TL;DR: Small lumps after a cat's vaccination are usually normal immune reactions that resolve within four weeks; however, you should contact a vet if the lump persists for three months or grows larger than two centimeters.
Why has a lump formed where my cat received their vaccination?
It can be worrying to find a small bump or lump under your cat’s skin after they have received their shots. In most cases, this is a normal localized reaction to the vaccine or the injection process itself. This small, firm, and usually painless mass is typically caused by the body's immune system responding to the components of the vaccine, which is how your cat builds the necessary immunity to stay healthy.
When should I contact a veterinarian about a post-vaccination lump?
The urgency level for a new lump at a vaccination site is generally low, as most are benign and disappear within two to four weeks. However, you should contact your veterinarian if:
- The lump remains present after three months.
- The lump grows larger than two centimeters in diameter.
- The lump continues to increase in size one month after the injection.
Consistent monitoring is essential because, while rare, a more serious condition called feline injection-site sarcoma can occur.
How does taking a photo of the lump help with a veterinary assessment?
Documenting the lump with a photo can be incredibly helpful for a triage assessment. We recommend following these steps to help your vet:
- Capture a clear image showing the exact location and surface appearance of the area.
- Place a common object, like a coin or a ruler, next to the lump to provide a clear sense of scale.
- Compare future photos to this baseline image to determine if the lump is shrinking or growing over time.
Clinical Context (Merck Veterinary Manual)
In cats, a lump appearing after vaccination may be indicative of a post-vaccination sarcoma, specifically a fibrosarcoma that develops at the injection site. These tumors appear as nodules or plaques in the soft tissues. While an association is better defined with rabies and feline leukemia virus vaccinations, aluminum hydroxide, commonly used in adjuvants, has been identified in vaccine-induced fibrosarcomas, where a prolonged proliferation of fibroblasts in response to the adjuvant may predispose these cells to undergo neoplastic transformation. Some chronic inflammatory reactions to long-acting feline vaccines may eventually lead to the development of a fibrosarcoma at the injection site.
Chapter: Infectious Disease, Preventative Medicine
Source: The Merck Veterinary Manual, 11th Edition (Page 796)