Parasitology: a conceptual approach / Eric S. Loker, Bruce V. Hofkin.

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
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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?
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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
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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
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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
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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
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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
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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
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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
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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