Histopathologic observations in weanling B6C3F1 mice and F344/N rats and their adult parental strains.

Weanling Fischer 344/N (F344) rats and the first filial hybrid of C57BL/6 x C3H (B6C3F1) mice and retired breeders from the parental stocks of these strains were monitored over a 5-yr-period by examining the histopathology of selected organs and comparing those results to viral and mycoplasmal serology and the intestinal tract bacterial flora of each animal on an individual basis. Serology gave no evidence of viral infection, but Mycoplasma arthriditis antibodies were detected. Reactivity of serum of adult C57BL/6 female mice with control cells or media (tissue culture, TC) was seen in a significant number of mice. TC reactivity correlated positively with lymphoid perivascular infiltrates, predominantly of the lungs, suggesting an allergic response in development of the lesions. Other lesions of note consisted of Harderian gland inflammation of rats, focal necrotizing lesions of the liver of both species, and thickening of the pleura and adjacent pulmonary interstitium of weanling rats. Embolization of bacteria from the gastrointestinal tract to the liver was considered a possible cause of the liver necrosis in both species. Although lesions of the lung and Harderian gland of the rats are similar to those caused by known viral agents, the cause of the latter could not be determined as these animals were negative for viral antibodies and the former was considered to be related to incomplete pulmonary development in the young rat. Features differentiating the lesions observed in animals of this survey from those caused by viral infection are discussed.