1. Parasite/Host Relationships
Please notice on many slides there will be additional teacher information in the notes section.
Copyright 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!
Be sure to keep time limits and encourage all kids to write something. Then get them to trade and be ready to share!!!
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.
*Teachers should fill in point value with what they think is appropriate or change the reward to a program used in your class.
Teacher can also erase the “See Notes” comment on slide.
Parasitism. Classical definition - intimate relationship between two organisms in which one (the parasite) lives on, off or at the expense of the other (host). This implies that one of the partners benefits, the other is harmed.

4. 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
*Take time here to let students give you their ideas/examples or ask questions about parasite/host relationships minutes These are very basic answers. The following is more specific with terms they need to know:
Parasites can be called: Eukaryotes that live on or in the body of another eukaryote species and exploit that species for sustenance. The host-parasite relationship is typically species specific

5. A More Formal Definition for Parasites:
Eukaryotes that live on or in the body of another eukaryote species and exploit that species for sustenance.
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.
Ask students what “species specific” means. Let them come up with examples to be sure they understand.
Answer: Limited to or found only in one species.
NOTE: Bacteria and viruses are also parasites in the technical sense, but by convention most people to parasites as eukaryotes.

6. 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?
Allow students time again to give you their thoughts, if they are not correct, prompt them to rethink their answer with well guided questions and see if they can come up with a correct response.
Fleas need the body of dog and cat for shelter and it bites it to eat its blood. This is why the animal itches. Then the flea lays its eggs on the animal and when the animal bites at the flea it eats the eggs. The eggs then hatch in the dog and are released in the feces. Once it the larva become an adult flea again it jumps on to a new host to start the cycle all over again.
Same with worms, they need the animal to live and to reproduce more eggs.

7. 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:
Acute Invasions/Infestations
Chronic Invasions/Infestations
Acute – when a parasite causes an acute infestation or invasion, it must move on to a new host quickly before it kills the host or is killed by the host.
Chronic – in this case, the parasite survives for long periods without killing its host right away or being killed by host.

8. 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!
Parasites have special adaptations to accomplish these requirements. Allow students to discuss or ask questions about things in their life that may be relevant to this. Such as, fleas on their dog/cats – what are they eating?

9. 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

10. Example Life Cycle:
4. Parasite returns to intestine to lay eggs to be released again in feces, continuing its life cycle.
1. Lets start here, eggs are released in feces.
This is the life cycle of one type of human parasite. Parasite eggs that pass in the feces hatch into free living larvae and can survive for a short time in moist, warm soil. These larvae can then invade a host via skin (in this case penetrating bare feet). Once inside the intestines of the host, the worms mature and lay eggs to perpetuate the cycle. Note that this particular parasite has no intermediate host.
3. The parasite moves to many different places inside horse as it grows and feeds off the animal host, sometimes making animal sick.
2. The animals eats the eggs that are now on the pasture grass.

11. Different Parasites have Different Ways of Surviving!
EWWWW!!!
Tick shown on lower left, tapeworm top middle, and roundworm on the far right.
Each parasite plays a role in a food chain/web. They find their NICHE in order to survive!!

12. Survival:
Below: example of evolution of horse.
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!
Evolutionary adaptations! Sounds like a big statement, take this time to ask students what this means. They have had exposure to adaptations, so they should be able to generate thoughts as to why this also applies to parasites and their hosts. Why would each need to change? Prompt students to think about how environmental factors may change and therefore so do the species living within them in order to survive. Medications for parasite treatment may also cause the parasite to change and mutate over time, becoming more resistant to the current medications.

13. 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?
Allow students time to discuss or ask questions about niche and understand it completely before moving on.
Have students explain or discuss the Niche of the three pictures on slide. Use terms like heterotroph, autotroph, predator, prey, consumer, producer…
Go to this link for more detailed information on niche:

14. Parasites Niche:
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.
Where would the tick live on the deer? What would it do to survive? Let students discuss this and this will help them see that the parasite finds a niche for itself on/in the animal to ensure its survival.

15. 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!!
Roundworms – dogs, cats, sheep, cows, humans! Basically almost all mammals can get some type of roundworm!
Tapeworms – basically all mammals again
Hook worms – all mammals
Heartworms – dogs, cats
Dysentery carrying amoeba – most commonly contracted by people eating contaminated food and drinks by poor hygiene. (people not washing hands after going number 2!!)
Flies – all mammals, and many other kinds of animals
Fleas - all mammals and many other animals
Bugs, lice, mites, ticks – all animals

16. 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

17. 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.
Ask as a class discussion question: What do you think the effects of over-grazing might be? What do you think the effects of leaving a pasture idle for a long time might be?
Ans.: More exposure to parasite eggs and greater degree of infestation. Depending on the species of parasite, you might break the life cycle, because the  eggs or larvae would die. They can't survive indefinitely outside a host and at some point they must invade the primary host in order to sustain the cycle."

18. Entry into/on Host Continued:
2. Skin Contact
Biting insects, passes in to blood stream or,
Parasite can penetration through skin.
3. From the mother to the baby while in the womb.

19. 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

20. 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

21. Contact with Host: Starts and Restarts Life Cycle!
Passive Contact:
Animals come to the eggs or larva that can not move
Active Contact:
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

22. Survival in Host Evade the immune system long enough to reproduce.
Methods to avoid the immune system:
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

23. Reproduction Produce numerous numbers of offspring
Most of which will not complete their life cycles
Tapeworm eggs

24. 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

25. 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.
Intermediate Hosts are very common and essential to the survival and spread of the parasite. They allow the parasite to travel further in environment and be in the environment with less chance of dying before it reaches a new host. It will not reproduce on the intermediate host, it is a step used by the parasite to ensure survival until a new host is found.
Example of Intermediate hosts and their response to parasite:
Rats, infected with the intermediate stages of a tapeworm called Spirometra grow larger than uninfected rats. The tapeworm larva produces an analogue of vertebrate growth hormone-is the growth boost harming the host or is it good for the host? Similarly many molluscs, when infected with the intermediate stages of Digenetic flukes develop thicker, heavier shells, which could be deemed an advantage. On closer investigation some of the classic examples of mutualism seem more like an armed standoff than mutual benefit. Given the right conditions many organisms which harbour symbiotic algae - like for example green hydra will digest the algae and carry on quite happily. Many trees have associated with their roots fungal mycorrhiza. The fungi get organic nutrients from the plant via the phloem, and in nutrient poor soil the trees seem to benefit by increased nutrient uptake, particularly phosphate by the fungus. But if soil nutrient levels are good it appears much more like a parasitic invasion by the fungus with the tree attempting to wall off infected cells. Depending on external conditions, the association switches between mutualism and parasitism.

26. 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.

27. So Why are Life Cycles So Important
Ask students why they think it is useful to know the life cycle of a given parasite species. Knowing the life cycle and all the different stages and where the parasite likes to go in the host allows for better treatment and prevention methods. The medications are made to treat the parasite based on where it is in its life cycle.
Next slide will discuss further!

28. 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????
Answer: When you lived in the womb of your mother.

29. Importance of Life Cycles
If you understand the life cycle of a parasite it answers a lot of important questions
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?

30. 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

31. 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

32. 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.

33. Research in Parasite Treatment Extra Information:
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, 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

34. Research in Parasite Treatment
Vaccines
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

35. References