Fig. 6.7 A-C. Actinomycosis in the skeleton. A Affecting the frontal bone and cranial vault, extending back to the midparietal region. There is bone destruction with some patchy new bone formation, especially in the frontal bone near the frontal sinus. B Infection of the thoracic spine. T 7 has partially collapsed and there is a paravertebral abscess on the left side. There is probably more pus running along the spine above and below the affected vertebra. There is a periosteal reaction around the seventh left rib posteriorly. (Courtesy of Professor Harold Jacobson). C Oblique and AP projections of the left foot of an African from Zimbabwe. There is partial destruction of the fifth metatarsal digits. There is marked soft tissue swelling and sinus formation. The discharging sinuses with yellow grains made it possible to distinguish this from leprosy.

Epidemiology and Pathology

Nocardiosis occurs in all age groups, particularly in older patients. There are no occupational or racial susceptibilities, but the majority of patients with systemic infection will be male and immunocompromised in some way. Yet, nocardiosis is not common in AIDS patients.

The usual route of infection is through the respiratory tract; less often, infection occurs via ingestion or after tissue damage. In the lungs, patients with chronic pulmonary damage or alveolar proteinosis are most at risk. Suppurating abscesses develop, usually multiple with surrounding fibrous tissue: the pus is odorless and greenish-yellow in color. Histopathologically there are very few epithelioid or giant cells. The infection may be localized, as in the lungs, but may spread to almost any part of the body, most frequently to the brain, peritoneum, muscle, and subcutaneous tissues. In the lungs there is usually chronic pneumonia and multiple abscesses, often involving the pleura and the chest wall. There is no caseation. The partially acid-fast gram-positive filaments are in the pus, sometimes with macrophages, and frequently have invaded adjacent granulation tissue.

The characteristic feature of nocardiosis is multiple abscesses, interlinked by communicating fistulae. There is very little fibrosis and scar tissue: the reaction is mostly inflammatory and suppurative. Less commonly, the fungal colonies (grains) are found in the microabscesses with a surrounding inflammatory edema. This type of infection will probably remain localized and become a nocardial mycetoma. There is no vasculitis and little fibrous tissue proliferation. Nocardia asteroides and N. brasiliensis are partially acid-fast and can be recognized by this and by the discrete slender branching hyphae (Fig. 6.8). There is an accurate complement fixing test which also can be used for monitoring treatment, but cross-reaction with antigens of some mycobacteria can occur. These are specialized tests and usually require a dedicated laboratory. Culture is easy on Sabouraud's medium. Staining with hematoxylin and eosin (H & E) can be unreliable because individual filaments may not be seen clearly. As with some other fungal and actinomycotic infections, it can be difficult to decide whether the nocardia are causal or a contaminant. Infection by N. asteroides may resemble that caused by Mycobacterium tuberculosis, particularly when the branching hyphae have broken up, but the tissue reaction and form of the bacteria are distinct and can be recognized histopathologically.