Chronic widespread dermatophytosis due to Trichophyton rubrum: a syndrome associated with a Trichophyton-specific functional defect of phagocytes

Tinea Incognito: Challenges in Diagnosis and Management

Tinea incognito is a dermatophyte infection with atypical features, due to the use of topical or systemic steroids or other immunosuppressive medications. Delayed diagnosis, spread of the infection to critical body surfaces, resistance to antifungal drugs, and increased costs due to prolonged hospitalization and multiple treatment regimens often complicate tinea incognito. It can affect individuals of all ages and genders, but it is more common in children. Atypical clinical appearance often necessitates differentiation from other diseases such as eczema, seborrheic dermatitis, lupus erythematosus, psoriasis, or other non-fungal skin conditions. The treatment of tinea incognito usually involves discontinuation of topical steroids or other immunosuppressive medications. Preventive measures and management of the underlying fungal infection are necessary and can be achieved with antifungal drugs. Patients should wear loose cotton clothes, use boiling water for laundry, and iron their clothing before wearing them. Additionally, they should avoid sharing bed linens, towels, clothes, and shoes. This review aims to raise awareness of tinea incognito among health practitioners, provide tips for detecting the disorder, include it in the differentials, and evaluate the available diagnostic procedures.

Dermatophyte Infections Worldwide: Increase in Incidence and Associated Antifungal Resistance

The increase in incidence of superficial fungal infections combined with the emergence of antifungal resistance represents both a global health challenge and a considerable economic burden. Recently, dermatophytes, the main culprit causing superficial fungal infections, have started to exhibit antifungal resistance. This can be observed in some of the most common species such as Trichophyton rubrum and Trichophyton mentagrophytes. Importantly, the new subspecies, known as Trichophyton indotineae, has been reported to show high resistance to terbinafine, a first-line treatment for dermatophyte infections. Compounding these issues is the realization that diagnosing the causative infectious agents requires using molecular analysis that goes beyond the conventional macroscopic and microscopic methods. These findings emphasize the importance of conducting antifungal susceptibility testing to select the appropriate antifungal necessary for successful treatment. Implementing these changes may improve clinical practices that combat resistant dermatophyte infections.

Revisiting the pathogenesis of dermatophytosis: A cross-sectional analytic study of serum levels of interleukins-2, 8, 10 and 17

BACKGROUND

Given the current epidemic-like scenario of dermatophyte infections, it is prudent to revisit the immunopathogenesis of dermatophytosis. Comprehending the intricate interactions among interleukins can aid in understanding the recent trends in infection. There is a paucity of literature on the various cytokine levels observed in the serum of patients suffering from various dermatophytoses.

AIM

To study serum cytokine levels of interleukins 2, 8, 10 and 17 in patients with dermatophytosis.

METHODS

A cross-sectional analytic study was conducted on 64 cases of clinical dermatophyte infections (KOH confirmed) and 64 controls. The clinico-epidemiological profile of the cases was studied. By using a solid phase sandwich ELISA (enzyme-linked immunosorbent assay), the serum levels of interleukins 2, 8, 10 and 17 were measured and compared between cases and controls. Serum interleukin-2, 8, 10 and 17 levels were studied among cases based on mode of onset, duration of illness, treatment history, site of infection and multiple other morphological characteristics of the infection.

RESULTS

The cases had statistically higher levels of interleukins-8, 10 and 17 in comparison with controls. The levels of interleukin-8 were significantly lower (p < .05) among those who had received oral antifungals. In cases where the lesion had scaling, the serum levels of interleukin-10 were significantly higher (p < .05). The lesional hyperpigmentation was significantly (p < .05) associated with low levels of interleukin-17. Also, interleukin-17 was significantly (p < .05) elevated in patients with lesions in the abdomen.

CONCLUSION

It is the first time that serum interleukin levels are studied in dermatophytosis. There is an immunological dysfunction specific to dermatophytoses initiated by their infection. Key factor in this dysfunction is the elevation of IL-10, contributing to persistent infection. In turn, causing an increase in IL-17, promoting inflammation and tissue damage. This cycle of elevated IL-10 and IL-17 can further exacerbate the infection and lead to chronicity. The activity of IL-2 and the Th1 immune pathway is reduced by two opposing immune pathways: the Th17 and Th2 axes.

Efficacy and Safety of Adding Low-Dose Isotretinoin to Itraconazole in the Treatment of Chronic Recurrent Dermatophytosis among Sample of Iraqi Patients: An Open-Labelled Therapeutic Clinical Comparative Study

Background

An increasing number of dermatophytosis is seen now in daily clinical practice, with unusual presentations, running a chronic recurrent course and seems to be more resistant to systemic and topical conventional treatments, which necessitate the use of other treatment options that help to cure these challenging clinical conditions such as isotretinoin along with itraconazole.

Objectives and Aim

This is a prospective randomised open-label therapeutic comparative clinical trial to assess the efficacy and safety of low-dose isotretinoin along with itraconazole to treat and reduce the recurrence of this distressing chronic recurrent dermatophytosis.

Methods

Eighty-one patients with chronic recurrent dermatophytosis with positive mycological examination were recruited, all of them received itraconazole for 7 days per month for 2 consecutive months duration; half of them were randomly chosen to receive low-dose isotretinoin every other day for 2 months in addition to itraconazole. All patients were followed up at monthly intervals for 6 months.

Results

The patients who received isotretinoin along with itraconazole showed earlier and complete clearance in 97.5% with a significantly low recurrence rate (12.8%) in comparison with those who received itraconazole alone where the cure rate was relatively slower reported in 53.7% of the patients with a relapse rate of 68.1% with no significant side effects.

Conclusion

Low-dose isotretinoin with itraconazole seems to be safe, effective and promising choice in the treatment of chronic recurrent dermatophytosis as it induced earlier complete cure with a significant reduction of recurrence rate.

The Dermatophyte Trichophyton rubrum Induces Neutrophil Extracellular Traps Release by Human Neutrophils

Neutrophils are the first leukocytes recruited to the site of infection and are thought to be responsible for fungal elimination from the skin such as dermatophytes. Neutrophils are able to secrete reactive oxygen species (ROS) and neutrophil extracellular traps (NETs) that can kill different fungi, including Aspergillus, spp., Candida albicans, and Phialophora verrucosa. However, NET production in response to Trichophyton rubrum, the main etiologic agent of dermatophytosis, has yet to be studied. We demonstrated that human neutrophils produce NETs against different morphotypes of T. rubrum in a dose-dependent manner and NET formation is dependent on ROS production. In addition, ROS production by human neutrophils in response to T. rubrum is dependent on NADPH oxidase, but not on fungal viability. NETs mediated killing of T. rubrum. Collectively, these results demonstrate that T. rubrum was able to trigger the production of NETs, suggesting that these extracellular structures may represent an important innate immune effector mechanism controlling physiological response to T. rubrum infection.

Nitric Oxide-Releasing Nanoparticles Are Similar to Efinaconazole in Their Capacity to Eradicate Trichophyton rubrum Biofilms

Filamentous fungi such as Trichophyton rubrum and T. mentagrophytes, the main causative agents of onychomycosis, have been recognized as biofilm-forming microorganisms. Nitric oxide-releasing nanoparticles (NO-np) are currently in development for the management of superficial and deep bacterial and fungal infections, with documented activity against biofilms. In this context, this work aimed to evaluate, for the first time, the in vitro anti-T. rubrum biofilm potential of NO-np using standard ATCC MYA-4438 and clinical BR1A strains and compare it to commonly used antifungal drugs including fluconazole, terbinafine and efinaconazole. The biofilms formed by the standard strain produced more biomass than those from the clinical strain. NO-np, fluconazole, terbinafine, and efinaconazole inhibited the in vitro growth of planktonic T. rubrum cells. Similarly, NO-np reduced the metabolic activities of clinical strain BR1A preformed biofilms at the highest concentration tested (SMIC₅₀ = 40 mg/mL). Scanning electron and confocal microscopy revealed that NO-np and efinaconazole severely damaged established biofilms for both strains, resulting in collapse of hyphal cell walls and reduced the density, extracellular matrix and thickness of the biofilms. These findings suggest that biofilms should be considered when developing and testing new drugs for the treatment of dermatophytosis. Development of a biofilm phenotype by these fungi may explain the resistance of dermatophytes to some antifungals and why prolonged treatment is usually required for onychomycosis.

Immunopathogenesis of Dermatophytoses and Factors Leading to Recalcitrant Infections

The pathogenesis of dermatophytic infections involves the interplay of three major factors: the dermatophyte, the inherent host defense, and the adaptive host immune response. The fungal virulence factors determine the adhesion and invasion of the skin while the immune response depends on an interaction of the pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMP) with pattern recognition receptors (PRRs) of the host, which lead to a differential Th (T helper) 1, Th2, Th17, and Treg response. While anthropophilic dermatophytes Trichophyton rubrum and now increasingly by T. interdigitale subvert the immune response via mannans, zoophilic species are eliminated due to a brisk immune response. Notably, delayed-type hypersensitivity (Th1) response of T lymphocytes causes the elimination of fungal infection, while chronic disease caused by anthropophilic species corresponds to toll-like receptor 2 mediated IL (interleukin)-10 release and generation of T-regulatory cells with immunosuppressive potential. Major steps that determine the ultimate clinical course and chronicity include genetic susceptibility factors, impaired epidermal and immunological barriers, variations in the composition of sebum and sweat, carbon dioxide tension, skin pH, and topical steroid abuse. It is important to understand these multifarious aspects to surmount the problem of recalcitrant dermatophytosis when the disorder fails conventional therapeutic agents.

Host immune responses in dermatophytes infection

Dermatophytosis is a skin infection caused by keratinophilic, filamentous fungi. These are highly prevalent, common mycoses, affecting approximately 20% of the population. These fungi invade the stratum corneum, and other keratinised tissues, like nails and hair, where they grow by secreting enzymes and degrading keratin to obtain nutrients. Clinical presentation is variable and may depend on many factors, such as the infection site, the host's immunity and the dermatophyte's virulence. Generally, patients with acute superficial dermatophytosis mount cell-mediated immune responses. However, those suffering from chronic or recurrent infections are unable to develop this response, for reasons yet unknown. Several reports have described severe and occasionally life-threatening invasive diseases (deep dermatophytosis) associated with genetic mutations in the innate immunity-associated molecule CARD9, displaying the need to better understand its immune response. These dermatoses have substantial clinical consequences, producing chronic and difficult to treat skin lesions. They also lead to a decline in the patient's quality of life and impact their self-esteem. This review summarises findings on the immune response against dermatophytes.

A review of recent research on antifungal agents against dermatophyte biofilms

Dermatophytoses are inflammatory cutaneous mycoses caused by dermatophyte fungi of the genera Trichophyton, Microsporum, and Epidermophyton that affect both immunocompetent and immunocompromised individuals. With therapeutic failure, dermatophytoses can become chronic and recurrent. This is partly due to their ability to develop biofilms, microbial communities involved in a polymeric matrix attached to biotic or abiotic surfaces, contributing to fungal resistance. This review presents evidence accumulated in recent years on antidermatophyte biofilm activity. The following databases were used: Web of Science, Medline/PubMed (via the National Library of Medicine), Embase, and Scopus. Original articles published between 2011 and 2020, emphasizing the antifungal activity of conventional and new drugs against dermatophyte biofilms were eligible. A total of 11 articles met the inclusion criteria and were reviewed - the studies used in vitro and ex vivo (fragments of nails and hair) experimental models. The articles focused on reports of antibiofilm activity for conventional antifungals, natural drugs, and new therapeutic tools. The strains reported on were T. mentagrophytes, T. rubrum, T. tonsurans, M. canis, and M. gypseum. Between the studies, the wide variability of experimental conditions in vitro and ex vivo was observed. The data suggest the need for methodological standardization (at some minimum). This review systematically presents current studies involving agents that present antibiofilm activity against dermatophytes; and an overview of the ideal in vitro and ex vivo experimental conditions to guarantee biofilm formation that may assist future research.

LAY ABSTRACT

This review presents the current studies on the antibiofilm activities of drugs against dermatophytes and ideal experimental conditions, which might guarantee in vitro and ex vivo biofilm formation. It can be useful to examine the efficacy of new antimicrobial drugs against dermatophytes.

Skin Immunity to Dermatophytes: From Experimental Infection Models to Human Disease

Dermatophytoses (ringworms) are among the most frequent skin infections and are a highly prevalent cause of human disease worldwide. Despite the incidence of these superficial mycoses in healthy people and the compelling evidence on chronic and deep infections in immunocompromised individuals, the mechanisms controlling dermatophyte invasion in the skin are scarcely known. In the last years, the association between certain primary immunodeficiencies and the susceptibility to severe dermatophytosis as well as the evidence provided by novel experimental models mimicking human disease have significantly contributed to deciphering the basic immunological mechanisms against dermatophytes. In this review, we outline the current knowledge on fungal virulence factors involved in the pathogenesis of dermatophytoses and recent evidence from human infections and experimental models that shed light on the cells and molecules involved in the antifungal cutaneous immune response. The latest highlights emphasize the contribution of C-type lectin receptors signaling and the cellular immune response mediated by IL-17 and IFN-γ in the anti-dermatophytic defense and skin inflammation control.

Advances in Understanding Human Genetic Variations That Influence Innate Immunity to Fungi

Fungi are ubiquitous. Yet, despite our frequent exposure to commensal fungi of the normal mammalian microbiota and environmental fungi, serious, systemic fungal infections are rare in the general population. Few, if any, fungi are obligate pathogens that rely on infection of mammalian hosts to complete their lifecycle; however, many fungal species are able to cause disease under select conditions. The distinction between fungal saprophyte, commensal, and pathogen is artificial and heavily determined by the ability of an individual host's immune system to limit infection. Dramatic examples of commensal fungi acting as opportunistic pathogens are seen in hosts that are immune compromised due to congenital or acquired immune deficiency. Genetic variants that lead to immunological susceptibility to fungi have long been sought and recognized. Decreased myeloperoxidase activity in neutrophils was first reported as a mechanism for susceptibility to Candida infection in 1969. The ability to detect genetic variants and mutations that lead to rare or subtle susceptibilities has improved with techniques such as single nucleotide polymorphism (SNP) microarrays, whole exome sequencing (WES), and whole genome sequencing (WGS). Still, these approaches have been limited by logistical considerations and cost, and they have been applied primarily to Mendelian impairments in anti-fungal responses. For example, loss-of-function mutations in CARD9 were discovered by studying an extended family with a history of fungal infection. While discovery of such mutations furthers the understanding of human antifungal immunity, major Mendelian susceptibility loci are unlikely to explain genetic disparities in the rate or severity of fungal infection on the population level. Recent work using unbiased techniques has revealed, for example, polygenic mechanisms contributing to candidiasis. Understanding the genetic underpinnings of susceptibility to fungal infections will be a powerful tool in the age of personalized medicine. Future application of this knowledge may enable targeted health interventions for susceptible individuals, and guide clinical decision making based on a patient's individual susceptibility profile.

Trichophyton rubrum Elicits Phagocytic and Pro-inflammatory Responses in Human Monocytes Through Toll-Like Receptor 2

Dermatophytosis is a superficial fungal infection mostly restricted to keratinized tissues such as skin, hair, and nails but with potential to cause invasive or even systemic disease in immunocompromised patients. Trichophyton rubrum is the main etiologic agent, accounting for approximately 80% of the cases. Mononuclear phagocytes respond to pathogens through phagocytosis followed by production of several antimicrobial molecules, such as reactive oxygen and nitrogen species, and failure in doing so may contribute to development of chronic fungal infections. Toll-like receptors (TLRs) located on the surface of phagocytic cells bind either directly to target particles or through opsonizing ligands and trigger an actin-mediated ingestion. Even though the mechanisms involved in TLR-mediated cytokine responses are well established, the contribution of TLR in the recognition of T. rubrum by adherent monocytes remains unclear. Here, we report that phagocytosis of T. rubrum conidia by adherent monocytes is mediated by TLR2. Blockade of TLR2 by neutralizing antibodies impaired the fungicidal activity of monocytes as well their secretion of tumor necrosis factor (TNF)-α, but neither nitric oxide (NO) production nor interleukin (IL)-10 secretion was disturbed. So far, our data suggest that TLR2 is required for efficient conidial phagocytosis, and the absence of TLR2 signaling in human monocytes may impair the subsequent inflammatory response. These findings expand our understanding of phagocyte modulation by this important fungal pathogen and may represent a potential target for interventions aiming at enhancing antifungal immune responses.

In situ immune response in human dermatophytosis: possible role of Langerhans cells (CD1a+) as a risk factor for dermatophyte infection

Dermatophytosis is a cutaneous mycosis caused by a plethora of keratinophilic fungi, but Trichophyton rubrum is the most common etiological agent. Despite its high prevalence worldwide, little is known about the host defense mechanisms in this infection, particularly the in situ immune response. Using an immunohistochemistry approach, we investigated the density of CD1a+, factor XIIIa+ and CD68+ cells in the skin of dermatophytosis patients. Langerhans cells (CD1a+ cells) were significantly decreased in the epidermis of patients, both in affected and unaffected areas. In the dermis, however, no differences in the density of macrophages (CD68+ cells) and dermal dendrocytes (factor XIIIa+ cells) were observed. These results suggest that the decreased number of Langerhans cells may be a risk factor for development of dermatophytosis.

Host- and pathogen-dependent susceptibility and predisposition to dermatophytosis

Dermatophytes are a highly specialized group of keratinophilic and keratinolytic filamentous fungi causing a ringworm disease called dermatophytosis or superficial mycoses. Although dermatophyte infections do not threaten the host's life, they lower its quality in humans by causing discomfort related to cosmetic problems and through their epidemiological significance, whereas in farm animals they are responsible for economic losses and constitute a source of the spread of spores. Evidence from countless observational studies that have been conducted over the last 90 years indicates that dermatophytes infect humans of every age, race, gender and socioeconomic status with strikingly high rates, as well as both farmed and wild animals in various health conditions and with various epidemiological statuses. However, the prevalence of superficial fungal infections is highly variable, since it depends on several parameters associated with the infected individual and the dermatophyte, their mutual interactions, and epidemiological and geographical factors. The curious disparity in dermatophyte infection patterns has prompted many investigators to search for a link between the host, the host's predispositions and susceptibility to the disease, and the dermatophyte species and virulence. Thus, the question arises as to whether, in addition to the generally recognized factors predisposing hosts to diseases, there are some other predispositions to dermatophyte infections in a species-specific host. In this review, we describe recent findings about the mechanism of dermatophyte infections, focusing on the adaptation of the fungi to the host and conditions predisposing each side to the disease.

Profile of Dermatophytosis in a Tertiary Care Center in Kerala, India

Background:

The incidence of dermatophytosis is increasing over the last few years and there are many cases which are recurrent and chronic.

Aim:

The aim was to study the host and pathogen factors in dermatophytosis, to identify the species responsible, and to study the histopathological features of chronic dermatophytosis.

Materials and Methods:

It was a descriptive study conducted in the Department of Dermatology for a period of 1 year and all patients who were clinically diagnosed as dermatophytosis were included. Isolated hair, and nail involvement were excluded from the study. Epidemiological parameters and treatment history were analyzed, scrapings, and fungal culture were done in all patients. Histopathological examination was done in patients with chronic dermatophytosis who had applied topical steroids.

Results:

Chronic dermatophytosis was seen in 68%; tinea corporis was the most common presentation; topical steroid application was seen in 63%; azoles were the most common antifungals used; varied morphologies such as follicular and nonfollicular papules, arciform lesions, pseudoimbricata were seen in steroid modified tinea. Trichophyton rubrum and Trichophyton mentagrophytes were the most common species isolated in culture, but rare species such as Trichophyton tonsurans, Trichophyton schoenleinii, Epidermophyton floccosum, and Microsporum audouinii were also isolated from chronic cases. Histopathology showed perifolliculitis in steroid modified tinea. Minimal inhibitory concentration was lowest for itraconazole in susceptibility studies.

Conclusion:

Chronicity in dermatophytosis is due to various factors such as topical steroid application, noncompliance, and change in predominant species.

Profile of Dermatophytosis in a Tertiary Care Center

Background:

The incidence of dermatophytosis is increasing over the last few years and there are many cases which are recurrent and chronic.

Aim:

The aim was to study the host and pathogen factors in dermatophytosis, to identify the species responsible, and to study the histopathological features of chronic dermatophytosis.

Materials and Methods:

It was a descriptive study conducted in the Department of Dermatology for a period of 1 year and all patients who were clinically diagnosed as dermatophytosis were included. Isolated hair, and nail involvement were excluded from the study. Epidemiological parameters and treatment history were analyzed, scrapings, and fungal culture were done in all patients. Histopathological examination was done in patients with chronic dermatophytosis who had applied topical steroids.

Results:

Chronic dermatophytosis was seen in 68%; tinea corporis was the most common presentation; topical steroid application was seen in 63%; azoles were the most common antifungals used; varied morphologies such as follicular and nonfollicular papules, arciform lesions, pseudoimbricata were seen in steroid modified tinea. Trichophyton rubrum and Trichophyton mentagrophytes were the most common species isolated in culture, but rare species such as Trichophyton tonsurans, Trichophyton schoenleinii, Epidermophyton floccosum, and Microsporum audouinii were also isolated from chronic cases. Histopathology showed perifolliculitis in steroid modified tinea. Minimal inhibitory concentration was lowest for itraconazole in susceptibility studies.

Conclusion:

Chronicity in dermatophytosis is due to various factors such as topical steroid application, noncompliance, and change in predominant species.

In vitro activity of azole derivatives and griseofulvin against planktonic and biofilm growth of clinical isolates of dermatophytes

As shown by recent research, most of the clinically relevant fungi, including dermatophytes, form biofilms in vitro and in vivo, which may exhibit antimicrobial tolerance that favour recurrent infections. The aim of this study was to determine the minimum inhibitory concentrations (MICs) of itraconazole (ITC), voriconazole (VCZ) and griseofulvin (GRI) against Trichophyton rubrum, Trichophyton tonsurans, Trichophyton mentagrophytes, Microsporum canis and Microsporum gypseum in planktonic and biofilm growth. For the planktonic form, susceptibility testing was performed according to the Clinical and Laboratory Standards Institute (CLSI), document M38-A2, while biofilm susceptibility was evaluated using the XTT colorimetric essay. The planktonic growth of all strains was inhibited, with MIC values ranging from 0.00195 to 0.1225 μg/mL for VRC, 0.00195 to 0.25 μg/mL for ITC and <0.0039 to 4 μg/mL for GRI, while a 50-fold increase in the MIC was required to significantly reduce the metabolic activity (P < .05) of dermatophyte biofilms. In brief, the ability of dermatophytes to form biofilms may be a contributing factor for the recalcitrance of dermatophytoses or the dissemination of the disease.

The Role of Phagocytes and NETs in Dermatophytosis

Innate immunity is the host first line of defense against pathogens. However, only in recent years, we are beginning to better understand the ways it operates. A key player is this branch of the immune response that are the phagocytes, as macrophages, dendritic cells and neutrophils. These cells act as sentinels, employing specialized receptors in the sensing of invaders and host injury, and readily responding to them by production of inflammatory mediators. They afford protection not only by ingesting and destroying pathogens, but also by providing a suitable biochemical environment that shapes the adaptive response. In this review, we aim to present a broad perspective about the role of phagocytes in dermatophytosis, focusing on the mechanisms possibly involved in protective and non-protective responses. A full understanding of how phagocytes fit in the pathogenesis of these infections may open the venue for the development of new and more effective therapeutic approaches.

Mechanistic Insights into the Role of C-Type Lectin Receptor/CARD9 Signaling in Human Antifungal Immunity

Human CARD9 deficiency is an autosomal recessive primary immunodeficiency disorder caused by biallelic mutations in the gene CARD9, which encodes a signaling protein that is found downstream of many C-type lectin receptors (CLRs). CLRs encompass a large family of innate recognition receptors, expressed predominantly by myeloid and epithelial cells, which bind fungal carbohydrates and initiate antifungal immune responses. Accordingly, human CARD9 deficiency is associated with the spontaneous development of persistent and severe fungal infections that primarily localize to the skin and subcutaneous tissue, mucosal surfaces and/or central nervous system (CNS). In the last 3 years, more than 15 missense and nonsense CARD9 mutations have been reported which associate with the development of a wide spectrum of fungal infections caused by a variety of fungal organisms. The mechanisms by which CARD9 provides organ-specific protection against these fungal infections are now emerging. In this review, we summarize recent immunological and clinical advances that have provided significant mechanistic insights into the pathogenesis of human CARD9 deficiency. We also discuss how genetic mutations in CARD9-coupled receptors (Dectin-1, Dectin-2) and CARD9-binding partners (MALT1, BCL10) affect human antifungal immunity relative to CARD9 deficiency, and we highlight major understudied research questions which merit future investigation.