ZOOL 452: January 28, 2014
Protozoan parasites that live in the GI tract
• Host finding
o Passive. Don't have any means of developing finding strategies.
o They do have passive strategies that let them
Making huge numbers of infectious forms that contaminate the
Durable cyst forms. Environmentallyresistant forms
Tend to be directly transmitted with simple life cycles and no
intermediate stages. Direct transmission allows direct hosttohost
transmission to be possible. Eg: human to human through
contaminated hands (feces)
• Establishment in the host
o Boundaries to establishment
Salivary enzymes and mechanical grinding of teeth. Can damage
the infectious agents
Digestive enzymes, acid in the stomach
Gradients of fluids in the GI tract = different habitats.
o Protozoans use site selection to find where they should be.
Eg: Giardia site selects to be in the upper half of the intestines.
That is where the majority of the food is. This is also very close to
bile ducts. Bile is required for Giardia to continue its life cycle.
o Attachment structures are used to remain in the gut.
Eg: Giardia have sucking disks that allow them to remain attached.
Most intestinal protozoans stay in the mucus. Mucins in the GI
tract protect the intestinal protozoans from enzymes and stuff. The
mucins can also act as a barrier to the infection…
o GI tract has huge numbers of bacteria as well. Microbiome modifies the
GI tract to a specific physiological environment. Includes changes in the
bile levels, pH levels, etc. Protozoans have to deal with sharing the space
with the bacteria. Differences in susceptibility to intestinal protozoa
parasites depends on the differences in the composition of the microbiome.
o if there is more than one protozoan in an area, it is possible for the more
dominant protozoan to push that other one off.
o It is possible for a section of the GI tract to be partitioned to allow
different organisms to coexist. Mechanisms of the partitioning are not well
Lots of gradients affect this. Differences in the types of columnar
epithelial cells. Eg
o Extracellular existence in the GI tract is “easier” than intracellular. You
have to figure out how to do this. Extracellular parasites are more recent in
terms of evolution o Could be argued that parasite with direct life cycles are more recent
evolutionarily than complex life cycles
• Living in the host
o Use the host’s metabolic pathways to get what they want. Get the things
that they need from enzyme products in the GI tract
o Most protozoans multiply asexually.
o Very few infectious forms are needed to start an infection. Then, the
protozoan parasites are able to multiple to HUGE numbers of infectious
forms. Huge problems for parasite
o Parasites must adapt to anaerobic environments. May be microaerophilic.
o Parasites have lost their ability to see. Most parasites that live inside a host
o How do the protozoa parasites sense the chemical cues?
Don’t have olfactory structures.
Look for a particular nutrient that is required for their metabolism.
o How do they take their food into their cells?
Balantidium coli has an organ that is required for food particle
Protozoans use pinocytosis to get stuff in. Create a phagolysosome
(phagosome + lysosome) to break down their food.
o What else do the parasite have to deal with?
Mucus produced by goblet cells.
Goblet cell hyperplasia happens when there is a GI protozoan
infection. Increased mucus secretion during the infection is really
hard for the parasites to deal with.
Can stain the GI tract with Periodic Acid Schiff. Will see LOTS of
mucus that is produced by them.
o In the duodenum
Bile is secreted through the bile duct
o Liver = detoxifying organ in the body.
Problems with liver ▯jaundice. Jaundice may also be caused by
o Lots of nutrients are here.
o Large intestine: reabsorption water and electrolytes.