The genus Nocardia is a group of bacteria that typically causes infection only when the body’s defenses are compromised, classifying it as an opportunistic pathogen. The disease it causes, Nocardiosis, is a relatively rare but serious infection primarily affecting individuals with underlying health conditions. This bacterium is found widely in the natural environment, meaning exposure is common, but a healthy immune system is usually effective at preventing the organism from establishing an infection.
Characteristics and Environmental Source
Nocardia species are Gram-positive, aerobic bacteria that exhibit a distinctive filamentous and branching appearance under a microscope. This morphology often causes them to be mistaken for fungi, but they are true bacteria belonging to the actinomycetes group. They are also partially acid-fast due to the presence of intermediate-length mycolic acids in their cell walls.
The bacteria are ubiquitous, found in natural settings including soil rich in organic matter, dust, and both fresh and salt water. Human infection is acquired primarily by inhaling airborne dust particles, leading to a pulmonary infection. Alternatively, the bacteria can be introduced directly through the skin via cuts or puncture wounds that contact contaminated soil or water. The infection is not generally spread from person to person.
How Nocardiosis Affects the Body
Nocardiosis commonly presents in three major forms, depending on the initial site of entry and whether the infection spreads. The most frequent manifestation is pulmonary nocardiosis, which occurs after the bacteria are inhaled into the lungs. This form often presents with symptoms mimicking respiratory illnesses like tuberculosis or bacterial pneumonia, including fever, a persistent cough, and chest pain.
The infection in the lungs can lead to the formation of abscesses or cavitary lesions. The second form, primary cutaneous nocardiosis, develops when the bacteria enter through a break in the skin, often in otherwise healthy individuals. This localized infection typically results in the formation of skin ulcers, shallow sores, or nodules at the site of inoculation.
The most severe presentation is disseminated nocardiosis, which occurs when the infection spreads from its primary site, usually the lungs, to other organs via the bloodstream. The central nervous system, particularly the brain, is a frequent target for this spread, leading to the formation of brain abscesses. Symptoms of central nervous system involvement include severe headache, confusion, seizures, or focal weakness.
Identifying Vulnerable Populations
Nocardia is classified as an opportunistic pathogen because it exploits a weakened immune system; approximately 60% of cases occur in individuals with a pre-existing immune compromise. T-cell-mediated immunity is the primary defense against this organism, and impairment of this function increases susceptibility. Individuals who have received solid organ transplants, especially lung transplants, are at a higher risk due to the necessary long-term use of immunosuppressive medications.
Patients with HIV/AIDS, particularly those with advanced disease, are also highly susceptible to Nocardiosis. Other conditions that suppress the immune response include cancer, chronic high-dose corticosteroid therapy, and long-term chronic lung diseases such as chronic obstructive pulmonary disease. While primary cutaneous infection can occur in healthy individuals, the pulmonary and disseminated forms are far more common in those with underlying health issues.
Treatment Protocols
Treatment for Nocardiosis requires antibiotics and is known for its protracted duration. The primary class of antimicrobial agents used is the sulfonamides, with trimethoprim-sulfamethoxazole (TMP-SMX) being the preferred initial medication. While generally effective, the choice of antibiotic may need adjustment based on the specific Nocardia species identified and its drug susceptibility profile.
Treatment is typically administered for an extended period to prevent disease relapse, which is common if medication is discontinued prematurely. Localized pulmonary infections require a minimum of six months of therapy. Infections that have disseminated or involve the central nervous system require an even longer course, often extending to twelve months or more.