Diagnosis And Management of Nocardia Keratitis
Nocardia keratitis: An Introduction
Nocardia keratitis, the corneal infection caused by Nocardia species. Though Nocardia keratitis is rare, it is potentially sight-threatening in humans. The diagnosis of Nocardia keratitis is often missed or delayed due to the rarity and the clinical profile’s resemblance to fungal keratitis. Now, with advances in the understanding of its pathogenesis, new diagnostic methods, and treatment options, the overall visual outcome in Nocardia keratitis has improved.
First described in 1888 by Edmond Nocard, the genus Nocardia is a gram-positive, aerobic Actinomycete that contains 90+ recognized species. Nocardia is ubiquitous, representing the microflora of air, water, soil, and decaying vegetation. Nocardia spp. may also be present in clinical specimens as opportunistic pathogens (OPs), say the studies.
- asteroides and N. brasiliensis are isolated from the ocular tissue of individuals with Nocardia keratitis. Other species such as N. abscessus, N. amikacinitolerans, N. amamiensis, N. beijingensis, N. cyriacigeorgica, N. exalbida, N. kruczakiae, N. otitidiscaviarum, N. puris, N. shinanonensis, N. transvalensis, and N. thailandica have also been found clinically significant in the cases of Nocardia keratitis, thanks to recent advances in molecular biology. N. farcinica is another species that needs special mention due to its resistance to most of the common topical ophthalmic antibiotics. Most reported cases of Nocardia keratitis by N. farcinica are in immunocompromised patients who had or recovered from primary pulmonary infection.
Though Nocardia is found worldwide, some species are more prevalent in specific geographic areas. N. cyriacigeorgica, N. farcinica, and N. nova are more prevalent in the soils of non-tropical countries whereas N. brasiliensis is found in tropical and subtropical regions of America and N. otitidiscaviarum is more prevalent in the US, India, and Japan. That said, clinicians are of the opinion that Nocardia keratitis is more prevalent in South Asia.
Studies from Nepal concluded that Nocardia constituted 0.3% of all isolates from keratitis cases. A South Indian study by Francis Char DeCroos et al. concluded that about 1.7% of all bacterial keratitis was due to Nocardia. Another study from Madurai in South India by M Srinivasan et al. reported the isolation of Nocardia from 8.34% of samples collected at Aravind Eye Hospital, Madurai.
Nocardia keratitis of all etiologies tends to occur sporadically with a majority reported in males and young adults.
- Ocular Trauma
Ocular trauma is an increasingly common predisposing factor for Nocardia keratitis. Trauma can be surgical or vegetal.
According to a study published in the Journal of cataract and refractive surgery, Nocardia keratitis occurred in 4 out of 3 patients who had undergone photorefractive keratectomy (PRK). Inadequate sterilization during surgery was concluded as the cause of this outbreak. Some studies also documented Nocardial keratitis following myopic keratomileusis (MKM) and penetrating keratoplasty (PK).
Ocular exposure to vegetative matter, soil, dirt, small insects as well as fingernail injury also serves as the inciting factor for Nocardia keratitis.
- Contact lens abuse
Contact lens wear is the second most common risk factor. Contact lens abuse, especially extended wear, poor lens hygiene, contaminated lens solution, ‘topping off’ lens solution, or sharing cosmetic contact lenses increase the risk for developing Nocardia keratitis. Extended or overnight wear is identified as a predisposing factor for infections by N. arthritidis, N. amikacinitolerans, N. farcinica, and N. exalbida.
- Topical Corticosteroid Usage
The use of topical steroids post corneal grafting may favor the development of Nocardia keratitis, say the studies. Scientists found that a 14-day course of topical prednisolone acetate (1%) suspension, after endothelial corneal graft rejection, was aggravated by Nocardia asteroides.
- Diabetes mellitus
People with diabetes are at an increased risk of infectious complications of Nocardia keratitis. Xian-Ning Liu et al. documented a case of Nocardia brasiliensis keratitis in a 32-year-old diabetic woman in China.
Nocardia keratitis: Physical examination & Clinical Manifestation
A comprehensive ocular examination is the first stage. This includes vision tests, ocular motility test, pupillary assessment, and recording intraocular pressure. This is followed by a slit-lamp exam where lids, lashes, cornea, iris, and lens are systematically assessed. A pupil dilation test should be performed to assess the optic nerve and retina.
The clinical course of Nocardia keratitis is usually progressive with pain and redness in one or both eyes being the first symptoms. Signs include photophobia, blepharospasm, lid, and conjunctival edema, eye discharge, and blurry vision. The conjunctiva may show chemosis and a mild to a severe papillary reaction.
Slit-lamp photography shows patchy anterior stromal infiltrates in a wreath-like arrangement. These infiltrates are usually seen in the mid-periphery of the cornea and extends along the limbus. Usually, there’s a prominent ciliary flush and corneal pannus.
Stromal infiltrates often show feathery margins with a necrotic or fluffy surface – the signs unique to fungal keratitis. This mimicking is one of the reasons why Nocardia keratitis diagnosis is challenging.
Satellite lesions, hypopyon, anterior chamber inflammatory reaction, keratic precipitates (KPs), and Descemet’s membrane folds are other clinical manifestations reported.
Topical application of prednisolone acetate 1% can induce Nocardia keratitis, concludes the study by Jain et al. White patchy infiltrates in the supranasal corneal, hypopyon of 1mm thickness, Descemet’s membrane folds, and conjunctival congestion appeared in individuals treated with topical steroids. Smear and culture revealed the presence of beaded branching filaments, characteristic of N. asteroids.
Peripheral deep neovascularization is also seen. Hypesthetic cornea and corneal ulceration resulting from the coalescence of superficial granular infiltrates are also reported.
History taking is the first step in making an Nocardia keratitis diagnosis. Ophthalmologists collect information on previous eye injury or surgery, and treatments used. A history of infection may suggest that Nocardia keratitis is secondary to a pathogen.
- Laboratory diagnosis
The laboratory diagnosis starts with corneal scraping and/or biopsy where a Kimura spatula or a No.15 surgical blade is used to take samples from the cornea. The eyelid margins – where normal flora is seen – are usually avoided for the diagnosis to be accurate. Calcium alginate swabs moistened with tryptic soy broth are also used to collect corneal samples for their higher sample recovery. The corneal scrapings are then taken for inoculation onto media plates and slides.
- Microscopic evaluation
The corneal scrapings are smeared, stained, and then examined under a confocal microscope. Gram stain, Giemsa stain, and Calcofluor-white (CFW) stain are the most widely staining methods to detect Nocardia species.
Nocardia spp. shares morphologic and histochemical staining features with Actinomycetes. However, as distinct from Actinomycetes, Nocardia is weakly acid-fast and can be distinguished with 1% acid-fast (Ziehl-Neelsen or modified Kinyoun’s method). Actinomycetes do not fluoresce under ultraviolet light in a Calcofluor-white (CFW) stain. All actinomycetes produce the characteristic black color in the Grocott’s Methenamine Silver (GMS) staining.
Fungal keratitis may mimic Nocardia keratitis and vice versa. Nocardia appears as highly reflective filaments on a confocal microscope, similar to many fungi. Nocardia keratitis may be distinguished from fungal keratitis by specific stains. For example, Nocardia spp. cannot be visualized on a Hematoxylin and Eosin (H&E) stain or Periodic acid–Schiff (PAS) stain for fungi.
Moraxella and nontuberculous Mycobacteria also mimic Nocardia infections. Moraxella, a gram-negative diplococcus, produces corneal ulcers in alcoholics, diabetic patients, and malnourished people. Both Nocardia and Moraxella grow on blood agar and chocolate agar; however, Nocardia stains gram-positive and develop filaments in culture whereas Moraxella stains gram-negative.
Mycobacteria, the acid-fast bacilli, causes progressive corneal infections. Both Nocardia and Mycobacteria grow in blood agar and stain positive with Ziehl-Nielson stain. However, the corneal infiltrates of Mycobacteria are more like fluffy, snow-flakes. Furthermore, unlike Nocardia, Mycobacteria grow on Lowenstein Jensen (LJ) media, a selective medium.
- Microbial culture
Cultures are preferred to smears as they are more specific and information yielding in terms of Nocardia infection. Nocardia spp. is slow-growing. However, they can grow as small, white dry colonies on a variety of non-selective and selective media. The colonies may take a yellowish shade and may or may not exhibit branching.
Typically, growth takes within 2-3days on blood agar, chocolate agar, and Sabouraud agar; microbiologists recommend at least 15-days for isolation to avoid false negatives.
Antibiotic-containing selective medium, such as modified Thayer-Martin agar is also used to culture Nocardia especially when specimens are suspected to be contaminated with other pathogens.
- Biochemical tests
Some of Nocardia species can be identified using biochemical tests, including hydrolysis of adenine, casein, tyrosine, xanthine, and hypoxanthine. For example, biochemically, N. brasiliensis can be differentiated from the other Nocardia species by its ability to hydrolyze urease, casein, esculin, hypoxanthine, and tyrosine.
Growth rate at 45 °C and acid production from carbohydrates are also used to differentiate Nocardia species. For example, N. farcinica can grow rapidly at 45°C within 72 hours by utilizing acetamide as a sole carbon and nitrogen source.
With the identification of new Nocardia strains, biochemical tests are proved to be non-specific, showing accurate, positive results for only a few species including N. brasiliensis, N. farcinica, and N. pseudobrasiliensis.
- Antimicrobial susceptibility tests
Antimicrobial susceptibility patterns are used to predict the presence of certain Nocardia spp. – N. brasiliensis, N. pseudobrasiliensis, and N. farcinica, for example. However, this method is limited by the ‘n’ number of species now known to cause keratitis.
- Molecular identification of Nocardia
Advances in molecular biology have enabled a better understanding of the phylogenetic relationship between Nocardia and related microbes. Molecular techniques have also made identification of many Nocardia spp. more rapid and accurate that is possible with conventional methods.
Gene sequencing: An invaluable tool for the identification of clinical isolates of Nocardia. The 16S rRNA gene of Nocardia is believed to be highly conserved, containing extensive regions identical for all species as well as species-specific variable regions. Patel et al. studied 500-bp 16S rRNA gene sequencing patterns to identify correlations between 11 Nocardia species. They identified sequence heterogeneity within the partial 16S rRNA region of N. nova, N. otitidiscaviarum, and N. transvalensis. Cloud et al. reported a 90% correlation in the sequences of the first 999-bp region of the 16S rRNA gene in the 10 Nocardia species sampled and studied.
Ribotyping: The molecular technique that uses DNA probes to recognize ribosomal genes in bacteria. McNeil et al. used ribotyping and showed that N. farcinica has a ribotype identical to that of N. farcinica but different from the strains of N. nova and N. asteroids.
Pyrosequencing: Touhy et al. used pyrosequencing of the hypervariable region of the 16S rRNA gene to identify clinically relevant Nocardia species. Pyrosequencing involves DNA sequencing by detecting the pyrophosphate released during DNA synthesis.
PCR and PCR-RFLP molecular analysis (PRA): One of the first molecular techniques used for the characterization and identification of Nocardia species. Brown et al. used N. farcinica-specific 314-bp primer to identify other members of the species.
Nocardia keratitis: Treatment
Treatment for Nocardia keratitis is mostly medical therapy, although surgery is required in the case of advanced infections. Some acute infections resolve spontaneously, with the use of topical antibiotics, or discontinuation of extended contact lens wear.
- Medical therapy
The popular drugs of choice for treating Nocardia infection are topical forms of chloramphenicol, amikacin, ciprofloxacin, levofloxacin, moxifloxacin, ofloxacin, gatifloxacin, and tobramycin. Oral clarithromycin is also recommended.
Recent research showed that Nocardia species, especially, N. amamiensis, N. thailandica, N. levis, and N. puris are susceptible to amikacin and sulphonamides. Varying doses of aminoglycosides such as gentamicin and tobramycin are also found to be effective in managing Nocardia infection. Fluoroquinolones such as ciprofloxacin and clarithromycin are also used in Nocardia keratitis treatment however the susceptibility varies across species.
Gulpczynski et al. showed that N. nova, N. cyriacigeorgica, N. abscessus, and N.brasiliensis are resistant to ciprofloxacin. Certain Nocardia strains exhibit resistance to vancomycin, amikacin, clarithromycin, and trimethoprim.
Corticosteroids are employed in treating bacterial keratitis at a cellular level. Besides limiting the scarring, they also help reduce decrease pain and burning feeling in the eyes; long-term usage of corticosteroid may increase intraocular pressure and lead to cataracts, though.
Limited doses of trimethoprim-sulfamethoxazole are also found to be effective in Nocardia treatment.
- Surgical methods
Nocardia keratitis is usually well treated with medications. However, surgery is warranted in the case of irreversible scarring, progressive corneal thinning, or when the infection extends beyond the limbus. The most recommended surgical options include therapeutic lamellar keratectomy, penetrating keratoplasty, and conjunctival flap.
Rahimi et al. used Amniotic Membrane Transplantation (AMT) as adjunctive therapy to treat persistent epithelial defect (PED) and corneal vascularization in Nocardia keratitis case extending beyond limbus; PED healed, yet corneal vascularisation persisted.