Since 1993, the cause-and-effect relationship between vaccination and soft tissue sarcoma in some cats has been well established. Yet, despite the proliferation of articles describing various treatment techniques for these aggressive tumors, new information on ways to mitigate risk for tumor formation, short of not vaccinating cats, has been conspicuously absent from the literature. A prospective, multicenter, population-based study involving over 1300 cats was designed to examine the potential extrinsic and intrinsic risk factors believed to be associated with tumor development in cats. Veterinarians submitting biopsy specimens to 6 different laboratories in which soft tissue sarcomas or basal cell tumors were confirmed were contacted for more patient medical history. The cats with basal cell tumors and those with sarcomas at sites not commonly used for vaccination served as the control group.

During the study period of January 1, 1998, to June 15, 1999, partial or complete information was obtained on 1347 cats. Cats lived in 42 states and 4 provinces. Several methods of data analysis were used, and not all cats were included in each analysis. The authors of this complex study concluded that no individual manufacturer or vaccine brand had a significantly higher or lower association with tumor development. They also concluded that vaccine is a "component cause" of tumor formation, meaning that all cats are not at equal risk for tumor formation subsequent to vaccination. This conclusion supports the hypothesis that some cats may, in fact, have a genetic predilection for tumor development. Investigating the possible link between tumorigenesis and needle gauge, use and shaking of multidose vials, mixing vaccines in a single syringe, and even syringe type, the authors report such associations to be either nonexistent or inestimable. Of particular interest is the finding that injectable medications, not just vaccines, are likely to contribute to tumorigenesis in some cats. Two classes of parenteral medications, long-acting corticosteroids and long-acting penicillin preparations, were cited.

The role of vaccine adjuvant in sarcoma tumorigenesis has been the focus of concern for several years. While the authors did not report a higher-than-average likelihood of tumor formation among cats receiving adjuvanted vaccine brands, data for this study was collected before introduction of recombinant (nonadjuvanted), and purportedly less inflammatory, vaccines.

COMMENTARY: The authors appropriately conclude that objectively defining risk factors for vaccine-associated sarcoma is difficult. Low tumor incidence, introduction of new biologicals, medical record accuracy, and minimal adverse event reporting are just a few of the obstacles involved in determining how any change in practice strategy might minimize or eliminate the risk for sarcoma formation in cats.

Multicenter case-control study of risk factors associated with development of vaccine-associated sarcomas in cats. Kass PH, Spangler WL, Hendrick MJ, et al. JAVMA 223: 1283-1292, 2003.