Pathologic Features of Legionnaires' Disease 



John A Blackitioii. Fnincis W. Cliaiuller. and Manui D Ilicklin 



Both autopsy and biopsy specimens from patients with Legionnaires' disease (LD) have been 

 examined in the Pathology Division laboratory at the Center for Disease Control. The only con- 

 sistent pathologic findings in autopsies of patients dying with acute LD were in the lungs where 

 a pneumonia was present. This pneumonia was frequently contTuent. often involving one or more 

 lobes in their entirety. In lung specimens the diseased tissue has appeared gray and granular. 

 Fibrinous pleuritis has frequently been present. 



Microscopically, the histologic pattern of tissues examined at autopsy from patients with 

 LD has been principally that of an acute fibrinopurulent pneumonia which resembles the hepati- 

 zation stages of lobar pneumonia. Of particular note is the exudation of neutrophils, macro- 

 phages, and large amounts of fibrin into the alveolar spaces (Figs. 1 and 2). Often this material 

 can be seen passing through pores of Kohn. In some cases there is extensive necrosis of the in- 

 flammatory exudate. The extent of inflammatory cell necrosis varies, and the observer should 

 not be surprised to find areas and sometimes entire lungs in which the cells are quite intact. The 

 underlying lung structure remains relatively undisturbed in acute pneumonias caused by the 

 Legionnaires' disease bacterium (LDB). However, focal interstitial necrosis, which is virtually 

 nonexistent in uncomplicated pneumococcal lobar pneumonia, is occasionally seen in LD. The 

 degree to which red blood cells extravasate into the alveoli varies. Most sections contain few or 

 no intra-alveolar erythrocytes, but areas of moderate to severe hemorrhage are occasionally 

 present. In LD pneumonia, the inflammatory process has not been noted to involve blood vessel 

 walls or large bronchi. Interstitial infiltration during the acute stage of the disease is minimal. 



The staining quahties of the LDB are unusual. It does not stain with hematoxylin and eosin 

 and stains only rarely with standard tissue stains for bacteria, such as the Brown-Brenn, Brown- 

 Hopps, and MacCallum-Goodpasture procedures. However, the organism can be demonstrated 

 consistently with the Dieterle silver impregnation procedure (Fig. 3). a fact which makes it the 

 staining method of choice for the LDB in our laboratory. Other investigators have reported to us 

 that the Warthin-Starry and other silver impregnation techniques are also effective. Some institu- 

 tions have used various modifications of the Giemsa stain with some success in demonstrating 

 the LDB in paraffin sections, but in our hands this procedure has not been satisfactory. Even 

 when the organisms can be stained with this method, it is difficult to obtain adequate color 

 differentiation between the bacterium and the inflammatory background. The Gimenez stain 

 is useful for impression smears and frozen sections but is unsatisfactory for paraffin-embedded 

 tissue. 



In tissue sections stained by the Dieterle selver impregnation procedure the LDB's appear 

 as short, pleomorphic rods (Fig. 4) measuring 2 to 4 microns in length and up to 1 micron in 

 diameter. Some of these rods appear bipolar, and beaded forms are noted. Organisms are dif- 

 fusely distributed throughout the areas affected by the acute pneumonia, but concentrations 

 vary from patient to patient. The degree of necrosis of the inflammatory exudate is directly 

 related to the number of organisms that can be demonstrated. Clusters of organisms are commonly 

 observed within macrophages in Dieterle-stained sections. In some cases, by focusing up and 



10 



