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Fig. 6.6A-H. Pelvic abscesses have many causes but when there is an IUCD, a possibility of actinomycosis should be considered. This 41-year-old woman was febrile and had a left lower quadrant tender mass. A Ultrasonography showed that the mass contained fluid and debris (*) and involved the bladder (arrows). B Transvaginal ultrasonography showed an IUCD (Lippes loop) (arrows). C, D Scans of the liver showed hypoechoic masses in the right (**) and left (*) lobes with extensions though the liver capsule into the abdominal wall (arrows). E The left kidney (*) was hydronephrotic due to ureteric involvement. F CT scans with contrast showed gas in the bladder (solid straight arrow) and the irregular thickened left wall of the bladder, involved with the pelvic mass (arrowheads). The IUCD is again shown (open arrow). The curved arrows show barium in the displaced sigmoid colon. G The mass was predominantly fluid (*) and has thick septa (arrows). The gas is shown again (curved arrow). H A liver scan shows a mass in the right lobe (arrowheads) with enhancing septa and walls. It extends to the tissues of the abdominal wall (arrows). No fistula was demonstrated with other contrast examinations but histopathology confirmed the diagnosis of actinomycosis. (Courtesy of Drs. J. G. Hochsztein, M. Koenigsberg, D. A. Green, and Radiographics, 1996)


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Thoracic actinomycosis often first appears on a chest radiograph as peripheral consolidation, frequently in a lower lobe. There are usually multiple small radiolucent areas, due to the abscesses. These may eventually coalesce into one large lung abscess, with thick walls. Actinomycosis spreads rapidly from the lung into the pleura, causing pleural thickening and eventually an empyema. The infection does not stop there, but spreads into the chest wall, causing osteomyelitis of the ribs, demonstrated radiographically as periosteal reaction and bone destruction (see Fig. 6.4). External sinuses on the chest wall will eventually develop. There is almost always abundant fibrosis, causing loss of lung volume, dense pleural thickening, and restriction of respiratory movement. If healing eventually occures, there may be calcification in the pleura and, less commonly, in the lung.

Both hilar lymphadenopathy and direct extension from the lung into the mediastinum may occur, sometimes with abscess formation. Like blastomycosis, actinomycosis may present as a hilar mass simulating bronchogenic carcinoma. However, outward spread to the chest wall is more characteristic of thoracic actinomycosis.


Any bone may be involved by direct spread from actinomycosis of the soft tissue (Fig. 6.7): hematogenous spread to bone is unusual. Characteristically there is marked soft tissue thickening beneath which the bone shows marked periostitis and local destruction. When there is intraosseous infection, periosteal expansion occurs. Cartilage is affected less often. Actinomycosis of the vertebrae may cause extensive paravertebral thickening, which may be recognized on scanning before an actual abscess has formed. Multiple vertebrae are commonly involved and show numerous small lytic defects: collapse of the vertebral bodies is common and the intervertebral disc spaces are usually preserved. When healing occurs, bone sclerosis develops and sometimes there is ossification of the intervertebral ligaments.

Unless the involvement of the neighboring soft tissues, with fibrosis and sinus formation, is recognized, actinomycosis cannot be differentiated by imaging from most other bone infections.

Aerobic Actinomycetes

The aerobic filamentous bacteria may form grains in tissue only and are divided into four genera. Clinically, however, it is easier to consider two main groups, the Nocardia spp. and the Streptomyces spp. and to include the very similar Actinomadura within this latter group. Laboratory identification can be difficult and is based largely on cell wall differences: taxonomy is complicated because of the similar organisms. The nomenclature adds to the confusion, because the same organism has often been given many names (see Synonyms in each section). The clinical history and imaging also have many variations, but it is often possible with careful imaging to suggest which main group is likely to be responsible.



Streptothricosis: in the past some authorities have called this actinomycosis. Nocardia asteroides has been called Cladothrix asteroides and Actinomyces gypsoides. Nocardia brasiliensis has been called Actinomyces mexicana and Nocardia transvalensis (which some believe is a separate species). Ger: Nocardiose (Aktinomykose durch Nocardia asteroides). Fr: Nocardiose. Sp: Nocardiosis.


Nocardiosis is an infection with one of the Nocardia spp. within the aerobic schizomycete species. This is usually N. asteroides, although N. brasiliensis and N. otitidiscavarium also infect humans. Even more uncommon is infection with N. transvalensis.

Geographic Distribution

The first human infection was described in 1890 and there were few subsequent reports until the last 30 years, when nocardiosis has been recognized more frequently, particularly in patients who are immunocompromised. N. asteroides is the most common cause in Japan, but is also found in Africa and tropical America. N. brasiliensis is most frequent in South America, and in Mexico may cause 85% or more of all mycetomas. It has also been identified in Africa and Asia.




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