Microscopically, the central zone of these abscesses shows extensive tissue necrosis with an exudate of neutrophils and varying amounts of fibrin; the latter becomes more prominent as the lesions coalesce and enlarge (Fig. 23.4). Coarse clumps of fibrin may be seen in the alveoli surrounding a lung abscess. Necrosis is a prominent feature of B. pseudomallei abscesses even in early lesions, probably as a result of toxins produced by the organisms. Bacteria are usually present within the abscesses, often in large numbers, but are infrequently encountered in the surrounding normal tissue.

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Fig. 23.4 A-D. Acute melioidosis: microscopic features. A Acute abscess with central necrosis and numerous neutrophiles. AFIP 68-797. B Discrete B. pseudomallei abscess in lung. This would be seen radiographically as one of numerous small pulmonary nodules in the acute septicemic form of the disease. AFIP 69-5781. C Necrosis of myocardial fibers in a patient who died of acute septicemic melioidosis involving the heart, lungs, and other organs. Necrosis is a prominent feature of even the early lesions of acute melioidosis. AFIP 69-5780. D Acute melioidosis abscess in skeletal muscle. AFIP 69-5784.

In the lungs, small lesions often have a surrounding narrow zone of hemorrhage; diffuse hemorrhage may be seen in larger, more confluent lesions produced by the coalescence of once small discrete abscesses, especially in patients who have survived for 4 or more days. In the subacute and subclinical forms of the disease, cavitation of the pulmonary lesions is frequent; at times, melioidosis may present as a solitary lung abscess or cavity.

Unlike the acute septicemic phase of the disease, chronic melioidosis is often extrapulmonary and is usually localized to a single organ or site. In many of these patients, only the skin is involved with cutaneous vesicles, nodules, or indolent ulcers which have raised, nodular edges. These ulcers usually break down and drain. Spread of infection may occur along tissue planes, yet regional lymphadenopathy is unusual even though there are occasionally abscesses in the lower neck. In addition to the skin, and occasionally the lymph nodes, bone and lungs are most frequently affected in this stage. This can occur either by direct infection from overlying soft tissue lesions or by septicemia. Pathologically, the lesions in the chronic stage of the disease show a combination of necrosis and granulomatous inflammation, featuring epithelioid cells and foreign body or Langhans' giant cells, often with a surrounding zone of fibrosis (Fig. 23.5). The central zone of necrosis frequently contains a purulent exudate, but it may also show a caseating appearance resembling tuberculosis. Few bacteria are identified in tissue sections of chronic melioidosis lesions, although they may be grown on culture.

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Fig. 23.5 A-D Chronic melioidosis: microscopic features. A A granulomain chronic melioidosis with epithelioid cells, foreign body giant cells, and numerous small inflammatory cells. Such lesions are often surrounded by a zone of fibrosis. AFIP 68- 798. B Lesion showing central necrosis surrounded by epithelioid cells. These such caseating lesions resemble tuberculosis. B. pseudomallei bacilli are seldom identified in chronic melioidosis lesions. X38. AFIP 69-5778. C Abscess with central necrosis in a lymph node from a patient with chronic melioidosis. The lesions may resemble those of lymphogranuloma venereum, tularemia, or cat-scratch disease. X11.5. AFIP 68-796. D Osteomyelitis in chronic melioidosis showing a combination of necrosis and granulomatous inflammation with erosion of bone trabeculae. The caseous necrosis resembles tuberculosis. X38. AFIP 56-20123.