PARA Lecture 8 09/25/2026

What is less problematic for ectoparasites

Ability to “home in” (site specificity) less problematic for ectoparasites

Example of site specificity feather lice

Feather lice attach to feathers with barbs of a certain diameter, matching the width of their appendages

Example of site specificity trematodes

Attach to gill filaments of their host fish

How do parasites find their preferred location (host environment)

Even though a host’s environment is complicated, it is usually predictable:

• Anatomical similarities between hosts

• Homeostatic mechanisms in hosts maintain relatively stable conditions

How do parasites find their preferred location (migrations)

Parasites that have complex migrations within their hosts utilize a very small number of environmental stimuli:

• Respond to specific environmental signals

• Stimulus threshold in host = genetically programmed response by parasite

How does fasciola hepatica (liver fluke) migrate from intestine to liver

• Fecal-oral transmission

• When cyst hits stomach it reaches a low pH starting excystment

• In duodenum violent spasms due to bile and excystation completed

• Continued spasms and release of lytic enzymes allow penetration of intestinal lining into peritoneal cavity

• Larvae adhere to lining with suckers and begin moving- because cavity is like a sphere any movement in any direction leads to liver

• Once liver found, feed upon liver tissue

What is of high importance to protozoan parasites that are not highly mobile

reliance on affinity between surface molecules of host tissues and parasites

Example of reliance on affinity between surface molecules of host tissues and parasites

Toxoplasma gondii microneme protein 2 (mic2) binds to host protein Intracellular Adhesion Molecule-1 (ICAM-1) on intestinal epithelium of intermediate host

What happens after Toxoplasma mic2 binds ICAM-1

leads to diapedesis allowing parasite to infect macrophages and dendritic cells (invasion with apical complex) which then further disseminate parasite thru out host

How does Toxoplasma gondii have host specificity

felines are definitive hosts, felines lack delta-6-desaturase which breaks linoleic acid into arachidonic acid, thus T. gondii recognize linoleic acid in definitive host

When is it more common for migrating stages of parasites to get lost

more common in accidental hosts where normal host environmental cues are not there

Example of unsuccessful parasite migration

Human infection with hookworms of other animals- cutaneous larval migrans

How do cutaneous larval migrans work

Infective L3 larvae of canine or feline hookworms penetrate human skin and can’t reach circulatory system- as they migrate aimlessly they provoke a trail of inflammation

What are examples of difficult sites endoparasites must hold their positions in

digestive tract- peristalsis

circulatory system- rushing blood

What are examples of difficult events ectoparasites must hold their positions in

Environmental factors-

• Wind

• Rain

• Host grooming

How do intestinal parasites hold their positions in host tissues

• invasion of epithelial linings

• adhere with adhesive disks or hook on with proboscis or scolex

How do trematodes hold their positions in host tissues

• strong, ventral suckers

• sexual reproducers- males and females form pairs and use male’s powerful musculature coupled with suckers

What protein does plasmodium falciparum produce to help with adherence

inside erythrocytes, parasite produces protein Plasmodium falciparum Erythrocyte Membrane Protein 1 (pfEMP1)

What does pfEMP1 do

causes formation of “knobs” that bind to capillary epithelial cells

How do the knobs formed by pfEMP1 help plasmodium falciparum survive

avoids transport to the spleen, where parasite- infectedcells might be detected and destroyed

Why are the infected-erythrocytes bad for host

infected erythrocytes accumulate in the capillaries of the central nervous or renal system, resulting in much of the severe pathology associated with P. falciparum

Example of ectoparasite maintaining positioning

Pennella balaenopterae (parasitic copepod)- External segmentation lost and instead has largely vestigial locomotory appendages- females have developed anterior anchors that bifurcate allowing her to embed tightly into blubber

What kind of reproductive rate is common for parasites

High reproductive rate

Reproduction investment vs progeny survival relationship

High investment in reproduction common when chances of progeny survival low

What allows for high reproductive rates

High egg-laying capacity

High reproductive rate hack by cestodes and some trematodes

Cestodes and some trematodes (flukes) are monoecious- proglottids of tapeworms equipped with both testes and ovaries so both sexes don’t have to be present