Human fusariosis: An emerging infection that is difficult to treat

Abstract Fusarium spp. has been associated with a broad spectrum of emerging infections collectively termed fusariosis. This review includes articles published between 2005 and 2018 that describe the characteristics, clinical management, incidence, and emergence of these fungal infections. Fusarium solani and F. oxysporum are globally distributed and represent the most common complexes. Few therapeutic options exist due to intrinsic resistance, especially for the treatment of invasive fusariosis. Therefore, the use of drug combinations could be an important alternative for systemic antifungal resistance. Increase in the number of case reports on invasive fusariosis between 2005 and 2018 is evidence of the emergence of this fungal infection.


INTRODUCTION
Fusariosis is an infection that affects plants, animals, and humans, and is caused by various fungi of the genus Fusarium 1,2 . Fusarium spp. is responsible for significant economic losses in the agricultural field worldwide 3 due to difficulties in management of diseases caused by this species 4 . Similarly, in the medical field, different Fusarium species have been related to local or invasive infections in both immunodepressed and immunocompetent individuals 5,6,7 .
However, infections are difficult to treat because of the lack of consensus regarding treatment protocols for fusariosis in humans caused by multi-drug resistant isolates 2,8,9 . In addition, it is possible that environmental isolates from Fusarium spp. acquire resistance due to previous exposure to fungicides that were used in the agricultural fields 10,11 , and these isolates may disseminate and consequently infect humans 12,13 . Perhaps this process of infection may be avoided by implementing public control policies regarding the sale and use of fungicides.
As such, the aim of this study is to review the literature to demonstrate the characteristics, clinical management, incidence, and emergence of fungal infections caused by Fusarium species. The lack of attention on these cases by public health institutions and the insufficient research on the development of novel antifungal agents as therapeutic options emphasize the need to address the main factors involved in fusariosis, such as clinical forms, treatment, and lack of epidemiological control. To address this problem, articles published between 2005 and 2018 were analyzed, and 23 publications were obtained that included important conclusions regarding this proposition ( Table 1). EMERGENCE  In vitro combined use of natamycin and voriconazole was found to be synergistic against most Fusarium strains, thereby significantly reducing the concentrations required to inhibit fungal growth.

Dalhoff 2016 Journal of Global Antimicrobial Resistance
Fusariosis is difficult to treat and the use of antimycotics in agriculture and horticulture facilitates the acquisition of antifungal resistance. Despite global distribution, endemic regions are tropical and subtropical in nature 7,16 . Although fusariosis is associated with specific climatic conditions, environmental and clinical isolates have been reported to cause infections outside previously established borders 14 . This fungus has efficient mechanisms of dispersion, and its conidia reach considerable distances 9 . Moreover, genetic similarities between clinical isolates and environmental isolates of the same species may be related to infections in patients by Fusarium spp. in the environment 16 .

CLINICAL ASPECTS OF FUSARIOSIS
Fusarium species cause a wide spectrum of infections in humans, ranging from superficial and locally invasive to disseminated, with the most prevalent infections being onychomycosis, skin infections, and keratitis 15 .
Invasive infections can be widespread involving the skin, brain, bloodstream, lungs, eyes, and bones 14,17,18 . Patients with severe and prolonged neutropenia, especially those with hematological malignancies, are most susceptible to prevalent infections 9,18 .
In their epidemiological study, Garnica and Nucci described the worldwide incidence of Fusarium spp. as the main non-dermatophyte filamentous fungus that causes onychomycosis 19 . These infections are presented as subungual distal and total dystrophic infections that are often associated with paronychia and characterized by purulent periungual inflammation. The most commonly involved complexes are F. oxysporum and F. solani 20 . Treatment is difficult and prolonged, usually lasting more than 4 or 6 months, even with the use of topical and systemic antifungal agents 21 .
Keratitis is one of the most common infections caused by Fusarium spp. and primarily develops from trauma to the eye, contact lens wear, and use of corticosteroids 9,19,22 . Trauma is the key predisposing factor and occurs in 40-60 % of patients 19 .
Skin infections are the result of dissemination of the fungus primarily in immunocompromised patients 14 . The most common pattern of disseminated disease is the combination of multiple painful erythematous papules or nodules, commonly with central necrosis. Such occurrences spread throughout the body and continuously release fungal cells, thereby resulting in a positive blood culture, and often pulmonary involvement, with or without involvement at other sites 9,17 .
The airways represent the main gateway to infection, followed by the skin at the site of the tissue or onychomycosis, contact lens wear, and possibly mucous membranes 9,18,22 . Prior to initiating immunosuppressive therapy-given the severity associated with disseminated fusariosis-signs of the presence of skin or nail infections should be carefully investigated, as they comprise the focus of fungal dissemination and are often neglected upon initial physical examinations 5,9,22 .

IMMUNE RESPONSE AGAINST FUSARIOSIS
The immune system impedes the establishment of invasive infections by various species of fungi as high mortality is seen in immunosuppressed individuals 23 . However, in terms of the emerging pathogenic species of the genus Fusarium, the lymphocyte response via Th2 may facilitate the invasiveness of this disease and explain the self-limiting difficulty related to its complex mycosis 24 .
When the infective structures of Fusarium spp. reach the mucous membranes, the innate cellular immune response of the host is activated, which includes dendritic cells, macrophages, monocytes, neutrophils, and soluble mediators of the complement system 25 . These responses are initiated by pattern recognition receptors (PRRs), which recognize a series of common and constant molecular patterns that are present in nearly all microorganisms, denominated as pathogen-associated molecular patterns (PAMPs). The activation of PRRs plays a dual role: it initiates processes of the innate immune system, such as phagocytosis, and establishes a link between innate and adaptive immunity via MHC type I and type II expressions 25 .
The most important PAMPs in filamentous fungi are mannan, β-glucan, and chitin. The primary soluble PRR is pentraxin-3, whereas cellular PRRs are lectins, Toll-like-receptors, and NOD receptors. Fusarium species are recognized by type 2 Toll-likereceptors, which are generated in response to the production of anti-inflammatory cytokines (IL4 and IL10), and thus promote an adaptive immune system response that is mediated by Th2 lymphocytes 26,27 . Thus, invasive Fusarium infections stimulate a Th2-type lymphocyte response, in which anti-inflammatory cytokines are produced, thereby leading to an inadequate response by the host to the infection and high morbidity and mortality 28 .
Despite their minor importance, various humoral factors also participate in the innate response, as the complement is activated by their associated classical and alternating pathways 25 . However, the predisposing factors of invasive mycoses relate to the dysfunction of the immune system of phagocytosis, rather than defects in humoral immunity. More knowledge on humoral immunity activity in response to fungal infections is required, although some studies have attempted to demonstrate a specific marker of invasive diseases caused by Fusarium spp. 29 .

ANTIFUNGAL RESISTANCE AND THERAPEUTIC OPTIONS
Fusarium spp. exhibit intrinsic resistance to echinocandins 2 . Moreover, some isolates exhibit resistance to azoles that are associated with a third analogue of the CYP51 gene 30 . On the other hand, the intrinsic resistance of echinocandins is linked to the Y639 region of the FKS1 gene, which is responsible for encoding the catalytic subunit of β-1-3 glucan synthase 31 . These fungi also exhibit mechanisms that contribute to acquiring resistance to most diverse antifungal agents, such as changes in amino acid sequences, overexpression of the CYP51 gene, and overexpression of genes that encode efflux pumps 32 .

Minimal inhibitory concentrations and minimum effective concentrations have not been established for Fusarium species 20 .
To present this missing knowledge, Espinel-Ingroff 8 defined the epidemiological breakpoints for amphotericin B, posaconazole, and itraconazole in relation to the main Fusarium species that cause fusariosis. In this scenario, a few options exist to combat this infection, and the frequently used antifungal agents include natamycin, amphotericin B, voriconazole, and posaconazole 5 . Therefore, depending on the clinical case, amphotericin B and voriconazole are the drugs of choice 9,33 . In vitro and in vivo tests also reveal natamycin and voriconazole as drugs of choice to treat keratitis induced by Fusarium spp. 2 In the case of resistance, the use of combinations of drugs may be an important alternative to combat various Fusarium species, increase the efficacy and spectrum of action of antifungal agents, and lower drug dosage and thus reduce toxic side effects 34,35 . Moreover, in vitro drug combinations have demonstrated the ability to control fungal biofilms in other fungal species 36 , and studies focused on Fusarium sp. remain scarce. Combinations of antifungal and nonantifungal agents have also been tested in vitro and the results are promising, especially in fusariosis, as a strong association with the inflammatory response has been found 37,38 . Despite promising results in an in vitro context, the use of combinations requires clinical studies to verify its effectiveness in vivo. A few reports have been conducted on treating patients with fusariosis using more than one drug. Tortorano et al. (2014) have reported an association between the use of lipid-based amphotericin B and voriconazole, as well as the use of up to three antifungals in the same patient 33 .
Factors that contribute to the severity of fusariosis include increased incidence of multidrug resistance to Fusarium spp. 39 and the lack of research relating to the development of new therapeutic options for treatment. In general, these infections progress with a severe prognosis, especially in terms of ophthalmology, in which cases of fungal keratitis led to negative outcomes, such as loss of vision, in affected individuals. Currently, isavuconazole, characterized as a second generation triazole antifungal, is being studied as an alternative for its potential treatment of fungal diseases in patients with hematological diseases 40 .

FUNGICIDES AND RESISTANCE IN PHYTOPATHOGENIC FUSARIUM SPECIES
Fungicides are specific substances that are used in the agricultural field to combat and prevent fungal diseases. Waste from the use of these substances is considered a pollutant with potential risk to the human body, as well as more commonly to the environment 41 . Demethylation inhibitors are abundantly used in the agricultural field. Moreover, demethylation inhibitors change the fungal population after multiple applications, thus requiring the application of new fungicides. A substitute used is triazole, and its time of permanence in the soil depends on the concentration used and generally ranges from 67 days to more than 1688 days, with a trend of accumulation based on the frequency of use 42 .
A risk factor that may be associated with fungicides in the environment is the development of microbial resistance 4 similar to that associated with the overuse of antifungals in humans 30 . Azoles are the most commonly used of all groups for both pest control and treatment of human infections. Therefore, the potential development of resistance to this specific class is of increasing concern 4 . Some benefits of the azole class include low cost and high efficacy, thereby rendering it the first-choice antifungal for use as a fungicide agent in crops since the 1970s 32 .
Proper fungicide management in agricultural fields is a current demand in terms of the economics related to agricultural practices, as well as in terms of negative environmental impact 43 .

INCIDENCE OF HUMAN FUSARIOSIS
Cutaneous lesions have been observed due to the spread of fungi in patients with hematological diseases. In Brazil, from 2007 to 2009, invasive fusariosis was proved to be the most frequent or probable invasive fungal disease, with 23 episodes among 937 patients with hematologic diseases 19 . Based on the information discussed thus far, a bibliographical search was conducted on the PubMed and Science Direct platforms using the term "fusariosis in human," including case reports published between 2005 and 2018. In this review, we included data from articles published only in 2005 and 2018, comprising 14 publications, with the aim to observe possible changes in both the etiology of infections and treatment ( Table 2). The factors for inclusion of the case reports involve the presence of relevant information on etiological agents, predisposing factors, and treatments. The exclusion factor was defined as the lack of any required information, as previously cited.
We observed that F. solani prevails as the etiological agent of fusariosis. The treatment also did not change over the years, indicating that amphotericin B, voriconazole, and posaconazole are prophylactic agents and treatment options for fusariosis 44 . The clinical forms of the disease in the case reports focused more on infections that present cutaneous lesions, which is characterized by the spread of the disease in patients with hematological dysfunctions.
The increased incidence of fusariosis from 2005 to 2018 can be observed in Figure 1 (A-B), which graphically shows the increase in the number of articles published on the PubMed and Science Direct platforms in this time period.

CONCLUSION
The efficient mechanisms of the dispersion of Fusarium spp. have led to the global distribution of clinical and environmental isolates. F. solani and F. oxysporum are the most common complexes. Infections in humans range from superficial to disseminated, and patients with hematological malignancies are the most susceptible. Dissemination of the fungus is seen mainly in immunocompromised patients because of the ease of infection related to the portal of entry of the fungus in the host, such as via the airways or the rupture of tissues and mucous membranes.
Invasive Fusarium infections stimulate an inadequate response by the host towards the infection, which accounts for the high mortality caused by this fungus. As such, biofilm formation renders treatment more difficult. Fusarium spp. exhibit intrinsic resistance to echinocandins, and some isolates exhibit resistance to azoles. In this scenario, the drugs of choice are amphotericin B and voriconazole, and drug combinations are an important means to combat multi-drug resistance. Just as the determination of the minimum inhibitory concentration provides an overview on in vitro resistance, it can also be considered strong evidence for selecting an antifungal treatment. Low investment by the pharmaceutical industry towards developing drugs to combat these infections was observed.
Risk factors of individuals contribute to the occurrence of new cases and F. solani continues to be the main etiological agent of fusariosis. Treatment also has not changed over the years, because of the lack of research in the development of new therapeutic options for the treatment of this infection. The increased incidence of fusariosis, as reported in the articles published between 2005 to 2018, is evidence of the emergence of this fungus.