Vaccine-Associated Sarcoma

Also know as: vaccine site-associated sarcoma — injection-site sarcoma — vaccine-induced sarcoma

Introduction

Since 1992, an association between certain types of feline vaccines and the development of connective tissue tumours has been recognised. Current thought suggests that it is not only certain types of vaccines, but also any injection which produces localised inflammation which can lead to the development of sarcomas in genetically predisposed and susceptible cats.

The incidence of vaccine-associated sarcomas is difficult to estimate, however figures of 1 in 1000 to 1 in 10,000 vaccines administered have been reported.

The vaccines most often implicated in tumour development are the FeLV and the Rabies vaccines. The likelihood of sarcoma development increases with the number of vaccines administered in one anatomical location. Most tumours arise within a few months, however the process may take years in some cats.

The pathogenesis of the disease is thought to be that the localised inflammatory response elicited by certain vaccines may lead to uncontrolled fibroblast and myofibroblast proliferation in susceptible cats. This later evolves into an aggressive tumour.

The role of adjuvants in the development of the tumour has been the subject of much speculation, and aluminium, which is commonly used in adjuvants, has been found in vaccine-induced fibrosarcomas. However there have been no significant differences found between the incidence of tumours using adjuvanted or non-adjuvanted vaccines.

Clinical signs

The most notable clinical finding is a swelling which appears at the site of a previous vaccination or injection, usually in the interscapular space, but sometimes on the hindlimb, flank or over the scapula. This rarely occurs sooner than 2-3 months after the vaccine. The swelling then develops into a hard, non-painful mass and slowly becomes less mobile as it attaches to the underlying muscle and bone.

If it is left to grow large and outgrow its blood supply, the surface may become ulcerated and necrotic.

Diagnosis

Clinical signs will raise suspicion.

Cytology is not suitable to make a diagnosis, and a biopsy is required.

The biopsy should be a core needle or wedge biopsy to collect sufficient material for diagnosis.

A minimum database including haematology and biochemistry should be performed to determine the overall health of the cat.

Radiography: the affected area can be examined for evidence of bone lysis, a thoracic radiograph should be taken to check for metastasis.

CT or MRI will more accurately describe the lesion and are very accurate at determining tumour infiltration.

The major differentials for a mass in that area include:

vaccine granuloma

cat bite abscess

Treatment

Three modalities are used to treat this disease and together are the most likely to bring about a successful outcome:

Surgery: wide, deep surgical margins are essential due to the local aggressive nature of the tumour. Any bone in the area should also be removed if possible. If surgery is performed alone, tumours can recur within 2 months.

Radiotherapy: this can be performed in conjunction with surgery and will help control local spread.

Chemotherapy: preoperative chemotherapy can reduce tumour size and facilitate surgery. Various protocols have been used, using doxorubicin, cyclophosphamide, vincristine and carboplatin.

Pain relief should be given if necessary and buprenorphine is useful in cats.

Prognosis

Prognosis is guarded because of the local recurrence and aggressiveness of these tumours. Up to 24% of the tumours will also metastasise to distant sites.

One radical surgical excision combined with radiation or chemotherapy is the most likely treatment to succeed.

Prevention

Some recommendations for preventing or reducing the incidence of vaccine-associated tumours include:

Changing the vaccination site location

Not giving vaccinations intra-muscularly: subcutaneous tumours will be noticed sooner

Not vaccinating a cat with a history of vaccine-associated sarcomas.

Giving rabies and FeLV vaccinations in the distal hindlimbs

Keeping accurate records of the site of vaccine administration, the vaccine serial and lot number.

However these recommendations are controversial and do not prevent the disease but rather enable an earlier diagnosis and a higher surgical cure rate.

Vaccine-Associated Sarcoma Learning Resources
Flashcards Test your knowledge using flashcard type questions	Feline Medicine Q&A 17

References

Withrow, S. (2001) Small animal clinical oncology Elsevier Health Sciences

Merck and Co (2008) Merck Veterinary Manual Merial

Norsworthy, G. (2011) The Feline Patient John Wiley and Sons