INFECTIOUS DISEASES OF MICE 385 



the first 2 to 4 days, slow enlargement of the solitary lymphoid follicles, 

 Peyer's patches, and mesenteric lymph nodes occurs, with catarrhal inflam- 

 mation of the mucosa appearing during the latter part of this period. 

 After blood stream invasion and the appearance of symptoms, the mucosa 

 becomes progressively red and swollen, mucus appears in increasing amounts 

 and hemorrhages into the mucosa and lumen are found. Ulceration of 

 lymph follicles appears. Similar but less marked changes are found in the 

 stomach, especially in the pyloric portion. Microscopically, the picture is 

 that of a severe .enteritis — capillary injection, denuding of epithelium, 

 ulceration of lymphoid follicles, and infiltration with polymorphonuclear 

 leukocytes and histiocytes. The mesenteric lymph nodes are swollen, con- 

 gested, hemorrhagic, and often show focal necrosis. Cellular infiltration 

 and bacteria may be found from the first few days of the infection. 



The spleen is regularly enlarged to three or four times its normal size. 

 In color, it is dark red or reddish-purple; its capsule is tight and its consist- 

 ency firm. Rarely, yellowish-white nodules may be seen under the capsule. 

 On sectioning, the pulp protrudes and the cut surface is mottled with 

 irregular hemorrhagic and gray areas. Histologically, congestive hypere- 

 mia, increase in pulp cells, degeneration of lymphoid cells in Malpighian 

 corpuscles, infiltration with inflammatory cells, and occasionally focal 

 areas of necrosis are the chief findings. Intra- and extracellular bacteria 

 are present after the fourth day. 



The liver enlarges progressively up to twice its normal size during the 

 course of the disease. It varies from a deep red to a brownish-yellow color 

 and is friable in consistency. The capsule is usually smooth but, when peri- 

 tonitis is present, it may be covered with a fibrinous exudate containing 

 bacteria. From the fifth or sixth day on, small, yellow, pin-head sized 

 lesions appear and increase in size and number. Microscopically these 

 consist of foci of lymphoid-like cells which begin to form about the second 

 day. The foci increase in size and then become necrotic. Liver cells sur- 

 rounding them also become necrotic, but retain their normal structural 

 alignment. The areas remain fairly well circumscribed, but are infiltrated 

 at the periphery with granulocytes and histiocytes. Occasionally, fibrinous 

 thrombi are found in the hepatic capillaries, and capillary hemorrhage into 

 the parenchyma is irregularly present. Bacteria are found extracellularly 

 in the tissues, intracellular}^ in neutrophiles and histiocytes, and frequently 

 in clumps in the capillaries. 



The lungs may be entirely free from involvement, may show punctate 

 hemorrhages, or may be congested and hyperemic. Histologically, the 



