INFECTIOUS DISEASES OF MICE 451 



duced in mice by the viruses of influenza (9, 80, 250, 262) and the possibiHty 

 of mistaking its identity. 



Inclusion bodies in the salivary glands and liver of the mouse. — Cellular 

 inclusions have presented something of a problem to investigators for a 

 number of years. Certain of them occur frequently in abnormal or malig- 

 nant cells, but are artefacts due to intracellular necrosis and the action of 

 ingredients in the fixatives (42). In the past two decades inclusion bodies 

 of a different type have been found in cells of the salivary glands and liver of 

 a number of animal species, including mice (68, 137, 273, 274, 168). They 

 occur quite constantly in some stocks or breeds of mice. Transmission to 

 normal young or adult mice is readily accomplished, is species-specific, and 

 no bacteria or parasites are found in association with the bodies. Foci of 

 chronic inflammatory cells are present in the affected organs. With the 

 possible exception of an epidemic mentioned by Thompson (273, 275; see 

 section on ectromelia), animals harboring them appear to be perfectly 

 healthy. These characteristics suggest that an infectious agent of low 

 pathogenicity is responsible for their production. The nature of the 

 inclusion bodies — whether degenerative, metabolic, mutative, or infectious — 

 is not known. Since the appearance of inclusions is concomitant with 

 infection by many of the known viruses, however, it is logical by analogy to 

 consider a virus as the causal agent here. Filterability, moreover, is 

 reported in one instance fi68). 



Inclusion bodies in the salivary glands. — The incidence of salivary 

 gland disease varies between 20 and 60 per cent in adult albino mice of 

 certain stocks. Other colonies may be entirely free from the disease regard- 

 less of the age of the animals, but in general mice less than i month of age 

 do not show the lesions. Spontaneous illness has not been described. The 

 natural method of transmission has not been determined, but once a colony 

 has been infected the disease continues for generations. 



Histopathologically, lesions are found only in the salivary glands. 

 Acidophilic intranuclear inclusions, usually large but of varying size, occur 

 in acinar cells of the serous and mucous portions of the glands, occasionally 

 in duct cells, and rarely in alveolar cells of the parotid. Such cells arc 

 hypertrophied and irregular in shape with granular, basophilic cytoplasm. 

 The nuclear contents may be completely replaced or distorted by inclusions 

 which are composed of minute spherules and are often surrounded by a 

 halo. Scattered foci of mononuclear cells are present throughout the tissue, 

 often without any apparent relationship to the aft'ected acinar cells. 



