A vaccine-associated sarcoma (VAS) is a type of malignant tumor found in cats that has been linked to certain vaccines. VAS has become a concern for veterinarians and cat owners alike and has resulted in changes in recommended vaccine protocols. These tumors have been most commonly associated with rabies and feline leukemia virus vaccines.
History and pathology
VAS was first recognized at the University of Pennsylvania School of Veterinary Medicine in 1991. An association between highly aggressive fibrosarcomas and typical vaccine location (between the shoulder blades) was made. Two possible factors for the increase of VAS at this time were the introduction in 1985 of vaccines for rabies and feline leukemia virus that contained aluminum adjuvant, and a law in 1987 requiring rabies vaccination in cats in Pennsylvania.
Inflammation in the subcutis following vaccination is considered to be a risk factor in the development of VAS, and vaccines containing aluminum were found to produce more inflammation. Furthermore, particles of aluminum adjuvant have been discovered in tumor macrophages. Similar examples of sarcomas developing secondary to inflammation include tumors associated with metallic implants and foreign body material in humans, and sarcomas of the esophagus associated with Spirocerca lupi infection in dogs and ocular sarcomas in cats following trauma. The incidence of VAS is between 1 in 1000 to 1 in 10000 vaccinated cats and has been found to be dose-dependent.
Diagnosis
VAS appears as a rapidly growing firm mass in and under the skin. The mass is often quite large when first detected and can become ulcerated or infected. Diagnosis of VAS is through a biopsy. The biopsy will show the presence of a sarcoma, but information like location and the presence of inflammation or necrosis will increase the suspicion of VAS. It is possible for cats to have a granuloma form after vaccination, so it is important to differentiate between the two before radical surgery is performed. One guideline for biopsy is if a growth is present three months after surgery, if a growth is greater than two centimeters, or if a growth is becoming larger one month after vaccination.
X-rays are taken prior to surgery because about one in five cases of VAS will develop metastasis, usually to the lungs but possibly to the lymph nodes or skin.
Treatment and prognosis
Treatment of VAS is through aggressive surgery. As soon as the tumor is recognized, it should be removed with very wide margins to ensure complete removal. Treatment may also include chemotherapy or radiation therapy. The most significant prognostic factor is initial surgical treatment. One study showed that cats with radical (extensive) initial surgery had a median time to recurrence of 325 days versus 79 days for cats with marginal initial excision.
Precautionary measures
New vaccine protocols have been put forth by the American Association of Feline Practitioners that limit type and frequency of vaccinations given to cats. Specifically, the core vaccines for feline panleukopenia, feline viral rhinotracheitis, and feline calicivirus should only be given every three years to adult cats and the vaccine for feline leukemia virus should only be given to kittens and high risk cats. Also, vaccines should be given in areas making removal of VAS easier. A new canarypox vector rabies vaccine is on the market that is non-adjuvanted and creates little inflammation at the injection site.
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