defra told Pets Magazine that 172 adverse events involving cats had been reported since compulsory microchipping came into force in June 2024.
The majority related to microchip failure. Nine reports concerned adverse reactions, and none specified a suspected sarcoma.
The British Veterinary Association and Cats Protection both support compulsory microchipping because of its role in reuniting missing cats with their families.
BVA President Dr Rob Williams said: “While any veterinary intervention involves a very small chance of something going wrong, thousands of cats are microchipped in the UK each year and side effects such as feline injection-site sarcoma are incredibly rare.”
Izzy Hocking, Cats Protection’s Senior Veterinary Engagement Manager, said: “Microchipping is widely regarded as a safe and effective procedure and complications associated with implantation are extremely rare.
“We would encourage anyone with questions or concerns about microchipping to speak to their vet so they fully understand what the procedure involves.”
Mark Gill, CEO of pet-identification platform Petasure, also expressed his sympathy.
“My sincere condolences to Dr Mayhew on the loss of Princess,” he said. “Losing a cat you’ve shared your life with is a real bereavement, and I hope his campaign brings some good out of it for other owners.”
Dr Mayhew insists that he does not want to deter owners from microchipping their cats.
“This isn’t about stopping microchipping,” he said. “It’s about making sure every owner has the information they need before making an informed decision.
“If Princess’ story helps even one family ask the right questions, then something positive will have come from our loss.”
Sign the petition
The petition needs 10,000 signatures by August 24 to receive a formal response from the Government.
Read and sign the petition for Princess’ law.
What is FISS?
Feline injection-site sarcoma, or FISS, is a rare but aggressive cancer that develops at a site of inflammation beneath a cat’s skin.
It is most commonly associated with vaccinations and other injected medicines. Sarcomas have also been reported beside implanted microchips, although only a very small number of feline cases have been published.
The presence of a microchip within or beside a tumour does not, by itself, prove that the implant caused the cancer. The risk specifically associated with microchipping cannot currently be reliably quantified.
A 2013 UK study estimated an annual incidence of between one FISS case in every 16,000 and 50,000 cats registered with the participating veterinary practices. The researchers cautioned that the practices studied might not be representative of the wider cat population.
These figures relate to FISS generally and should not be interpreted as estimates of the risk from microchipping.
FISS is particularly serious because it can invade surrounding tissue. Treatment may require radical surgery with wide margins, sometimes accompanied by radiotherapy or chemotherapy. Even after extensive treatment, the cancer can return.
What owners should watch for
Owners should check injection and microchip sites for persistent or growing lumps and contact their veterinary practice if a lump:
- remains present three months after an injection;
- is larger than 2cm; or
- continues to grow one month after an injection.
This is known as the 3-2-1 rule. Early veterinary assessment offers the best chance of effective treatment.
Sources: Shirley van Lelyveld, EBVS European Veterinary Specialist in Small Animal Oncology; Dean, Pfeiffer and Adams, BMC Veterinary Research; European Advisory Board on Cat Diseases; AAHA/AAFP Feline Vaccination Guidelines.
