Nocardiosis in the tropical Northern Territory of Australia, 1997-2014

Background: Nocardia is an opportunistic pathogen that can cause life-threatening disease. We aimed to characterise the epidemiological, microbiological and clinical features of nocardiosis in the tropical north of Australia.

Methods: We conducted a retrospective cohort study of nocardiosis diagnosed between 1997 and 2014. Population-based incidences were calculated using district population data.

Results: Clinically significant nocardiosis was identified in sixty-one patients. The unadjusted population-based annual incidence of nocardiosis was 2.02 (95% confidence interval [CI] 1.55-2.60) per 100,000 people and was 1.7 (95% CI 0.96-2.90) fold higher in Indigenous compared with non-Indigenous persons (p=0.027). Of 61 patients, 47 (77%) had chronic lung disease, diabetes and/or hazardous alcohol consumption; 22 (36%) were immunocompromised; and 8 (13%) had no identified comorbidities. Disease presentations included pulmonary (69%; 42/61), cutaneous (13%, 8/61) and disseminated nocardiosis (15%, 9/61). The most commonly identified species were N. asteroides and N. cyriacigeorgica (each 11%). Linezolid was the only antimicrobial to which isolates were universally susceptible; 89% (48/54), 60% (32/53) and 48% (26/54) of isolates were susceptible to trimethoprim-sulfamethoxazole, ceftriaxone and imipenem, respectively. 18 patients (30%) required intensive care unit (ICU) admission and one-year mortality was 31%.

Conclusions: The incidence of nocardiosis in tropical Australia is amongst the highest reported globally. Nocardiosis occurs in both immunocompromised and immunocompetent hosts, and is associated with high rates of ICU-admission, 1-year mortality and resistance to commonly-recommended antimicrobials. Diagnosis should be considered in patients with consistent clinical features, particularly if they are Indigenous or have chronic lung disease.