Dear Christopher Cat
My cat has a lump where she was recently vaccinated. Should I do anything about it?
Yes. Keep an eye on it, and ask your veterinarian to biopsy it if: 1) its size is increasing one month after vaccination, 2) it is larger than two centimeters (three-fourths of an inch) in diameter, or 3) it continues to be present three months after vaccination.
I once had a post-vaccination lump, but it disappeared within a month or two, so it did not fit into the “1-2-3” guideline.
Persistent lumps at vaccination sites are of concern because they can be cancerous.
We cats can develop malignant tumors, called sarcomas, at injection sites. These cancers are rare – they occur in perhaps one in 10,000 vaccinated cats – but they are very aggressive.
Early detection is important, so it’s best to examine your cat after every injection, and keep in mind the “1-2-3” guideline.