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.
Treatment of Injection-Site Sarcoma (Vaccine-Site Sarcoma)
Because of the aggressive nature of the injection-site sarcoma, no single treatment has proved effective. However, treatment may include one or more of the following:
The most important thing you can do at home is to observe the area of concern carefully. Record when you first noticed the mass and seek advice from your veterinarian.
Regular visits to your veterinarian are critical to monitor your cat and treat this problem if it arises. Your veterinarian will examine the mass, measure and record its size and location, and discuss biopsy procedures with you.
After a veterinary pathologist has examined the biopsy specimen and given a diagnosis, appropriate treatment can be discussed and implemented by your veterinarian.
The only prevention is to eliminate vaccinations. However, since the incidence of injection-site sarcomas is low, it is wise to continue your immunization schedule. Consider vaccination every 3 years (rather than yearly) for rabies and panleukopenia. Limit vaccination for FeLV (feline leukemia virus) and FIP (feline infectious peritonitis) to at-risk cats as needed.
After immunizations, monitor your cat for swellings that may develop in body regions of previous injection or vaccination and see your veterinarian as soon as possible for early diagnosis and treatment.
In-depth Information on Vaccine-Assocaited Sarcoma in Cats
Other conditions both benign and malignant can occur and are often confused with injection-site sarcoma. Following is a partial list of such swellings:
Diagnostic tests must be performed to confirm the diagnosis of injection-site sarcoma and exclude other types of masses. Tests may include:
The current recommendations for management of injection-site sarcomas are as follows:
Consider any mass that develops in the area of a previous injection or vaccination to be malignant until proven otherwise. A mass should be treated aggressively if it meets the following criteria:
If the lesion meets one or more of these criteria, a biopsy is recommended before surgical excision. A biopsy is a procedure in which a small portion of the mass is removed and sent to a laboratory for microscopic examination by a veterinary pathologist. Biopsy samples can be collected by a special biopsy needle (called a Tru-Cut® needle) or by surgical incision of the mass to obtain a small wedge of tissue for microscopic examination. Fine needle aspiration and cytological examination using a conventional syringe and needle is not recommended because tumors of the sarcoma type generally do not readily shed cells during this procedure. Sedation or anesthesia may be required to obtain the biopsy sample.
Tumors typically are firm, well-demarcated, and gray to white in appearance. Many tumors are 4.0 cm (about 1 1/2 inches) in size when first noticed.
If the diagnosis of neoplasia (cancer) is determined by microscopic examination of the biopsy specimen:
Diagnostic procedures to evaluate for metastasis include:
Treatment of injection-site sarcomas must be individualized based on the extent of the mass, distant spread of the tumor (metastasis) and other factors that must be analyzed by your veterinarian. Treatment options include some combination of surgery, radiation treatment and chemotherapy. Currently, no cure for injection-site sarcomas exists, and research is ongoing regarding the best recommendations for prevention and treatment.
Treatment recommendations may include:
Recommendations may include the following drugs:
Prognosis for Injection Site Sarcoma in Cats
The prognosis for this type of cancer depends on the location and stage of the tumor. For example, tumors on the tail, on the distal legs or that are small and localized have a very good prognosis. Tumors that are large and treated aggressively with radiation, surgery and chemotherapy still have a fairly good prognosis; however some cats can have either reoccurrence or metastasis. Cats that are treated with aggressive surgery at the first diagnosis have the best chance of survival. Ask your surgeon about the specifics about your cat’s tumor.
Optimal treatment for your pet requires a combination of home and professional veterinary care. Follow-up can be critical. Administer as directed all medications and contact your veterinarian if you are experiencing problems treating your pet.
Have your pet rechecked by your veterinarian on a regular basis. Examinations are recommended monthly for the first three months, and then every three months for one year.