Recent advances in the understanding and management of mucormycosis

Introduction

Mucormycoses are life-threatening fungal infections mostly occurring in hematology, solid organ transplant, or diabetic patients, it may also affect immunocompetent patients following a trauma or burn¹. Nosocomial or community outbreaks have been described². Mucormycosis is characterized by host tissue infarction and necrosis resulting from vasculature invasion by hyphae starting with a specific interaction with endothelial cells. Most common clinical presentations are rhino-orbito-cerebral and pulmonary. Multicenter and single-center studies have reported an increasing incidence probably due to an increase in the at-risk population and improved diagnostic tools^3,4. In a French study, mucormycosis incidence increased by 7.3% per year, especially in patients with neutropenia⁵.

These infections are difficult to manage for several reasons⁶. Firstly, diagnosis is difficult because of clinico-radiological similarities with invasive aspergillosis and historical lack of diagnostic tools. However, new tools in serum and tissue as well as the recognition of highly suggestive radiological signs recently modified diagnostic possibilities. Secondly, treatment is an emergency and combines surgery, which is frequently required owing to the angioinvasive and necrotic character of infection⁷, and antifungal treatment. Primary in vitro resistance to several antifungal drugs limits therapeutic options⁸. However, recent data enlarge the antifungal armamentarium with the US Food and Drug Administration’s and European Medicines Agency’s approval of the new triazole isavuconazole. However, comparative clinical data are lacking, and the respective places of polyenes and different azoles need to be discussed.

Moving towards improved mucormycosis understanding

Human mucormycoses are caused by a wide range of pathogenic species. Mucormycosis location is linked to the mucorales species; Rhizopus arrhizus (Figure 1) is present in 85% of rhino-cerebral forms, compared with only 17% of non-rhino-cerebral forms in the French RetroZygo study⁹. This finding could be explained by virulence differences between Mucorales species. In experimental studies, ketoacidosis has been found to predispose mice to Rhizopus spp. but not Lichtheimia spp. infection^10,11. In parallel, corticosteroid treatment enhanced the susceptibility of mice to lung infections caused by Lichtheimia corymbifera or Lichtheimia ramosa^10,11.

Figure 1. Typical features of Rhizopus arrhizus.

(a) Sporangiophore branching and rhizoids (stereomicroscope); (b) grey-brownish colony on malt 2% medium; (c) melanized sporangium and sporangiospores.

Mucormycosis’ clinical presentation is also related to underlying conditions. Rhino-cerebral mucormycosis is the most common form in patients with diabetes mellitus¹, while pulmonary mucormycosis occurs most often in patients with hematological malignancies¹². Radiological findings in patients with pulmonary mucormycosis are also related to immunological status¹³. Although unusual, lately there have been diagnoses made in the gastrointestinal system. The stomach is more frequently involved, and then the colon. Symptoms are abdominal pain and gastro-intestinal bleeding¹⁴. Diagnosis is suspected on endoscopic findings with necrotic lesions that can lead to perforation and peritonitis¹⁵.

Mucorales can gain entry to a susceptible host through inhalation, ingestion of contaminated food, or abraded skin. These routes result in rhino-orbito-cerebral, pulmonary, gastrointestinal, or cutaneous/wound infections. One of the characteristic features of mucormycosis is its angioinvasive property, resulting in vascular thromboses and ultimately tissue necrosis. Ketoacidosis and deferoxamine are known to predispose to mucormycosis, revealing the importance of hyperglycemia, iron, and acidifying ketone bodies in mucorales virulence. Angioinvasion was reported to be related to the interaction between a spore-coating protein family (CotH) on Rhizopus spp. surface and endothelium glucose regulator protein 78 (GRP78) expressed at the surface of endothelial cells. This interaction triggers host cell injury and subsequent fungus hematogenous dissemination¹⁶. Elevated levels of serum glucose, iron, and ketone bodies increase fungal growth and induce the expression of GRP78 and CotH, resulting in increased ability of Rhizopus to invade host tissues and explaining the susceptibility of diabetic and deferoxamine-treated patients to mucormycosis. However, it should be noted that the majority of studies on virulence and the association between ketoacidosis and the occurrence of mucormycosis have been conducted with Rhizopus species¹⁷. Decreased numbers and impaired function of monocytes and neutrophils are important mucormycosis risk factors, since they are known to inhibit Mucorales spore germination. This includes patients with hematological disorders, AIDS, or liver cirrhosis, those who have undergone solid organ transplant, and those being treated with high-dose steroids^18,19. Finally, victims of natural disaster are also at risk²⁰ owing to wounds contaminated with water, soil, or debris²¹, such as after the 2004 Indian Ocean tsunami²² or after the 2011 Missouri tornado²³.

Computed tomography

The most common radiological pattern of lung mucormycosis on initial computed tomography (CT) scan is a halo sign and then nodule or mass^13,24. However, when studied very early and on serial follow-up, sequential morphologic changes could be observed as (i) reversed halo sign (Figure 2) followed by (ii) consolidation or nodule or mass with halo sign and, finally, (iii) central necrosis and air-crescent sign. For pulmonary mucormycosis, a recent study showed that there was a significant increase in the prevalence of reversed halo sign in neutropenic (79%) and non-neutropenic (31%) patients (P <0.05)²⁵.

Figure 2. Inversed halo sign.

Inversed halo sign.

Major steps in mucormycosis: towards earlier diagnosis (Figure 3)

Mucormycosis diagnosis is challenging, as it is associated with high mortality, especially in hematological patients. Early distinction from invasive aspergillosis is of utmost importance, as antifungal treatment may differ, whereas underlying conditions and clinical presentation are often similar. Until recently, mucormycosis diagnostic tools were based on limited basic microbiology and frequently led to diagnosis delay.

Figure 3. Mucormycosis: from suspicion to treatment.

PCR, polymerase chain reaction.

Unlike invasive aspergillosis, the detection of circulating antigen such as galactomannan and β-D-1,3-glucan provides no help for mucormycosis diagnosis. Therefore, samples from the infection site are highly required to diagnose mucormycosis based on the microscopic detection of typical hyphae or on a positive culture²⁶. Recently, the development of molecular biology tools has allowed the non-invasive diagnosis of mucormycosis. Million et al. designed a quantitative multiplex polymerase chain reaction (qPCR)-based 18S rRNA targeting Mucor/Rhizopus, Lichtheimia, and Rhizomucor. This PCR assay was evaluated with the aim to detect Mucorales DNA early in the course of the infection in the blood (serum)²⁷. The authors were able to detect Mucorales DNA in serum samples from 90% of patients up to three days before mucormycosis diagnosis²⁷. Negative serum PCR was also associated with better outcome as compared to patients with persistently positive PCR. Furthermore, a study among severely ill burn patients found that circulating Mucorales DNA was detected 11 (4.5–15) days before standard diagnosis for invasive wound mucormycosis²⁸. Other studies have evaluated the use of real-time PCR targeting Mucorales on tissue or respiratory samples in patients with hematological malignancy suffering from proven and probable mucormycosis^29–32.

Consequently, it is currently necessary in patients with hematological malignancies to include the value of reverse halo sign on CT combined with serum qPCR targeting Mucorales in the early diagnosis of pulmonary mucormycosis³³.

Several antifungals now available for mucormycosis treatment

Conclusion

Mucormycosis is a life-threatening fungal infection characterized by host tissue infarction and necrosis that occurs mostly in immunocompromised patients and is associated with an increasing incidence and mortality despite the availability of therapeutic tools. Determining whether the patient has invasive aspergillosis or mucormycosis could be challenging at the bedside. In this context, new tools of molecular biology have been developed to obtain earlier diagnosis and start optimal medico-surgical treatment. Comparative studies are needed to better optimize induction and consolidation treatment.