Infectious Disease and Microbiology - Absidia corymbifera

BASICS

Genus

Absidia

Species

Absidia corymbifera is the principal species of Absidia historically recognized as pathogenic to humans.

It was previously referred to as:

Absidia ramosa

In more recent taxonomic classifications, organisms historically called Absidia corymbifera have been reclassified, but the older name is still encountered in medical literature.

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MICROBIOLOGIC CHARACTERISTICS

Absidia corymbifera is a:

• Filamentous fungus

• Mold

• Member of the order Mucorales

Its morphology resembles other mucormycetes such as Rhizopus.

Characteristic microscopic features include:

• Broad hyphae

• Hyaline appearance

• Pauciseptate or nonseptate hyphae

• Hyphal diameter approximately 6–15 μm

The hyphae commonly branch at relatively wide angles, as is typical of mucormycetes.

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CULTURE CHARACTERISTICS

The fungus grows rapidly in culture.

Typical growth may be visible within:

Approximately 4 days

Colonies are often:

• Light gray

• Grayish-brown

• Woolly or cotton-like

It can grow over a relatively broad temperature range, approximately:

25–45°C

Rapid growth helps distinguish mucormycetes from many slower-growing molds.

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EPIDEMIOLOGY

Absidia corymbifera is:

Ubiquitous in the environment.

It can be found in:

• Soil

• Decaying organic matter

• Plant material

• Environmental debris

Because it is widely distributed, its isolation from a nonsterile specimen does not always indicate invasive disease.

It may occasionally represent:

Environmental contamination or colonization.

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MODE OF ACQUISITION

The most common route of acquisition is believed to be:

Inhalation of airborne sporangiospores.

Other routes can include:

• Direct inoculation into damaged skin

• Traumatic implantation

• Contamination of wounds

The route of exposure influences the clinical syndrome that develops.

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RISK FACTORS

Severe invasive infection occurs predominantly in patients with impaired host defenses.

Important risk factors include:

• Profound neutropenia

• Hematologic malignancy

• Hematopoietic stem-cell transplantation

• Solid-organ transplantation

• Prolonged corticosteroid therapy

• Severe immunosuppression

• Advanced HIV infection

• Uncontrolled diabetes mellitus

• Diabetic ketoacidosis

• Major trauma or burns

• Iron overload

• Deferoxamine exposure

The risk rises substantially when several of these factors coexist.

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PATHOGENESIS

Like other mucormycetes, Absidia can invade blood vessels.

Angioinvasion produces:

• Thrombosis

• Tissue ischemia

• Infarction

• Necrosis

This explains the rapidly progressive and destructive nature of mucormycosis.

Vascular invasion also facilitates:

Hematogenous dissemination to distant organs.

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INFECTIONS

Absidia corymbifera is an uncommon cause of:

Mucormycosis

Clinical disease may involve many organ systems.

Major forms include:

• Rhinocerebral mucormycosis

• Pulmonary mucormycosis

• Cutaneous mucormycosis

• Disseminated mucormycosis

Less common manifestations include:

• Central nervous system infection

• Meningitis

• Post-traumatic infection

• Deep soft-tissue infection

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RHINOCEREBRAL MUCORMYCOSIS

Rhinocerebral disease typically begins in the:

Nasal cavity or paranasal sinuses

and may extend rapidly into:

• Orbit

• Facial structures

• Skull base

• Brain

It is classically associated with:

Poorly controlled diabetes and diabetic ketoacidosis.

Possible symptoms include:

• Facial pain

• Headache

• Nasal congestion

• Fever

• Periorbital swelling

• Diplopia

• Visual loss

Black necrotic tissue involving the nasal mucosa or palate is a major warning sign.

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PULMONARY MUCORMYCOSIS

Pulmonary disease occurs especially in:

• Neutropenic patients

• Patients with hematologic malignancies

• Transplant recipients

Symptoms may include:

• Fever

• Cough

• Dyspnea

• Pleuritic chest pain

• Hemoptysis

Pulmonary infection can rapidly invade blood vessels and cause:

• Pulmonary infarction

• Massive hemorrhage

• Dissemination

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CUTANEOUS MUCORMYCOSIS

Cutaneous infection may follow:

• Trauma

• Burns

• Contaminated dressings

• Surgery

• Direct inoculation

Lesions may begin as:

• Erythema

• Swelling

• Painful plaques

and progress to:

• Ulceration

• Necrosis

• Black eschar

Localized cutaneous disease can extend deeply into:

• Subcutaneous tissue

• Fascia

• Muscle

• Bone

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DISSEMINATED MUCORMYCOSIS

Dissemination occurs most often in patients with severe immunosuppression.

The infection may spread hematogenously to:

• Brain

• Liver

• Spleen

• Kidneys

• Heart

• Skin

Disseminated disease carries a very high mortality rate.

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CENTRAL NERVOUS SYSTEM DISEASE

CNS involvement may occur by:

• Direct extension from rhinocerebral infection

• Hematogenous dissemination

• Rarely, traumatic inoculation

Meningitis after severe head injury has been described but is uncommon.

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DIAGNOSIS

Diagnosis requires a combination of:

• Clinical suspicion

• Histopathology

• Culture

Because mucormycosis progresses rapidly, treatment should not be delayed while waiting for definitive culture results when clinical suspicion is high.

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CULTURE

Culture can identify the organism.

However:

A positive culture alone does not always prove invasive infection because the organism may be an environmental contaminant.

Conversely:

A negative culture does not exclude mucormycosis.

Tissue examination is therefore very important.

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HISTOPATHOLOGY

Characteristic tissue findings include:

• Broad hyphae

• Irregular width

• Pauciseptate or nonseptate appearance

• Wide-angle branching

• Angioinvasion

• Tissue necrosis

Demonstration of fungal invasion into tissue or blood vessels strongly supports true invasive disease.

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DIFFERENTIAL DIAGNOSIS

Important fungal differentials include:

• Rhizopus

• Mucor

• Lichtheimia

• Cunninghamella

• Aspergillus

• Fusarium

A particularly important distinction is:

Mucormycetes:

→ Broad, irregular, pauciseptate hyphae with wide-angle branching

Aspergillus:

→ Narrower, regularly septate hyphae with acute-angle branching

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TREATMENT

Mucormycosis is a medical and surgical emergency.

Successful management depends on three major principles:

1. Prompt antifungal therapy

2. Aggressive surgical debridement when feasible

3. Correction of underlying predisposing factors

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FIRST-LINE ANTIFUNGAL THERAPY

A lipid formulation of amphotericin B is generally preferred for invasive mucormycosis.

Typical dosing:

Liposomal amphotericin B approximately 5 mg/kg/day IV

Higher doses may be considered in selected severe infections, particularly CNS disease.

Historically, amphotericin B deoxycholate was used at:

Approximately 1–1.5 mg/kg/day

However, lipid formulations are generally preferred because they permit higher dosing with less nephrotoxicity.

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POSACONAZOLE

Posaconazole has activity against many mucormycetes.

It may be used as:

• Step-down therapy

• Salvage therapy

• An alternative when amphotericin B cannot be continued

Older regimens used:

Posaconazole approximately 800 mg/day

Modern dosing depends on formulation and should follow current prescribing guidance.

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ISAVUCONAZOLE

Isavuconazole is another important agent active against mucormycetes.

It may be considered for:

• Primary treatment in selected patients

• Step-down therapy

• Salvage treatment

Choice between amphotericin B, posaconazole, and isavuconazole depends on:

• Disease severity

• Site of infection

• Renal function

• Drug interactions

• Antifungal susceptibility

• Clinical response

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SURGICAL MANAGEMENT

Surgery is often crucial.

Aggressive debridement is especially important in:

• Rhinocerebral disease

• Cutaneous disease

• Necrotic soft-tissue infection

• Localized pulmonary disease in selected patients

Necrotic tissue has poor blood supply, limiting delivery of systemic antifungal therapy.

Therefore:

Antifungal therapy alone may be insufficient when extensive necrotic tissue remains.

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REVERSAL OF UNDERLYING RISK FACTORS

Whenever possible:

• Correct diabetic ketoacidosis

• Control hyperglycemia

• Reduce or discontinue corticosteroids

• Reduce immunosuppression

• Treat neutropenia

• Discontinue deferoxamine

• Correct severe metabolic abnormalities

Recovery of neutrophil function can be particularly important for survival.

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DURATION OF THERAPY

There is no single fixed duration.

Treatment is generally continued until:

• Clinical signs resolve

• Radiographic abnormalities improve substantially

• Surgical disease is controlled

• Immunosuppression has improved when possible

Therapy frequently lasts:

Several weeks to months

rather than a predetermined total cumulative dose.

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GENERAL PREVENTION

There is no vaccine.

Preventive strategies focus on reducing susceptibility and exposure.

Important measures include:

• Avoid unnecessary immunosuppression

• Optimize management of HIV infection

• Control diabetes

• Avoid prolonged severe neutropenia when possible

• Minimize corticosteroid exposure

• Avoid deferoxamine in patients at risk when alternatives are appropriate

• Protect wounds from environmental contamination

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PROGNOSIS

Prognosis depends strongly on:

• Site of infection

• Speed of diagnosis

• Extent of tissue invasion

• Degree of immunosuppression

• Ability to surgically remove infected tissue

• Reversal of underlying risk factors

Localized cutaneous disease generally has a better prognosis than:

• Pulmonary disease

• Rhinocerebral disease with CNS extension

• Disseminated mucormycosis

Delayed therapy markedly worsens outcomes.

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HIGH-YIELD MICROBIOLOGY

Absidia corymbifera:

• Mold

• Mucorales

• Broad hyphae

• Pauciseptate or nonseptate

• Hyaline

• Rapidly growing

• Environmental organism

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HIGH-YIELD PATHOLOGY

Broad, irregular, pauciseptate hyphae

+

Wide-angle branching

+

Angioinvasion

→ Think mucormycosis

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HIGH-YIELD CLINICAL PATTERN

Immunocompromised patient

+

Rapidly progressive necrotic infection

+

Broad nonseptate fungal hyphae

→ Think Mucorales infection

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HIGH-YIELD RISK PATTERN

Diabetic ketoacidosis

+

Facial pain/sinus disease

+

Black necrotic nasal or palatal lesion

→ Rhinocerebral mucormycosis

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EXAM ESSENTIALS

Organism:

→ Absidia corymbifera

Older name:

→ Absidia ramosa

Organism type:

→ Filamentous mold

Order:

→ Mucorales

Hyphae:

→ Broad, hyaline, pauciseptate/nonseptate

Typical hyphal width:

→ Approximately 6–15 μm

Growth:

→ Rapid

Major route of infection:

→ Inhalation of spores

Major disease:

→ Mucormycosis

Common forms:

→ Rhinocerebral, pulmonary, cutaneous, disseminated

Major risk group:

→ Immunocompromised patients

Important pathogenic mechanism:

→ Angioinvasion with thrombosis and tissue necrosis

Diagnosis:

→ Tissue histopathology + culture

Preferred major antifungal class:

→ Amphotericin B, especially lipid formulations

Important alternatives/step-down agents:

→ Posaconazole or isavuconazole

Critical additional treatment:

→ Surgical debridement

Major management principle:

→ Reverse underlying immunosuppression or metabolic risk factors whenever possible