Mucorales

Background

• Order of ubiquitous environmental fungi that are rare causes of aggressive necrotizing infections

Microbiology

• Large, aseptate dematiaceous molds with ribbon-like appearance on microscopy
• May or may not have rhizoids, depending on the species
• Human pathogens within the order include:
  • Rhizopus
    • Rhizopus oryzae, the most common cause of mucormycosis
    • Rhizopus microsporus
    • Rhizopus stolonifer
  • Mucor
    • Mucor circinelloides
  • Rhizomucor
    • Rhizomucor pusillus
  • Others
    • Cunninghamella bertholletiae, which is associated with higher mortality
    • Apophysomyces elegans
    • Saksenaea vasiformis, which has a flask-shaped sporangium
    • Absidia corymbifera
    • Syncephalastrum racemosum
    • Actinomucor elegans
    • Cokeromyces recurvatus
    • Mortierella wolfii

Identification

• Microscopy:
  • Sporangia on an apophysis on a sporangiophore that comes off of a stolon. May have rhizoids at base.
  • Sporangia may be globose (round) or pyriform (teardrop-shaped), may have collarettes.

Characteristic	Rhizopus	Mucor	Rhizomucor	Absidia
Growth at 37ºC	+(–)	–(+)	+	+
Growth at 54ºC	–	–	+	–
Sporangium	Globose	Globose	Globose
Rhizoids	Well-developer	Absent	Primitive	Present but inconspicuous
Apophysis	Inconspicuous	Absent	Absent	Present
Growth on cycloheximide	–(+)	+(–)	–	+(–)

Epidemiology

• Ubiquitous environmental fungi often found in decaying organic substances i.e. bread, fruits, vegetables, soil, compose and animal feces.
• It can infect anyone, but highest prevalence risk factors are:
  • Poorly-controlled diabetes, especially susceptible to the rhinocerebral form
  • Metabolic acidosis, e.g. DKA, which also changes the iron into a form that is more readily absorbed.
  • High-dose steroids/TNF-α inhibitors
  • Penetrating trauma/burns
  • Persistent neutropenia
  • Chronic transfusion dependence
  • Chelation therapy with deferoxamine in patients on dialysis, which increases free iron availability.
  • Hemochromatosis, since invasive growth requires iron in the tissue or bloodstream
  • Malnutrition, where it can cause necrotizing enterocolitis

Pathogenesis

• Transmission:
  • Inhalation: from environmental sources (most common)
  • Cutaneous routes via trauma or direct injection/inoculation (most common in immunocompetent hosts)
  • Gastrointestinal: from ingestion of spores in immunocompromised patients
  • Injection drug use
• Immunology
  • Strong innate immunity and cell-mediated immunity is required and predisposed if prolonged neutropenia
  • To establish infection, spores must overcome killing by phagocytes to germinate into their hyphal forms (the angioinvasive form of the infection). Larger spores may lodge in the nasal turbinates and cause local sinusitis. If you inhale a large spore inoculum, this can lead to a slowly progressing pulmonary mucormycosis even in the immunocompetent host
  • Cutaneous mucormycosis occurs secondary to direct inoculation of spores in the context of trauma
• Invasiveness
  • Mucorales have an exceptional capacity to invade blood vessels resulting in necrosis of vessel walls and mycotic thrombi. This may lead to infarction and hematogenous dissemination

Clinical Manifestations

Rhinocerebral Mucormycosis

• Rhinosinusitis
  • Present with sinus pain, congestion, headache, mouth or facial pain, hyposmia. Involved tissues are first red, then violaceous then black with thrombosis and tissue necrosis. May see necrotic eschar.
• Rhinocerebral
  • The only sign that there has been brain invasion might be bloody nasal discharge.
  • Can present with epidural/subdural abscesses and cavernous and sagittal sinus thrombosis.
• Rhino-orbital
  • Usually occurs as a result of invasion from nasal/sinus infection, resulting from ethmoid sinus disease.
  • Patients with extensive disease may present with trigeminal and facial cranial nerve palsies.
• For diagnosis, ENT scope for tissue biopsy (+/- debridement) is important.

Pulmonary Mucormycosis

• More common in patients with prolonged neutropenia, solid organ or hematologic transplants.
• Often occurs concomitantly with sinus infection.
• Difficult to distinguish from invasive pulmonary aspergillosis and often present with refractory fever despite prolonged broad spectrum antimicrobials, especially if already on anti-mold prophylaxis.
• Less commonly associated with classic “halo sign” and may actually see “reverse halo sign” which is a focal round area of ground-glass attenuation surrounded by ring consolidation.

Skin and Soft Tissue Infection

• Usually occur as a result of contaminated trauma.
• Initially starts with erythema and induration of the skin at the puncture site then progresses to necrosis with a black eschar. It then extend into the deep fascia and muscle layers.

Gastrointestinal Mucormycosis

• Rare
• Has been described in malnourished patients and premature infants where it presents as necrotizing enterocolitis
• This may lead to peritonitis after invaded through the gastric mucosa and bowel wall
• Liver abscess have been described after ingestion of herbal products contaminated with mucor
• Can also be associated with peritoneal dialysis.

Disseminated Mucormycosis

• Initial source is usually pneumonia
• Blood cultures/BALs are almost always negative, biopsy is most helpful for diagnosis
• Diagnosis is almost always made too late

Prognosis and Complications

• Overall mortality ranges from 40 to 80% depending on underlying risk factors, but is even higher in disseminated or CNS disease

Investigations

• For pulmonary disease, a CT chest can show:
  • Halo sign: a nodular infiltrate surrounded by a ring of ground glass, representing ischemia and angioinvasive disease
  • Reversed halo signs: an area of ground glass surrounded by a ring of consolidation
  • Vascular occlusion sign: termination of a vessel at the edge of a focal lesion, seen on CTPA
• For suspected rhinocerebral disease, a CT or MRI, with MRI preferred for eye or brain involvement

Diagnosis

• Culture is gold standard
  • Mincing, not grinding, is recommended for tissue samples
  • False negatives in 50%
  • Growth at 30ºC, since some strains do not grow at 37ºC
    • Temperature tolerance can help differentiate, with Mucor at ≤37ºC, Rhizopus 40-50ºC, and Rhizomucor 38-58ºC
  • Cyclohexamide susceptible, so need to be careful with selection of media
  • Microscopy
    • Wide, aseptate or pauci-septate hyaline mold with non-dichotomous branching
    • Sporangiophores branching from stolon
    • Sporangium at the end of the sporangiophore, and may be attached by an apophysis (a conical neck at the end of sporangiophore)
      • May instead have a sporangiolum, which produces spores more discretely, so it there are countable spores then it's a sporangiolum, as seen in Cunninghamella
      • May instead have finger-like merosporangia, each finger of which has a countable number of spores
    • May have columella at the neck
    • May have rhizoids at base of sporangiophore
• Non-culture methods
  • Histopathology
    • GMS or PAS are best stains, and should be requested STAT
  • Galactomannan is usually negative; if positive, it can help rule in Aspergillus in the right clinical context, and therefore help to rule out mucormycosis
  • β-D-glucan is usually negative, and guidelines recommended against use
    • Rhizopus sometimes has enough to be detectable
  • Molecular methods are not standardized, and guidelines recommend against use
    • May be available in Canada through Hospital for Sick Children in Toronto

Management

• Surgical Debridement
  • Necessary to remove as much of the necrotic tissue as possible
• Antifungals
  • Mucorales are inherently resistant to ketoconazole, fluconazole, voriconazole, flucytosine and echinocandins (since no beta-D glucan in cell wall)
  • First-line: liposomal amphotericin B 5 to 10 mg/kg daily
    • Liposomal preferred to deoxycholate
    • Use higher end of dose range for CNS involvement or solid-organ transplant
    • Dose-reduce if renal toxicity, but ideally stay above 5 mg/kg
  • Alternatives that can be used as first-line or as salvage:
    • Isavuconazole 200 mg IV q8h for 6 doses followed by 200 mg IV daily
    • Posaconazole 300 mg IV/PO q12h for 2 doses followed by 300 mg IV/PO daily
  • Synergy: may have some evidence for addition of rifampin to amphotericin or terbinafine and amphotericin. Also some suggestion regarding use of caspofungin.
  • Can step down to oral isavuconazole or posaconazole if improving on repeat imaging
  • Duration is until immunosuppression is reversed and complete radiographical resolution of the infection
• Hyperbaric oxygen: may have benefit in diabetic patients with rhinocerebral disease, but only for the duration of hyperbaric oxygen.

Further Reading

• Global guideline for the diagnosis and management of mucormycosis: an initiative of the European Confederation of Medical Mycology in cooperation with the Mycoses Study Group Education and Research Consortium. Lancet Infect Dis. 2019;19(12):e405-e421. doi: 10.1016/S1473-3099(19)30312-3.