Fungi form a huge kingdom of eukaryotes, of which only a few are pathogenic. This review focuses on fungi which can cause respiratory disease by tissue invasion in situations of immunosuppression. Thus, diseases resulting from a disordered immune response (e.g. allergic bronchopulmonary aspergillosis) and endemic mycoses (less common and less dependent of the immune status) are not considered in this review.

Opportunistic mycoses are caused by ubiquitous fungi, including those present in commensal flora. Mycotic invasion is controlled by intact skin and mucous membranes, neutrophil activity, and cell-mediated immune response led by CD4 cells and macrophages. There is no common response to all fungi, and distinguishing clinical features have been identified: low CD4 levels are associated with pathogens such as Pneumocystis and Cryptococcus, while neutropenia can lead to Aspergillus and Mucoralean infections.

Types of immunosuppression associated with these mycoses are listed in Table 1. It is worth noting that many of the risk factors mentioned are associated with the chronic use of high-dose corticosteroids, underlining the need for careful adjustment of immunosuppression and consideration of therapeutic alternatives.

Clinical manifestations of pulmonary mycoses are not specific, and while it is easy to suspect mycosis in the context of known immunosuppression, the opposite approach must not be forgotten: a diffuse, slowly progressing pulmonary syndrome should alert to a possible fungal infection and lead to an investigation of any possible underlying immunosuppression. Detailed history and examination may guide diagnosis: involvement of paranasal sinuses may be suggestive of Mucor, skin lesions in disseminated disease may be a good source for biopsy, etc.

With regard to diagnosis, chest X-ray may be normal, so computed tomography is the imaging test of choice. Diffuse bilateral infiltrates with areas of ground glass and/or multimodal patterns are common, but not specific,1 and microbiological culture or the detection of fungal antigens may be positive in a colonization setting, reducing their diagnostic value for invasive disease. The yield of serological testing in immunocompromised patients is poor. Biopsy has been conventionally considered as the unequivocal test for demonstrating invasive fungal proliferation, but in many cases, the clinical status of the patient prevents sampling. Moreover, the extreme seriousness of these diseases means that appropriate empirical treatment must be started as soon as possible.

The European Organization of Research and Treatment of Cancer/Mycoses Study Group (EORTC/MSG) has defined invasive mycoses as: proven (consistent histological results or positive culture in sterile medium); probable (lower grade microbiological evidence in a patient with risk factors and consistent clinical syndrome); and possible (patients with clinical syndrome and risk factors, despite absence of microbiological confirmation).2 These guidelines reflect the effort made in unifying criteria to assist the clinician in dealing with diagnostic uncertainty.

It is important to point out that treatment does not depend exclusively on the administration of antifungals. Immunosuppression must also be addressed and reduced or reversed to achieve a level of balance that remains poorly defined. In a situation of irreversible immunosuppression, there will be little response to antifungal treatment, while recovery of the immune response may be associated with clinical worsening, due to the inflammatory response. This phenomenon is well established in HIV patients, but has also been described in non-HIV patients.3

Aspergillosis is the primary pulmonary mycosis among critically ill patients,4 the most common species being A. fumigatus, A. flavus, A. niger and A. terreus. They are acquired by inhalation of conidia, which in an immunocompetent individual are eradicated by alveolar macrophages and neutrophils. Disease develops when this line of defense breaks down, or, more rarely, in situations of excessive inhalation of conidia, such as occurs during landslides or great catastrophes.5 Several clinical pulmonary forms have been described, the most important of which are invasive pulmonary aspergillosis (IPA), chronic aspergillosis, and aspergilloma.6 The type and severity of aspergillosis is determined by the characteristics of the patient (Fig. 1). Severe, generalized immunosuppressive states, such as prolonged neutropenia, are associated with acute invasive disease, while moderate, localized immunosuppression, such as preexisting cavities, favors the development of forms such as aspergilloma.

Fig. 1.

Association between degree of immunodeficiency and type of pulmonary aspergillosis.

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Mucormycosis is disease caused by Mucorales fungi, the most commonly involved genera being Rhizopus, Mucor and Rhizomucor.

They are acquired by inhalation or via damaged skin and mucous membranes. They cause rapid vascular invasion, thrombosis and secondary necrosis.32 The risk factors for mucormycosis are similar to those of aspergillosis, but diabetes mellitus with poor metabolic control, iron overload, and the use of desferoxamine should also be taken into account. Controversy persists on whether voriconazole prophylaxis can increase the risk of mucormycosis32–35; the reason for this association has not been clarified, but it is posited that it may be due to Aspergillosis being displaced, allowing the positive selection of Mucorales, which are mostly resistant to voriconazole. Paradoxically, it is rare in HIV, except in cases associated with the use of intravenous drugs and corticosteroid therapy.32

The most common sites of infection are the paranasal sinuses, the lungs and the skin. Rhino-orbital mucormycosis is mostly seen in diabetics, while in hematological patients the predominant forms are pulmonary. Lung involvement behaves similarly to very severe and rapidly invasive micronodular interstitial pulmonary disease. Mortality is around 70%–75%.32,36 Clinical symptoms include dyspnea, cough, chest pain, and hemoptysis, which on occasions can be massive. It also has a high capacity for causing cavitation and progressive invasion, which may affect the pericardium, pleura, mediastinum, and chest wall. Similarly to Aspergillosis, endobronchial involvement and airway obstruction may also occur.20 Radiological findings are very varied and include infiltrates, nodules, cavities, atelectasis, pleural effusion, and mediastinal lymphadenopathies. Like Aspergillus, it produces the halo sign, and more characteristically, the reverse halo sign.32,37

The presentation, then, is similar to that of IPA, but some factors guiding the diagnosis of mucormycosis are the following: greater rhinosinusal, palatine or facial involvement, or the presence of thoracic subcutaneous tissue.38 Radiological findings of more than 10 nodules and pleural effusion are factors more often associated with mucormycosis than IPA.39 Moreover, galactomannan and 1,3-β-d-glucan are negative in this entity. A history of voriconazole use and lack of response to this drug support the suspicion. None of these findings is specific, but are useful as guidance when waiting for microbiological results.

Diagnosis is made from respiratory cultures or biopsies. Since its presence in respiratory samples is rare, isolation in a consistent clinical context is considered diagnostic. Blood cultures are negative, even in invasive forms.37,38

Treatment must be initiated promptly,40,41 with resolution of risk factors, surgical debridement of accessible areas and administration of antifungal agents.32 Only amphotericin B and posaconazole are known to be effective. Amphotericin B is the first line treatment, while posaconazole is used in cases of renal dysfunction and in sequential oral treatment in the maintenance phase. Duration of treatment has not been established and should be individualized according to clinical response and resolution of lesions.20,21,38,42,43 Surgery plays a fundamental role in the resolution of local forms. The effectiveness of granulocyte stimulating factors in neutropenic patients has been defined.42,44

The most important pathogens of the Cryptococcus genus are Cryptococcus neoformans (C. neoformans) and the emerging Cryptococcus gattii (C. gattii). They are ubiquitous and are isolated from soils, particularly in areas frequented by birds and bats.45 While the distribution of C. neoformans is universal, C. gattii is more common in subtropical regions, and its presence has increased in recent years in some areas of the North Pacific.46

The invasive capacity of Cryptococcus is due to its expression of a polysaccharide capsule with antiphagocytic properties. It enters by the airways and causes 2 distinct clinical syndromes: meningitis and meningoencephalitis. Its pulmonary transit, however, can be blocked by a cell-mediated immune reaction and the development of granulomas; in an immunocompromised host, invasion can occur.47–49

The major risk factor is HIV infection, particularly when CD4 counts are below 100cells/mm3, CNS involvement being the predominant clinical manifestation.50 In other immunosuppressive states, such as chronic lung diseases, patients on long-term corticosteroids, or solid organ transplant recipients, the resulting syndrome is predominantly respiratory.21,51C. gattii affects the lungs, even in immunocompetent patients.46

Pulmonary cryptococcosis is characterized by fever, with dyspnea, dry cough, night sweats, and progressive, occasionally fulminant, respiratory failure.47,52,53 Severe pulmonary presentations involve a risk of dissemination to the CNS and to other regions, particularly the skin.54

The most common radiological pattern is multiple nodular involvement. Reticulonodular forms are more typical of invasive syndromes. Involvement in the form of lobar or segmentary consolidation is also possible. Thoracic lymphadenopathies may also occur. Pleural effusion and cavitation are exceptional.55

Cryptococcus in colonizations is unusual; therefore, isolation in a sputum or BAL culture consistent with clinical signs is sufficient for diagnosis. Cryptococcus antigen determination in plasma is very useful, but it can be negative in some limited pulmonary forms.53,56 Antigen detection in sputum or BAL is unreliable.47 Cerebrospinal fluid (CSF) can be studied by India ink staining, antigen detection and/or culture. Routine lumbar puncture is not recommended for all pulmonary forms, but rather in the presence of neurological symptoms, severe immunosuppression, signs of dissemination, or high serum antigen levels.57

The treatment of pulmonary forms has not been widely studied, and the principal recommendations are based on the experience of treatment of HIV patients with CNS involvement. For mild or moderate respiratory syndromes, fluconazole for 6–12 months is recommended.20,21,47,58 In severe manifestations or CNS involvement, at least 2 weeks combined treatment with amphotericin B and flucytosine is recommended.21,58,59 This treatment should be followed sequentially by a 6–12 month course of fluconazole. Treatment duration must be individualized on the basis of risk factors and recovery of immunosuppression status. For example, in HIV subjects, maintenance treatment should continue until viral replication is controlled and a stable CD4 count>100cells/mm3 has been achieved. Moreover, initiation of antiretroviral treatment should be delayed for at least 4–8 weeks to avoid immune reconstitution inflammatory syndrome, underlining once more the difficulties in achieving the immunological balance required in the management of opportunistic mycosis.56,60

Pneumocystis jirovecii is an atypical fungus, acquired by the airborne route, even in childhood.61 The main risk factor for developing this mycosis is HIV infection, particularly when CD4 levels are below 200cells/mm3. It can, however, also occur in other cell-mediated immunosuppressive states common to other mycoses.62,63 It is important to note that the immunosuppressive agents most frequently cited are corticosteroids, particularly in association with dose increases or reductions from previously high doses.64,65

The clinical syndrome in patients with HIV infection is subacute, with fever, dry cough, and progressive dyspnea, and generalized asthenia and weight loss. However, in immunosuppressed patients without HIV infection, the clinical picture is more acute, prognosis is poorer and the rates of acute respiratory failure, need for ventilatory support and mortality are higher.64,66–69 Some authors have associated this increased mortality in non-HIV patients with more severe immunosuppression status, and with delay in diagnosis and treatment, since the suspicion of pneumocystosis is not so strong.67

Diffuse bilateral interstitial infiltrates may be seen on X-ray. CT provides a more defined image of bilateral ground glass opacities, possible pneumatoceles and pneumothorax due to local parenchymal damage. It is rare to see lobar infiltrates, nodules, cavities, or pleural effusion. If the CT is normal, the diagnosis should be questioned.70

During pneumocystosis, 1,3-β-d-glucan levels are raised, leading many authors to suggest that a positive finding in a high risk patient with consistent signs and symptoms may be sufficient to reach a diagnosis. However, in view of the association with pneumocystosis with other intercurrent processes, the recommendation is to obtain a more specific diagnosis.71,72 Since it cannot be cultured, visualization using immunofluorescence techniques is needed, and yield is high. Immunofluorescence can be used in induced sputum, with sensitivity ranging between 55% and 90%, or BAL with a sensitivity of around 90%–100%. Biopsy is rarely necessary. PCR values have yet to be defined.73

If there is a clinical suspicion of pneumocystosis, treatment must be initiated without waiting for sampling of respiratory specimens, as eradication of pathogens from the respiratory secretions will take several weeks.74,75

In HIV patients, the treatment of choice is cotrimoxazole for 21 days. Alternatives include pentamidine, dapsone, atovaquone, or combined clindamycin and primaquine, depending on severity. Co-administration of corticosteroids is recommended in severe cases.76 Antiretroviral treatment must also be delayed for at least 2 weeks.68,76,77

Treatment of non-HIV patients is less widely studied, and most of the recommendations for antimicrobials apply in these cases. Corticosteroid treatment is more controversial in these patients. Some small studies were not able to show clear efficacy in terms of mortality, so corticosteroids should be continued in patients previously receiving this treatment, and initiated in cases of severe lung inflammation.21,78,79 Cotrimoxazole is the agent of choice for secondary prophylaxis and should be administered until immunological recovery. Cotrimoxazole is also given for primary prophylaxis. Alternatives include atovaquone, dapsone, or aerosolized pentamidine. The main indications are listed in Table 3.1,27,80,81

The authors state that they have no conflict of interests.