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Veterinarian administering a vaccine to a multicolored cat

Monitor cat’s vaccination sites for lumps

Q: My cat, Jade, has a lump on the outside of her right thigh. What is it? Does it warrant an immediate exam, or may I wait until Jade’s annual exam in six months?

A: You should schedule an appointment now. Jade’s veterinarian will be able to tell you what the lump is after an examination and probably a biopsy.

If another animal bit Jade, the lump may be a pus-filled abscess. If that’s the case, your vet may simply lance the abscess and give Jade an antibiotic to fight the infection.

If the lump feels firmer, it may be something more serious, particularly if it’s at the site of a vaccination or other injection.

When it comes to masses, the 1-2-3 rule helps determine when to biopsy an injection-site mass:

1. if the lump is growing one month after the injection, or

2. if the mass is at least 2 centimeters (3/4 of an inch) in diameter, or

3. if the lump persists three months after the injection.

Very rarely, an injection-site mass is an aggressive cancer called a sarcoma.

An injection-site sarcoma can occur after any injection, including an antibiotic, steroid or subcutaneous fluids. However, the risk seems greatest with certain vaccines, particularly leukemia vaccines and rabies vaccines that contain an ingredient called adjuvant that prolongs immunity.

Still, the risk of sarcoma formation is low. The incidence, while unknown, appears to be about one case in 10,000 cats.

To improve detection, veterinarians give rabies vaccines in the right hind leg, leukemia in the left hind leg, and distemper in the right front leg. If Jade received a rabies vaccination in her right thigh, and if the 1-2-3 rule applies to her mass, she may have an injection-site sarcoma and needs to see her veterinarian now.


Lee Pickett, V.M.D. practices companion animal medicine in Pennsylvania. Contact her at askdrlee@insurefigo.com

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