Acanthamoeba Antigens

Acanthamoeba is a microscopic, free-living ameba, or amoeba (single-celled living organism). It is the causative agent of granulomatous amebic encephalitis (AGE) and amebic keratitis (AK) and have been associated with cutaneous lesions and sinusitis. The ameba is found worldwide in the environment in air, soil, and water. Most people will be exposed to Acanthamoeba during their lifetime, but very few will become sick from this exposure. Based on rRNA gene sequences, the genus Acanthamoeba is divided into 17 different genotypes to date (T1 - T17). Studies in which clinical isolates have been identified based on sequence types have shown that the majority of strains causing keratitis belong to T4.

• Acanthamoeba granulomatous encephalitis (AGE)
  GAE is a disease which is characterized by a chronic protracted slowly progressive CNS infection which also may involve the lungs. Serological laboratory diagnosis has indicated that several Acanthamoeba species are associated with GAE. The route of infection is thought to be by inhalation of amebae through the nasal passages and lungs or introduction through skin lesions. Access to the CNS may be by hematogenous spread from a primary site in the lungs or skin or directly through the olfactory neuroepithelium.
• Acanthamoeba keratitis (AK)
  AK is a painful progressive sight-threatening corneal disease. Several species of Acanthamoeba have been reported to cause AK. Unlike debilitated patients with GAE or cutaneous acanthamebiasis, individuals with AK generally are immunocompetent. Nevertheless, these individuals do not develop protective immunity, and reinfection can occur. It is now recognized that the wearing of contact lenses is the leading risk factor for AK.

Fig.1 The transmission electron micrograph of Acanthamoeba trophozoite (R Siddiqui & NA Khan. 2012)

The ameba's life cycle includes an active feeding trophozoite stage and a dormant cyst stage. During the trophozoite stage, Acanthamoeba feeds on organic particles as well as other microbes and divides mitotically under optimal conditions (food supply, neutral pH, ~30°C) and 50-80mOsmol. Exposure to harsh conditions result in cellular differentiation into a double-walled cyst form. The outer walls consists of proteins and polysaccharides, while the inner wall possesses cellulose. Both walls are normally separated by a space, except at certain points where they form opercula in the centre of ostioles (exit points for excysting trophozoite).

Fig. 2 The life cycle of Acanthamoeba spp (R Siddiqui & NA Khan. 2012)

Infection can be diagnosed by direct microscopy of wet mounts of cerebrospinal fluid or stained smears of cerebrospinal fluid sediment, light or electron microscopy of tissues, in vitro cultivation of Acanthamoeba, and histological assessment of frozen or paraffin-embedded sections of brain or cutaneous lesion biopsy material. Immunocytochemistry, chemifluorescent dye staining, PCR, and analysis of DNA sequence variation also have been employed for laboratory diagnosis.