Pathology and pathogenesis of bioterrorism-related inhalational anthrax

Abstract

During October and November 2001, public health authorities investigated 11 patients with inhalational anthrax related to a bioterrorism attack in the United States. Formalin-fixed samples from 8 patients were available for pathological and immunohistochemical (IHC) study using monoclonal antibodies against the Bacillus anthracis cell wall and capsule. Prominent serosanguinous pleural effusions and hemorrhagic mediastinitis were found in 5 patients who died. Pulmonary infiltrates seen on chest radiographs corresponded to intraalveolar edema and hyaline membranes. IHC assays demonstrated abundant intra- and extracellular bacilli, bacillary fragments, and granular antigen-staining in mediastinal lymph nodes, surrounding soft tissues, and pleura. IHC staining in lung, liver, spleen, and intestine was present primarily inside blood vessels and sinusoids. Gram's staining of tissues was not consistently positive. In 3 surviving patients, IHC of pleural samples demonstrated abundant granular antigen-staining and rare bacilli while transbronchial biopsies showed granular antigen-staining in interstitial cells. In surviving patients, bacilli were not observed with gram's stains. Pathological and IHC studies of patients who died of bioterrorism-related inhalational anthrax confirmed the route of infection. IHC was indispensable for diagnosis of surviving anthrax cases. The presence of B. anthracis antigens in the pleurae could explain the prominent and persistent hemorrhagic pleural effusions.

Figure 1.

Pathology and immunohistochemisty of mediastinal lymph nodes. A: Photograph of mediastinal compartment showing an enlarged, hemorrhagic lymph node and edematous, hemorrhagic soft tissues. B: Photomicrograph of mediastinal soft tissues showing hemorrhage. C: Photomicrograph of mediastinal lymph node showing necrosis and pyknotic nuclei. D: Photomicrograph of mediastinal lymph node showing hemorrhage, immunoblasts, and pyknotic nuclei. E: Photomicrograph of mediastinal lymph node showing abundant granular antigen-staining and bacillary fragments (B. anthracis cell-wall antibody). F: Photomicrograph of the same mediastinal lymph node, note the different pattern of granular antigen-staining when using the B. anthracis capsule antibody. H&E stain (B, C, D); immunohistochemical assay using naphthol/fast red substrate with hematoxylin counterstain (E, F). Original magnifications: B, ×5; C, D, E, and F ×40.

Figure 2.

Photomicrograph showing pathology and immunohistochemistry of lung. A: Lung showing thickened pleura with inflammation and fibrin. B: Lung showing intraalveolar edema and inflammation in the alveolar septa. C: Pleura showing gram-positive bacilli. D: Pleura showing silver staining bacilli. E: Lung showing abundant granular antigen-staining in the pleura, interalveolar septa, and intra alveolar macrophages. F: Lung showing granular antigen-staining of intraalveolar macrophages. H&E stain (A and B); gram’s stain (C); Steiner stain (D); immunohistochemical assay using the B. anthracis capsule antibody and naphthol/fast red substrate with hematoxylin counterstain (E and F). Original magnifications: A, ×10; B, ×20; C, ×100; D, E, and F, ×40.

Figure 3.

Pathology and immunohistochemistry of biopsy specimens obtained from surviving patients. A: Photomicrograph of parietal pleural biopsy specimen showing reactive mesothelium and mixed inflammation. B: Photomicrograph of parietal pleural biopsy specimen showing abundant granular antigen-staining. C: Photomicrograph of pleural effusion cell block showing bacillary fragments and granular antigen-staining. D: Photomicrograph of transbronchial biopsy showing granular antigen-staining in histiocytes of the submucosa. H&E stain (A); immunohistochemical assay using the B. anthracis capsule antibody (B) or the B. anthracis cell-wall antibody (C, D) naphthol/fast red substrate with hematoxylin counterstain. Original magnifications: A and B, ×40; C and D, ×63.

Figure 4.

Photomicrographs showing immunohistochemistry of abdominal organs from patients who died. A: Liver showing bacilli in the sinusoids and bacillary fragments in Kupffer’s cells. B: Spleen showing bacillary fragments and granular antigen-staining. C: Intestinal serosa showing granular antigen-staining. D: Intestinal submocosal blood vessel showing granular antigen-staining. Immunohistochemical assay using the B. anthracis capsule antibody (B, C, and D) or the B. anthracis cell wall antibody (A) naphthol/fast red substrate with hematoxylin counterstain. Original magnifications: A and B, ×100; C, ×63; D, ×40.