Parasitism Test 3

Parasitism

A parasite consumes part of a host and lowers its fitness; the host usually survives.

Ectoparasite

Parasite living on the outside of a host.

Endoparasite

Parasite living inside a host.

Chigger

Ectoparasitic mite larva that can transmit the bacterium causing scrub typhus.

Scrub typhus

Disease caused by Orientia tsutsugamushi; symptoms include fever, headache, and ulcers.

Image: chigger life cycle

Egg, larva, nymph, adult; the diagram also shows feeding at the skin surface.

Ascaris lumbricoides

Parasitic roundworm used as an example of an endoparasite.

Image: Ascaris life cycle

Eggs leave in feces; infective eggs are ingested; larvae migrate and adults live in the intestine.

Ectoparasite vs. endoparasite: environment

Ectoparasites face higher exposure to the external environment and natural enemies.

Ectoparasite vs. endoparasite: immunity

Endoparasites face greater exposure to the host immune system.

Ectoparasite vs. endoparasite: feeding

Endoparasites generally have easier access to feeding than ectoparasites.

Ectoparasite vs. endoparasite: transmission

Moving to and from a host is generally more difficult for endoparasites.

Host–parasite population cycles

Host and parasite populations can fluctuate in recurring cycles.

Graph: forest tent caterpillar outbreaks

Defoliation intensity rises in repeated outbreak peaks across sites and years.

Graph: host–parasite population cycles

Host increases can precede parasite increases; parasite pressure may then reduce hosts.

Chiggers: parasitic life stage

The larval stage depends on a host for food; nymphs and adults are free-living.

Chiggers: preferred attachment areas

They often attach where clothing fits tightly, such as sock lines and waistbands.

Chiggers: how they feed

They do not drink blood; they release digestive enzymes that break down skin cells and consume the digested tissue.

Image: chigger feeding on skin

Larva attaches at skin surface and feeds on liquefied skin tissue using digestive enzymes.

Chigger lifecycle image: host-dependent stage

Only the larval stage feeds on a host; nymphs and adults are free-living.

Chigger image: stylostome

A feeding tube forms in skin as larval chiggers digest tissue; they do not suck blood.

Chigger feeding: tight clothing

Larvae often attach around tight clothing edges, including socks and waistbands.

Exposure to natural enemies

Ecto: High Endo: low

Exposure to external environment

Ecto: high Endo: low

Difficulty of movement to and from the host, for parasite or its offspring

Ecto: low Endo: high

Exposure to hosts immune system

Ecto: low Endo: high

Ease of feeding on host

Ecto: low Endo: high