Parasite/Host
Relationships
Copyright 2010. PEER.tamu.edu
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!

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)

A HOST is any organism that allows
another organism or bacteria to live in it
or on it.

The PARASITE is the organism living
off of the HOST.
*Lets think about and discuss some examples
of a parasite/host relationship
A More Formal Definition for
Parasites:
Parasite:
 Eukaryotes that live on
or in the body
of another eukaryote
species and exploit
that species for
sustenance.

http://www.aber.ac.uk/~mpgwww/Edu/Pa
ra_ism/PaIsmTxt.html this is a useful
sight with definitions and explanations of
symbiotic relationships.

This means the
parasite benefits at the
expense of the host.

The host-parasite
relationship is
typically species
specific.
Some Examples:

Fleas on a dog or cat – why is this a
parasite host relationship?

Intestinal worms in horses or cows – why
is this a parasite/host relationship?
Relationships Between Parasite and
Host Can Vary:

Like a “Hit and Run” – parasites live on or in the
host for a brief period of time then must move on, they
may or may not kill the host.

Parasites may invade/infest the host in two ways:

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Acute Invasions/Infestations
Chronic Invasions/Infestations
What a parasite MUST HAVE to
survive:

Nutrition – food! The host
provides nourishment for
the parasite.

Protection from the
environment –The host
provides shelter, warm and
snug!

Ability to Reproduction –
make more! The parasite can
not survive without its host!
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.
YUCK!!!

Let’s take a look at one example of a parasite
life cycle….next slide
Example Life
Cycle:
4. Parasite returns to
intestine to lay eggs to be
released again in feces,
continuing its life cycle.
3. The parasite moves to
many different places inside
horse as it grows and feeds off
the animal host, sometimes
making animal sick.
1. Lets start here,
eggs are released in
feces.
2.The animals eats the
eggs that are now on
the pasture grass.
Different Parasites have Different Ways of
Surviving!
EWWWW!!!
Each parasite plays a role
in a food chain/web. They
find their NICHE in
order to survive!!
Survival:

Mutual evolutionary
adaptations of parasite and
host may lead to a parasite
becoming less damaging and the
host becoming more resistant!

This means that both the host
and parasite change over time!!!
Just like many other organisms
on the planet!
Below: example of
evolution of horse.
What is the Niche of a Parasite?

You must first know what a niche is.

the role of an organism within its natural
environment that determines its relations with
other organisms and ensures its survival – in
other words, the organisms JOB! What does it
do to survive?
Parasites Niche:
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
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The parasite must find a place within the host
to survive.
A particular parasite may occupy a specific
site and utilize certain host resources and this
defines its niche.
So a niche is a description of the parasites
requirements, not of host attributes.
Common Types of Parasites:
Roundworms
Tapeworms
Hook worms
Heart worms
Amoeba
Flies, Fleas, Lice,
Mites,Ticks, and
Spiders.
Each causes different
symptoms and different
infections.
As host is treated for
parasite, both can build up
immunities. The host to the
parasite and the parasite to
the treatment.
That is why new methods of
treatment must constantly be
created!!
Parasites in a food chain or food web:
6 Links of the Chain
1. Contact and Entry Into Host
2. Gaining a suitable niche
In the host (where it lives to
gain nutrition)
6. Development in environment
or intermediate host.
3. Continued survival in Host.
May have to move to
different areas within
host body.
5. Escape from host,
released back into
environment.
4. Reproduction
Entry into/on Host

Three Routes of entry
1. Host can eat eggs that are on grass or other plant
life, or if parasite is on grass or other plant life, it
can crawl on to host.
Entry into/on Host Continued:
2. Skin Contact

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Biting insects, passes in to blood stream or,
Parasite can penetration through skin.
3. From the mother to the baby while in the
womb.
Parasite Finds its Niche in Host:


Must evade host
defenses and,
Must Migrate =
Movement
 Through tissue
 Through systems,
from organ to organ
 Most must return to
intestines to be
released in feces.
Human lesions
from hookworm
migration
Examples of Specific Niche Needed for
Protection and Nutrition
The babesia parasite
is only found within
red blood cells
Human head lice only
reproduce on humans and
prefer areas of skin with lots
of hair, like the head
Contact with Host: Starts and Restarts Life
Cycle!
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Passive Contact:

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Animals come to the eggs or larva that can not move
Active Contact:
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
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Some parasites move to areas where they are more likely
to come into contact with an animal. E.x. ticks on trees,
grass, bushes will crawl or jump on to animal.
Fleas actually detect vibration and carbon dioxide and will
move towards it because it means that a potential host is
near by
One species of roundworm larvae crawl up vegetation
onto the leaves to increase their chances of being eaten
Survival in Host


Evade the immune system
long enough to reproduce.
Methods to avoid the
immune system:



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Live inside cells of the host
Mimic host substances so
they are not recognized
Live in the gastrointestinal
tract of the host out of
reach of the immune system
Form a protective cyst
which cannot be penetrated
by the host immune system
Roundworms in Intestines
Reproduction

Produce numerous
numbers of offspring

Most of which will
not complete their
life cycles
Tapeworm eggs
Escape from Host



The larva, eggs, or cysts have to have a way
to leave the host
Most common way is through the feces
Can also leave through urine, saliva, semen,
milk, mucous membranes, or blood
Development in Intermediate Host



Some parasites require
another host to complete
their life cycle, this is called
an Intermediate Host.
This particular parasite has
the cow or sheep as the
direct host and a snail as
the indirect host
An intermediate host often
helps to increase the
spread of the parasite and
protect the parasite from
spending too much time in
the environment.
Development in Environment

Generally hotter weather
leads to faster
development/reproduction.

Colder weather leads to
longer environmental
survival, parasites can lay
dormant during cold
weather.
So Why are Life Cycles So Important
One Last Point:


No definition of parasitism is ever going to be
completely satisfactory, if we try hard enough we
can always find an exception and there are always
going to be grey areas where parasitism, mutualism
and commensalism overlap.
It has been estimated that more than 50% of all
known species are parasitic at some stage of their
life cycle.

If the above statement is true, can you think of a time
when you acted like a parasite????
Importance of Life Cycles

If you understand the life cycle of a parasite it
answers a lot of important questions

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Who is infested?
What stage of the life cycle is parasite in and how does
this affect the host? (
It is an acute or chronic condition?
What time of year is parasite most common in
environment?
What kind of infestation/sickness does this organism
cause?
Where are the vulnerable points of the life cycle for the
best treatment and prevention?
Prevention/Treatment


A thorough understanding of the life cycle
helps in the control and prevention of
parasites
Know when and how to expose parasites to
the maximum dose of medication
Drug Resistance

Resistance is a real problem

In Texas we have multiple species of parasites that
have developed resistance to many treatments

Reasons resistance develops


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.
Can be difficult to get drugs to the parasite depending
where they are in the body
Research in Parasite Treatment

Current Drugs
 Treatment is sometimes difficult, 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.
Research in Parasite Treatment
Extra Information:

New Drugs

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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, so animal research does not
translate well to human parasites. In animals, 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

Vaccines

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So far have had 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