Fungal keratitis caused by Pyrenochaeta unguis-hominis: Identification and microbiological characteristics of a rare case

Abstract

Fungal keratitis, a sight-threatening ocular infection, can lead to severe complications if not diagnosed and treated promptly. This case report documents the first documented case of fungal keratitis caused by Pyrenochaeta unguis-hominis in a Chinese male patient following ocular trauma and surgical intervention. The study elucidates the microbiological characteristics, genomic features, and antifungal susceptibility profiles of this rare pathogen, offering critical insights for clinical diagnosis and therapeutic management. Furthermore, this case underscores the necessity of enhancing clinical awareness and leveraging advanced diagnostic techniques to identify uncommon fungal species in ocular infections, thereby ensuring timely and accurate medical interventions for patients.

Introduction

Fungal keratitis, a severe ocular infection, may lead to corneal scarring, visual impairment, and even blindness if not treated promptly and effectively [1], [2]. The diagnosis, treatment, and prognosis of fungal keratitis vary significantly depending on the etiological species involved. Therefore, understanding the microbiological characteristics, clinical presentations, and emerging diagnostic tools for different fungal pathogens is critical to improving patient outcomes.

Currently, the primary etiological agents of fungal keratitis include filamentous fungi (Aspergillus, Fusarium) and Candida [3], [4]. Aspergillus and Fusarium typically enter the eye through trauma or exposure to contaminated materials [5], whereas Candida infections are more common in immunocompromised patients or those with a history of ocular surgery [6]. Although detection techniques and treatment protocols for these pathogens are relatively well-established, diagnostic delays and treatment challenges persist in clinical practice, often leading to irreversible ocular damage. In recent years, the clinical diversity of fungal keratitis cases has expanded, with an increasing number of rare fungal pathogens being reported. These emerging pathogens exhibit distinct microbiological characteristics compared to traditional etiological agents, significantly complicating diagnosis.

This report presents a rare case of fungal keratitis caused by Pyrenochaeta unguis-hominis. Through detailed analysis, we explore the microbiological characteristics, genomic features, and antimicrobial treatment strategies associated with this pathogen. By documenting this case, we aim to raise awareness of this understudied fungal infection, promote the development of effective diagnostic and therapeutic approaches, and reduce vision loss caused by diagnostic delays or inappropriate management.

Case report

A 72-year-old male farmer presented with left eye pain, blurred vision, and progressive proptosis after being struck in the eye while chopping wood. Initial diagnosis included "secondary glaucoma in the left eye and bilateral cataracts." Topical therapy was initiated with tobramycin-dexamethasone, bromfenac sodium, and moxifloxacin hydrochloride eye drops, supplemented by tobramycin-dexamethasone ointment for ocular surface management. Over 48 h, the patient reported improved visual acuity (OS: 0.02; OD: 0.5), though intraocular pressure (IOP) in the left eye elevated to 36 mmHg. Adjunctive IOP-lowering therapy with timolol and brimonidine tartrate eye drops was added to the regimen. After four days of combined IOP control, corneal nutrition support, and anti-inflammatory treatment, the patient’s condition stabilized, and he was discharged with continued topical medications.

One month later, the patient presented with uncontrolled IOP (OS: 36 mmHg; OD: 22 mmHg), diagnosed as "acute bilateral angle-closure glaucoma (acute phase OS, preclinical phase OD), incomplete lens dislocation OS, and bilateral senile cataracts." Given significant visual impairment due to lens opacification, bilateral phacoemulsification with intraocular lens (IOL) implantation, abnormal lens extraction, and goniosynechialysis were performed. Postoperative care included prophylactic antibiotic eye drops (bromfenac sodium, tobramycin-dexamethasone, and moxifloxacin hydrochloride). At follow-up, BCVA improved to 0.6 OD and 0.25 OS, with mild conjunctival hyperemia, clear corneal incisions, deep anterior chambers (corneal edema resolved), intact IOL positioning, and vitreous liquefaction with turbidity. The Fundus examination revealed well-defined optic disc margins, normal retinal vasculature, and visible foveal reflex. IOP stabilized at 14 mmHg OD and 15 mmHg OS. The patient was discharged with bromfenacin sodium, tobramycin-dexamethasone, and bovine alkaline fibroblast growth factor eye drops for anti-inflammatory recovery.

Clinical diagnostic and microbiological studies

However, four months later, the patient was readmitted due to recurrent redness and a foreign body sensation in the left eye, which progressively worsened, accompanied by decreased vision, epiphora, and headache, significantly impacting daily life. Ophthalmic examination revealed no light perception in the left eye, mixed conjunctival hyperemia, and a white infiltrate (3 × 4 mm) at the nasal side of the cornea involving all corneal layers, with visible satellite lesions. Mild corneal endothelial edema and a 1-mm purulent hypopyon were observed, with the intraocular pressure (IOP) in the left eye recorded as "Tn" (touching the cornea).

Three-dimensional imaging of the anterior segment revealed no conjunctival hyperemia and a clear cornea in the right eye (Fig. 1a), while a grayish-white ulcerative infiltrate was observed in the central inferonasal region of the left cornea, with a hypopyon in the anterior chamber (Fig. 1b). Optical coherence tomography (OCT) was performed to assess the left eye further, revealing significant central corneal edema and thickening (Fig. 1c).

Fig. 1.

Three-dimensional imaging of the anterior segment of the right eye (a) and left eye (b). (c) Anterior segment optical coherence tomography (AS-OCT).

Microbiological characteristics

Direct Gram staining of the secretion demonstrated abundant septate, thick fungal hyphae with neutrophil infiltration (Fig. 2a). To clarify hyphal morphology, fungal immunofluorescence staining was performed, confirming extensive septate hyphae under microscopy (Fig. 2b). After 48 h of culture at 28°C, slow-growing colonies appeared on blood agar (Plate I), Sabouraud dextrose agar (SDA, Plate II), and chocolate agar (Plate III). By day 4, fungal hyphae grew on blood agar (Plate Ⅳ), SDA (Plate V) and chocolate agar (Plate VI) (Fig. 2c).

Fig. 2.

(a) Microscopic imaging after direct smear and Gram staining of the secretion. (b) Image of fungal hyphae after immunofluorescence staining. (c) Colonies after 2 days of incubation on blood agar (Ⅰ), Sabouraud agar plates (Ⅱ), and MacConkey agar (Ⅲ). (d) Colonies after 4 days of incubation on blood agar (Ⅳ), Sabouraud agar plates (Ⅴ), and MacConkey agar (Ⅵ), respectively.

The isolate was subcultured onto Sabouraud dextrose agar (SDA) plates, where it exhibited extremely slow growth. After incubation at 28°C for 4 days, grayish-white, woolly colonies began to appear. These colonies had a woolly to leathery texture, with a raised center and prominent radial furrows. The reverse side of the colonies was brownish in color (Fig. 3a). By day 8 of incubation, the colonies developed a more pronounced woolly to leathery appearance (Fig. 3b).

Fig. 3.

(a) Colonies on Sabouraud Dextrose Agar (SDA) plates after 4 days of incubation at 28°C, frontal view (Ⅰ) and posterior view (Ⅱ). (b) Colonies on Sabouraud Dextrose Agar (SDA) plates after 8 days of incubation at 28°C, frontal view (Ⅰ) and posterior view (Ⅱ). (c) Conidia (Ⅰ) and conidiophores (Ⅱ) after staining with lactic acid cotton blue.

A single colony was selected for lactophenol cotton blue staining. Microscopic examination revealed hyaline, cylindrical to elongated ovoid conidia, as well as olive-brown to black, globose to flask-shaped pycnidia measuring 100–400 μm in diameter. Each pycnidium had 1–3 ostioles. Setose structures were visible on the outer walls of the pycnidia (Fig. 3c).

Molecular biology identification

Due to the difficulty of precise identification through morphological examination, targeted DNA sequencing was first performed on this strain. The D1/D2 region of the fungal LSU rDNA is widely recognized as a reliable marker for identifying rare or cryptic fungal species [7]. In this study, we amplified and sequenced this region, and the results from BLAST analysis preliminarily identified the strain as Pyrenochaeta unguis-hominis (formerly known as Neocucurbitaria unguis-hominis, GenBank No. MW561576.1). To gain a more comprehensive understanding of the genomic characteristics of this strain, we conducted whole-genome sequencing. The genome assembly yielded 94 allelic sequences totaling 34 Mb (allele N50 = 1.02 Mb). BLAST analysis against the NCBI database showed that the assembled sequences shared 97.496 % similarity with Neocucurbitaria unguis-hominis. The whole-genome comparison showed high matches with Parastagonospora nodorum, Alternaria solani, and Pyrenophora teres f. maculata. Further genomic analysis included gene prediction, preliminary functional annotation, and phylogenetic analysis. Initially, we performed functional annotation of the assembled genome using eggNOG-mapper, which identified 2710 genes, 24 Clusters of Orthologous Groups (COG) categories, 11,104 Gene Ontology (GO) terms, and 383 Kyoto Encyclopedia of Genes and Genomes (KEGG) metabolic pathway annotations. These findings suggest that P. unguis-hominis possesses extensive metabolic capabilities that may aid its survival in diverse environments. Genes associated with carbohydrate and lipid metabolism indicated that P. unguis-hominins may utilize various nutrient sources, potentially enhancing its adaptability within the human host.

Antimicrobial susceptibility testing

In this study, antifungal susceptibility testing of Pyrenochaeta unguis-hominis was performed in vitro using the broth microdilution method to determine the minimum inhibitory concentrations (MICs). The MIC values of Pyrenochaeta unguis-hominis for commonly used antifungal agents were as follows: echinocandins such as anidulafungin, micafungin, and caspofungin: MIC > 8 μg/mL; flucytosine: MIC > 64 μg/mL; fluconazole: MIC > 256 μg/mL; voriconazole: MIC 1 μg/mL; isavuconazole: MIC > 32 μg/mL; posaconazole: MIC 0.25 μg/mL; itraconazole: MIC 0.5 μg/mL; and amphotericin B: MIC 2 μg/mL. Antifungal therapy was guided by susceptibility testing results, as summarized in Table 2.

Table 2.

Clinical medication regimen.

Medication	Dosage and Administration
Voriconazole Powder for Injection (0.1 g)	Dissolve in 10 mL of sterile water to prepare a 10 mg/mL eye drop solution, administered 3–5 times/day.
Moxifloxacin Hydrochloride Eye Drops	Administer 3 times/day, 1 drop each time.

Following antifungal treatment based on susceptibility testing, the patient's condition improved with no reported discomfort. Ophthalmologic evaluation showed hand motion vision in the left eye, normal color perception bilaterally, and full extraocular motility without proptosis. The right eye was normal, with a clear cornea, deep anterior chamber, reactive 3 mm pupil, well-positioned intraocular lens, and mild vitreous opacities. Fundus examination revealed a pale optic disc, tortuous vessels, and blurred foveal reflex. In the left eye, conjunctival hyperemia, mild corneal endothelial edema, and a fibrous membrane over the temporal iris were noted. The pupil was pharmacologically dilated to 4 mm with light reflex present; posterior segment visualization was limited. Intraocular pressure was 16 mmHg in the right eye and normal by palpation in the left. The patient showed continued improvement with symptomatic management. Ongoing antifungal therapy and gradual tapering of topical medications were recommended, along with regular follow-up.

Discussion

Fungal keratitis, also known as mycotic keratitis, is reported as the second most common cause of infectious keratitis after bacterial infections, characterized by more severe and destructive clinical outcomes [8]. In recent years, with the increased use of contact lenses and the misuse of antibiotics, there is growing evidence that some rare microorganisms can also cause ocular infections [9].

Pyrenochaeta unguis-hominis belongs to the class Dothideomycetes, order Pleosporales, family Cucurbitariaceae, and genus Pyrenochaeta, a group of dematiaceous fungi[10]. Members of this genus are commonly found in the environment as saprophytes in soil, plants, and wood, with some species causing severe plant diseases. However, human infections caused by Pyrenochaeta species are exceedingly rare. In 1980, a case of finger infection caused by P. unguis-hominis was reported [11], and in 2019, Shi's team reported a case of foot infection caused by P. unguis-hominis[12]. Ocular infections caused by P. unguis-hominis are rarer still, with the first corneal case reported by a Spanish team in 2023 [13], and a subsequent case from an Austrian team in 2024 [14]. Notably, next-generation sequencing identified the pathogen in the latter case, but antimicrobial susceptibility testing was precluded in both instances due to the failure of the isolates to grow in standard RPMI medium.

In this study, we describe a rare case of fungal keratitis caused by P. unguis-hominis in a Chinese male patient with a clear history of ocular trauma followed by surgical intervention. Despite initial empirical treatment with antibacterial agents and symptomatic management, the infection recurred and showed no signs of improvement. Corneal secretions were subsequently collected for microbiological and molecular analysis. Interestingly, a prior case reported by Nissan et al. involved P. unguis-hominis keratitis associated with contact lens use, in a patient with a history of herpetic keratitis and long-term corticosteroid therapy. That case progressed to surgical intervention and ultimately resulted in poor visual prognosis [15]. In contrast, our patient had a clear history of eye trauma from chopping wood, suggesting direct inoculation of fungal elements from wood debris. Although both cases involved the same pathogen, differences in antifungal susceptibility profiles imply potential genetic or phenotypic heterogeneity among P. unguis-hominis strains. Furthermore, while the case reported by Nissan highlighted the utility of MALDI-TOF MS for rapid pathogen identification, our use of whole-genome sequencing (WGS) enabled a more comprehensive understanding of the organism’s phylogenetic characteristics and resistance mechanisms. These findings suggest that both host-related factors (e.g., immune status, nature of trauma) and strain-specific traits may influence the pathogenicity and clinical course of P. unguis-hominis infections. Additional studies with larger sample sizes and genomic analyses are warranted to better define its epidemiology, virulence factors, and optimal therapeutic approaches.

Conclusion

In summary, this case report highlights the first documented instance of fungal keratitis caused by Pyrenochaeta unguis-hominis in a Chinese male patient with a history of ocular trauma and surgical intervention. The findings emphasize the critical need for heightened clinical awareness and timely identification of uncommon fungal pathogens in ocular infections. By integrating multidisciplinary approaches—ranging from conventional microscopy to genomic sequencing—clinicians can improve diagnostic accuracy and tailor effective treatment strategies. This case serves as a valuable reference for future encounters with P. unguis-hominis and other rare fungi, advocating for standardized protocols in pathogen identification and susceptibility testing to mitigate vision-threatening outcomes. Continued research and case reporting are essential to expand the understanding of such rare pathogens and optimize patient care in ophthalmology.

CRediT authorship contribution statement

Hua Wang: Methodology. Yueyang Li: Software. Zhigang Wang: Writing – original draft, Software, Methodology, Investigation. Yani Zhang: Formal analysis. Yating Cui: Writing – original draft, Validation, Software, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Zhijun Gao: Writing – original draft, Validation, Software, Methodology, Investigation. Qingyi Zhang: Validation. Haiying Li: Writing – review & editing, Visualization, Supervision, Resources, Project administration, Conceptualization. Hailan Xue: Data curation.

Ethical approval

Yes.

Consent

Written informed consent was ob tained from the patient and / or parent/legal guardian of the patient for publication of the details of their medical case and any accompanying images.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.