Nocardia asteroides and nocardiosis: a rare infection explained

Nocardia is a genus of bacteria found widely in soil and water. Patient.info, a UK clinical reference for professionals, explains that Nocardia can cause several illnesses and a disseminated, or invasive, disease known as nocardiosis. Disseminated nocardiosis is most commonly caused by Nocardia asteroides, while Nocardia brasiliensis is found in the tropics and causes skin infection. Other species include Nocardia otitidiscaviarum and Nocardia transvalensis.

Newer gene-sequencing methods have identified many more species, and the group has been reassessed and reorganised into complexes, so a laboratory report may name a species other than asteroides. In the London hospital series described below, Nocardia farcinica accounted for half of all the isolates identified.

Patient.info describes it as an opportunistic infection, one that takes advantage of reduced immune defences or damaged tissue. The sources read for this page are written for clinicians; there is no NHS patient information page on nocardiosis among them, so this guide should be read as background rather than as personal medical advice.

Patient.info lists groups at increased risk: people with pre-existing lung disease such as chronic obstructive pulmonary disease or alveolar proteinosis, people on chronic steroids or on azathioprine and methotrexate, transplant recipients such as those with a kidney transplant, and people with HIV or AIDS. It says adults are most commonly affected, especially men, there is no seasonality and outbreaks are rare.

The London case series, from St George's Hospital, covered 18 laboratory-confirmed cases between 2000 and 2018. Seventy-two per cent were male, all were UK residents and the median age was 56 years. Most had underlying pathology or risk factors, including cancer in 39% and immunosuppression in 33%.

Patient.info notes that prevention is mainly practical: avoiding walking barefoot in high-risk areas and covering cuts and grazes appropriately, along with clinicians keeping a high index of suspicion in people at risk.

Patient.info says Nocardia can affect any organ but mostly leads to five types of illness: pneumonia, skin involvement, disseminated nocardiosis, central nervous system involvement and eye involvement.

Lung infection may be indistinguishable from other pneumonias. It is usually subacute, developing over weeks, and should be suspected in any prolonged pneumonia that does not respond to the usual antibiotics. Features can include cough, fever, malaise, loss of appetite, night sweats and weight loss, with coughing up blood, chest pain and breathlessness usually less prominent. Chest X-rays may show nodules that can be mistaken for cancer spread, and about 30% of patients may have a pleural effusion.

Skin infection can appear as ulcers, nodules, abscesses, cellulitis, a lesion with a draining ulcer and enlarged local lymph nodes, or a mycetoma, a slowly progressive mass commonly on the hands and feet that Patient.info says is common in parts of Central and South America and Africa. Disseminated disease occurs when bacteria spread through the blood, with up to 50% of nocardial pneumonias disseminating according to Patient.info, to the kidneys, skin, gut and especially the brain.

The central nervous system is affected in more than 30% of cases of nocardial infection, usually as abscesses, with features including headache, fever and focal neurological deficits such as seizures, though some patients have no symptoms. Eye infection is rare and usually follows injury or surgery.

Patient.info says the diagnosis is confirmed by growing nocardia from tissue samples sent for staining and culture; specimens include sputum, bronchoscopy samples and skin or brain biopsies. No blood antibody tests are currently available for diagnosis. Nocardia is very slow growing, taking up to four weeks to produce a positive culture, so the microbiology laboratory must be informed if it is suspected so that samples can be incubated for longer.

The London series shows the practical difficulty. Routine 48-hour bacterial culture was positive for 15 of 18 isolates, and three culture-negative specimens were later confirmed as positive in a reference laboratory. The authors concluded that Nocardia should never be regarded as a contaminant or commensal organism in clinical specimens and that isolates should also be sent to a reference laboratory.

Patient.info says trimethoprim-sulfamethoxazole (co-trimoxazole) is the treatment of choice, with imipenem, amikacin and third-generation cephalosporins also used and combination therapy sometimes giving better results. Resistance has been described in some species.

Treatment is usually prolonged because of the risk of relapse: six to twelve months of antimicrobial therapy for people with normal immunity, and a minimum of twelve months for immunocompromised patients or those with brain involvement. Abscesses may need drainage, either surgically or radiologically, and empyemas should be drained. People who have been infected before or remain at risk may need long-term preventive therapy, for example people with HIV or transplant recipients.

In the London series, 55% of patients required surgical intervention and one co-trimoxazole-resistant isolate was identified.

Patient.info lists lung fibrosis with long-term breathing difficulty, scarring and deformity of infected skin, and long-term neurological deficits from brain abscesses as possible complications. Prognosis varies with the site of infection; central nervous system involvement, disseminated disease or involvement of more than one site carries a poorer outlook.

The London series illustrates how serious the illness can be in a referral hospital population: 78% required critical care, and more than half of patients (55%) died from refractory infection, including all of those with disseminated disease. These figures come from a small single-centre series of patients, many with cancer or immunosuppression, and should not be read as the outlook for every person with a Nocardia isolate.

The sources read give no patient advice on nocardiosis, so the following is general good sense rather than quoted guidance: see a GP for symptoms that persist or worsen, particularly a cough, fever, night sweats or weight loss that does not settle, and seek urgent help through NHS 111, A&E or 999 for severe breathlessness, confusion, seizures, severe headache or rapid deterioration. People who are immunosuppressed should tell their specialist team promptly about new infection symptoms.

Diagnosis and treatment of nocardiosis normally involve hospital microbiology, infectious diseases or respiratory specialists, because of the specialised laboratory work and prolonged antibiotic courses. In the UK this is usually organised through NHS hospitals, and private specialists can be consulted for opinions, but they rely on the same laboratory processes. If you have a weakened immune system and develop a persistent infection that is not responding to treatment, it is reasonable to ask your team whether unusual organisms have been considered.