Acanthamoeba

(endorsed 1996)

Guideline

No guideline value is set for Acanthamoeba species in drinking water.

General description

Acanthamoeba spp. are small free-living amoebae that are common in aquatic environments and are among the predominant protozoa in soil. Several of approximately 20 known species are virulent, causing the cerebral infection granulomatous amoebic encephalitis (GAE), or the corneal infection amoebic keratitis, or both. One or both diseases have occurred in most temperate and tropical regions of the world. Acanthamoeba spp. may also be significant as host cells for the proliferation and dispersal of Legionella species.

The relative importance of water as a source of infection is unknown. The wide distribution of Acanthamoeba in the natural environment makes soil, airborne dust and water all likely sources. Delays in the diagnosis of GAE and keratitis cases have made it difficult to investigate possible sources of infection, while the lack of a stable classification of Acanthamoeba inhibits identification of individual isolates, including the matching of amoebae from infections with organisms from the environment.

Regular monitoring for Acanthamoeba is not appropriate, but these organisms need to be considered when planning the maintenance of eyewash stations that use mains water.

Australian significance

Amoebic keratitis has been recorded in New South Wales, Queensland, South Australia, Victoria and Western Australia (e.g. Roussel et al. 1985). Currently, four cases of GAE have been diagnosed in Australia (Victoria and Western Australia, e.g. Harwood et al. 1988). Data have also been collected on the diversity and density of Acanthamoeba species in water and sediments, mainly in South Australia; the organisms are likely to proliferate over a wide temperature range in water where organic carbon levels promote significant bacterial production. Contamination of environments that may become sources of infection (swimming and spa pools, cooling towers etc) cannot be assumed to originate with organisms from the water supply, given the wide distribution of Acanthamoeba in the natural environment.

Treatment of drinking water

Acanthamoeba species are usually less numerous in surface source waters than Naegleria species, but often contaminate piped water supplies at a low level, even when chlorine is present. Their cysts are among the most resistant of protozoan cells to oxidative disinfectants, making removal difficult at the levels of disinfectant generally used for drinking water. In any case, control of Acanthamoeba may be most important in specialised uses of water: distribution in hospitals, renal dialysis or industrial eye-wash stations.

Method of identification and detection

Detection of amoebae, concentrated from water samples, requires relatively simple growth media and standard laboratory incubation facilities. Identification of Acanthamoeba species is more specialised. These amoebae are most likely to be significant in specific investigations of sources of infection, when comparison with reference strains would be essential to their identification.

Health considerations

Acanthamoeba species are opportunistic pathogens. GAE usually occurs in immunocompromised patients, secondary to infection of another organ (often lungs or subcutaneous tissue). Most cases have been recognised at post-mortem after protracted illness, making any investigation of the circumstances of infection difficult. Amoebic keratitis occurs in two groups of people: those who sustain a corneal lesion before or at the time of infection and who often have outdoor occupations (Roussel et al. 1985); and people who wear contact lenses. A specific source of infection has rarely been confirmed, but circumstances suggest that the first group are often infected by cysts from airborne dust or soil, while tap water, used incorrectly to wash lenses, may often be the source for the second group.

Derivation of guideline

No guideline value is proposed for Acanthamoeba species, given the uncertainty about sources of infection, but water authorities should be aware of the direct health significance of these organisms and their possible role in the ecology of Legionella.

References

Harwood CR, Rich GE, McAleer R and Cherian G (1988). Isolation of Acanthamoeba from a cerebral abscess. Medical Journal of Australia, 148, 47–49.

Roussel TJ, Badenoch PR, Chandraratnam E and Coster DJ (1985). Acanthamoebic keratitis in a healthy Australian man. Medical Journal of Australia, 143, 615–617.