Mucorales

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-alpha inhibitors
  • Penetrating trauma/burns
  • Persistent neutropenia
  • Chronic transfusion dependence
  • Chelation therapy with deferoxamine in patients on dialysis, which increases free iron availability.
  • Hemochromatosis - mucorales species require iron in the tissue or bloodstream for invasive growth
  • Malnutrition: can cause necrotizing enterocolitis.

Pathogenesis

• Transmission:
  • Inhalation: from environmental sources (most common)
  • Cutaneous routes via trauma or direct injection/inoculation
  • 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 Syndromes

• Rhizopus oryzae is the most common cause of invasive disease.

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

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)
  • Amphotericin B active (first-line treatment), up to 10 mg/kg if CNS involvement.
  • Posaconazole active (salvage therapy):
    • 10.1007/s11046-014-9792-y
    • Study from 2015 that looked at posaconazole as salvage therapy. 4/10 patients had proven mucor, 1/10 had IFI of unknown origin. Duration of therapy ranged from 15-355 days, median 47 days. Efficacy of salvage therapy for mucor specifically in this study was 60%
    • There is a recommendation that for mucormycosis, can consider step down to oral therapy alone with posaconazole after 3 weeks of IV liposomal Amphotericin
  • Posaconazole as monotherapy: (10.1016/j.ijantimicag.2019.01.002)
    • Systematic review of contemporary management of mucormycosis infections
    • Posaconazole oral suspension monotherapy was prescribed as an initial antifungal in 11 cases. 1/11 died from disseminated infection and there was 1 case of infection recurrence.
    • The dose used was 800mg po daily of the oral suspension
    • The posaconazole MR tablet achieves higher concentrations, but as of yet has only been used for cutaneous infections
  • Synergy: may have some evidence for addition of rifampin to amphotericin or terbinafine and amphotericin. Also some suggestion regarding use of caspofungin.
• Hyperbaric oxygen: may have benefit in diabetic patients with rhinocerebral disease, but only for the duration of hyperbaric oxygen.

Human pathogens

• Rhizopus oryzae
• Rhizopus microsporus
• Rhizopus stolonifer
• Mucor circinelloides
• Rhizomucor pusillus
• Cunninghamella bertholletiae
• Apophysomyces elegans
• Saksenaea vasiformis
• Absidia corymbifera
• Syncephalastrum racemosum
• Actinomucor elegans
• Cokeromyces recurvatus
• Mortierella wolfii

Species 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	–(+)	+(–)	–	+(–)

Rhizopus

• Clinical:
  • Most common agent of mucormycosis
  • Rapidly progressing infection characterized by tissue necrosis and products of infarcts in the brain, the lungs and the intestines. Patients with DKA, Malnutrition, severe burns or immunocompromising conditions are at highest risk
• Macroscopic:
  • Very rapid growth
  • Texture deeply cottony
  • Colour white becoming grey brown on the surface, reverse pale
• Microscopic:
  • Broad hyphae, scarcely septate
  • Brown sporangiophores which may be simple, branched.
  • They can arise singly or in a group/cluster on the stolons called rhizoids,
  • Round sporangia
• Temperature:
  • Usually grows at 37ºC
  • DOES NOT grow at 54ºC

Mucor

• Clinical:
  • Less commonly causes mucormycosis, usually in the severely ill.
• Macroscopic:
  • Very rapid growth
  • Wooly texture
  • Greyish-brown colour on the surface, reverse pale
• Microscopic:
  • Broad hyphae, scarcely septate
  • Branched sporangiophores
  • Sporangia with columellas
  • No apophysis (funnel-shaped swelling)
  • Sporangiospores round to ellipsoidal
  • Chlamydospores present
  • Rhizoids/stolons absent
• Temperature:
  • Grows at 37ºC
  • DOES NOT grow at 54ºC

Rhizomucor

• Clinical:
  • Occasionally an agent of pulmonary, rhinofacial, cerebral or disseminated mucormycosis. Usually seen in hematological malignancies
• Macroscopic:
  • Rapid growth
  • Wooly texture
  • Pale brown on the surface, reverse white
• Microscopic:
  • Broad hyphae, scarcely septate
  • Rudimentary rhizoids and stolons but RARE
  • Branched sporangiophores
  • Apophysis absent
  • Sporangiospores round or oval
• Temperature:
  • Grows at 54ºC which is a unique feature and allows it to be identified

Lictheimia (previously Absidia)

• Clinical:
  • Most commonly implicated in opportunistic pulmonary invasions, infections of the skin, the meninges and kidneys. Usually in the immunocompromised host.
  • Also causes mycotic abortion in cows
• Macroscopic:
  • Rapid growth
  • Wooly texture
  • White to grayish-brown on surface, clear reverse
• Microscopic:
  • Broad hyphae, aseptate
  • Branched Sporangiophores with APOPHYSIS (funnel-shaped swelling) beneath the sporangium
  • Few rhizoids
• Temperature:
  • Grows more rapidly at 37ºC compared to 25ºC
  • L. corymbifera (the only human pathogen) is able to grow up to 52ºC but no growth at 54ºC

Apophysomyces elegans

• Clinical:
  • Causes mucormycosis. Often via trauma involving inoculation of soil or vegetative matter.
  • Host is usually IMMUNOCOMPETENT.
  • Most commonly presents as necrotizing fasciitis, osteomyelitis and angioinvasion
• Macroscopic:
  • Similar in appearance to L.corymbifera but has white colonies
  • Fluffy and cottony
  • Does not sporulate on routine media
• Microscopic:
  • Bell-shaped apophysis
  • Foot-cell like hyphal segment
  • Rhizoids
• Temperature:
  • Grows at 37ºC
  • Grows rapidly at 42ºC and up to 50ºC

Cunninghamella bertholletiae

• Clinical:
  • Occasionally can cause pulmonary or disseminated mucormycosis in a severely ill patient
  • Angioinvasive
  • Sited to have the worst prognosis of all the mucorales
• Macroscopic:
  • Very rapid growth
  • Cottony texture
  • White to grey on surface, reverse pale
• Microscopic:
  • Broad hyphae, aseptate
  • Branched sporangiophores terminating in a vesicle
  • One spored sponrangioles formed on the denticles of the vesicle surface
• Temperature:
  • Non-pathogenic forms will grow at 40ºC

Basidiobolus and Conidiobolus

• Refer to entomophthoromycosis