Spontaneous occurrence of tumors in laboratory-reared arvicoline rodents

Data from more than 2500 laboratory-reared rodents of four arvicoline genera raised since 1966 indicated that the incidence of neoplasia increased spontaneously in a nearly linear manner from 1968 to 1974. The incidence was highest in Clethrionomys rutilus, with an age-adjusted rate of 32 tumors/100 animals in 1974. The various species of Dicrostonyx exhibited dissimilar rates (22 tumors/100 animals, average for 1972 to 1973) with lower rates observed in the genera Microtus and Lemmus. Tumors of epithelial origin predominated with four common types accounting for 69% of the 260 tumors observed. A trend toward development of multiple primary tumors corresponded with the increase in incidence. An agent, identified as an adenovirus, was isolated from Harderian and preputial gland adenocarcinomas, and a second virus, tentatively identified as a hamster type H partcle, was observed in mammary and salivary gland tumors of Dicrostonyx stevensoni and C. rutilus, respectively. These and other data presented suggest an infectious etiology.