Vaccine-associated sarcoma (VAS) is a malignant tumor of cats (also rarely seen in dogs and mountain weasels); they are related to some vaccines. VAS has become a worrisome problem for veterinaries and cat owners and as a result some changes were implemented in vaccinations. These tumors are mostly related to rabies and feline leukemia vaccines but some other vaccines and treatments via injection can also cause tumors.
VAS was first identified in 1991 at the Veterinary Faculty of University of Pennsylvania. They discovered a link between very aggressive fibrosarcomas and typical vaccination spots (between the scapulas). Two possible reasons for the increase in VAS cases are the rabies and feline leukemia virus (FeLV) vaccinations containing aluminium adjuvants which came into use in 1985 and the fact that rabies shots for cats have become compulsory for cats in 1987 in Pennsylvania. In 1993, a causation between VAS and aluminium adjuvants containing rabies and FeLV vaccinations was discovered with epidemiological methods and in 1996, a research team called Vaccine-Associated Sarcoma in Cats was formed.
In 2003, a study on mountain weasels revealed that VAS can also be seen in this species. Tumors were discovered on the areas injected frequently and these tumors showed similarities to VAS tissues in cats. Again in 2003, a study comparing fibrosarcomas on the injection areas in dogs and VAS seen in areas not injected in cats revealed many similarities between the two. This means that VAS can also be seen in dogs.
Subcutaneous inflammation is seen as a risk factor in the emergence of VAS and aluminium containing vaccines are known to cause relatively more inflammations. In addition, tumor macrophages revealed aluminium adjuvant parts. Rate of occurence of VAS in cats is between 1/1000 to 1/10000 and the rate depends on the vaccine dose. The formation of the tumors takes between 3 months to 11 years after the vaccination. Fibrosarcoma is the most frequently encountered VAS type. The other types are rhabdomyosarcoma, myxosarcoma, chondrosarcoma, malignant type fibrous histiocytoma and undifferentiated sarcoma.
Similar secondary sarcomas emerging after inflammation are related to metal implants and foreign matter placed in the body in humans. Esophagus sarcomas in dogs are related to Spirocerca lupi infection (a type of round worm) and post-traumatic occular sarcoma in cats. Cats are more prone to VAS as a species because they are more vulnerable to oxidative problems. This was proven by Heinz anemia and asetaminophen intoxications.
VAS appears on or under the skin as a solid mass which has a fast growth rate. When it is first discovered, mass is usually quite large and might be ulcerated or inflamed. Generally it contains cavities filled with liquid due to its rapid growth. VAS is diagnosed with biopsy. Biopsy will reveal that a sarcoma is present but the location of the sarcoma, inflammation and necrosis will raise suspicions about VAS before the biopsy. In cats, granular tumor after vaccinations is probable and that is why distinguishing between the two before a surgical operation is important. Some tips for the biopsy: if the growth continues 3 months after the operation, if the growth is more than 2 cms or if the growth rate increases a month after the vaccination.
An Xray is done before the operation because in 1 in 5 VAS cases, tumor metastasizes and spreads generally to lungs and sometimes to lymph nodes or the skin.
Treatment and Prognosis
Treatment for VAS is aggressive surgical intervention. As soon as the tumor is discovered, it should be removed completely along with the surrounding tissue. Treatment might also include chemotherapy or radiotherapy. The most important factor for prognosis is the initial surgical intervention. According to one research, relapse occurs on average in 325 days in cats who have undergone aggressive surgery and in 79 days in cats who have been operated on later. Tumor suppressor gene p53 is frequently seen in VAS and shows a slower progression of the disease.
American Feline Practitioners Association delimits the types and frequency of vaccinations for cats with their new vaccination protocols. Particularly feline leukemia virus vaccine should only be administered to kittens and cats under risk. Feline Rhinotracheitis/Panleukopenia/Calicivirus vaccines should be administered every 3 years. Non-adjuvanted rabies vaccines (annual) should be preferred over adjuvanted 3 year rabies vaccines. In addition, vaccines should be applied to areas where a VAS tumor can be easily removed from. Vaccine brand or producer, concurrent infections, past traumas and environment play no role in the occurrence of VAS.
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