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 proximitySmaller size than hostAbility to enter / associate with hostAttachment / interactive membranesPhysiological ability to survive in / on host and derive nutrientsPhysiological ability to reproduce in / on hostOffspring 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 stages-

Need 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

• 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 TRANSMISSIONA. MechanicalB. Biological

COMMUNICABLE or TRANSMISSIBLE

1. PORTALS OF ENTRYOral/AlimentaryOcularSkinRespiratoryVenereal/UrogenitalTransplacental/transovarial

2. PORTALS OF EXITRespiratoryVenereal/Urogenital (urine, semen, vaginal and abortion products)Blood (vectors)Fecal/oral (alimentary)Blood and blood productsTransplacental/transovarial/colostralOcular dischargeMilk, other secretionsCarcass ‑ 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 TransmissionThe parasite undergoes cyclical changes and multiplies within the vector, i.e., there are both developmental changes and multiplication of the parasite.B. Cyclodevelopmental TransmissionThe 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 TransmissionThe 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 TransmissionThis 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.