ParasiteHostRelationshipandLifeCycles

1. Parasite/Host
Relationships
Copyright 2011 PEER.tamu.edu

2. What is a HOST? What is a PARASITE?
□ 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!

3. 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.

4. Final Answer
(Should go something like this)
□ 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.
■ 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

5. Some Examples:

6. Common Types of Parasites:
□ Think fast! You have one minute to list as
many parasites as you can. Remember to
include what species they affect.

7. Parasite Life Cycle:
□ 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:
■ 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.

8. The Intermediate Host
□ 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?

9. 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.

10. Importance of Life Cycles
□ Understanding the life cycle of a parasite
provides valuable information concerning:
■ 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

11. Think It Through
□ 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.

13. Liver Fluke Life Cycle

14. Your Turn – Complete The Life Cycle
1.
2.
3.
4.
5.
6.
• 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

15. Parasite Transmission
□ Examine the images and determine how a
parasite might gain access to the host.

16. Diagnosis
□ 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?

17. Treatment
□ Why take the time to diagnose a parasite?
Can’t we just give an animal medicine to kill
any parasite they might have?

18. Drug Resistance
□ Multiple species of parasites have developed
resistance to many treatments.
■ 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.

19. Potential Side-effects
□ Often times parasites and their hosts have
similarities that may make the treatment
drug harmful to the host as well.
■ 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.

20. Treatment
□ Administer drugs strategically
□ Focus on prevention

21. Large Animal Case Study
□ 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!

22. Prevention
□ View the images below & identify management
practices that could lead to parasite infestation

23. Prevention
□ 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.

24. Research in Parasite Treatment
□ New Drugs
■ Constant need for new drugs for resistant parasites
□ 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
■ 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

25. Research in Parasite Treatment
□ Vaccines
■ 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

26. References
□ http://cal.vet.upenn.edu/projects/dxendopar/
parasitelists/pdlist.html
□ http://www.dpd.cdc.gov/DPDX/default.htm
□ www.nih.gov
□ http://users.rcn.com/jkimball.ma.ultranet/
BiologyPages/S/Symbiosis.html
□ http://www.peteducation.com/article.
cfm?c=2&aid=757