Bacterial pneumonia accounts for 20% to 50% of pneumonia cases in HSCT recipients.38 Gram-negative organisms, specifically Pseudomonas aeruginosa and Escherichia coli, were reported to be the most common pathologic bacteria in recent prospective trials, whereas previous retrospective trials showed that common community-acquired organisms were the most common cause of pneumonia in HSCT recipients.32,39 This underscores the importance of being aware of the clinical prevalence of microorganisms and local antibiograms, along with associated institutional susceptibility profiles. Initiation of immediate empiric broad-spectrum antibiotics is essential when bacterial pneumonia is suspected.
The prevalence of viral pneumonia in stem cell transplant recipients is estimated at 28%,32 with most cases being caused by community viral pathogens such as rhinovirus, respiratory syncytial virus (RSV), influenza A and B, and parainfluenza.39 The prevention, prophylaxis, and early treatment of viral pneumonias, specifically CMV infection, have decreased the mortality associated with early pneumonia after HSCT. Co-infection with bacterial organisms must be considered and has been associated with increased mortality in the intensive care unit setting.40
Supportive treatment with rhinovirus infection is sufficient as the disease is usually self-limited in immunocompromised patients. In contrast, infection with RSV in the lower respiratory tract is associated with increased mortality in prior reports, and recent studies suggest that further exploration of prophylaxis strategies is warranted.41 Treatment with ribavirin remains the backbone of therapy, but drug toxicity continues to limit its use. The addition of immunomodulators such as RSV immune globulin or palivizumab to ribavirin remains controversial, but a retrospective review suggests that early treatment may prevent progression to lower respiratory tract infection and lead to improved mortality.42 Infection with influenza A/B must be considered during influenza season. Treatment with oseltamivir may shorten the duration of disease when influenza A/B or parainfluenza are detected. Reactivation of latent herpes simplex virus during the pre-engraftment phase should also be considered. Treatment is similar to that in nonimmunocompromised hosts. When CMV pneumonia is suspected, careful history regarding compliance with prophylactic antivirals and CMV status of both the recipient and donor are key. A presumptive diagnosis can be made with the presence of appropriate clinical scenario, supportive radiographic images showing areas of ground-glass opacification or consolidation, and positive CMV polymerase chain reaction (PCR) assay. Visualization of inclusion bodies on lung biopsy tissue remains the gold standard for diagnosis. Treatment consists of CMV immunoglobulin and ganciclovir.
Early fungal pneumonias have been associated with increased mortality in the HSCT population.43 Clinical suspicion should remain high and compliance with antifungal prophylaxis should be questioned thoroughly. Invasive aspergillosis (IA) remains the most common fungal infection. A bimodal distribution of onset of infection peaking on day 16 and again on day 96 has been described in the literature.44 Patients often present with classic pneumonia symptoms, but these may be accompanied by hemoptysis. Proven IA diagnosis requires visualization of fungal forms from biopsy or needle aspiration or a positive culture obtained in a sterile fashion.45 Most clinical data comes from experience with probable and possible diagnosis of IA. Bronchoalveolar lavage with testing with Aspergillus galactomannan assay has been shown to be clinically useful in establishing the clinical diagnosis in the HSCT population.46 Classic air-crescent findings on chest CT are helpful in establishing a possible diagnosis, but retrospective analysis reveals CT findings such as focal infiltrates and pulmonary nodular patterns are more common.47 First-line treatment with voriconazole has been shown to decrease short-term mortality attributable to IA but has not had an effect on long-term, all-cause mortality.48 Surgical resection is reserved for patients with refractory disease or patients presenting with massive hemoptysis.
Mucormycosis is an emerging disease with ever increasing prevalence in the HSCT population, reflecting the improved prophylaxis and treatment of IA. Initial clinical presentation is similar to IA, most commonly affecting the lung, although craniofacial involvement is classic for mucormycosis, especially in HSCT patients with diabetes.49Mucor infections can present with massive hemoptysis due to tissue invasion and disregard for tissue and fascial planes. Diagnosis of mucormycosis is associated with as much as a six-fold increase in risk for death. Diagnosis requires identification of the organism by examination or culture and biopsy is often necessary.50,51 Amphotericin B remains first-line therapy as mucormycosis is resistant to azole antifungals, with higher doses recommended for cerebral involvement.52
Candida pulmonary infections during the early HSCT period are becoming increasingly rare due to widespread use of fluconazole prophylaxis and early treatment of mucosal involvement during neutropenia. Endemic fungal infections such as blastomycosis, coccidioidomycosis, and histoplasmosis should be considered in patients inhabiting specific geographic areas or with recent travel to these areas.