Nonneoplastic lesions in the lungs

This chapter covers the range of nonneoplastic lesions occuring in the lungs of aging specific-pathogen-free (SPF) rats. The question of age association will be adressed in selected instances, but for many of the lesions there is insufficent evidence of a strong inherent tendency for an increasing prevalence with age. Reports, for instance, of an increase with age of two commonly reported lesions, namely alveolar histiocytosis and interstitial lymphoid accumulations ( Flodh et al. 1974; Kroes et al. 1981; Anver et al. 1982; Ward et al. 1983; Shibuya et al. 1986), can be attributed largely to documented or probable infection of the rodent colonies by respiratory pathogens. Our experience is that in colonies maintain free from known respiratory pathogens routine evaluation does not reveal an age association for these two lesions. This same conclusion was reached earlier by Burek (1978). The fact that much of the earlier information was derived from colonies of rats that were infected wit h respiratory pathogens is one of the major reasons why there can be no useful tabulation of the prevalence of the various lesions and no accurate comparison among commonly used strains of rats. The other major reason is the imprecesion with which nonneoplastic lesions, often of insignificant degree, are recorded. Specific infectious diseases are not included in this chapter since the emphasis is on SPF colonies. Castleman (in this volume) refers to the most important confounding agents, particulary Mycoplasma pulmonis, cilia-associated respiratory bacillus, an Sendai virus, and provides access to the relevant literature.