Epidemiologic evidence for a causal relation between vaccination and fibrosarcoma tumorigenesis in cats

Abstract

Summary Within the past 2 years, a putative causal relationship has been reported between vaccination against rabies and the development of fibrosarcomas at injection sites in cats. A retrospective study was undertaken, involving 345 cats with fibrosarcomas diagnosed between January 1991 and May 1992, to assess the causal hypothesis. Cats with fibrosarcomas developing at body locations where vaccines are typically administered (n = 185) were compared with controls (n = 160) having fibrosarcomas at locations not typically used for vaccination. In cats receiving FeLV vaccination within 2 years of tumorigenesis, the time between vaccination and tumor development was significantly (P = 0.005) shorter for tumors developing at sites where vaccines are typically administered than for tumors at other sites. Univariate analysis, adjusted for age, revealed associations between FeLV vaccination (odds ratio [or] = 2.82; 95% confidence interval [CI] = 1.54 to 5.15), rabies vaccination at the cervical/interscapular region (or = 2.09; 95% CI = 1.01 to 4.31), and rabies vaccination at the femoral region (or = 1.83; 95% CI = 0.65 to 5.10) with fibrosarcoma development at the vaccination site within 1 year of vaccination. Multivariate analysis, adjusted for age and other vaccines, also revealed increased risks after FeLV (or = 5.49; 95% CI = 1.98 to 15.24) and rabies (or = 1.99; 95% CI = 0.72 to 5.54) vaccination. The risk of cats developing fibrosarcoma from a single vaccination in the cervical/interscapular region was almost 50% higher than in cats not receiving vaccines at that site; the risk in cats with 2 vaccinations was approximately 127% higher and the risk with 3 or 4 vaccines was approximately 175% higher. However, the frequency of fibrosarcomas in the population is low (estimated at 20/100,000 cats). In approximately half of the cats in our study, fibrosarcomas were at sites where vaccines conventionally are given, and of those cats, approximately half (depending on vaccine) had previously received a vaccine at the tumor site. Although we believe that veterinarians should not alter their vaccination protocols, precautions in vaccine administration (administering different vaccines at varied sites) and further informed consent (presenting current information in a proper benefit/risk context) appear to be advisable.