parasitology lecture 6: a parasite's way of life pt.2

what is an indirect life cycle?

Requires two or more hosts

what is a definitive host?

a host in which a parasite is able to reach sexual maturity and sexual reproduction may take place

what is an intermediate host?

a host in which paradite undergo required developmental steps but do not reach sexual maturity (asexual reproduction can occur, but no sexual reproduction)

what is a paratenic host?

nonessential hosts that increase the liklihood of transmission to the definitive host (no reproduction or development)

what is an example of an indirect life cycle?

Acanthocephalan (thorny-headed worm)

Simple, two-host life cycle

describe the Acanthocephalan life cycle

eggs passed in feces, eggs consumed by arthropod (intermediate host), eggs hatch in gut of arthropod and penetrate into hemocoel, eggs from the cystacanth, definitive host consumes the arthropod and becomes infected

what is another example of an indirect life cycle?

Alaria americana

Trematode that is

monoecious.

Common in North

America (temperate

regions near water)

describe the life cycle of alaria americana

eggs released in feces by definitive host-> miracisium hatches in freshwater and penetrartes the first intermediate host-> asexual reproduction in first intermediate host-> cercariae released from first intermediate host and penetrate second intermediate host-> second intemediate host can either be directly consumed by definitive host or can be consumed by paratenic host (which is then consumed by the definitive host)

QOTD: Why might a parasite species that has a paratenic host as part of its life cycle have an advantage over one that does not?

fills trophic gaps

allows transmission of parasite to species in different environments

allows for greater efficiency

the paratenic host does not necessarily suffer from the infection, rather they increase efficiency of parasite transmission-> increasing numbers in the environment

what is transmission?

the passage of a parasite to a host

Almost all organisms (this includes free-living) must disperse

from site of origin to successful grow and reproduce

what is a mode of transmission

the way a parasite moves from host to host

what types of parasites use the fecal-oral mode of transmission?

most intestinal protozoa and helminths

how does fecal-oral transmission work?

propagules (the stage responsible for achieving transmission) are released in the host's feces

Food and water can be contaminated → consumption by host

◦ Parasite usually in a metabolically inactive stage (egg or cyst)

These are hardy and can withstand harsh environmental conditions.

◦ Little energy expended by parasite

what is an example of fecal-oral transmission?

giardia lamblia

what are the parasite stages of giardia lamblia?

Trophozoite - feeding stage

in intestines that replicate by

binary fission.

◦ Cyst - dormant form of

parasite released in feces.

Physiological conditions cause

some trophozoites to become

cysts.

describe the life cycle of giardia

Cysts are ingested in feces-contaminated food/water

Cysts -> trophozoite in the duodenum, they attach to the epithelium, undergo binary fission, and encyst in the large intestine

what are two potential mechanisms of cyst formation for transmission?

1) Decreased lipid concentrations &

increased pH in host intestine.

2) Density of trophozoites in host. Quorum sensing - mechanism of regulating gene expression that depends

on the density of cells in each area.

what happens during cyst formation of giardia?

Flagella lost

Cyst wall proteins excreted via

exocytosis

Once in environment, can survive for months until a host consumes it.

what is trophic transmission?

Parasite takes advantage of an established

predator-prey relationship.

◦ Definitive host consumes

intermediate host

this can be coupled with other modes of transmission (like fecal-oral)

what is an example of trophic transmission?

the rat tapeworm

(Hymenolepis diminuta )

describe the life cycle of the rat tapeworm

(Hymenolepis diminuta )

Rat defecates and eggs in feces

◦ Insect intermediate host ingests eggs in feces

◦ Larvae hatch in insect

◦ Back to definitive host when

rat eats the insect.

what is an example of host manipulation during trophic transmission?

Pomphorhynchus laevis (a thorny-headed worm) uses the

aquatic amphipod Gammarus pulex as its intermediate host

and a freshwater fish (like perch) as its definitive host.

Uninfected vs. infected amphipods:

◦ Uninfected - stay in darker, rocky water and are hidden from

fish

◦ Infected - swim in open, light water and are easy prey for fish

what is direct penetration?

Once the parasite finds

their host, they bore

their way in!

what is an example of direct penetration?

the itch mite (Considered an

ectoparasite even though

the adult female burrows

into skin to lay eggs)

what is an example of a sexually transmitted parasite?

trichomonas vaginalis or pthirus ppubis (pubic lice/crabs)