Acanthamoeba sp.

Several species of these free-living amoebae have been described: Acanthamoeba castellanii, A. culbertsoni, A. polyphaga, A. astronyxis, A. hatchetti, A. rhysodes. The trophozoite measures 15-45 m and exhibits fine thorn-like protrusions, acanthapodia. The name of the genus is derived from this feature (Gr. acanth = thorn). The cysts have a double wall. The outermost wall is wrinkled. The protozoa occur in numerous places (water, dust, waste). Unlike with Naegleria, infection of the central nervous system progresses slowly and occurs where there is immune suppression or in the course of a severe general illness. Generally it presents as a subacute granulomatous meningo-encephalitis with signs of a brain abscess, and develops in two to three weeks. Abscesses in other locations, and granulomatous skin lesions in which histological investigations show amoebae, have also been observed. Treatment is very difficult. Rifampicin has been used. With pentamidine, cure can be achieved in systemic infections with Acanthamoeba rhysodes.

Keratitis is more common than cerebral inflammation. This was first described in 1974. These amoebae may infect small wounds of the cornea and then trigger a dangerous ulcerative keratitis which may develop into panophthalmitis. Initially this diagnosis is often missed and the lesion is considered to be a herpetic or fungal keratitis. Acanthamoeba infection may follow upon a herpetic keratitis. The amoebae are often scarce in corneal smears, culture is possible but often not available and sometimes the diagnosis is made purely on anatomopathological grounds, e.g. during a cornea transplantation. The number of cases has grown in recent years as the result of increased use of contact lenses and the practice of rinsing these with tap water. Possibly bacteria in the biofilm on dirty contact lenses constitute a good source of nutrition for the amoebae. If there is a superficial epithelial defect, whether or not caused by the contact lens, the amoebae may become invasive in the corneal stroma.

Treatment of Acanthamoeba keratitis is difficult. Sometimes a combination of propamidine isethionate eye drops (Brolene®), topical neomycin, polyhexamethylene biguanide eye drops (Lavasept®) and/or topical chlorhexidine (Hibitane®) is given. Brolene®, available in Great Britain, is an antiseptic which is moderately toxic for the corneal epithelium. The use of topical steroids is controversial but probably beneficial. Oral itraconazole is probably also active. Topical miconazole is sometimes also used. Pentamidine (a diamidine related to propamidine) is being evaluated.

Category: Microbiology Notes