Parasitology: a conceptual approach /
Eric S. Loker, Bruce V. Hofkin.
- xv, 560 pages : color illustrations ; 28 cm
Includes bibliographical references and index.
CHAPTER 1 AN INTRODUCTION TO PARASITISM 1.1 BUILDING AN UNDERSTANDING OF THE BASICS OF PARASITISM Parasites live in or on their hosts and cause them harm Opinions vary on how to define some of the key aspects of parasites and their biology The residence time for a parasite in or on a host is highly variable There are many additional ways to categorize parasites 1.2 HOSTS—ESSENTIAL LIFELINES FOR PARASITES Hosts also fall into several different categories 1.3 APPRECIATING PARASITISM'S PLACE IN NATURE Parasitism is one of several categories of symbiotic associations Parasitoids straddle the boundary between predation and parasitism Our understanding of parasitism is enhanced by an appreciation of its relationship to another ubiquitous type of symbiosis, mutualism REVIEW QUESTIONS REFERENCES CHAPTER 2 AN OVERVIEW OF PARASITE DIVERSITY 2.1 THE DIVERSITY OF PARASITE SPECIES What constitutes a parasite species requires some explanation Given these considerations, how many species of parasites inhabit the Earth? Evolutionary trees are used to visualize evolutionary relationships and to display parasite diversity Efforts are well underway to reveal the overall tree of life Horizontal gene transfer (HOT) has been pervasive throughout the evolution of life Many bacteria are parasites Eukaryotes are a very diverse group that includes many different kinds of parasites HOT has also played a role in the evolution of eukaryotic parasites The Apicomplexa is a huge, important, nearly exclusive parasitic group of organisms Many well-known parasites belong to familiar groups of multicellular organisms 2.2 INSIGHTS INTO PARASITISM FROM THE STUDY OF DIVERSITY The phylogenetic affinities of enigmatic parasites can be revealed Some groups of parasites remain "persistent problematica" Studies of parasite diversity reveal how particular parasites came to infect humans Studies of diversity can help reconstruct the historical biogeography of parasites 2.3 THE ONGOING QUEST TO REVEAL AND UNDERSTAND PARASITE DIVERSITY DNA barcoding is one way to catalog parasite diversity Some parasites exist in complexes of cryptic species Whole lineages of unapparent parasites may escape our attention Metagenomics provides a new way to reveal parasite diversity Studies of parasite diversity help provide a better foundation for taxonomy 2.4 OTHER WAYS TO CONSIDER PARASITE DIVERSITY Diversity within parasite species is extensive and important Do parasites give rise to free-living organisms? REVIEW QUESTIONS REFERENCES CHAPTERS THE PARASITE'S WAY OF LIFE 3.1 A HISTORICAL PERSPECTIVE OF THE PARASITE LIFE CYCLE Early medical and natural history studies gave rise to an understanding of parasite life cycles Mosquito transmission was first demonstrated for filarial worms Arthropod transmission for filarial worms suggested that other diseases may be similarly transmitted 3.2 AN OVERVIEW OF PARASITE LIFE CYCLES Parasites with direct life cycles use only a single host Two or more hosts are necessary for those parasites with indirect life cycles 3.3 THE PARASITE'S TO DO LIST Effective transmission is essential for all parasites High reproductive rates are common in many parasite life cycles Both sexual and asexual reproduction are used by apicomplexans such as Toxoplasma gondii Parasites may use strategies other than high fecundity to achieve transmission Many factors can complicate an understanding of parasite transmission Mathematical models provide a useful tool to predict transmission rates Many parasites must migrate to specific sites or tissues within the host The evolution of complex migration within a host is not always clear Parasites are adapted to maintain their position on or within the host Finding a mate is a requirement for many sexually reproducing parasites Parasite genomes reflect their adaptations to a parasitic lifestyle The relationship between parasitism and genome size is not always clear Propagules are released through a portal of exit Parasites undergo complex developmental changes in response to environmental cues Epigenetic phenomena and co-opting of host signaling molecules may be important in parasite development REVIEW QUESTIONS REFERENCES CHAPTER 4 HOST DEFENSE AND PARASITE EVASION 4.1 AN EVOLUTIONARY PERSPECTIVE ON ANTI-PARASITIC IMMUNE RESPONSES Prokaryotes have developed remarkable immune innovations during their billions of years encountering parasites Many kinds of parasites compromise the health of plants so it is important to know how plants defend themselves Although plants lack specialized immune cells, they still can mount effective, long-term responses to parasites Many nematode species are specialized to parasitize plants Invertebrates have distinctive and diverse innate immune systems Invertebrates, including vectors and intermediate hosts, mount immune responses to contend with their parasites Invertebrates also adopt distinctive behaviors to supplement their anti-parasite immune responses Parasites suppress, manipulate, and destroy invertebrate defense responses Some parasites rely on symbiotic partners to subvert the immune responses of their invertebrate hosts Some invertebrates enlist symbionts to aid in their defense Researchers hope to manipulate invertebrate immune systems to achieve parasite control 4.2 AN OVERVIEW OF VERTEBRATE DEFENSE 4.3 IMMUNE RESPONSES TO EUKARYOTIC PARASITES Recognition of PAMPS initiates the immune response to protozoa Immune responses to protozoa include both humoral and cell mediated components Protective immunity to malaria develops as a consequence of repeated exposure Immune responses are generated against each stage m the Plasmodium life cycle Helminth parasites provoke a strong Th-2 response ° Extensive changes to the intestinal epithelium occur m response to intestinal helminths Immunocompromised hosts are more vuln. rable to parasitic infection and increased patholo-y 4.4 PARASITE EVASION OF HOST DEFENSES Many parasites are able to evade complementmediated innate immune responses Intracellular parasites have evolved mechanisms to avoid destruction by host cells Parasites may interfere with intracellular signaling pathways Some parasites interfere with antigen presentation, resulting in an impaired immune response Some parasites regularly change their surface antigens to avoid immune responses Parasites frequently suppress or alter host immune responses by interfering with cell communication Some parasites render themselves invisible to immune detection Various parasites are able to undermine the effector functions of antibodies REVIEW QUESTIONS REFERENCES CHAPTER 5 PARASITE VERSUS HOST: PATHOLOGY AND DISEASE 5.1 PATHOLOGY RESULTING FROM PARASITIC INFECTIONS Parasites can induce pathogenesis in various ways Pathology can be categorized as one of several general types Parasites can cause direct trauma to host cells, tissues, and organs Mechanisms underlying the pathogenicity of Entamoeba histolytica remain obscure Parasitic infection can alter host-cell growth patterns Many parasites adversely affect host nutrition Plasmodium infections can result in host iron deficiency Toxins are a less frequent component of parasite pathology Pathology often results from immune-mediated damage to host cells and tissues Immunopathology is an important component of the pathology observed in malaria Granulomas formed in response to parasite antigen are both protective and pathological Parasites may serve as a trigger for autoimmunity Toxoplasma gondii may both contribute to and help to prevent artherosclerosis 5.2 PARASITES AND HOST BEHAVIOR Some parasites may modify host behavior to facilitate transmission The mechanisms that parasites use to alter host behavior are obscure Infected hosts may display unusual neurotransmitter profiles in their central nervous systems 5.3 PARASITE-MEDIATED AMELIORATION OF PATHOLOGY Parasitic infection may be required for proper immune system development Certain intestinal helminths may reduce the host inflammatory response Intestinal helminth infection results in activation of regulatory T cells Intestinal helminths can be administered therapeutically REVIEW QUESTIONS REFERENCES CHAPTERS THE ECOLOGY OF PARASITISM 6.1 DEFINING THE HABITATS OF PARASITES Parasites occupy multiple habitats in succession Parasites have microhabitat preferences and occupy specific sites within their hosts Host specificity is one of parasitisms most distinctive properties Encounter and compatibility filters determine the range of host species used by a parasite The origins and consequences of host specificity are debated Underlying mechanisms dictating specificity are also often not known 6.2 PARASITE POPULATION BIOLOGY Parasite populations are complex Parasites often show aggregated (ovei dispersed) distributions in their hosts Both density-independent and density-dependent factors influence parasite population size Intraspecific competition can regulate parasite populations in different ways Parasite population studies often require a long-term perspective and detailed sampling 6.3 PARASITE COMMUNITIES The richness of parasite communities varies among host species for reasons that are still debated Most studies suggest parasite communities are stochastic in nature Parasite species within infracommunities engage in negative and positive interactions with one another Generalizable patterns are also elusive in component communities of parasites Human parasites have a distinctive community ecology 6.4 THE ROLE OF PARASITES IN FOOD WEBS AND ECOSYSTEMS Parasites can be a food source for other organisms 6.5 GLOBAL PAHERNS IN PARASITE DIVERSITY 6.6 PARASITE EFFECTS ON HOST ECOLOGY Hosts try both to avoid infection and to actively remove parasites if they do become infected Hosts also change their diets and engage in self-medication when infected Parasites influence host migratory behavior Parasites can regulate host populations, but examples are few Parasites influence competitive interactions among hosts Parasites can manipulate their hosts to affect the likelihood of predation 6.7 ECOLOGICAL IMMUNOLOGY 6.8 THE METABOLIC THEORY OF ECOLOGY AND PARASITES 6.9 EPIDEMIOLOGY AND ITS RELATIONSHIPS WITH ECOLOGY Modeling is an invaluable approach to the study of infectious diseases Microparasites exemplify basic modeling approaches that estimate population size and clarify transmission Models of macroparasite populations and transmission involve keeping track of individual parasites Models for parasites with complex life cycles involving vectors become more complex New models open the black box and estimate microparasite populations within hosts and the influences on them Models need to take spatial and temporal factors into account Some individual hosts may serve as superspreaders REVIEW QUESTIONS REFERENCES CHAPTER? EVOLUTIONARY BIOLOGY OF PARASITISM 7.1 MICROEVOLUTION IN PARASITES The subdivided nature of their populations influences the evolution of parasites The effective population size, Ne, influences parasite evolution The mode of parasite reproduction affects microevolutionary change Stability of the host environment influences parasite microevolution The mobility of parasites impacts their evolution, as exemplified by bird lice Parasite microevolutionary change is strongly impacted by host mobility A parasites life cycle also affects the potential for evolutionary change 7.2 COEVOLUTION OF PARASITE-HOST INTERACTIONS Parasites and hosts reciprocally affect each other's evolution Parasites and hosts engage in arms races In parasite-host relationships, there can be an advantage to being rare Parasites and hosts can be locally adapted, or maladapted, to one another Some factors conspire to limit strong coevolutionary dynamics between parasites and hosts 7.3 THE EVOLUTION OF VIRULENCE Virulence and transmission biology of parasites are linked The trade-off hypothesis requires a nuanced approach The mode of transmission influences virulence 7.4 MACROEVOLUTIONARY PARASITOLOGY New parasite species are potentially formed in at least three different ways Different outcomes can be expected when parasites or their hosts diversify What does the evidence suggest about how parasites have speciated? Does sympatric speciation occur in parasites? Host switches can enable radiations in parasites Parasites go extinct, sometimes along with their hosts Macroevolutionary patterns among parasites are not yet very clear 7.5 SOME DISTINCTIVE ASPECTS OF PARASITE EVOLUTION Organisms have repeatedly adopted parasitism by more than one route Some parasites are derived from their hosts Selection can favor the evolution of complex parasite life cycles Sometimes complex life cycles are simplified secondarily Parasites often have simplified bodies or genomes but also have other talents not seen in free-living organisms 7.6 PARASITE EFFECTS ON HOST EVOLUTION Parasites select for genetic changes and genetic diversity in their hosts Parasites affect the evolution of host MHC genes Parasites play a role in host selection of mates Host speciation may be facilitated by parasites Can infection directly cause speciation? Parasites are believed to favor the evolution of sexual reproduction in their hosts Parasites can cause extinction of host species REVIEW QUESTIONS REFERENCES CHAPTERS PARASITES AND CONSERVATION BIOLOGY 8.1 SOME THEORY ABOUT PARASITES AND CONSERVATION BIOLOGY Theory often predicts parasites will not extirpate their hosts, but by no means always Persistent parasite infectious stages may also favor demise of host populations The presence of a parasite-tolerant host species may endanger a susceptible one 8.2 PARASITES INFLUENCE EFFORTS TO PRESERVE HOSTS Parasites can cause extinction of host species Parasites work in concert with other stressors to affect hosts The impact of parasitism is influenced when hosts occur in small or fragmented populations. Parasites can strongly affect hosts with reduced genetic variation Captive host populations are often very vulnerable to parasites Parasites are frequently transferred from abundant host species to rare relatives, including from humans to our great ape cousins Farming can pose parasite problems for wild host species Parasites of an iconic symbol—the giant panda— point out our need to know more 8.3 DANGERS RESULTING FROM SPECIES INTRODUCTIONS Parasites can be introduced with their hosts and have spillover effects Introduced hosts can favor indigenous parasites and cause spillback effects Sometimes introduced nonhost organisms can influence indigenous parasite transmission Invading hosts can benefit by leaving their natural enemies, such as parasites, behind Invasive hosts can potentially be controlled by parasites from their original range Introductions of parasites or hosts often fail Translocations of endangered host species can have unforeseen consequences Can invasional meltdown occur? 8.4 PARASITES AS INDICATORS OF ENVIRONMENTAL HEALTH Parasites can help us monitor ecosystem integrity 8.5 PARASITES AS INFERENTIAL TOOLS TO PRESERVE HOST BIODIVERSITY Parasites can provide information useful to preserving their hosts 8.6 THE NEED TO PRESERVE PARASITE DIVERSITY Parasites play key roles in maint.lining ecosystem health Parasites are drivers of biodiversity Parasites are a source of pharmacological and therapeutic novelties Slip sliding away—parasite diversity is being lost REVIEW QUESTIONS REFERENCES CHAPTER 9 THE CHALLENGE OF PARASITE CONTROL 9.1 STRATEGIES TO REDUCE PARASITE TRANSMISSION Parasite transmission may be reduced in various ways Parasites using trophic transmission can be controlled by insuring food safety Proper sanitation is the key to controlling parasites transmitted via the fecal-oral route Various other factors influence the success of control efforts The control of vector-borne diseases focuses on reducing human-vector contact The discovery of DDT radically altered vector control efforts Newer insecticides provide alternatives to DDT Transmission of vector-borne parasites can be reduced through environmental manipulation Biological control offers the possibility of low-cost, sustainable control The production of transgenic vectors provides hope as a means to reduce vector capacity 9.2 ANTI-PARASITIC DRUGS Various factors influence the selection of the best anti-parasitic drug in different situations Different drugs may be appropriate for treatment and for prophylaxis Drugs may be used to either treat or protect individuals or to protect a population Certain drugs are active only against specific parasite life-cycle stages Drug use may affect the immune status of the population The use of anti-parasitic drugs can lead to resistance Genetic alterations can cause resistance in diverse ways Resistance poses a considerable problem for disease control programs Drug resistance can be prevented or reversed Concerns about resistance highlight the need for new anti-parasitic drugs New drugs are also needed to replace older more toxic medications The manner in which new drugs are discovered has changed considerably Potentially effective drugs usually require chemical modification prior to their use Economic issues often affect the rate at which new drugs are developed 9.3 VACCINES Vaccines must be safe and inexpensive, while inducing long-term immunity Vaccines against eukaryotic parasites are particularly problematic Vaccines can be categorized into several types An effective malaria vaccine has been the object of intensive investigation Vaccines against different life-cycle stages offer different potential benefits Recent candidate vaccines may be used clinically in the near future A few anti-eukaryote vaccines are available for veterinary use Vaccines against helminth parasites are being investigated REVIEW QUESTIONS REFERENCES CHAPTER 10 THE FUTURE OF PARASITOLOGY 10.1 OUR FUTURE WORLD 10.2 SOME FUTURE CHALLENGES FOR PARASITOLOGISTS There is always something new to be found under the parasitological sun We need to better understand the ecological and evolutionary roles of parasites Revealing how parasite and host molecules interact is needed to clarify many fundamental aspects of parasitism Climate change will affect parasites, but we know little about how 10.3 CONTROLLING PARASITES IN THE FUTURE Improved understanding of immunity should enable development of new anti-parasite vaccines, but so far the parasites are winning Chemotherapy-based control is an arms race between human ingenuity and parasite evolvability Integrated control may provide the best prospects for sustainable parasite control and is built on a thorough knowledge of parasite biology Major programs are underway to eliminate many parasites as public health problems We will need improved methods to detect low levels of parasite infection and transmission in the future Provision of improved living conditions, including education, wiU further discourage parasite transmission