Parasites are found in all
groups of organisms
Parasites a very diverse set of
eukaryotic pathogens
• Parasitology as a scientific discipline historically covers a
diverse collection of multi- and unicellular organisms
• Protozoa: unicellular eukaryotes (this is a historic term,
protozoans are not really a monophyletic group)
• Platyhelminthes: flatworms these include flukes and tape
worms
• Nematodes: elongated worms with rigid cuticula
• Arthropods: insects, ticks and mites which either are
parasitic or transmit parasites as vectors
Terminology
• Prepatency: infected but parasite presence can not be
detected yet
• Patency: established infection, parasite stages can be
detected (malaria parasites in blood smears, worm eggs
in feces etc.)
• Incubation period: time between infection and the
development of symptoms
• Acute disease can lead to crisis which can resolve in
spontaneous healing, chronic infection or death
• Convalescence: Period after healing, absence of
infectious agents, no symptoms, in certain case
immunity to reinfection
Interactions between organisms
Symbiosis: “living together”- 2 organisms living in/on each other
Phoresis: travelling together: the phorant carried by other (spores on legs)
Mutualism: both symbionts benefit: usually an obligate relationship
termite + intestinal protozoan, lichens (alga + fungus), bacteria in
guts of leeches
Commensalism: one symbiont benefits, other: no impact
facultative: ciliates on crystaceans
obligate- epiphytic plants (bromeliads)
Parasitism: one symbiont benefits at the expense of the other
-mechanical injury, inflammation reaction, competition for
resources
facultative: mosquito (micro-predator)
obligate: tapeworm
Definitions are general and arbitrary. Some cannot be easily
assigned to these categories. What we call commensalism may be
parasitism that we cannot recognize, and vice versa.
Are all parasites bad?
Requirements for Parasitic relationships
Live in close proximity
Smaller size than host
Ability to enter / associate with host
Attachment / interactive membranes
Physiological ability to survive in / on host and derive nutrients
Physiological ability to reproduce in / on host
Offspring must be able to contact a potential host (eggs released that pass
out of or off of host, ability to infect another host or be ingested by next host)
No strong immune response by host to symbiont
Reproductive strategies: R strategists
-large numbers of offspring with no density-dependent restraints, little/no
competition (liver flukes: 20 k eggs/day)
-some act more like k strategists
Tsetse fly produces one egg at a time, lice produce 1-few eggs and invest
much energy into offspring
Reproduction continued:
Common for parasites to have sexual and asexual reproductive cycles
Plasmodium (causes malaria) alternates asexual and sexual cycles
Trematodes have asexual cycles in snail hosts, sexual cycles in vertebrates
Some Tapeworms have asexual and sexual cycles and have replication of sex
organs in each segment
Some parasites live in/on 1 host
Some trematodes have 5 hosts/life cycle, and up to 5 developmental stagesNeed asexual and sexual reproduction to keep up numbers to maintain
transmission
Estimates of parasite fecundity
What are the factors involved in the evolution of specific traits,
or the co-evolution between parasites and hosts?
Throughout this course think not only about what factors and
structures exist, but also how their development may have
come about
PARASITOLOGY TERMS AND DEFINITIONS
1. Obligatory parasite - one that is physiologically and metabolically
dependent on the host.
2. Facultative parasite - establishes a relationship with a host if the
opportunity presents itself.
3. Ectoparasite - lives on the outer surface of its host.
4. Endoparasite - lives inside its host.
5. Accidental parasite - one that is found in other than its normal host
(=incidental parasite).
6. Permanent parasite - lives its entire adult life within or on a host.
7. Temporary parasite - contacts its host only to feed and then
leaves (=intermittent parasite).
8. Aberrant parasite - characteristic of the host, but found in an
unusual location within the host.
Obligate/facultative, and
permanent/intermittent parasites
• Most parasites are
obligate parasites
• In some species only
some life cycle stages,
e.g. the larvae are
parasitic, in others
parasitic and free living
generations can alternate
depending on
environmental conditions
(Strongiloides stercoralis) .
Indirect
Direct
Ecto- & Endoparasitism
• Ectoparasites live on, but
not in their hosts (they can
nevertheless cause severe
illness). Ich a protozoan
ectoparasite of the skin of
a fish.
• Endoparasite live within
the body and tissues of
their hosts. Trypanosomes
(which cause sleeping
sickness) within the blood
of an infected animal.
HOSTS:
1. Definitive host - one in which the parasite reaches sexual maturity, or the
most important host if no sexual reproduction occurs.
2. Intermediate host - one in which some development or asexual reproduction
occurs, but sexual maturity does not occur.
3. Vector - intermediate host that actively transmits a disease organism.
4. Paratenic host - host which the parasite enters, does not undergo any
development or reproduction, but remains infective to the definitive host.
5. Reservoir host - a "living source" of the parasite; not host of primary
concern.
Hosts and life cycles
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The definitive host is by definition the
one in which the parasite reproduces
sexually
Additional hosts are then designated
intermediate hosts
Host which actively transmit parasites
to humans are often called vectors
In paratenic or transport hosts no
parasite development occurs
Reservoir host are alternate animal
host from which the parasite can be
transmitted to humans (zoonosis) or
domestic animals
Accidental host, not suitable for
parasite development, but severe
disease might ensue nonetheless
LOCATION OF PARASITES WITHIN THE HOST:
1. Coelozoic - lives in cavities of the host; i.e., peritoneal cavity, lumen
of the gut, blood vessels, etc.
2. Histozoic - lives in the tissues of the host, but not within cells.
3. Cytozoic - intracellular parasites; live within host cells.
LIFE CYCLES:
1. Direct life cycle - one host cycle (= monoxenous parasite), commonly
called "hand-to-mouth" cycles.
2. Indirect life cycle - one requiring two or more hosts (= heteroxenous
parasite).
Adaptations for parasitic life:
Attachments to adhere to host- internally or externally
Nutrition- often feed on food digested by host (gut parasites, liver parasites)
Reproduction: often use hosts' GI / urinary tract to release specific stages into
the environment. Others may use resistant stages ie cysts to survive periods in
environment
Survival: dependent on their ability to withstand digestive juices, local and
systemic host immunological responses
Some parasites need several different hosts for completion of life cycle- ie
Dicrocoelium dendriticum a trematode) completes life cycle successively in
sheep, snails, and ants. The modifications needed to live in vertebrate and
invertebrate hosts and external environment are tremendous.
Transmission: Depends on the number of hosts used in life cycle.
Passive: infective stages of parasites contaminate or infect the food/water of
the host and are swallowed. (eggs of Ascaris, cysts of Entamoeba)
Active: Parasites actively seek out and enter specific hosts- cercariae of
trematodes swim in the water looking for a host, attach, and enter.
Inoculative: parasite stage develops in other organism and is transmitted
when that organism bites another (avian malaria transmitted with the bites of
mosquitoes)
Characteristics: Extremely high reproductive capacity to compensate for
the tremendous losses in each transmission stage- high egg production in
cestodes (tapeworms) and trematodes (flukes), duplication of sex organs in
some cestodes, asexual reproduction in some stages (trematodes).
MODES OF TRANSMISSION
1. HORIZONTAL AND VERTICAL TRANSMISSION
2. CONTACT TRANSMISSION:
3. VECTOR TRANSMISSION
A. Mechanical
B. Biological
COMMUNICABLE or TRANSMISSIBLE
1. PORTALS OF ENTRY
Oral/Alimentary
Ocular
Skin
Respiratory
Venereal/Urogenital
Transplacental/transovarial
2. PORTALS OF EXIT
Respiratory
Venereal/Urogenital (urine, semen, vaginal and abortion products)
Blood (vectors)
Fecal/oral (alimentary)
Blood and blood products
Transplacental/transovarial/colostral
Ocular discharge
Milk, other secretions
Carcass - Scavenging, predation, decomposition
Parasites transmitted by vectors
Often very specific vector-parasite relationships
Biomphalaria sp. - Schistosoma mansoni
Anopheles sp. – Plasmodium falciparum
Simulium sp. – Onchocerca volvulis
Some more general
Rhodnius sp / triatoma sp. - Trypanosoma cruzi
TRANSMISSION OF PARASITES BY VECTORS:
Biological Transmission
I.
A. Cyclopropagative Transmission
The parasite undergoes cyclical changes and multiplies within the
vector, i.e., there are both developmental changes and multiplication
of the parasite.
B. Cyclodevelopmental Transmission
The parasite undergoes cyclical changes within the vector but does
not multiply, i.e., there are only developmental changes of the parasite
without multiplication.
C. Propagative Transmission
The parasite multiplies within the vector without any cyclical
changes, i.e., the parasite increases in number within the vector but
does not undergo any developmental changes.
II. Mechanical Transmission
This is similar to a "flying syringe" where transmission from one host
to another is accomplished because the parasite contaminates the
mouthparts of an arthropod and is physically carried to another host.