CHAPTER 3 PARA

Direct life cycle

Requires only ONE host. Examples: Ascaris lumbricoides, Giardia lamblia, Trichuris trichiura.

Indirect life cycle

Requires TWO or MORE hosts. Examples: Plasmodium, Fasciola, Schistosoma, Toxoplasma, Alaria.

Definitive host

Host where the parasite undergoes sexual reproduction.

Intermediate host

Host where the parasite develops and/or reproduces asexually.

Paratenic host

Host used for parasite transport or survival; usually no development occurs.

Reservoir host

Host that maintains the parasite in nature.

Vector

Organism that transmits a parasite to another host.

Host memory: D-I-P-R-V

Definitive = sex; Intermediate = development; Paratenic = transport; Reservoir = maintains; Vector = transmits.

Basic Alaria americana life cycle

Egg → Miracidium → Snail → Cercaria → Frog → Mesocercaria → Paratenic host → Canid → Adult.

where does Adult Alaria live?

Intestine of a canid.

Alaria eggs

Leave the canid in feces.

Alaria miracidium

Hatches from eggs in water and infects a snail.

First intermediate host of Alaria

Snail.

Alaria cercaria

Leaves the snail and infects frogs/tadpoles.

Alaria mesocercaria

Develops in frogs/tadpoles and paratenic hosts.

Alaria paratenic hosts

Reptiles, birds, rodents, and other mammals.

Alaria definitive host

Canids, including dogs and wild canids.

Cercaria vs. mesocercaria

Cercaria leaves the snail; mesocercaria develops in frogs and paratenic hosts.

Alaria MC trap

Cercaria leaves the snail; mesocercaria is the stage in frogs/paratenic hosts.

Digenetic trematode

Trematode requiring two or more hosts to complete its life cycle.

Typical digenetic trematode sequence

Egg → Miracidium → Sporocyst → Redia → Cercaria → Metacercaria → Adult.

Do all trematodes have every larval stage?

No. Not every trematode has every stage.

Miracidium

Free-swimming larva that usually infects a snail.

Sporocyst

Developmental sac found inside the snail.

Redia

Larval stage capable of producing more larval stages.

Cercaria

Larval stage that leaves the snail.

Metacercaria

Encysted stage usually infective to the definitive host.

Adult trematode

Usually the sexually mature stage in the definitive host.

Trematode first intermediate host

Usually a snail or other mollusk.

Trematode additional intermediate hosts

May include fish, crustaceans, and amphibians.

Trematode definitive host

Usually a vertebrate such as a mammal, bird, or human.

Parasite's basic to-do list

Find → Enter → Survive → Locate → Feed → Grow → Reproduce → Exit → Transmit.

Fecal-oral transmission

Parasite leaves in feces and enters another host through the mouth.

Fecal-oral parasite examples

Giardia, Cryptosporidium, Entamoeba, and Ascaris.

Giardia fecal-oral transmission

Cyst → feces → contaminated food/water → swallowed → new host.

Environmental stages for fecal-oral transmission

Cysts and oocysts help parasites survive outside the host.

Giardia encystment

Environmental conditions such as intestinal pH and bile-related conditions help trigger encystment.

Fecal-oral memory

Feces → environment → mouth.

Trophic transmission

Host becomes infected by eating an infected host.

Trophic transmission example

Trichinella spiralis can spread through infected meat.

Trophic transmission adaptation

Resistant stages survive inside prey and remain infective when eaten.

Trophic transmission memory

Eat infected host → parasite survives → infects predator.

Direct penetration

Parasite actively penetrates the host's skin or body surface.

Direct penetration examples

Schistosoma cercariae and hookworms.

Schistosoma transmission

Cercariae in freshwater directly penetrate human skin.

Parasite penetration adaptations

Motility, penetration enzymes, specialized structures, environmental sensing, and chemotaxis.

Direct penetration memory

Parasite goes through the skin itself.

Biological vector

Vector where the parasite develops and/or multiplies.

Biological vector example

Anopheles mosquito → Plasmodium.

Mechanical vector

Vector that simply carries the parasite; no development occurs.

Mechanical vector example

Housefly carrying parasite eggs.

Biological vs. mechanical vector

Biological = parasite develops; mechanical = parasite is only carried.

Mechanical vector memory

Mechanical = taxi.

Anopheles mosquito

Transmits Plasmodium.

Mosquito

Can transmit Wuchereria bancrofti.

Tsetse fly

Transmits Trypanosoma brucei.

Sand fly

Transmits Leishmania.

Black fly

Transmits Onchocerca.

Vector competence

Vector's intrinsic ability to acquire, maintain, develop, and transmit a parasite.

Vector capacity

How effectively the vector population transmits the parasite in nature.

Factors affecting vector capacity

Abundance, longevity, host contact, feeding behavior, competence, and environmental conditions.

Competence vs. capacity

Competence = Can it transmit? Capacity = How effectively does it transmit?

Primary vector

Major or important vector maintaining parasite transmission.

Secondary vector

Can transmit the parasite but is less important epidemiologically.

Trypanosoma brucei transmission

Tsetse fly bite.

Trypanosoma cruzi transmission

Primarily triatomine bug feces contaminating a bite wound or mucous membrane.

Other Trypanosoma cruzi transmission routes

Congenital transmission, blood transfusion, organ transplantation, and contaminated food.

T. brucei vs. T. cruzi

T. brucei = tsetse bite; T. cruzi = triatomine bug feces.

Sexual transmission

Transmission through sexual contact.

Main sexually transmitted parasite

Trichomonas vaginalis.

Vertical transmission

Transmission from parent to offspring.

Examples of vertical transmission

Toxoplasma gondii and Trypanosoma cruzi.

Three routes of vertical transmission

Transplacental/congenital, perinatal, and lactational.

Vertical transmission memory

Placenta → Birth → Milk.

Tissue specificity

Parasite's tendency to prefer a specific tissue or organ.

Why parasites prefer specific tissues

They provide suitable nutrients, temperature, pH, oxygen, receptors, and protection.

Wuchereria bancrofti preferred tissue

Lymphatic vessels.

Fasciola hepatica preferred tissue

Liver and bile ducts.

Adult Ascaris preferred tissue

Small intestine.

Schistosoma mansoni preferred tissue

Mesenteric veins.

Schistosoma haematobium preferred tissue

Vesical/pelvic venous plexus.

Trichinella spiralis larvae preferred tissue

Skeletal muscle.

Plasmodium preferred tissues

Liver and red blood cells.

Cues used to locate tissues

pH, temperature, oxygen, chemical gradients, hormones, receptors, and nutrients.

Do parasites have a literal GPS?

No. They respond to environmental and host-derived cues.

Why parasites take unusual migration routes

Their migration is shaped by evolutionary life-cycle and developmental requirements.

Ascaris migration

Intestine → Blood → Liver → Heart → Lungs → Trachea → Swallowed → Intestine.

Key idea about parasite migration

Evolutionary pathway does not necessarily equal the shortest pathway.

Wrong host

Host where the parasite cannot normally complete its life cycle.

What usually happens in a wrong host?

The parasite cannot reproduce normally and may die or be eliminated.

Toxocara in humans

Larvae cannot complete normal development and may cause disease.

Toxocara diseases

Visceral larva migrans and ocular larva migrans.

Dead-end host

Host where the parasite cannot normally complete its life cycle or continue transmission.

Wrong host memory

Wrong host → dead end.

Trematode attachment structure

Suckers/acetabula.

Cestode attachment structure

Scolex with suckers/hooks.

Acanthocephalan attachment structure

Hooked proboscis.

Parasite attachment memory

Trematode = suckers; Cestode = scolex; Acanthocephalan = hooked proboscis.

r-selection/r-strategy

Producing many offspring because many die before reproducing.

Why parasites often produce many offspring

Many parasite stages die during transmission.