Feline Injection-Site Sarcoma (Vaccine-Site Sarcoma)
An injection-site sarcoma, also known as vaccine site sarcoma, vaccine-associated fibrosarcoma, and vaccine associated sarcoma, is a tumor thought to be induced by an injection – most often a vaccination. Post-vaccinal sarcomas are very rare but may occur in cats as a consequence of an overzealous inflammatory or immune system reaction to the vaccine.
A sarcoma is a malignant tumor composed of cells derived from connective tissue. These tumors often develop quickly and can spread (metastasize) to distant locations in the body. These tumors often are not responsive to treatment and result in serious illness and ultimately death of the animal. Recurrence of such tumors is common after surgical removal.
Injection-site sarcomas were first recognized in the late 1980’s when some changes occurred in the vaccine manufacturing process. At that time, manufacturers changed from production of modified live virus vaccines to killed virus products as directed by the United States Department of Agriculture (USDA). This change in manufacturing process resulted in the inclusion of aluminum into vaccines. It is this aluminum component of the vaccines that is suspected to be associated with development of post-vaccinal sarcomas. The feline leukemia virus and rabies vaccines are most frequently suspected in pets that develop post-vaccinal sarcomas.
The actual incidence of injection-site sarcomas is not known with certainty. Some investigators estimate that post-vaccinal sarcomas occur in as many as 1 of every 1,000 to as few as 1 in every 10,000 cats vaccinated. Injection-site sarcomas are recognized primarily in cats. The average age for onset of vaccine-site sarcomas is 7 to 9 years. There is no known breed predisposition. It is believed that tumors develop week to years after injection.
Despite the localized appearance of these tumors, microscopic branches of the tumor extend like fingers into the surrounding healthy tissue. During surgery to remove the tumor, these microscopic branches can remain and contribute to re-growth of the tumor. According to one study, as many as 62 percent of post-vaccinal sarcomas recur within 6 months after surgical removal.
There are several types of injection-site sarcomas:
What To Watch For
You should watch for a firm, painless swelling in a subcutaneous (under the skin) location in the region of the body in which the cat was vaccinated or received an injection. It is wise to run your hand over your cat’s shoulders, back and rear legs periodically to monitor for development of abnormal lumps or tumors. The mass maybe hairless or ulcerated in some cats.
Diagnosis of Injection-Site Sarcoma (Vaccine-Site Sarcoma)
A swelling, nodule or mass under your cat’s skin in a region of the body that was previously used for injection or vaccination should be taken very seriously. If this occurs, you should schedule an appointment with your veterinarian to have your cat examined and the mass evaluated. This approach is recommended especially if the mass has persisted for 3 or more months, is larger than 2 cm (just under one inch) in diameter, or if you notice that the mass has been increasing in size during the one month after injection or vaccination. Have your veterinarian examine your cat as soon as possible in this situation.
Diagnostic tests are needed to recognize injection-site sarcomas and exclude other diseases.
Biopsy is a procedure in which a small portion of the mass is removed, preserved in a fixative solution, and sent to a laboratory for microscopic examination by a veterinary pathologist. Biopsy specimens can be obtained by an instrument called a Tru-Cut® needle (which collects a very small core of tissue) or by surgical incision of the mass to obtain a small wedge of tissue.