A comprehensive review of pathogenesis of mucormycosis with implications of COVID-19: Indian perspective

Causative agents

In earlier times, the infection was termed as “Zygomycosis” with fungi belonging to the phylum Zygomycetes as causative agents. Zygomycetes are aseptate thermotolerant molds ubiquitously present in the environment typically thriving on decaying matter such as dead leaves, wood, and soil.^[11,12] However, the phylum Zygomycetes involve different orders of fungi which cause distinct types of infections, and thus, the terminology was considered to be very broad. Similarly, “Entomophthoromycosis” and “Phycomycosis” have been used synonymously with mucormycosis, but the infections are caused by taxonomically different groups of fungi even though their clinicopathological characteristics overlap.^[13] Over the years, the etiological fungi associated with mucormycosis have been methodically narrowed down to certain species belonging to the family Mucoraceae.^[14] Among various species, Rhizopus oryzae has been most commonly isolated from patients with mucormycosis infections accounting for 70% of the cases.^[15] Other species commonly associated with the infection include Mucor spp. and Lichtheimia spp., followed by less common ones such as Rhizomucor, Actinomucor, Cokeromyces, Cunninghamella, Apophysomyces, Saksenaea, and Syncephalastrum which are found in <5% of cases.^[16] The fungi characteristically exhibit aseptate, broad hyphae of dimensions averaging 3–30 μm that branch at right angles. The hyaline appearance of the fungal hyphae on histopathological examination can be attributed to the presence of chitinous component within the fungal wall coupled with lack of pigmentation or septae.^[17]

Modes of entry

Despite their ubiquitous presence, the organisms are seldom present among normal commensal microflora of skin or gastrointestinal tract. The fungi are generally incapable of suffusing through intact skin or mucous membranes. Breach in the mucocutaneous integrity because of trauma, wounds, mechanical procedures, or tooth extraction sockets creates passageways for entry of the fungi. In addition, the fungal spores may enter through the nose or may be introduced directly into the oral cavity with the use of contaminated tongue depressors, wooden applicators, surgical dressings, non-sterile adhesive tapes, and other equipment used in dentistry such as forceps, burs, or airotor handpiece.^[18,19]

The present COVID-19 pandemic scenario provides further opportunities for entry of the pathogens due to prolonged hospital stays, shortage of health care workers, and subsequent aseptic conditions. The most common mode of entry has been described as through the mechanical ventilators used to deliver oxygen in hypoxemic patients.^[20] Direct access into the bloodstream through intravenous catheters or subcutaneous injections used for drug delivery in the treatment of COVID-19 is another possibility.

Risk factors

Once the spores bypass mucocutaneous barriers and manage to enter the body, they are encountered by polymorphonuclear lymphocytes as well as mononuclear phagocytes.^[21] Exposure of neutrophils to the fungal hyphae and spores elicits a pro-inflammatory response due to upregulation of Toll-like receptors present on macrophages and dendritic cells with a resultant induction of NF-KB pathway.^[22] Consequently, oxidative metabolites and cationic peptides such as defensins are generated; and along with chemotaxis of natural killer cells, these are able to actively destroy the invading pathogens.^[21] Besides, the rich vasculature in the maxillofacial region further restricts the incidence of opportunistic infections.^[23] In this manner, an immunocompetent host is able to ward off the infection caused by R. oryzae or other related species.

As a result, existence of a condition that compromises the host’s immune system such that it is unable to fight against the fungal invasion is necessitated to establish the infection. The underlying conditions associated with mucormycosis that accounted for majority of the cases were identified as diabetes (36%) and malignancy (17%) in a comprehensive review (n = 929). Other risk factors for mucormycosis include solid organ or bone marrow transplantation, injection drug use, trauma, and burns.^[23] Studies have reported that patients with AIDS do not exhibit an increased affinity to develop mucormycosis, thus, suggesting that T-lymphocytes may not play an active role in resistance against the infection.^[24] Even so, role of polymorphonuclear leukocytes in combat against mucormycosis is undeniable, and thus, patients suffering from neutropenia or defects in neutrophils are predisposed to develop the infection.^[25] Although hematological malignancies are more common in western countries, uncontrolled diabetes and trauma predominate as the fons et origo of mucormycosis in developing countries such as India.^[26,27]

Role of iron

Unlike other opportunistic fungi, R. oryzae require iron for vital processes of the cell contributing to the cell growth and development.^[36] Normally, iron is present in the serum of mammals in bound forms such as transferrin, ferritin, and lactoferrin circumvents the effects that may arise due to toxicity of free iron.^[37] Therefore, procurement of free iron from the host is a prerequisite for the growth of R. oryzae for which the organism has developed distinct mechanisms.

Fungi inherently secrete low-molecular-weight iron chelators, termed as “siderophores,” that possess affinity for iron surpassing that of transferrin and lactoferrin.^[38,39] R. oryzae secretes one such polycarboxylate siderophore known as “rhizoferrin” that acquires iron from host through a receptor-mediated process dependent on energy.^[40] However, rhizoferrin has been deemed as inefficient in acquiring iron from the host serum.^[41] Consequently, the role of endogenous siderophores in virulence of the organism may be limited which is also highlighted by the fact that it has adapted to utilize xenosiderophores such as deferoxamine.

Deferoxamine is included in the treatment regimen for patients at risk of developing toxicity due to iron overload such as those on renal dialysis. It directly chelates the bound iron in transferrin resulting in iron-ferrioxamine complex which liberates free ferrous iron by reduction at the fungal cell surface through Fob receptors.^[42] Compared to other opportunistic fungi such as A. fumigatus or Candida albicans, it has been demonstrated that Rhizopus spp. can acquire 8–40 times the amount of iron through deferoxamine.^[43] The ferrous iron is then ingested by the fungus by means of a high-affinity plasma membrane iron permease (FTR1) after reoxidization by copper oxidase.^[44] The FTR1 also facilitates release of ferric iron from host hemoglobin through degradation by means of heme oxygenases.^[18] Therefore, anti-FTRP1 targeted therapy may play a key role in mitigating mucormycosis infection in future.

The extent of availability of free iron has been directly linked to severity of infection caused by Rhizopus spp. Although deferoxamine in itself is not a pathogenetic factor for the infection, it serves to provide iron to the fungi which was unavailable to it previously. Animal models have also demonstrated that administration of deferoxamine or free iron has reduced the survival of the organism infected with Rhizopus spp.^[45] Thus, conditions, such as acidosis that impair iron-binding capacity of transferrin by proton-mediated displacement of ferric ions, tend to create favorable environments in which the fungi thrive.^[46] Patients undergoing repeated blood transfusions due to underlying conditions such as myelodysplastic syndrome tend to develop iron overload which predisposes them to mucormycosis.^[47]

Modes of presentation

Depending on the areas or organs infected by the fungi, various clinical types of mucormycosis have been observed such as rhinocerebral, pulmonary, cutaneous, gastrointestinal, and disseminated forms. These forms obviously correspond to the mode of entry and other pathoanatomical factors. Since the spores are generally acquitted through inhalation, rhinocerebral is the most common type observed clinically (39%) followed by pulmonary infection (24%). Gastrointestinal type is observed in patients with malnutrition and cutaneous type is observed in those having history of trauma or burns. In established cases, wherein the pathogen has managed to invade the blood vessels and spread to other organs, disseminated type of infection is noted.^[12,18,48] The clinical types were also shown to be related with the patient’s underlying conditions as a reflection of the host’s immune function and other related factors. For instance, patients with malignancy tend to develop pulmonary infections while those suffering from uncontrolled diabetes present with sino-orbital or rhinocerebral forms.^[23]

In a long-term analysis of mucormycosis cases in India (n = 372), rhino-orbito-cerebral involvement was found to be most frequent (44.2%).^[49] Considering the fact that trauma is the most common predisposing factor for mucormycosis, second to diabetes in India, the corresponding cutaneous type was found to be relatively more common (15.5%). An upsurge in the incidence rate of mucormycosis cases in India was reported in the past two decades that could be attributed primarily to the rising epidemic of diabetes. In addition, the relatively hot and humid climate in most parts of the country along with a significant portion of malnourished population produce optimal settings for the fungi to thrive and contribute to its incidence.^[8,49]

In contrast to other common fungal infections such as Candidiasis (20–50%) and Aspergillosis (35–45%), the mortality rate of mucormycosis is higher ranging from 50% to 100% depending on the clinical type.^[15,23] The mortality rate is even higher for patients with underlying conditions such as diabetes mellitus or those with extensive trauma and burns. Disseminated forms of the disease are almost inevitably fatal REF. Naturally, rhino-occulo-cerebral forms have higher mortality rate than early cases limited to rhinomaxillary area. The alarmingly high mortality rates indicate that the disease cannot be trifled with and warrants early diagnosis and reversal of the condition before it progresses to the disseminated form.

Implications of diabetes

Diabetes and mucormycosis are almost inseparable considering the fact that majority of the reported cases have an underlying history of diabetes mellitus. Diabetic ketoacidosis (DKA) hinders the ability of phagocytes and polymorphonuclear leukocytes to effectively respond to the infection.^[50] The systemic acidosis significantly increases the proportion of free iron available in the serum by releasing it from the binding proteins, thus, creating a rich source of nutrition for the fungi.^[21] Furthermore, Rhizopus arrhizus also produce the enzyme ketoreductase by means of which the fungi can utilize ketone bodies available in the serum of a patient with DKA to enhance their growth.^[51] As mentioned previously, the expression of GRP78 receptors present on the surface of endothelial cells, that facilitate angioinvasion, is also increased in the sinus, lungs, and brain of individuals with DKA.^[40] Overall, the defective function of immunomodulatory cells, increase in availability of free iron, and increased expression of virulence factors account for higher morbidity and mortality in mucormycosis patients with diabetes mellitus [Figure 2]. The dreadful combination is even higher in India which comprises a significant portion of diabetic individuals.

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Figure 2:

Factors predisposing to mucormycosis in a susceptible host.

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Implications of COVID-19

Besides claiming millions of lives worldwide by itself, the aftereffects of COVID-19 infection and its treatment have created a unique scenario that is in favor of mucormycosis. Umpteen cases of fungal osteomyelitis developing in patients previously diagnosed with COVID-19 have come into light in recent times and have been the topic of interest for various academic groups as well as online and offline media.^[9] The presentation of these cases, although anecdotal, has more or less been identical to the rhinomaxillary form of mucormycosis.^[52]

Current guidelines have recommended use of remdesivir and glucocorticoids in treatment regimen of COVID-19 patients, especially in those requiring oxygen support.^[53] Although the drugs are effective in resolving the hypoxemic condition of the patients, the fact that they create dysregulation of immune system cannot be overlooked. Glucocorticoids add to the already existing glycemic load in diabetic patients, consequently worsening their hyperglycemia and DKA.

In addition, the SARS-CoV-2 virus in itself is able to cause impairment of cell-mediated immune response by reducing the count of T lymphocytes.^[3] Acute respiratory dysfunction syndrome and multiorgan dysfunction linked with cytokine storm further add to compromise of the immune capability of the patients suffering from severe forms of COVID-19. Overall, a combination of multiple associated factors constructs a cascade of events that ultimately cause immune exhaustion and predispose the patient to secondary bacterial and fungal infections [Figure 3].

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Figure 3:

Cascade of events in COVID-associated mucormycosis.

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The hospitals are pre-occupied with booming number of cases of COVID-19 and majority of health-care resources are shifted to combat the perilous effects of the pandemic which might account for delay in diagnosis of mucormycosis cases. Since the organism possesses virulence factors that enable it to grow and disseminate very rapidly, the “window of opportunity” to diagnose and resolve the condition is extremely short, particularly in cases of COVID-19 patients. Delay in commencement of therapy has been reported to result in a doubled mortality rate in patients with mucormycosis infection.^[54]

The mortality in cases of mucormycosis associated with COVID-19 patients has been reported as much higher (87.5%).^[5] However, patients may refrain from reporting to the hospitals post-recovery from COVID-19 due to psychological or financial reasons, and thus, the actual mortality rate of COVID-associated mucormycosis might be much higher.^[55]