ZOOL 452 January 14, 2014
• Parasitic life cycle stages living outside hosts I
o Free living stages of different kinds of parasites
• Freeliving adults
o Why are the freeliving stages important to us?
Used to maintain the organism in the lab
Used to study transmission rates and how transmission works
o Features of the environment that would be important to freelivers
• Humidity can control the rate at which water leaves the
organisms. Very species specific. Eg: some nematode
larvae do well in liquid water.
• Some parasites do well when they are frozen. Good for
• Varies depending on the parasite
• pH, toxins, salinity
• Solar radiation (UV) can be very damaging to parasites Oxygen
• Freeliving forms require oxygen. Parasitic forms tend not
to use it.
• Can act in conjunction with moisture levels to affect rates
o When looking at freeliving stages in the environment, consider the effects
from where they are and not from the human perspective
o Why do we study the freeliving stages of a parasite?
Impractical to study them in nature.
You wouldn’t let to host organisms shed in the environment. You’d
have to bring them into the lab.
o Environmental resistance. How do these freeliving parasites survive in
the environment, considering the kinds of stuff that they have to
Some of these parasites do feed. (eg: 1 and 2 larval stages of
nematodes). That is critical to how they feed.
For stages that do not feed, people study how the parasite manages
its energy reserves. Eg: cercariae do not feed.
For the motile stages, people study their locomotion behaviour.
Critical for how they will find their host.
• How do they recognize that a host is there and target it?
This is also studied.
o How do you get freeliving stages?
From parasites that are already in the lab.
• Advantage: you know exactly what you are getting when
the host is kept in the lab.
• Disadvantage: this procedure can select out natural
variations. What you would get in the lab would be things
that have adapted to the specific lab conditions.
Researchers select for rapidly reproducing parasites. This
does not give you a good idea about what is actually seen in
the wild. If you use very biasing conditions, your
conclusions will be wrong, because they would only apply
to the strain that is being grown in the lab.
o Sometimes, one lab is able to maintain a parasite.
Sends it to another lab that is not able to maintain
the parasite. Could be related to the water
composition in the area, the type of food that you
Get the wild animal hosts from the environment
• You can go and grab it from the wild host. • Disadvantage: that host that you found in the wild has been
exposed to other parasites.
o The conditions of the wild animal hosts will change
when you bring them into the lab environment. That
will change the way in which the parasites grow.
Can’t exactly depend on the results from that study.
Get the parasite from the wild host
• Disadvantages: you’ll have a more heterogeneous set of
parasites. Not easy to get results that can be reproduced
o When do you get the material?
Not a problem in the wild
If you are getting the parasites from the wild, you have to consider
the life cycles of th