Invited reviewFree-living amoebae as opportunistic and non-opportunistic pathogens of humans and animals
Introduction
The distinction between parasitic and free-living protozoa is generally sharply drawn with organisms falling readily into one or the other category. Some of the free-living amoebae are unusual in that they straddle the line separating the two groups of organisms and yet are as destructive as any of the classic parasitic protozoa. Unlike their parasitic counterparts, they are not well adapted for parasitism. They almost invariably kill their hosts instead of evolving, as have many parasites, to a state of détente with their hosts. Furthermore, being free-living and widely distributed in nature, they are not dependent upon a host for transmission and spread, nor does host-to-host transmission of these amoebic diseases occur. The amoebae include Acanthamoeba spp., Balamuthia mandrillaris, Naegleria fowleri, Sappinia diploidea, and several other representatives that, by virtue of their ability to survive within a mammalian host, are preadapted for a pseudo-parasitic and potentially pathogenic life-style. These organisms have been called amphizoic amoebae in recognition of their ability to live endozoically, yet they are capable of free-living existence (Page, 1974).
It was due to the prescient observations of Culbertson that the pathogenic potential of free-living amoebae was first realised (Culbertson et al., 1958). Cytopathology produced in monkey kidney tissue cultures used for growing poliovirus was shown to be caused by Acanthamoeba and not by a simian virus as originally thought (Culbertson, 1971). Culbertson (1971) later confirmed that, when inoculated into mice or monkeys, the amoebae killed the animals. The first human case of amoebic meningoencephalitis was reported from Australia, but the etiologic agent, first thought to be Acanthamoeba, was later identified as Naegleria (Fowler and Carter, 1965). Not long after, however, the first human infection by Acanthamoeba was described (Jager and Stamm, 1972). Acanthamoeba infections were characterised as opportunistic infections by Martinez, who recognised their occurrence in debilitated or chronically ill patients, and who distinguished between the pathologies caused by Acanthamoeba and Naegleria (Martinez, 1980). Keratitis cases caused by Acanthamoeba were diagnosed in the United Kingdom and in the United States by Naginton et al., 1974, Jones et al., 1975, respectively. The first recognised case of a Balamuthia infection was identified by Visvesvara et al. (1990) in a baboon that died in a zoological park, but the amoeba was detected in humans soon after (Visvesvara et al., 1993).
It should be noted that earlier reports of human infections by free-living amoebae have appeared in the literature, but they were ascribed to parasitic or commensal amoebae (e.g. Iodamoeba bütschlii) and, only upon later review, were they found to be caused by free-living amoebae (Section 3.4). Some reports, particularly early ones, of Acanthamoeba infections referred to the amoeba as Hartmannella, or used the two genus names interchangeably (Cleland et al., 1982, Culbertson, 1971, Jager and Stamm, 1972). Hartmannella is, however, a distinctly different amoeba (Page, 1967), and no human pathologies have been reliably associated with hartmannellid amoebae.
At present, the taxonomy of the free-living amoebae is unsettled and subject to change, reflecting new data arising from genomic sequencing studies (De Jonckheere, 2004, Stothard et al., 1998). The amoebae are a polyphyletic group, with stocks arising from different branches of the protozoal ancestral tree. Acanthamoeba and Balamuthia, based on 16S rRNA sequencing data, are closely related (Amaral Zettler et al., 2000, Booton et al., 2003a), but phylogenetically distant from Naegleria and Sappinia. The pathophysiology of the diseases they cause and the immunocompetence of the hosts also helps to distinguish between them. Opportunistic infections by Acanthamoeba and Balamuthia occur in immunocompromised or immunosuppressed individuals, and the HIV/AIDS epidemic gave rise to a number of cases of acanthamoebiasis and balamuthiasis. Acanthamoeba also causes amoebic keratitis (AK), a non-opportunistic ocular infection in otherwise healthy individuals. Naegleria fowleri causes a non-opportunistic, but devastating and rapidly fatal meningoencephalitis (primary amoebic meningoencephalitis) in immunocompetent children and young adults. Sappinia, based on a single case reported in the literature, was described from an immunocompetent individual. The encephalitides caused by Acanthamoeba and Balamuthia are of a granulomatous type that develop insidiously over an extended period of time. There is no host specificity for these amoebic diseases, and they have been described in a variety of animals as well as humans. A comparison of the four genera of amoebae and of the diseases they cause can be found in Table 1.
Section snippets
Acanthamoeba spp.
Acanthamoeba is the most common amoeba, if not the most common protozoon, to be found in soil and water samples (Page, 1988). It has a cosmopolitan distribution and has been isolated from a wide variety of habitats including fresh, brackish, and seawaters, beach sands, sewage, and soils ranging from tropical to arctic regions. It can also be readily isolated in the home environment from flowerpot soils, home aquaria, humidifiers, water taps and sink drains, and has been recovered from the
Naegleria fowleri
Naegleria fowleri is the causal agent of primary meningoencephalitis (PAM). Naegleria spp. are amoeboflagellates found in soil and water, but they are not as ubiquitous as Acanthamoeba. In general, they are more sensitive to environmental conditions such as drying and pH extremes, and cannot survive in seawater. Although, some 30 species of Naegleria have been recognised based upon sequencing data (De Jonckheere, 2004), N. fowleri is the only one that has been isolated from cases of amoebic
Balamuthia mandrillaris
Following the recognition of Acanthamoeba and Naegleria encephalitides, infections by amoebae were turning up that were caused by neither of these two pathogens, or of any other known free-living or parasitic amoebae. In the late 1980s, an amoeba was isolated from the brain of a pregnant mandrill baboon that died in the San Diego (California) Wildlife Park (Visvesvara et al., 1990). Once the amoeba was established in culture and antibodies against it raised in rabbits for immunofluorescence
Sappinia diploidea and other amphizoic amoebae
A single case of encephalitis caused by the amoeba S. diploidea was reported in an immunocompetent male (Gelman et al., 2001, Gelman et al., 2003). In addition to environmental isolations of the amoeba, it had been previously identified from feces of humans, elk, bison and perhaps cattle, but was never before implicated in pathology (Levine, 1961). The amoeba is relatively large with indistinct pseudopodia and a pellicle that may wrinkle as the amoeba moves (Fig. 13). A distinctive feature of
Conclusions
Based on the relatively small number of infections that occur in humans and animals, the free-living amoebae are not a major public health menace at the level of malaria, Entamoeba amoebiasis, trypanosomiasis, etc. Because of the lack of effective antimicrobial therapy for treating amoebic encephalitides and the resultant often-fatal consequences of infections, they are a cause for concern. Systemic amoebic diseases are difficult to diagnose and it is likely that many cases go unrecognised and,
Acknowledgements
We thank Dr Rolf Michel (Central Institute of the Armed Forces Medical Service, Koblenz, Germany) for providing the culture of Sappinia diploidea that was used for photomicrography.
References (230)
- et al.
A molecular reassessment of the leptomyxid amoebae
Protist
(2000) - et al.
Survival of Escherichia coli 0157 in a soil protozoan: implications for disease
Fed. Eur. Microbiol. Soc. Microbiol. Lett.
(1999) - et al.
Successful medical management of Acanthamoeba keratitis
Am. J. Ophthalmol.
(1990) Characterization of Naegleria species by restriction endonuclease digestion of whole-cell DNA
Mol. Biochem. Parasitol.
(1987)Geographic origin and spread of pathogenic Naegleria fowleri deduced from restriction enzyme patterns of repeated DNA
BioSystems
(1988)Sequence variation in the ribosomal internal transcribed spacers, including the 5.8S rDNA, of Naegleria spp
Protist
(1998)Molecular definition and the ubiquity of species in the genus Naegleria
Protist
(2004)- et al.
Non-Acanthamoeba amoebic infection
J. Infect.
(1998) A fatal case of generalized amoebiasis due to a protozoon closely resembling, if not identical with, Iodamoeba bütschlii
Trans. R. Soc. Trop. Med. Hyg.
(1948)- et al.
The epidemiology and control of primary amoebic meningoencephalitis with particular reference to South Australia
Trans. R. Soc. Trop. Med. Hyg.
(1983)
Acid-active neuraminidases in growth media from cultures of pathogenic Naegleria fowleri and in sonicates of rabbit alveolar macrophages
Biochim. Biophys. Acta
Comparative sensitivity of Naegleria fowleri to amphotericin B and amphotericin B methyl ester
R. Soc. Trop. Med. Hyg.
Brain abscesses caused by free-living amoeba probably of the genus Hartmannella in a patient with Hodgkin's disease
Lancet
Survival and growth of Francisella tularenssis in Acanthamoeba castellanii
Appl. Environ. Microbiol.
Incidence of free-living amoebae in the nasal passages of local population in Zaria, Nigeria
J. Trop. Med. Hyg.
Resistance of Acanthamoeba castellanii cysts to physical, chemical and radiological conditions
J. Parasitol.
Apoptosis as a mechanism of cytolysis of tumor cells by a pathogenic free-living amoeba
Infect. Immun.
Amebic meningoencephalitis in a patient with AIDS caused by a newly recognized opportunistic pathogen. Leptomyxid ameba
Arch. Pathol. Lab. Med.
Nasal carriage of free-living amoebae
Microbiol. Ecol. Health Dis.
Pathogenicity of Acanthamoeba and a Corynebacterium in the rat cornea
Arch. Ophthalmol.
Amebic encephalitis caused by Balamuthia mandrillaris: report of four cases
Pediatr. Infect. Dis.
Detection of Naegleria spp. and Naegleria fowleri: a comparison of flagellation tests, ELISA and PCR
Water Sci. Technol.
Production of respirable vesicles containing live Legionella pneumophila cells by two Acanthamoeba spp
Appl. Environ. Microbiol.
18S ribosomal DNA typing and tracking of Acanthamoeba species isolates from corneal scrape specimens, contact lenses, lens cases, and home water supplies of Acanthamoeba keratitis patients in Hong Kong
J. Clin. Microbiol.
Genotyping of Balamuthia mandrillaris based on nuclear 18S and mitochondrial 16S rRNA genes
Am. J. Trop. Med. Hyg.
Identification of Balamuthia mandrillaris by PCR assay using the mitochondrial 16S rRNA gene as a target
J. Clin. Microbiol.
Balamuthia mandrillaris: identification of clinical and environmental isolates using genus-specific PCR
J. Euk. Microbiol.
Molecular and physiological evaluation of subtropical environmental isolates of Acanthamoeba spp., causal agent of Acanthamoeba keratitis
J. Euk. Microbiol.
Acanthamoeba keratitis: synergy between amebic and bacterial cocontaminants in contact lens care systems as a prelude to infection
J. Clin. Microbiol.
Interaction of Legionella pneumophila with Acanthamoeba castellanii: uptake by coiling phagocytosis and inhibition of phagosome-lysosome fusion
Infect. Immun.
Viabilitiy of Acanthamoeba cysts in ophthalmic solutions
Appl. Environ. Microbiol.
Observations by immunofluorescence microscopy on the cytopathogenicity of Naegleria fowleri in mouse embryo-cell cultures
J. Med. Microbiol.
Assessing the risk of primary amoebic meningoencephalitis from swimming in the presence of environmental Naegleria fowleri
Appl. Environ. Microbiol.
Meningoencephalitis due to pathogenic free-living amoebae. Report of two cases
J. Am. Med. Assoc.
Hartmannella vermiformis isolated from the cerebrospinal fluid of a young male patient with meningoencephalitis and bronchopneumonia
Arch. Med. Res.
Studies of limax amoebae in a swimming pool
Hydrobiologia
Acanthamoeba culbertsoni and Naegleria fowleri: occurrence of antibodies in man
J. Hyg. Epidemiol. Microbiol.
Amoebic meningoencephalitis: sixteen fatalities
Science
Isolation of limax amoebae from the nasal mucosa of man
Folia Parasitol. (Praha)
Legionella effectors that promote nonlytic release from Protozoa
Science
Protein kinase activation and protein phosphorylation in Naegleria fowleri amebae in response to normal human serum
J. Euk. Microbiol.
Subgenus classification of Acanthamoeba by ribotyping
Korean J. Parasitol.
Interaction of Mycobacterium avium with environmental amoebae enhances virulence
Infect. Immun.
Chronic amebic meningoencephalitis
Arch. Neurol.
The pathogenicity of soil amebas
Annu. Rev. Microbiol.
Acanthamoeba: observations on animal pathogencity
Science
Use of cell cultures as an indicator of pathogenicity of free-living amoebae
J. Clin. Pathol.
Virulence of pathogenic free-living amebae
J. Parasitol.
Immunity to pathogenic free-living amoebae: role of humoral immunity
Infect. Immun.
A case of primary amoebic meningoencephalitis: North Island, New Zealand
N.Z. Med. J.
Cited by (634)
The gene expression and proteomic profiling of Acanthamoeba isolates
2023, Experimental ParasitologyIdentification and genotyping of Acanthamoeba spp. in the water resources of western Iran
2023, Parasite Epidemiology and ControlIdentification and characterization of novel marine oxasqualenoid yucatecone against Naegleria fowleri
2023, International Journal for Parasitology: Drugs and Drug Resistance