Nocardia transvalensis keratitis: an emerging pathology among travelers returning from Asia

Case presentation

A 23-year-old man presented with redness, pain and decreased visual acuity in his right eye following an injury caused by dust while driving a motorcycle in Thailand. Despite topical treatment with prednisolone and neomycin prescribed by a local ophthalmologist, the ocular condition did not improve after four weeks. Ten weeks later, the patient was referred to our ophthalmology department for further management of a persistent corneal ulcer. Upon initial examination, visual acuity was limited to "counting fingers" in the right eye and was 20/20 in the left eye. Slit-lamp examination of the right eye showed a well-defined, dense, whitish infiltrate in the central cornea with an epithelial defect of the same size (Figure 1A). Discrete patches of keratitis with pinhead satellite stromal infiltrates in the margin were also observed, but there was no anterior chamber reaction. Multiple scrapings of the ulcer bed and margins were obtained after topical anesthesia. N. transvalensis was cultivated by inoculating 5%-sheep blood agar with the corneal scraping. The culture was incubated at 37°C in a 5% CO₂ atmosphere. The definite identification was based on partial sequencing of the 16S rRNA gene (over a length of 1.439 nucleotides), which showed a 99.79% sequence similarity with the N. transvalensis stricto sensu reference sequence (GenBank accession number GQ217496.1) and a lower 98.96% relatedness to Nocardia blacklockiae sequence (GenBank accession number GQ376162.1) and 98.55% to Nocardia wallacei sequence (GenBank accession number GQ853074.1). Additional microbiological analyses, including fungal culture and the molecular detection of amoebas and herpes simplex virus (based on the 18S rDNA and DNA polymerase genes respectively), were negative. The patient was given topical amikacin (50 mg/ml) to be applied at hourly intervals in conjunction with oral ketoconazole (200 mg a day) and oral doxycycline (100 mg a day). After a four-week treatment period, the corneal infiltrate decreased so that only a faint subepithelial opacity remained (Figure 1B). Topical corticosteroids were then administered, and the patient recovered a visual acuity of 20/80 three months after the beginning of the treatment. There was no evidence of recurrence during the one-year follow-up period.

In the case reported herein, temporal evidence links the infection and the airborne dust that the patient suddenly felt in his eye while motorbiking in Thailand. Moreover, the patient was not a contact-lens wearer, and he did not have any history of ocular problems. We thus concluded that the patient acquired the infection in Thailand. In this patient, N. transvalensis infection was firmly documented by culture and subsequent sequence-based identification. This second reported case of N. transvalensis ocular infection [3] indicates that N. transvalensis must be added to the list of Nocardia species associated with infectious keratitis [2, 4]. Of a total of 73 reported cases of Nocardia spp. keratitis over the last five years, 67 (92%) have been clearly acquired in individuals with direct links to Asia [2, 3, 5–16]. Whereas Nocardia spp. keratitis is a well-described clinical entity in Asia [1], it is seldom diagnosed in countries outside Asia. A recent visit to Asia therefore provides a clue for clinical diagnosis while waiting for PCR-based confirmation. Laboratory techniques have to be used to analyze every case of infectious keratitis because simple microscopic examination may mistakenly identify the case as fungal keratitis when the histopathology reveals acute-branching septate hyphae similar to those found in fungi. The most frequently noted predisposing factor for Nocardia keratitis is trauma, with surgery being the second most common factor. A few cases of Nocardia keratitis have also been reported in contact lens wearers, after refractive surgery and after implantation of intracorneal ring segments [1, 17].

Topical amikacin is commonly recommended to treat Nocardia keratitis [1] based on its in vitro antibacterial activity against Nocardia organisms [18], its demonstrated corneal penetration and its safety profile [19]. Several authors previously reported that the species N. transvalensis in fact comprises of an heterogeneous spectrum of organisms including both amikacin-susceptible and amikacin-resistant organisms, a hallmark of the N. transvalensis complex also incorporating the two newly described reported N. blacklockiae and N. wallacei [20]. In the patient herein reported, intensive tropical application resulted in complete resolution of the infection. One previously published case of N. transvalensis keratitis showed a decreased sensitivity to amikacin [3]. In fact, amikacin susceptibility and resistance have been determined on the basis of amikacin concentration achievable in serum during systemic Nocardia infections; as for Nocardia keratitis, amikacin is used as a topical antibiotic, achieving local concentrations far higher than those achievable during parenteral administration. While amikacin susceptibility profile could be used for the identification of Nocardia isolates, it is not useful for the topical treatment of Nocardia keratitis.