Parasite/Host
Relationships
Copyright 2011 PEER.tamu.edu
What is a HOST? What is a PARASITE?
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Before we even begin, I
want each student to
write down on a piece of
paper what they think a
HOST is and what they
think a PARASITE is.
1-2 minutes, GO!
After you have written down your
thoughts, fold your paper and trade it with
someone else who is done in the class. 30
seconds, GO!

NEXT:

Read your piece of
paper, if you think your
paper has a correct
answer raise your hand
and read it out loud.
If it is correct, you and
the person who wrote the
answer will receive *___
pts on your next quiz.

Final Answer
(Should go something like this)
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
A HOST is any organism that another organism or
bacteria live in or on.
The PARASITE is the organism living inside
(endoparasite) or on (ectoparasite) the HOST.
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This means the parasite benefits at the expense of the
host.
The host-parasite relationship is typically species specific.
*Lets think about and discuss some examples of a
parasite/host relationship
Some Examples:
Common Types of Parasites:
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Think fast! You have one minute to list as
many parasites as you can. Remember to
include what species they affect.
Parasite Life Cycle:
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The life cycle is a never-ending chain of events
that lead to the parasite constantly reproducing
and re-infesting another animal.
There are two types of lifecycles:
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Indirect - require different types of hosts or a
vector (intermediate host) for various stages of
their life cycle.
Direct - a life cycle in which a parasite is transmitted
directly from one host to the next without an
intermediate host or vector of another species.
The Intermediate Host
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Required by some parasites
to complete their life cycle,
Advantages of an
intermediate hosts include:
increasing the spread of the
parasite & protecting the
parasite from spending too
much time in the
environment.
What is the intermediate
host in the life cycle of the
Heartworm?
Why do I need to know this?

Take a moment to explain
to a neighbor the
importance of
understanding life cycles of
common parasites.

Hint: think about
controlling and preventing
disease.
Importance of Life Cycles
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Understanding the life cycle of a parasite
provides valuable information concerning:
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The individual infested
How the particular stage in the life cycle affects the
host
The most common time of year for the parasite to
be in the environment
Types of infestation/sickness caused by the organism
Vulnerable points in the life cycle for the best
treatment and prevention
Think It Through
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
The next two slides show diagrams and
descriptions of two different parasite
lifecycles.
Determine whether the depicted lifecycle is
direct or indirect and give justification for
your answer.
Liver Fluke Life Cycle
Your Turn – Complete The Life Cycle
5.
• Infected adult flea
ingested by dog
• Eggs ingested by larval
flea
• Segments of adult
worm passed in feces
• Adult worms develop
in small intestine
• Segments & egg
packets in feces & on
fur
• Larval flea develops
into adult flea and
contains immature
tapeworm
Parasite Transmission

Examine the images and determine how a
parasite might gain access to the host.
Diagnosis
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Why is a fecal examination a useful tool to
diagnose a large variety of parasites?
What other methods might be used to make a
diagnosis?
Treatment
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Why take the time to diagnose a parasite?
Can’t we just give an animal medicine to kill any
parasite they might have?
Drug Resistance
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Multiple species of parasites have developed
resistance to many treatments.
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Parasites with genomes that allow them to survive
treatment can pass this DNA to their offspring.
Thus, in following generations, the whole population
becomes resistant.
Potential Side-effects
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Often times parasites and their hosts have
similarities that may make the treatment drug
harmful to the host as well.
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Example: the only proven method to kill adult
heartworms in the dog is to use arsenic, a highly
toxic compound that is potentially deadly to the
host (dog) as well.
Treatment
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Administer drugs strategically
Focus on prevention
Large Animal Case Study
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Dr. Brown was called to Ag farm to examine
Scout, a 1000 pound Thoroughbred. Scout is
unable to put on weight and has a dull, coarse
coat. A fecal exam reveals large strongyle and
ascarid eggs. Dr. Brown recommends a broad
spectrum anti-parasitic given at 1 mg per 5 kg of
body weight. What dose should Scout be given?

Don’t forget to convert from pounds to
kilograms!
Prevention
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View the images below & identify management
practices that could lead to parasite infestation
Prevention
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Over-crowding, over-grazing, and poor sanitary
conditions lead to increased exposure to parasite eggs
and a greater degree of infestation.
Pasture management (mowing, rotation, etc…)
exposes eggs & larvae reducing their chance of survival.
Parasites can't survive indefinitely outside of a host and
at some point they must invade the primary host in
order to sustain the cycle.
Research in Parasite Treatment
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New Drugs
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Constant need for new drugs for resistant parasites
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Common problem with fleas. Companies are coming out with new
products every few years because the old products are ineffective
Special concerns with drugs given to food producing
animals because of potential accumulation of drugs in
tissues for human consumption
Animal Research
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Many parasites are host specific, testing can only be done in
susceptible species.
Even if a parasite infects different species, it often behaves
differently in the different hosts
Research in Parasite Treatment
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Vaccines
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limited success
Parasites have complex antigens that are difficult
to characterize
Vaccine research is expensive and it is difficult to
get funding
There are some new vaccines for parasites on
the horizon but only time will tell if these prove
to be effective treatments
References
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http://cal.vet.upenn.edu/projects/dxendopar/p
arasitelists/pdlist.html
http://www.dpd.cdc.gov/DPDX/default.htm
www.nih.gov
http://users.rcn.com/jkimball.ma.ultranet/Biol
ogyPages/S/Symbiosis.html
http://www.peteducation.com/article.cfm?c=2
&aid=757