ZOOL 452 – February 4, 2014
• Host finding
o Passive through eggs, ingested larvae (Trichinella), etc.
o Active through penetration of the skin. (eg: hookworms, Strongyloides
o Autoinfection ▯ Trichuris trichiura and Enterobius (pinworms)
o Intestinal helminthes ▯evolved recently to be parasites of hosts.
• Establishing in the host
o Site selection is involved.
o Most of the GI helminths like the upper half of the small intestines
o Diurnal migration may happen to follow the food bolus or reselect the
primary site of occupation
o Males and females try to find each other in order to mate.
o Most helminthes do not reproduce in the host. The dose of the infection is
regulated by the number of organisms to which you are exposed to.
Parasite load depends on the number of organisms that can
establish successfully. Exception: Trichinella
Inoculum size governs the degree of pathology that would be seen
in the GI tract.
o How do helminthes find their niche?
Olfactory structures and chemoreceptors
Males and females find each other using pheromones. Chemical
sensing of the gradients that are present.
Aggregation pheromones allow communication between the
worms. Eg: Proteocephalus parasites of fish (similar life cycle to
the Diphyllobothrium latum, which infects humans, fish, and
• Proteocephalus pinguis : native to North America. cestode
that uses the northern pike, Esox lucius, and the chain
pickerel, Esox reticulus, as hosts.
o Adults are found in the intestines and the villi of the
pike. They have 4 suckers on the scolex that act as
holdfast structures. They also have a 5 sucker that
is the rostellum.
o Life cycle: Begins as a floating egg in the water and
is ingested by the copepod. Once ingested, the egg
hatches and forms a procercoid. The copepod is
then ingested by the second intermediate host,
typically a small fish. Procercoid develops into a
plerocercoid. Small fish is eaten by a pike. Within
the pike, the plerocercoid develops into an adult. There is displacement within the developmental stages of the
worms and the displacement within the different species of worms.
There is also some competition between helminthes and
protozoans. Not related to chemical signals. Related to the damage
of primary sites of the other competing parasite. Trichinella’s
damage of the GI tract makes it hard for Giardia to establish in
certain places. Giardia is displaced and has to move to another
o How do the helminthes stay in place?
Attachment structures/Holdfast structures (bothria, hooks, suckers,
Need to develop these structures to prevent being moved out by
peristaltic action of the gut.
Ascaris maintains its position in the gut because of its size. Worm
occupies a large portion of the intestines. Hard to dislodge them.
Hymenolepis diminuta moves with the food bolus and back to its
Acanthocephalans ▯ burrow into the gut. Hard to dislodge them
• Living in the host
o Glycolysis ▯primordial metabolic pathway used by intestinal helminthes
o Krebs cycle generates way more ATP. Problem is that it requires lots of
oxygen. Not a lot of that in the intestines. Hard to do this.
o If a host does not feed for a long time, and