Breakthrough nocardiosis emerged despite standard prophylaxis in a renal transplant recipient
A renal transplant recipient in India developed disseminated nocardiosis while receiving trimethoprim/sulfamethoxazole prophylaxis, and the infection extended to both the lungs and brain. The organism proved resistant to the very drug class commonly relied on to prevent or treat nocardial disease in transplant recipients. In practice, that creates a dangerous diagnostic trap: prophylaxis can suppress susceptible strains while leaving resistant organisms free to spread.
The case began after a 39-year-old woman, 14 months removed from renal transplantation, presented with 45 days of fever, hemoptysis, anorexia, and weight loss. She was taking tacrolimus, mycophenolate sodium, prednisolone, and trimethoprim/sulfamethoxazole prophylaxis. Chest computed tomography showed right upper lobe consolidation with pleural effusion, and lung biopsy culture grew Aspergillus fumigatus; a serum galactomannan index of 8.16 supported invasive pulmonary aspergillosis under 2020 European Organization for Research and Treatment of Cancer and Mycoses Study Group criteria.
The pulmonary fungal diagnosis did not explain the full clinical picture.
Despite voriconazole and liposomal amphotericin B, the patient developed altered sensorium and bilateral lower-limb weakness and was referred to a higher center. On examination, she had asymmetric upper motor neuron weakness in the lower limbs, neck stiffness, and coarse bilateral crackles. Brain imaging then revealed multiple ring-enhancing supratentorial lesions with central diffusion restriction and surrounding edema, a pattern strongly consistent with abscesses in an immunocompromised host.
Mixed pulmonary and central nervous system disease forced a broader microbiologic search
Repeat transbronchial lung biopsy gave the decisive clue. Histopathology showed suppurative inflammation, and Fite staining demonstrated weakly acid-fast, branching, beaded filamentous bacteria. Culture on Sabouraud dextrose agar yielded dry, chalky colonies suggestive of Nocardia species. Species-level confirmation could not be completed because matrix-assisted laser desorption/ionization time-of-flight mass spectrometry failed twice and 16S rRNA sequencing was unavailable.
Susceptibility testing therefore carried real bedside weight. Using disc diffusion on cation-adjusted Mueller-Hinton agar and Comité de l’Antibiogramme de la Société Française de Microbiologie thresholds, the isolate appeared resistant to cotrimoxazole and ciprofloxacin, yet susceptible to linezolid, minocycline, imipenem, and amikacin. In a transplant patient already receiving trimethoprim/sulfamethoxazole prophylaxis, this profile explains how nocardiosis can advance silently before neurological symptoms reveal dissemination.
The final diagnosis was disseminated trimethoprim/sulfamethoxazole-resistant nocardiosis with pulmonary and cerebral involvement, occurring alongside invasive pulmonary aspergillosis. Coinfection with Nocardia and Aspergillus remains uncommon in posttransplant disease, but when it appears, overlapping radiology and immunosuppression can delay recognition long enough for the infection to spread.
Drug selection changed after cytopenias and seizure concern
Immunosuppression was reduced by withholding tacrolimus and mycophenolate, while low-dose prednisone was continued. Imipenem and linezolid were started with voriconazole, and the patient improved clinically within 1 week. The response was rapid enough to support the microbiologic diagnosis, yet the treatment course still required multiple adjustments because of toxicity and neurologic risk.
After 1 month, cytopenias developed and linezolid was switched to minocycline. Because of seizure risk, imipenem was changed to meropenem, and meropenem was continued for 3 months together with minocycline. Voriconazole was stopped after 6 months. By the end of therapy, imaging showed complete radiologic resolution of the pulmonary and intracranial lesions, and graft function was preserved. The patient remained well at 1-year follow-up and continued secondary prophylaxis with minocycline.
The treatment sequence matters because nocardiosis in transplant recipients is rarely a one-drug problem. Deep-seated disease in the lung and brain often requires prolonged combination therapy, and the antimicrobial plan must be adjusted to toxicity, tissue penetration, and evolving neurologic status rather than to a fixed protocol.
Resistance patterns make prophylaxis less reliable than clinicians expect
Trimethoprim/sulfamethoxazole resistance in Nocardia is increasingly reported worldwide, but the prevalence varies by geography, isolate source, and susceptibility method. One study found 42% of United States isolates resistant to trimethoprim/sulfamethoxazole and 61% resistant to sulfamethoxazole, whereas another study of 552 isolates across six reference laboratories reported only 2% resistance. The contrast likely reflects inter-laboratory variability and misreading of trailing minimum inhibitory concentration endpoints as much as any true biologic difference.
Indian transplant cohorts have shown similarly unsettling resistance rates. In one renal-transplant series of 16 nocardiosis cases, nine patients were already taking trimethoprim/sulfamethoxazole prophylaxis and four N. farcinica isolates were resistant. A South India 10-year analysis reported 20.8% resistance among 48 cases. A North India center found nocardiosis in 12 of 1.1% renal transplant recipients, with 60% of isolates resistant and 58.3% of cases representing breakthrough infection during prophylaxis.
These data fit the biology of the organism. Nocardia can survive and disseminate under the selective pressure of prophylaxis when resistance is present, and the consequences are severe in patients already held in check by chronic immunosuppression. Once the infection reaches the central nervous system, the clinical course becomes far more dangerous.
Brain imaging should not wait for neurologic symptoms
This case reinforces a direct diagnostic principle: brain imaging is indicated in pulmonary nocardiosis even when neurologic symptoms are absent, because central nervous system involvement occurs in 20% to 44% of cases. In transplant recipients, disseminated nocardiosis carries a reported mortality of 64% to 100%. Those numbers help explain why a pulmonary presentation cannot be treated as isolated lung disease when the host is immunosuppressed.
Species-level identification remains difficult. Morphologic and biochemical methods are often inconclusive, and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry may be unavailable or fail to classify the isolate. When susceptibility testing can be performed, it becomes central to management, especially when standard prophylaxis has already failed.
This case documents a transplant recipient who developed pulmonary aspergillosis and disseminated nocardiosis despite trimethoprim/sulfamethoxazole prophylaxis, with the nocardial isolate resistant to cotrimoxazole. The core clinical lesson is not abstract: in immunosuppressed patients with pulmonary disease and new neurologic findings, nocardiosis must stay on the differential even when prophylaxis is already in place, and susceptibility testing should guide definitive therapy.