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:

Fibrosarcoma – a malignant tumor arising from cells called fibroblasts in connective tissue. This is the most common type.

Histiocytoma – a malignant tumor arising from tissue cells called histiocytes

Osteosarcoma – a malignant tumor arising from bone

Chondrosarcoma – a malignant tumor arising from cartilage

Rhabdomyosarcoma – a malignant tumor arising from muscle

Myxosarcoma – a malignant tumor arising from loose connective tissue

Liposarcoma – a malignant tumor arising from fat

Neurofibrosarcoma - a malignant tumor arising from cells called fibroblasts in connective tissue and nerve tissue.

Hemangiopericytoma – a malignant tumor that arises from the pericytes, cells that are located around vessels.

Schwanoma –are tumors of the nerve sheath

Leiomyosarcoma – a tumor arising from smooth muscle.

Nerve sheath tumor – a tumor arising from the deep soft tissue, usually in close proximity of a nerve trunk

Myofibroblastic sarcoma – a malignant soft tissue tumor in which myofibroblasts are quantitatively the predominant cell type.

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.