Mucormycosis: causative agents, pathogenesis, symptoms, diagnosis and treatment

May 19, 2021 Gaurab Karki Microbiology, Mycology 0




Mucormycosis: causative agents, pathogenesis, symptoms, diagnosis and treatment
Mucormycosis: causative agents, pathogenesis, symptoms, diagnosis and treatment

Mucormycosis

  • Mucormycosis is an opportunistic fungal infection which is rare but serious infection.
  • It is also called as black fungus infection which has been reported recently in the recovered patients of COVID-19.
  • Mucormycosis is sometimes called as zygomycosis.
  • It is caused by mucormycetes which is group of fungi.
  • Several fungi belonging to the phylum Glomeromycota cause it. They are the saprophytic fungi and are present ubiquitously in the soil and environment.
  • The first authentic human case of mucormycosis was reported in 1855. It was described by Kurchenmeister in a patient of neoplastic lung on the basis of histopathology.
  • Pulmonary mucormycosis was described by Furbringer in 1876 for the first time.
  • Mycology:
    • Mucormycetes is a group of lower fungi.
    • Their hyphae are generally non-septate, sparsely septate or pauci-septate.
    • Reproduction occurs asexually by sporangiospores and/or by means of conidial development.
    • Reproduction occurs sexually by the formation of zygospore.

Common causative agents of Mucormycosis:

  • Rhizopus arrhizus ( Old name Rhizopus oryzae )
  • Rhizopus microspores var. rhizopodiformis
  • Mucor racemosus
  • Rhizomucor pusillus
  • Lichtheimia corymbifera ( Mycocladus corymbiferus or Absidia corymbifera )
  • Apophysomyces elegans
  • Cunninghamella bertholletiae
  • Saksenaea vasiformis
  • Cokeromyces recurvatus
  • Syncephalastrum recemosum
  • Absidia corymbifera

Modes of Transmission of Mucormycosis:

  •  Mucormycosis is caused by:
    • Inhalation of spores present in air.
    • Ingestion from contaminated food
    • Inoculation into skin amd soft tissues by trauma Trauma inoculates in skin and soft tissue.
    • Person to person and between animals and persons transmission doesn’t occur.

Virulence factor of Rhizopus arrhizus

  • Rhizopus arrhizus is the commonest encountered mucormycetes which has several virulence factors such as:
    • Angioinvasive nature
    • Growth at the body temperature or above
    • Destructive enzymes production
    • Dormant spores
    • Active ketone reductase system
    • Hydroxamate siderophores

Pathogenesis of Mucormycosis:

  • Infection can occur by either inhalation, percutaneous inoculation or ingestion of fungal spore.
  • Phagocytes, polymorphonuclear neutrophils and macrophages play a critical role in patients of mucormycosis.
  • When the spores are inhaled into lungs then alveolar macrophages ingest it.
  • After ingestion, macrophages inhibit the germination of spores but the activity to kill them is limited. So, they can escape out from the antifungal activity of the macrophages and germinate into mycelia form .
  • Then the expected to work against the fungi are polymorphonuclear neutrophils and peripheral monocytes.
  •  The leukocytopenic patients are susceptible to this disease because they lack the sufficient cell like these which perform the antifungal activity.
  • There is also the risk of mucormycosis to the patients of ketoacidosis ( metabolic acidosis due to accumulation of ketone bodies in the blood).
  • It may be due to the release of iron bound to the protein.
  • The low serum pH due to the ketoacidosis decreases the phagocytic effect of macrophages, chemotactic and oxidative burst of neutrophils.

Risk factor for Mucormycosis

  1. Diabetes patients with diabetes kitoacidosis:
    • The patients of diabetes are at risk of mucormycosis when they have got ketoacidosis.
    • It is because diabetic patient have the high glucose level in their blood and Rhizopus  can thrive in it too.
    • Dye to the presence of active ketone reductae system Rhizopus can survive in high glucose and acidotic conditions.
    • Due to the impaired glutathione pathway, these patients also have decreased phagocytic activity.
    • Though the exact phenomenon is unknown, it might be due to the metabolic abnormalities in combination with diabetic patients.
    • Similarly, the invivo growth of fungus is not supported solely by hyperglycemia o acidosis.
    • Acidosis without hyperglycemia has been reported with the invasive mucormycosis.
    • Rhizopus gets inhibited at the normal serum but the growth is stimulated in the patients of diabetic ketoacidosis.
    • It has been found that the patients on dialysis and iron overload, who are being treated with deferoxamine, an iron chelator are susceptible to mucormycosis.
    • It may be due to Mucorales which to obtain more iron use the siderophore as the chelator.
  2. Other risk factors includes:
    • Neutropenia ( abnormal decrease of the neutrphil in circulating blood )
    • High-dose systemic steroids
    • Protein-calorie malnutrition
    • Solid organ and bone marrow transplantation
    • Immunodeficiency
    • Leukemia (blood cancer caused by increase in White blood cell )
    • Intravenous drug use

Histopathology of Mucormycosis:

  • The primary histopathological features of mucormycosis are:
    • Angioinvasion: ( spread of tumor into a blood vessel, also called as vascular invasion)
    • Thrombosis: (formation of the fibrinous clot in any part of the circulatory system)
    • Ischemia: (deficiency of blood supply to an organ or tissue)
    • Tissue necrosis: (death of tissue in the living body)

Clinical features of Mucormycosis:

  • It is the fatal disease which progresses rapidly due to its involvement in blood vessels and being angioinvasive in nature.It is of six types:
    • Rhinocerebral Mucormycosis
    • Pulmonary Mucormycosis
    • Cutaneous Mucormycosis
    • Gastrointestinal Mucormycosis
    • Isolated Renal Mucormycosis
    • Disseminated Mucormycosis

i) Rhinocerebral Mucormycosis:

  • It is the most common and fulminating type of mucormycosis.
  • If it is left untreated it may lead to fatal consequences within a week.
  • It occurs when the inhaled spores germinates in the nasal passage then making spreading from the nasal mucosa to the turbinate bones, paranasal sinuses, orbit and palate.
  • It then spreads upto the brain where its invades blood vessels massively causing the major infarct (area of necrosis due to deficient blood supply )
  • The imaging techniques like CT or MRI shows the destruction of bones.
  • Mostly it is caused by Rhizopus arrhizus and others etiological agents are also reported.
  • Symptoms includes:
    • Facial pain
    • Head-ache
    • Lethargy
    • Loss of vision
    • Brownish, bloodstained nasal discharge
    • Black eschar on palate ( due to hemorrhage and tissue necrosis )
    • Fixed and dilated pupil
    • Global proptosis and ptosis
    • Dysfunction of cranial nerves ( especially 5th and 7th nerves )
    • Extensive and rapid destruction to the surrounding tissues.
    • Sometimes the infection may spread to other parts like lungs, gastrointestinal tract, skin.
    • Symptoms of orbital (related to orbit which is the socket in the skull where eyeball is present) mucormycosis:
    • Chemosis (oedema of the conjunctiva producing the pronounced ring around the cornea)
    • Periorbital cellulitis
    • Ophthalmoplegia ( partial or total paralysis of the muscles that moves the eye)
    • Proptosis ( forward displacement of any organ )
    • Ptosis ( drooping or falling or lowering out of eyelids or other organs )
    • Abrupt visual loss
    • Orbital pain
    • Facial hypoesthesia ( decreased sensitivity particularly of touch )
    • Infection may spread from orbit to brain which may lead to frontal lobe necrosis and abscesses formatin.
    • Invasion of the fungi in blood vessels causes necrotizing (causing the death of tissues ) vasculitis ( inflammation of blood vessels ) which results in the thrombosis (formation of fibronous clot in any part of circulatory system ) of vessel lumen.

ii) Pulmonary Mucormycosis :

  • It occurs through the inhalation of the sporangiospores.
  • The invasion of the blood vessel may lead to the destruction of lung parenchyma.
  • From the onset of the illness to the terminal stage of illness it takes about 1 to 4 weeks.
  • Symptoms includes:
    • Chest pain
    • Dyspnea
    • Hemoptysis
    • Infiltration can be seen in the chest x-ray which shows the progressive infectiom from an anatomical site to the multiple adjoining areas on the same lung.
  • The pulmonary mucormycosis and its extension is best determined by the HRCT (High Resolution Computed Tomography) scan.
  • When the patients have a reverse halo sign on CT of the chest, this entity is suspected.
  • Rarely the members of the order Mucorales are involved in forming the fungus ball like that of aspergilloma.
  • It has been reported that the Rhizopus caused the hypersensitivity pneumonitis in Scandinavian Sawmill workers (so-called Wood trimmer’s disease) and in farm workers.

iii) Cutaneous Mucormycosis:

  • It occurs in the patients of severe burns which spread to the underying tissue.
  • The severe underlying necrosis develops.
  • In the diabetic patient cutaneous lesions may occur at the site of injection.
  • It may occur when the contaminated surgical dressings or the splints are applied in the skin.
  • Clinical manifestation may vary from the pustules or vesicles to wounds with wider areas of necrotic zones.
  • Lesions in the early stages resembles ecthyma gangrenosum. In this condition, cotton like growth can be seen over the surface of the tissue. This clinical sign is called as “hairy pus”.
  • Cutaneous mucormycosis can be primary infection or it can be secondary to the disseminated form.
  • For the primary cutaneous mucormycosis, Saksenaea and Apophysomyces should be suspected as the causative agent. It occurs when there is contamination of the wound and traumatic injured areas with dust or soil. After few days blistering and necrotic lesions occurs.
  • Apophysomyces tends to invade the vascular lumen causing thrombosis leading to ischemia and finally tissue necrosis.

iv) Gastrointestinal  Mucormycosis:

  • Rarely occurring mucormycosis which is about 7 % of all the mucormyccosis.
  • It most often involves in stomach.
  • It occurs in patients with extreme malnutrition.
  • Ingestion of foods like fermented milk, porridge, and alcohol made from corn and herbal products  contaminated with fungal spores causes gastrointestinal mucormycosis.
  • Lesions occurs  in stomach which are followed by colon, ileum and esophagus.
  • Invasive mucormycetes either colonize or invade gastric mucosa.
  • Non-specific symptoms include:
    •  Abdominal pains
    • Diarrhea
    • Hematemesis
    • Melena
  • Patients on dialysis who are undergoing treatment with desferrioxamine are also reported to develop mucormycosis.
  • Causative agents of gastrointestinal mucormycosis:
    • Lichtheimia corymbifera
    • Basidiobolus ranarum

v) Isolated Renal Mucormycosis:

  • It is one of the emerging clinical entity.
  • It is an unusual cause of renal infarction, if not detected in time may be fatal.
  • It infects the kidney.
  • Symptoms includes:
    • Flank pain
    • Fever
    • Pyuria
    • Infarct of renal tissue leading hematuria
    • Initially patents are assumed to be infected with bacterial pyelonephritis and does not respond to the antibiotics.
    • Acute pyelonephritis occur in immunocompromised patients.

vi) Disseminated Mucormycosis:

  • Portal of entry is skin which may be from trauma. It then invades blood vessels and disseminates.
  • Dissemination may occur to different parts of body which affects lungs, kidney, gastrointestinal tract, heart and brain.
  • Lungs is the most commonly involved site.
  • Symptoms includes:
    • Pneumonia
    • Stroke
    • Subarachnoid hemorrhage
    • Brain abscess
    • Cellulitis
    • Gangrene of skin
    • Gastrointestinal bleeding
    • Peritonitis
    • Acute myocardial infarction
    • Hepatitis
  • Disseminated Mucormycosis is caused by:
    • Apophysomyces
    • Saksenaea
    • Rhizopus
  • Cerebral infection is distinct from the rhinocerebral mucormycosis because it leads to the focal neurological signs.
  • Disseminated infections by Mucor species:
  • Endophthalmitis
  • Prosthetic mitral valve mucormycosis

Laboratory diagnosis of mucormycosis:

  • Diagnosis is slightly difficult because of rapid and fulminant course of disease.
  • Doubtful significance of isolates because they are usually encountered as laboratory contaminants.

Specimens:

  • Nasal discharge
  • Biopsy
  • Sputum
  • Bronchoalveolar lavage
  • Necrotic lesions
  • CSF

Direct Examinations: by Microscopy

  • KOH Wet mount preparation of the nasal discharge or biopsy material shows  broad, non-septate, thick walled ribbon-like hyphae with wide-angle or right-angle branching at irregular intervals which are the characteristics of  hyphal elements of mucormycetes. It is  different  from other fungi. For example: Hyphae of Aspergillus, Fusarium and/or Scedosporium spp.have  slender hyphae with regular dichotomous branching and frequent septation.
  • Microscopic morphology of Saksenaea vasiformis shows a typical trumpet-shaped sporangiophore.
  • Calcofluor white staining
  • Well stained in H & E but poorly stained with PAS
  • Other staining: Gridley, Gram’s and GMS
  • Immunochemical staining methods
  • Frozen section evaluation while patient is undergoing operative procedure

Fungal Culture:

  • Specimens should be directly inoculated in the culture media avoiding grinding. It is because the hyphal elements are prone to physical damage.
  • Mucormycetes grows at conventional media like Sabouraud Dextrose Agar (SDA) with antibiotics at 25°C and 37°C.
  • BHI broth can be used for biopsy material.
  • Portion of tissue can be kept in water which is added with  malt yeast broth. Then molecular techniques can be done directly from the sample.
  • Fibrous and cotton-candy growth in media. Also called as “lid-lifters”.
  • Apophysomyces spp. , Saksenaea spp. and Mortierella wolfii under routine cultural conditions on SDA, CMA or PDA may produce only sterile hypahe without any spores.
  • For the induction of sporulation various induction techniques are designed:
  • Czapek Dox agar and slide culture in humid atmosphere
  • Padhye and Ajello technique: use of saline agar (1%) supplemented with grass and hay
  • Soil extract medium

Serological test:

  • No routine serological test

Treatment of Mucormycosis:

  • Rapid correction of underlying predisposing factor of the host like diabetic ketoacidosis
  • Surgical debridement of necrotizing tissue
  • Antifungal therapy
  • Consideration of adjunctive therapy such as hyperbaric oxygen
  • Drugs: Intravenous amphotericin B, Posaconazole and Isavuconazole