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Acanthamoeba keratitis is a serious and vision threatening disease. It is commonly associated with contact lens wear (up to 93%).1 Early diagnosis and treatment are essential to improve the visual outcome.2 Devastating ocular damage can be attributed to various factors such as misdiagnosis,3 incorrect treatment, excessive topical steroid before diagnosis,4 and resistance.5
Acanthamoeba keratitis in non-contact lens wearers is rare and poses a diagnostic challenge. We present a case of acanthamoeba keratitis in a non-contact lens wearer following accidental exposure to bird seed dust. The strain of acanthamoeba obtained from this patient appeared to show in vivo and in vitro resistance to poly(hexamethylene biguanide, PHMB) and chlorhexidine after a good clinical response initially.
A 57 year male patient presented with pain, blurring of vision, and photophobia of his left eye. Two weeks before the presentation he had an accidental exposure to bird seed dust (brand name Trill, manufactured by Master Foods, Hungary) for his budgies while cleaning the seed pot. It was a seemingly trivial injury, not likely to have caused a breach of epithelium. Examination revealed a visual acuity of 6/6 of the right eye and 6/18 for the left eye. The left eye showed multiple punctate epithelial erosions with epithelial and stromal infiltrates. There was no retained debris at the time of presentation. Initially he was treated as a case of viral keratitis with topical aciclovir and steroid. Although there was an early improvement, the keratitis relapsed after 2 weeks. At that stage a typical ring infiltrate suggestive of acanthamoeba keratitis developed and epithelial culture grew Acanthamoeba polyphaga. He was started on intensive treatment with PHMB, Brolene, and neomycin. His symptoms improved and his visual acuity recovered to 6/9 over a period of 3 weeks. Topical steroids were then added. The antimicrobial treatment was given for 2–3 months and withdrawn gradually over next 4–6 weeks after complete resolution. But following complete cessation of all drops he developed a recurrence with positive cultures. We restarted the intense treatment with PHMB and chlorhexidine, but the clinical response was poor. Pain was severe with intense limbal inflammation and signs of scleritis.
A corneal biopsy was performed which showed persistence of infection. Resistance to PHMB (minimum inhibitory concentration (MIC), 3.125 μg/ml) and chlorhexidine (MIC, 6.25 μg/ml) was demonstrated in the culture obtained from the biopsy. The strain showed in vitro sensitivity to propamidine (MIC, 3.9 μg/ml). A change of treatment to topical propamidine isethionate 0.1% (Brolene) and neomycin led to a rapid response with a decrease in symptoms. Six months after initial diagnosis he is comfortable on maintenance treatment with propamidine isethionate 0.1% and neomycin, but unfortunately has developed a dense central corneal scar (fig 1) and vision of hand movements.
Acanthamoeba keratitis not related to contact lens wear has been reported before and risk factors include trauma, dirty water splash, and exposure to leaf juice.6 Exposure to bird seed dust has to our knowledge not been reported previously as a known risk factor. Unfortunately, an attempt to culture acanthamoeba from the actual bird seeds and tray was unsuccessful. The second uncommon feature in our case is the demonstration of in vitro resistance of this strain of acanthamoeba to two of the modern first line acanthamoeba drugs (PHMB and chlorhexidine) while showing a good sensitivity to propamidine.
This is contrary to what has been reported by other authors.7,8 We are unable to say whether resistance developed during treatment or was pre-existent, as sensitivity profiles of the earlier isolates were not obtained. This patient’s initial good clinical response was achieved with a combination of PHMB and propamidine with the latter tapered early during the course of treatment, indicating at least partial in vivo sensitivity to PHMB in the earlier stages. A poor association between in vivo and in vitro resistance has been described for biguanides,7 but this case shows that in vitro MIC can be useful information in the management of persistent acanthamoeba keratitis.
Other authors have stressed the need for long term treatment2 and this case also underscores the importance of prolonged effective antimicrobial treatment in order to prevent recurrences.