The illness is biphasic and nonspecific clinically: three or four days of a viral-like syndrome, worsening, admission to the hospital, followed by rapid clinical deterioration and death one day later.⁹  The incubation period is variable; thus patients with simultaneous exposure may have onset after one or two days or even six weeks later.^{9, 12}  Early treatment and 60 days of prophylactic antibiotics are curative and preventive, respectively.¹¹  Therefore, timely diagnosis and rapid epidemiologic identification of the exposure could save thousands of lives.

In Sverdlovsk in 1979 an alert pathologist, Dr. Faina Abramova, recognized and documented that a case of hemorrhagic meningitis was caused by Bacillus anthracis .¹  Prophylactic tetracycline was given, and intensive care saved the lives of five victims of inhalation anthrax that was caused by spores that escaped from a biological weapons factory. More than 60 persons died.

Gross Pathologic Lesions
The key to the diagnosis of inhalational anthrax at autopsy is the gross examination. Marked hemorrhages into the tracheobronchial lymph nodes and massive hemorrhage into the mediastinum that dissects along the bronchi into the hila of the lungs occur in all cases (1-3, 5, 8, 11, 13, 19, 20, 22). Most cases have massive bilateral pleural effusions and boggy, gelatinous edema of the mediastinum and costal pleurae.⁹ 

A high proportion of cases will have lesions reflecting hematogenous spread of the bacilli including many submuocosal hemorrhages in the intestine and hemorrhagic meningitis (1, 3, 5, 8, 11, 13, 19, 20, 22). Some cases even have focal hemorrhages in the pulmonary parenchyma, apparently a manifestation of hematogenous spread back to the lungs.⁹  Do not expect to find anthrax pneumonia; this is an erroneous concept.

Microscopic Lesions
Microscopic examination reveals that the hemorrhages in the tracheobronchial lymph nodes, mediastinum, gastrointestinal submucosa, meninges, and lungs are accompanied by minimal-to-mild cellular infiltrate.⁹ 

Careful semiquantitative morphologic analysis by Jerry Smith and Lev Grinberg of the microscopic lesions in 41 autopsies from the Sverdlovsk event revealed vasculitis as an underlying, previously neglected, microscopic feature of disseminated B. anthracis infection.⁹  Some hemorrhages distorted and displaced tissues as if an artery had ruptured. Other hemorrhages had percolated through the tissues as if from the low pressure of a damaged vein or capillaries.

The areas of gelatinous edema show characteristic fibrin-rich exudates.

Morphologic Identification of Bacillus anthracis
Tissue gram stain usually detects large boxcar-shaped gram positive bacilli in the blood vessels and tissues of the hemorrhagic lesions if the patient has not been treated with an effective antibiotic.⁹  In some cases the bacteria stain gram variable with some organisms appearing gram negative, possibly an effect of partial treatment. In patients treated with effective antibiotics for more than 21 hours, bacilli might not be detectable at all. Electron microscopy reveals bacilli with a gram positive type cell wall and a capsule. Treated cases may contain bacilli with discontinuities in the cell wall structure.

Sherif Zaki and his group at CDC have developed immunohistochemical methods for the specific identification of B. anthracis by its cell wall antigens or its capsular antigens. These methods were more sensitive and specific than some of the other approaches applied during the anthrax attack of 2001, enabling the correct diagnosis to be made even in the face of strong opposing opinions.⁷  These techniques are also effective in evaluating liquid specimens such as pleural fluid.

Culture Diagnosis of Bacillus anthracis Infection
The most readily available specific diagnosis of anthrax is achieved by bacterial culture. Bacillus anthracis is recovered on sheep blood agar, appearing as flat, off-white, non-hemolytic colonies with an irregular margin. B. anthracis can be distinguished from B. cereus by the nonmotility of the anthrax bacilli. The isolate should be transferred as promptly as possible to a laboratory that performs specific identification (e.g, specific gamma phage lysis).

Molecular Diagnosis of Anthrax
Analysis of the genomes of B. anthracis and other bacteria have identified regions of unique DNA sequences from which PCR primers have been developed. The CDC can perform molecular diagnosis by this method, but the primer sequences have not been divulged, presumably to avoid a bioterrorist's genetic engineering of strains that would not be detected. Others have developed their own primers which function effectively to establish the diagnosis.

Pathogenesis of Inhalational Anthrax
The spores of the B. anthracis are incredibly stable and of the perfect size to reach the alveoli if dispersed as particles of a single spore or a small clump of spores of 5mm diameter or less. Fortunately for us the human lung is not a favorable location for germination of the spores. Apparently only 1 out of 10,000 inhaled spores germinates in most persons. However, in some individuals inhalation of only 1-10 spores is likely enough to establish infection that will lead to a fatal outcome unless treated early with appropriate antibiotics. The spores are phagocytosed by alveolar macrophages in which they are transported by lymphatic vessels to the hilar tracheobronchial lymph nodes where germination occurs and toxin secretion begins.^{4, 17}  The classic virulence factors of B. anthracis are encoded on two plasmids, pX02, which carries the genes for synthesis of the polyglutamic acid capsule, and pX01, which encodes three proteins, the receptor binding protein (protective antigen), lethal factor, and edema factor. The molecular pathogenesis includes the macrophage cell membrane receptor, the furin-protease processing of the protective antigen creating a binding site for lethal factor or edema factor, assembly into a heptamer of protective antigen molecules, endocytosis of the complex, mild acidification of the endocytic vacuole, insertion of the heptamer into the cell membrane forming a b-barrel, translocation of the lethal toxin or edema toxin into the macrophage, and activity of the lethal toxin as a metalloprotease with one or more of the MAP kinase kinases as a target and activity of edema toxin as a calmodulin-dependent adenyl cyclase.^{6, 15, 16, 21}  Despite knowledge of these intricate details, numerous other mechanisms remain unclear, such as the full list of targets of the lethal toxin metalloprotease and the effects of the inactivation of these target proteins and the mechanism by which activation of an adenyl cyclase in macrophages would lead to edema. Studies of the toxins in animals has led to the concept that they are the whole story of pathogenesis. This oversimplification is unlikely to be true. Consider for example the dogma that the sole target cell of the toxins is the macrophage and the observation of hemorrhages possibly due to vasculitis and the 10,000-fold difference in germination efficiency in the skin vs. the lung.¹⁰  How can these be explained?

Mechanisms of Death in Inhalational Anthrax
Analysis of the mechanism of death in the 41 autopsied patients in Sverdlovsk by a clinician who had participated in the care of the patients and a team of pathologists, who performed the autopsies and examined the microscopic lesions carefully, identified severe bilateral atelectasis owing mainly to massive pleural effusions as the primary mechanism of death in 39% and a major contributory mechanism of death in another 46%. Meningoencephalitis was the primary mechanism of death in 34% and a major contributory mechanism in 7%. Pulmonary consolidation owing principally to parenchymal hemorrhages was the primary mechanism of death in only 5% but was a major contributory mechanism in 27%. Septic shock, presumably due to the systemic effect of the toxins, was considered to be the primary mechanism of death in only one patient, although a major contributory mechanism of death in 51%. Potential life saving therapy would include not only treatment with an effective antibiotic but also drainage of the pleural effusions, treatment with neutralizing antibodies against protective antigen and lethal factor and molecules that competitively inhibit the function of protective antigen by disrupting the formation of the b-barrel pore, and inhibition of spore germination.^{14, 18} 

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