Merck Manual

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Larry M. Bush

, MD, FACP, Charles E. Schmidt College of Medicine, Florida Atlantic University

Last full review/revision Jun 2019| Content last modified Jun 2019
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Nocardiosis is an infection (typically of the lungs) caused by the gram-positive bacteria Nocardia.

  • Nocardia bacteria can infect the lungs when they are inhaled, and they can infect the skin when they enter the skin through a cut or scrape.

  • Symptoms depend on whether the bacteria infect the lungs (typically, causing a cough, fever, chills, chest pain, weakness, and loss of appetite) or the skin (abscesses, which may drain to the surface of the skin).

  • To diagnose nocardiosis, doctors examine a sample of infected tissue under a microscope or send the sample to a laboratory to be cultured.

  • Nocardiosis is treated with one or more antibiotics.

(See also Overview of Bacteria.)

Several species of Nocardia infect people. Nocardia asteroides usually causes lung and bodywide infections. Another species usually causes skin infections, particularly in people who live in tropical climates.

People become infected when

  • They inhale dust that contains Nocardia bacteria, resulting in a lung infection.

  • Soil or water that contains the bacteria enters the skin through a cut or scrape, resulting in a skin infection.

Nocardiosis rarely spreads from person to person.

Nocardia bacteria can spread to nearby tissue or through the bloodstream to infect any organ. They most commonly infect the brain, skin, kidneys, bone, or muscle. Abscesses (pockets of pus) may form in the infected organs.

Nocardiosis occurs worldwide in people of all ages. But it is more common among older people, especially men, and in people who have a weakened immune system. Any of the following conditions, which weaken the immune system, increases the risk of getting nocardiosis:

Nocardiosis is considered an opportunistic infection because it occurs in people with a weakened immune system. However, about 20 to 30% of people with nocardiosis have no disorder or condition that increases the risk.

In the United States, an estimated 500 to 1,000 new cases of nocardiosis occur each year.


Symptoms of nocardiosis may be short-lived (acute) or become long-lasting (chronic).

Lung infection

If the lungs are infected, symptoms usually include cough, fever, chills, chest pain, weakness, loss of appetite, and weight loss.

Skin infection

If the skin is infected, abscesses often form in the skin or the tissues under the skin. The affected skin may be firm, red, warm, and tender to the touch. A lump that contains pus may form and spread under the skin or through the lymphatic vessels. Channels may form between the lump and the skin, and the pus may drain through them to the skin's surface.

Brain infection

In up to half of people with nocardiosis, the infection spreads to the brain, causing a collection of pus (abscess) to form. A brain abscess causes severe headaches and symptoms such as weakness, confusion, and seizures.


  • Examination under a microscope or culture

Doctors may diagnose nocardiosis by identifying Nocardia bacteria in a sample of infected tissue examined under a microscope. Or they may send a sample of infected tissue to a laboratory where Nocardia bacteria, if present, can be grown (cultured) and identified. Doctors locate the infected tissue during a physical examination or on an x-ray or other imaging scan.

If Nocardia bacteria are identified, they are tested to see which antibiotics are effective (a process called susceptibility testing).


Without treatment, nocardiosis that involves the lungs or that has spread is usually fatal. With appropriate treatment, people with an abnormal immune system have about a 10% risk of dying, particularly if the infection affects only the lungs. But even with treatment, over half the people with a weakened immune system die.

Skin infections resolve in most people after treatment with antibiotics.


  • Antibiotics

Nocardiosis is treated with one or more antibiotics for several months or more. Trimethoprim/sulfamethoxazole is often used. If people have a weakened immune system or the infection has spread, they need to take a second antibiotic. Treatment is started before doctors get the test results. The antibiotics may later be adjusted based on the results of susceptibility testing.

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