Pathogenic and opportunistic free-living amoebae: Acanthamoeba spp., Balamuthia mandrillaris, Naegleria fowleri, and Sappinia diploidea

Among the many genera of free-living amoebae that exist in nature, members of only four genera have an association with human disease: Acanthamoeba spp., Balamuthia mandrillaris, Naegleria fowleri and Sappinia diploidea. Acanthamoeba spp. and B. mandrillaris are opportunistic pathogens causing infections of the central nervous system, lungs, sinuses and skin, mostly in immunocompromised humans. Balamuthia is also associated with disease in immunocompetent children, and Acanthamoeba spp. cause a sight-threatening infection, Acanthamoeba keratitis, mostly in contact-lens wearers. Of more than 30 species of Naegleria, only one species, N. fowleri, causes an acute and fulminating meningoencephalitis in immunocompetent children and young adults. In addition to human infections, Acanthamoeba, Balamuthia and Naegleria can cause central nervous system infections in animals. Because only one human case of encephalitis caused by Sappinia diploidea is known, generalizations about the organism as an agent of disease are premature. In this review we summarize what is known of these free-living amoebae, focusing on their biology, ecology, types of disease and diagnostic methods. We also discuss the clinical profiles, mechanisms of pathogenesis, pathophysiology, immunology, antimicrobial sensitivity and molecular characteristics of these amoebae.

Cultivation of pathogenic and opportunistic free-living amebas

Free-living amebas are widely distributed in soil and water, particularly members of the genera Acanthamoeba and NAEGLERIA: Since the early 1960s, they have been recognized as opportunistic human pathogens, capable of causing infections of the central nervous system (CNS) in both immunocompetent and immunocompromised hosts. Naegleria is the causal agent of a fulminant CNS condition, primary amebic meningoencephalitis; Acanthamoeba is responsible for a more chronic and insidious infection of the CNS termed granulomatous amebic encephalitis, as well as amebic keratitis. Balamuthia sp. has been recognized in the past decade as another ameba implicated in CNS infections. Cultivation of these organisms in vitro provides the basis for a better understanding of the biology of these amebas, as well as an important means of isolating and identifying them from clinical samples. Naegleria and Acanthamoeba can be cultured axenically in cell-free media or on tissue culture cells as feeder layers and in cultures with bacteria as a food source. Balamuthia, which has yet to be isolated from the environment, will not grow on bacteria. Instead, it requires tissue culture cells as feeder layers or an enriched cell-free medium. The recent identification of another ameba, Sappinia diploidea, suggests that other free-living forms may also be involved as causal agents of human infections.

Leptomyxid ameba, a new agent of amebic meningoencephalitis in humans and animals

Amebae belonging to the order Leptomyxida are regarded as innocuous soil organisms incapable of infecting mammals. We report here the isolation of a leptomyxid ameba from the brain of a pregnant baboon (Papio sphinx) that died of meningoencephalitis at the San Diego Zoo Wild Animal Park. By using rabbit anti-leptomyxid serum in the immunofluorescence assay, we have identified the leptomyxid ameba in the brain sections of a number of human encephalitic cases from around the world as well as a few cases of meningoencephalitis in animals in the United States, which suggests that the leptomyxid amebae are potential etiologic agents of fatal meningoencephalitis in humans and animals.

Do free-living amoebae in treated drinking water systems present an emerging health risk?

There is an expanding body of evidence that free-living amoebae (FLA) increase both the numbers and virulence of water-based, human-pathogenic, amoeba-resisting microorganisms (ARM). Legionella spp., Mycobacterium spp., and other opportunistic human pathogens are known to be both ARM and also the etiologic agents of potentially fatal human lung infections. However, comparatively little is known about the FLA that may facilitate ARM growth in drinking water. This review examines the available literature on FLA in treated drinking water systems; in total 26 studies from 18 different countries. FLA were reported to breakthrough the water treatment barrier and enter distribution systems, in addition to the expected post-treatment system ingress. Once in the distribution system there is evidence of FLA colonization and regrowth especially in reservoirs and in-premise plumbing storage tanks. At the point of use the average FLA detection rate was 45% but highly variable (n = 16, σ = 31) due to both differences in both assay methods and the type of water systems examined. This review reveals that FLA are consistently detected in treated drinking water systems around the world and present a yet unquantified emerging health risk. However, more research is urgently required before accurate risks assessments can be undertaken to assess the impacts on human health, in households and institutions, due to exposure to FLA facilitated pathogenic ARM.

Cocultivation of Legionella pneumophila and free-living amoebae

Studies of the interaction of Legionella pneumophila with free-living amoebae showed that Naegleria lovaniensis and Acanthamoeba royreba could use L. pneumophila as a sole food source. However, growth of the amoebae on nonnutrient agar plates seeded with L. pneumophila was slower than growth on nonnutrient agar plates seeded with Escherichia coli. On inoculation of L. pneumophila into axenic cultures of N. lovaniensis and A. royreba, 99.9% of the L. pneumophila was destroyed within 24 h. After several weeks, however, some amoeba cultures became chronically infected and supported the growth of L. pneumophila. Amoebae exposed to L. pneumophila and containing adhered L. pneumophila, L. pneumophila antigens, or both, showed no increased pathogenic potential on intranasal inoculation of weanling mice. Similarly, L. pneumophila propagated in chronically infected amoeba cultures showed no increase in virulence on intraperitoneal inoculation of guinea pigs relative to L. pneumophila grown in yeast extract broth.

Temperature tolerance of pathogenic and nonpathogenic free-living amoebas

Within tested strains of the genera Naegleria and Acanthamoeba the ability to grow at high temperatures seems directly related to virulence, with nonvirulent strains unable to grow at normal or elevated body temperatures. Outside these genera, nonvirulent Hartmannella and Tetramitus do grow at elevated temperatures, which suggests a barrier to pathogenicity other than temperature sensitivity. The high optimal temperature of pathogenic Naegleria apparently explains previous difficulty in obtaining isolates from the aquatic environment.

Growth characteristics, cytopathic effect in cell culture, and virulence in mice of 36 type strains belonging to 19 different Acanthamoeba spp

A total of 36 strains belonging to 19 different species of Acanthamoeba were compared for temperature tolerance, ability to grow in an axenic medium, cytopathic effect in Vero cell culture, and virulence in mice. Pathogenic strains appeared to belong to different species, whereas pathogenic and nonpathogenic strains occurred in one species. Although growth at high temperatures and readiness to grow axenically indicated a potential for pathogenicity, each such strain had to be tested in cell cultures or laboratory mice to determine whether or not it was virulent. This study was not intended to differentiate Acanthamoeba spp., but to provide methods to be used for the specific isolation and identification of pathogenic Acanthamoeba strains.

Parasites in cultured and feral fish

Parasites, causing little apparent damage in feral fish populations, may become causative agents of diseases of great importance in farmed fish, leading to pathological changes, decrease of fitness or reduction of the market value of fish. Despite considerable progress in fish parasitology in the last decades, major gaps still exist in the knowledge of taxonomy, biology, epizootiology and control of fish parasites, including such 'evergreens' as the ciliate Ichthyophthirius multifiliis, a causative agent of white spot disease, or proliferative kidney disease (PKD), one of the most economically damaging diseases in the rainbow trout industry which causative agent remain enigmatic. Besides long-recognized parasites, other potentially severe pathogens have appeared quite recently such as amphizoic amoebae, causative agents of amoebic gill disease (AGD), the monogenean Gyrodactylus salaris which has destroyed salmon populations in Norway, or sea lice, in particular Lepeophtheirus salmonis that endanger marine salmonids in some areas. Recent spreading of some parasites throughout the world (e.g. the cestode Bothriocephalus acheilognathi) has been facilitated through insufficient veterinary control during import of fish. Control of many important parasitic diseases is still far from being satisfactory and further research is needed. Use of chemotherapy has limitations and new effective, but environmentally safe drugs should be developed. A very promising area of future research seems to be studies on immunity in parasitic infections, use of molecular technology in diagnostics and development of new vaccines against the most pathogenic parasites.

Primary amoebic meningoencephalitis in Britain

Meningoencephalitis proved to be due to an amoeba (Naegleria) has been diagnosed in Great Britain for the first time. The first patient (a boy of 2) survived longer than any previously recorded cases, but in spite of early diagnosis and treatment he died 15 days after the onset of meningeal symptoms.Two other children who were exposed to the same possible source of infection (a warm, muddy puddle) had similar symptoms and developed mild meningitis. A naegleria was isolated from the cerebrospinal fluid of one of them. Both recovered after treatment with amphotericin.

Axenic growth and drug sensitivity studies of Balamuthia mandrillaris, an agent of amebic meningoencephalitis in humans and other animals

A cell-free growth medium for the opportunistic pathogenic ameba Balamuthia mandrillaris is presented. This represents an advance over the use of monkey kidney cells for growth of the amebas and can be helpful in isolation of these amebas from brain tissue from cases in which amebic meningoencephalitis is a diagnostic possibility, as well as for biochemical and molecular biological studies. Three isolates of Balamuthia have been cultured in this medium. The cell-free growth system was also used to screen cultures for sensitivity to a variety of antimicrobial agents. Of the various drugs tested, pentamidine isethionate was most effective against amebas (ca. 90% inhibition after 6 days of exposure), but the drug was amebastatic and not amebacidal in the axenic system at the highest concentration tested (10 micrograms/ml).

Is Acanthamoeba encephalitis an opportunistic infection?

Fifteen patients with granulomatous amebic encephalitis (GAE) resulting from Acanthomoeba were studied. GAE usually occurred in chronically ill and debilitated individuals, some of whom had received immunosuppressive therapy; none had a history of swimming or had been involved in aquatic activities. In contrast, primary amebic meningoencephalitis (PAM) caused by Naegleria fowleri usually occurs in healthy, young individuals who are exposed to polluted waters. The results of this study confirm that opportunistic amebic infections occur with increased frequency in patients treated with steroids, chemotherapeutic drugs, or broad-spectrum antibiotics. The mechanism of such infection may be depressed cell-mediated immunity.

Studies on pathogenic and non-pathogenic small free-living amoebae and the bearing of nuclear division on the classification of the order amoebida

A systematic study of small free-living amoebae has been made under standardized and reproducible cultural conditions, and their pathogenicity has been tested in mice. Naegleria aerobia, Hartmannella culbertsoni and H. rhysodes are pathogenic; H. castellanii, H. astronyxis, H. palestinensis, H. glebae, H. exundans, H. vermiformis, Schizopyrenus russelli, Didascalus thorntoni and Tetramitus rostratus are non-pathogenic. Strains of H. culbertsoni and H. rhysodes are present in Indian soils. A classification of the order Amoebida Kent into families Schizopyrenidae Singh, Hartmannellidae Volkonsky, 1931 emend. Singh, 1952, and Endamoebidae (Calkins), based on nuclear division, is proposed, and the relation of this classification to previously defined families and genera of amoebae and its bearing on phylogeny are discussed. Metronidazole and other anti-amoebic drugs are ineffective against N. aerobia and H. culbertsoni in vitro and in meningo-encephalitis in mice.

Pathogen-pathogen interaction: a syndemic model of complex biosocial processes in disease

There is growing awareness of the health implications of fact that infectious agents often do not act independently; rather their disease potential is mediated in diverse and significant ways by their relationships with other pathogens. Pathogen-pathogen interaction (PPI), for example, impacts various virulence factors in human infection. Although still in its infancy, the study of PPI, a form of epidemiological synergism, is emerging as an important arena of new research and new understanding in health and clinical care. The aims of this paper are to: 1) draw attention to the role of PPI in human disease patterns; 2) present the syndemics model as a biosocial approach for examining the nature, pathways, contexts, and health implications of PPI; and 3) suggest the utility of this approach to PPI. Toward these ends, this paper (a) reviews three of case examples of alternative PPIs, (b) describes the development and key concepts and components of the syndemics model with specific reference to interacting infectious agents, (c) contextualizes this discussion with a brief review of broader syndemics disease processes (not necessarily involving infections disease), and (d) comments on the research, treatment and prevention implications of syndemic interaction among pathogens.

Importance of nonenteric protozoan infections in immunocompromised people

There are many neglected nonenteric protozoa able to cause serious morbidity and mortality in humans, particularly in the developing world. Diseases caused by certain protozoa are often more severe in the presence of HIV. While information regarding neglected tropical diseases caused by trypanosomatids and Plasmodium is abundant, these protozoa are often not a first consideration in Western countries where they are not endemic. As such, diagnostics may not be available in these regions. Due to global travel and immigration, this has become an increasing problem. Inversely, in certain parts of the world (particularly sub-Saharan Africa), the HIV problem is so severe that diseases like microsporidiosis and toxoplasmosis are common. In Western countries, due to the availability of highly active antiretroviral therapy (HAART), these diseases are infrequently encountered. While free-living amoebae are rarely encountered in a clinical setting, when infections do occur, they are often fatal. Rapid diagnosis and treatment are essential to the survival of patients infected with these organisms. This paper reviews information on the diagnosis and treatment of nonenteric protozoal diseases in immunocompromised people, with a focus on patients infected with HIV. The nonenteric microsporidia, some trypanosomatids, Toxoplasma spp., Neospora spp., some free-living amoebae, Plasmodium spp., and Babesia spp. are discussed.

Effect of disinfectants on pathogenic free-living amoebae: in axenic conditions

The amoebicidal properties of chlorine, chlorine dioxide, ozone, and deciquam 222 were examined in axenic conditions. Naegleria spp. were found to be more sensitive to chlorine and chlorine dioxide than Acanthamoeba spp. No marked difference in sensitivity to ozone or deciquam 222 could be detected between the pathogenic (A-1) and nonpathogenic (1501) strains of Acanthamoeba and the pathogenic (MsT) and nonpathogenic (P1200f) strains of Naegleria. Methods of disinfection are discussed with reference to suitability of the disinfectants to real conditions.

A case of keratitis due to Acanthamoeba in New York, New York, and features of 10 cases

A man in New York, New York, contracted keratitis caused by Acanthamoeba castellanii. The diagnosis was delayed because amoebae were not initially suspected as the infectious organism. The culture isolate and the amoebae in corneal sections were identified as A. castellanii by immunofluorescence using antiserum to plasma membranes of this species. With the rapid agar disk diffusion method, the amoebae were shown to e susceptible to pimaricin (0.5%) and resistant to greater than 1,000-micrograms/ml levels of paromomycin, polymyxin B-bacitracin-neomycin, acriflavine, 5-fluorocytosine, amphotericin B, gentamicin, and trimethoprim-sulfamethoxazole. The infection responded to treatment with pimaricin administered with several other drugs. This infection is the eighth case reported in the literature of acanthamoebic keratitis and emphasizes the need for clinicians to consider acanthamoebic infection in the differential diagnosis of eye infections that fail to respond to bacterial, fungal, and viral therapy.

The diagnostic value of a ring infiltrate in acanthamoebic keratitis

During the past ten years it has become increasingly apparent that acanthamoebae can directly infect the cornea, usually after trauma, associated with contaminated water or soft contact lens wear. Thirteen cases of acanthamoebic keratitis have been published. In only three of these cases was the diagnosis first made by microbiologic methods, while in the others it was made only after pathologic examination of resected corneal specimens or enucleated eyes. We report three additional cases, two of which were accurately diagnosed by corneal scrape-smears and cultures before penetrating keratoplasty was performed. The reason for the accurate laboratory diagnosis in these cases was the presence of a diagnostic paracentral annular corneal infiltrate or abscess, a feature identified in over two-thirds of the earlier cases but one which has not been adequately emphasized or pursued for its early diagnostic value. We review the other clinical and epidemiological features of this entity, microbiological diagnostic techniques, the pathologic aspects, the role of topically and systemically administered medicaments, and finally point out the almost unavoidable role of penetrating keratoplasty after the temporizing effects of medical treatments have been achieved.

Observations by immunofluorescence microscopy and electron microscopy on the cytopathogenicity of Naegleria fowleri in mouse embryo-cell cultures

The destruction of secondary mouse-embryo (ME) cells by Naegleria fowleri was studied by indirect immunofluorescence with ME-cell antiserum as a specific label to trace the fate of mammalian-cell cytoplasm. The appearance of naegleria-induced cytopathic effect in the cultures coincided with the accumulation of discrete particles containing granules of ME-cell antigen within the cytoplasm of amoebae, suggesting that the organisms ingested host-cell material. In cultures containing cytochalasin B, a non-lethal inhibitor of phagocytosis by N. fowleri trophozoites failed to acquire any granular fluorescence and were not cytopathogenic. The engulfment of mammalian-cell cytoplasm by the organisms was confirmed when thin sections of naegleria-infected ME-cell cultures were examined by electron microscopy. Amoebae were seen in the process of detaching portions of cytoplasm from whole ME cells by means of distinctive ingesting pseudopodia, and fragments of mammalian-cell cytoplasm were identified within the food vacuoles of trophozoites. There was no evidence for cytotoxic disruption of ME cells before or during engulfment of these fragments. It is concluded that N. fowleri trophozoites attack and destroy cultured ME cells by a phagocytosis-like mechanism alone, without the aid of any amoeba-associated cytotoxic or cytolytic agents. The possible significance of these findings with respect to the in-vivo pathocity of N. fowleri is discussed.

Acanthamoeba, an opportunistic microorganism: a review

Granulomatous amebic encephalitis due to Acanthamoeba spp. usually occurs in chronically ill and debilitated individuals. Some of these patients may have received immunosuppressive therapy. Another infection due to Acanthamoeba spp. has been corneal ulcerations which usually occur after minimal trauma to the corneal epithelium (1). In contrast, primary amebic meningoencephalitis due to Naegleria fowleri usually occurs in healthy, young individuals with a history of swimming in heated swimming pools, in manmade lakes or with recent contact with contaminated water and practising water-related sports. Subclinical infections due to free-living amebas are probably common in healthy individuals with the protozoa living as "normal flora" in the nose and throat. It is possible that in humans, antibodies and cell-mediated immunity protect the host in such ordinary circumstances against invasive infection. In debilitated and chronically ill individuals, depressed cellmediated immunity may allow these protozoa to proliferate, allowing a fulminant "opportunistic" infection to develop. In the case of acanthamoebic keratitis, it is important to keep in mind that the temperature and moist environment of the eye serve as a good medium for the growth and proliferation of the amebas and is not necessarily associated with immunosuppression but rather with trauma. This review confirms that opportunistic free-living amebic infections occur with increased frequency in patients treated with steroids, radiotherapy, chemotherapeutic drugs or with broad-spectrum antibiotics and suggest that the mechanism of such infection may be depressed cell-mediated immunity or some other alteration of the immune system, like acquired immunodeficiency syndrome (AIDS).

Isolation of two strains of Acanthamoeba castellanii from human tissue and their pathogenicity and isoenzyme profiles

Two strains of amoebae, one (CDC:0180:1) from the lung tissue of a patient who died of granulomatous amoebic encephalitis and the other (CDC:0179:1) from the debrided tissue of a mandibular autograft, were isolated and identified as Acanthamoeba castellanii based on the morphological and immunofluorescent staining characteristics of the trophozoites and cysts. Both strains of amoebae caused cytopathic effects in mammalian cell cultures and destroyed the cell sheet. However, only the CDC:0180:1 strain, on intranasal instillation into mice, produced the disease manifested by ruffled fur and aimless wandering, followed by coma and death within 30 days. The CDC:0180:1 strain also differed consistently from CDC:0179:1 and another nonpathogenic A. castellanii strain (ATCC 30,011) in isoenzyme makeup, a dissimilarity which probably reflects its pathogenic potential.

The distribution of Naegleria fowleri in man-made thermal waters

The discharges of 16 thermal polluting factories were examined for the occurrence of Naegleria fowleri, the causative agent of primary amebic meningoencephalitis. Seven of these waters were shown to harbor this ameba. Of 22 N. fowleri isolates, 3 were highly virulent for mice when inoculated intranasally. The three pathogenic strains were isolated from three different places during summer. More N. fowleri were found during summer than in winter, which could be attributed to the lower temperature during winter. The distribution seems to be bound to the cooling waters of older factories and is not restricted to one type of factory. There are strong indications that the isolation of nonpathogenic seropositive strains from water is an indication for the occurrence of pathogenic N. fowleri.