Indicators for Waterborne Pathogens (2004) / Chapter Skim
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3 Ecology and Evolution of Waterborne Pathogens and Indicator Organisms
3 Ecology and Evolution of Waterborne Pathogens and Indicator Organisms
Pages 109-163
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From page 109... ...
Notable exceptions include various species of Vibrio and Legionella bacteria and protozoan parasites such as the free-living amoebae Naegleria and Acanthamoeba. Many microorganisms that are pathogenic to humans and animals enter ambient waters after import from various point and diffuse sources.
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From page 110... ...
of public health concern as an aid to better understand how selective forces may alter one's ability to assess the microbial quality of water. Indeed, indicators of microbial water quality can be the pathogenic organisms themselves, other microorganisms, or other physical or chemical aspects of the aquatic environment (see Chapter 4 for further information)
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From page 111... ...
Are there environmental conditions that promote genetic exchange or the acquisition of genetic elements that confer selective advantage under clinical conditions? For example, the increases in antibiotic and multiple antibiotic resistances may be influenced by physical conditions in the environment.
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From page 112... ...
Because the same species of microorganism from different animal hosts or environmental reservoirs can differ greatly in human infectivity and the ability to cause disease, determining risks to human health requires the use of advanced analytical methods that are often well beyond the methods currently used for their detection in environmental waters. Furthermore, the continuous movement of microorganisms through different hosts and abiotic environmental media exerts selective pressures that are opportunities for genetic change leading to the emergence of new strains with different traits and health risks.
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From page 113... ...
The combination of increased inputs of nutrients to eutrophic conditions and association of the organism with shellfish, fish, and zooplankton created environmental reservoirs that could persist for extended periods of time. Thus, reintroduction was not necessary.
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From page 114... ...
indicators have been historically effective in detecting and quantifying fecal contamination, they are not always reliable predictors of microbial water quality due largely to our lack of understanding of the basic ecology of waterborne pathogens and indicators. For example, total coliform counts and enterococci have been used as indicators of human fecal contamination for decades (see Chapter 1)
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From page 115... ...
In natural systems the residence times of introduced bacteria can be much longer than 24 hours, thus providing an opportunity for genetic changes either through acquisition of plasmids or by allowing mutations to take place under the selective pressures of the new habitat. Various natural history and environmental aspects of pathogens and indicator organisms also contribute to their ability to be detected and monitored.
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From page 116... ...
Viruses contain relatively small amounts of nucleic acid, usually from a few to several tens of nucleotide kilobases -- enough information to encode a few to several tens of proteins. Despite this relative paucity of genetic information, viruses are genetically diverse, sometimes highly genetically variable, and quite capable of adapting to the changing conditions of their host cells and the host environment.
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From page 117... ...
Despite the variability and uncertainty of predicting waterborne virus infection and disease outcomes, studies of virus properties, virus-host interactions, virus infection and disease outcomes, and viral ecology and epidemiology have all helped to elucidate the natural history of viruses and virus risks to their hosts. Virus strains that produce infectious viruses more rapidly and at higher yield are more likely to be successful if fitness is positively correlated with population size of the susceptible host.
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From page 118... ...
Such changes are often referred to as genetic drifts, and if they occur in an expressed gene these changes are referred to as antigenic drift. Genetic and antigenic drifts can occur in response to selective pressures from host populations, such as immunity and genetic changes in host cells and whole hosts such as animals and plants.
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From page 119... ...
Because virus mutation rates are high, there is rapid reversion of vaccine polioviruses to genotypes with neurovirulent properties among the excreted viruses of vaccine recipients. Serial transmission of vaccine strains of polioviruses among susceptible human hosts results in the accumulation of mutations, which can eventually lead to selection and further serial transmission of neurovirulent vaccine strains.
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. That is, genetic changes in viruses can occur when two or more viruses coinfect host cells and exchange genetic information during replication.
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From page 121... ...
. Viruses in Human and Animal Wastes and in the Aquatic Environment Enteric viruses found in human and animal feces, sewage, and fecally contaminated water include not only enteric pathogens but also viruses that infect bacteria residing in the intestinal tracts of humans and other warm-blooded mammals that are called enteric bacteriophages.
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From page 122... ...
The apparent animalhost specificity of these bacteriophages may be related to the bacterial host ranges of the phages themselves or to the animal host ranges of their bacteria, although the ecological aspects of these relationships have not been adequately studied. Regardless of the mechanisms, the diversity of enteric bacteriophages and their bacteria, and their occurrence in human and animal hosts, pose challenges to the development and application of bacteriophages as indicators of enteric viruses and fecal contamination.
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From page 123... ...
Because on-site wastewater treatment systems, typically septic tanks and subsurface drainfields, often inadequately reduce viruses and the wastes of feral, domestic, and agricultural animals are either untreated or inadequately treated, they can deliver substantial numbers of enteric viruses and other pathogens to ground- or surface waters (Borchardt et al., 2003; Powell et al., 2003; Scandura and Sobsey, 1996)
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From page 124... ...
In addition, viruses are so small that they are readily transported in water and wastes and can migrate through soils and other porous media. The persistence and transport of human enteric viruses in water and other environmental media constitute a public health concern because the viruses can retain their infectivity and cause human infection if humans ingest or otherwise come in contact with them in environmental media.
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From page 125... ...
. These microorganisms are widespread in aquatic environments and may be introduced into hospitals by patients, staff, or visitors and become established in microenvironments such as sinks, showers, and ice machines.
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The issue of antibiotic resistance in bacteria is discussed in the following section. Antibiotic Resistance Our understanding of the mechanisms that promote the selection and transmission of bacterial genes under various environmental conditions is critical to addressing long-term public health problems.
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They suggested that exposure to elevated levels of naturally occurring solvents may have resulted in the indirect selection of antibiotic resistance and that these bacteria are an environmental reservoir of antibiotic resistance genes. Selection acting on one set of genes (e.g., metal tolerance)
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From page 128... ...
. For example, endosymbiotic or parasitic relationships between Legionella bacteria and their free-living algal and protozoan hosts allow not only for bacterial proliferation but also for protection from disinfection, thereby increasing their survival and ability to reach human hosts through drinking, recreational, and cooling tower waters.
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. Natural transformation therefore appears to be a relatively efficient process under certain conditions and has the potential to allow the spread of genetic determinants such as metal and antibiotic resistance.
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Biological indicators of these pathogens that fail to identify these environmental reservoirs will be of little efficacy in tracking potential impacts or outbreaks. Ecology of Plasmids Bacteria in nature can and do acquire and lose genetic material through a variety of novel evolutionary mechanisms.
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From page 131... ...
Furthermore, if pathogens can become integrated into biofilms and retained for sufficient time, they would be subject to selection. Bacterial Persistence in the Environment and Detection by Culture Methods The extent to which pathogenic and bacterial indicator organisms persist outside a vertebrate host is highly variable and depends on the type of pathogen and the environmental conditions (Mitchell, 1972)
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From page 132... ...
A general concern is that many other waterborne bacterial pathogen and indicator species will be found that express this trait of non-culturablity and that this condition may confound the reliability of various microbial indicators that are based on culture techniques. However, as VBNC cells are metabolically active, indicators that measure some correlate or product of metabolism might be developed that are capable of monitoring these targets even when these cells cannot be cultured.
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. Because of the possibility of waterborne pathogens surviving and replicating in various environmental reservoirs however, an understanding of mechanisms of dispersal is important.
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From page 134... ...
. Summary Clearly, improved understanding of the ecology of waterborne bacterial pathogens is needed before more effective means of detecting them directly or through the use of indicator organisms can be implemented.
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From page 135... ...
Suprapopulation dynamics are influenced by both the density and the diversity of reservoir hosts. The impact of parasites on human hosts in a given geographical area will depend on the types and numbers of nonhuman hosts of the same parasite, such as feral, domestic, and agricultural animals.
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From page 136... ...
Human populations and public health can be strongly influenced by parasite population dynamics, such as the presence and proliferation of schistosomes (flukes causing "swimmers itch") in surface waters used by humans for aquaculture, agriculture, recreation, and other purposes.
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From page 137... ...
Host recognition of the invading parasite triggers a range of immune responses that influence infectivity and disease outcomes. Furthermore, these immune responses to parasite infection are influenced by the host's environment and other host-related factors, including genetics, age, gender, diet, physical environment, and behaviors.
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From page 138... ...
A number of parasites enter host cells as a feature of their pathology and a mechanism of virulence. Examples of protozoan parasites that intracellularly invade the cells of their hosts are the amoebae Entamoeba histolytica and Toxoplasma gondii.
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From page 139... ...
, and often spread through the fecal-to-oral route. As such, these are important organisms from a public health perspective and are associated with waterborne disease worldwide, including the United States (see also Chapters 1 and 2)
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From page 140... ...
. Although several species of waterborne protozoa are of public health concern, this section focuses on the ecology, evolution, and basic biology of the following groups and genera: the free-living amoebae Naegleria and Acanthamoeba; the enteric protozoa Giardia, Cryptosporidium, and Toxoplasma; and a relatively newly recognized group in human infections, the Microsporidia.
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cholerae were isolated from cysts of Naegleria. Naegleria fowleri is virtually ubiquitous in the aquatic environment and is considered an opportunistic pathogen of great public health concern.
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However, the basic ecology of this parasite is not well understood. Microsporidia are eukaryotic spore-forming obligate protozoan parasites that infect all animal groups (especially arthropods)
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. These parasites pose new and emerging threats because of their ability to survive in a variety of moist habitats including surface waters and because protozoan parasites were among the most frequently identified etiologic agents in waterborne disease outbreaks (see Figure 1-1; Marshall et al., 1997)
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From page 144... ...
Although fecal contamination of the drinking water was never detected, contamination from a nearby lake was suspected. In the United States there are minimal data on the occurrence of human strains of
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lamblia has environmental reservoirs such as beavers and possibly muskrats. Cryptosporidium parvum, also a zoonotic species, can infect an unusually wide range of mammals, including humans (O'Donoghue, 1995)
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In a 17month survey (Bodley-Tickell et al., 2002) of Cryptosporidium oocysts in surface waters draining a livestock operation, the parasites were found to be present yearround, with maximum concentrations and highest frequency of occurrence during autumn and winter.
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From page 147... ...
Cryptosporidium and Giardia in Water Most studies of waterborne protozoa of public health concern have focused on the occurrence of the enteric protozoa Cryptosporidium and Giardia. Although these two enteric protozoan parasites are not related taxonomically, they are closely related from an epidemiological, regulatory, and public health point of view.
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From page 148... ...
These interactions involve a considerable amount of genetic exchange and selection, as well as host-dependent phenotypic expression. For many parasites, the host immune response is a major factor influencing pathogen-host interaction and its effects on the health of the host.
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Ecology and Evolution of Fungi and Their Role as Human Pathogens Humans are continuously exposed to fungi from various environmental sources and often are colonized with fungi. On rare occasions, some fungi cause human infection and illness and most of these illnesses occur in immunocompromised hosts.
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From page 150... ...
For many fungi, other factors besides morphological changes play an important role in pathogenicity, as do the host's immune competence, immune response, and other physiological and constitutive states. As noted above, it is well established that the nature and extent of impairment of host defenses influences the pathology, severity, and outcomes of fungal infections, such that the clinical manifestations of disease are contingent upon the hosts' immune system and other host characteristics.
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From page 151... ...
A comprehensive analysis that considers the multiple properties of the fungus and the host and the complex manner in which they interact in various environmental settings is necessary in order to understand the host-parasite interaction and gain insights into the factors responsible for exposure, infection, disease, and the resulting health effects in humans. SUMMARY: CONCLUSIONS AND RECOMMENDATIONS The ecology and evolution of waterborne pathogens have important implications for the emergence and reemergence of those pathogens of public health concern.
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From page 152... ...
· Because some waterborne pathogens or indicator organisms may survive and replicate in various environmental reservoirs independently of each other, an improved understanding of the ecology and natural history of microbial indicators and pathogens and the mechanisms of their persistence, proliferation, and dispersal should be sought. · Advanced analytical methods should be used to help distinguish between introduced pathogenic and naturally occurring nonpathogenic strains of waterborne microorganisms and to characterize the emergence of new strains of pathogens as a result of genetic change.
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From page 153... ...
The factors that select for increased resistance to these agents in natural populations of bacteria need to be elucidated as do the factors influencing the natural transfer of these resistance traits to waterborne pathogens, indicators, and other aquatic microorganisms. · Research is needed to develop a better understanding of the ecology and natural history of both the environmental and infectious stages of pathogens and the parallel stages of indicator organisms to grasp how the organisms are distributed in nature; how they persist and accumulate in water, other environmental media, and in animal reservoirs; and how dissemination of the environmental form occurs, especially human exposures.
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Applied and Environmental Microbiology 69(2)
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Ap plied and Environmental Microbiology 64: 3332-3335.
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. Applied and Environmental Microbiology 64: 2736-2738.
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Applied and Environmental Microbiology 66: 4615-4619.
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in fil tered drinking water supplies. Applied and Environmental Microbiology 57(9)
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Applied Environmental Microbiology 66: 3722-3726. McFadden, G
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Applied and Environmental Microbiology 62(8)
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Applied and Environmental Microbiology 58(11)
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Applied and Environmental Microbiology 64: 2822-2830. Sobecky, P.A.
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Applied Environmental Microbiology 62: 2994-2998. Winiecka-Krusnell, J., and E
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