Adaptations of Organisms Upon Encounter

Death or Survival?

For a parasite to survive, it must be able to do two things, encounter the host, and be compatible with the host. To encounter the host, the parasite evolves special phenotypes which allow for a greater rate of encounter, or it simply lives in areas where the host must meet the parasite at some point. Unfortunately for the parasite, hosts also evolve to be able to either fight the parasite or avoid it. If the parasite has been able to latch onto or enter the host, it must now survive while draining nutrients from its victim. This is where being compatible with the host is essential. If the host is able to detect the parasite and remove it, the parasite dies and the host is able to keep its resources to itself. This is where either closing the encounter filter or the compatibility filter is important in the destruction of the parasite.[1]

Fleas Fleas Everywhere!

When a host is able to detect a parasite, it must decide whether to avoid it entirely or kill it off. Two similar hosts, the great tit and the blue tit, must make the decision whether to live in a nest with fleas or without. Normally, both tits prefer to live without fleas so that they my keep their resources and not share them with the fleas, but this is not always possible.[1] For multiple reasons, a tit may decide it is easier to contend with the fleas and must now compensate in a variety of ways for the resources the fleas are taking from both parents and nestlings. For both species of tits, the nestlings are greatly affected from the presence of fleas. They are smaller in size and weight which makes it harder for them to survive and mature. There begging rate also increases because they lack resources sufficient resources to allow development into a mature adult. This increased begging rate causes stresses on the father because he is focused on providing for the current generation of nestlings. To do this, he will leave the nest more frequently to find food which tires him and makes him more susceptible to disease like malaria.[1] While the father puts in effort to acquire more food, the mother tit usually bears a larger brood because she focuses on the future generations she will lay.[2,3] These behavioral changes were all the result of not closing the encounter and compatibility filters and resulted in a lifestyle which greatly reduced the health of both parent and nestling tit.

Changing Your Skin to Avoid Detection

When a parasite enters its host, it must either avoid detection or be able to withstand the persecution of the hosts’ immune response. Trypanosoma Brucei is a protozoan which causes African sleeping sickness.

http://www.cdc.gov/parasites/sleepingsickness/biology.html [7] 

 To be able to survive in mammals, it has developed the ability to change properties of antigens upon its membrane surface.[4] An antigen is a foreign substance which triggers an immune response which ultimately results in antibody development for that specific foreign substance.[5] The antigens of T. Brucei are concentrated on the surface of the protozoan in a thick glycoprotein known as the variable surface glycoprotein (VSG). What distinguishes T. Brucei from other microorganisms is that its VSG has over 1000 genes which allow it to adapt to immune responses.[6] When the immune system is able to detect the antigens expressed by the VSG and mount a response to kill T. Brucei, it mutates its VSG using a different gene which expresses another antigen the hosts immune system has not yet identified and formed antibodies for.

http://www.nature.com/nrmicro/journal/v4/n6/fig_tab/nrmicro1428_F2.html [8]

By mutating the VSG T. Brucei, it is able to thrive within its mammalian host for extended periods of time. This lengthy survival is possible because of the roughly one week period of time the body takes for an adapted immune response to target and kill T. Brucei. With over 1000 different genes in the VSG, it can avoid detection of the mammalian adaptive immune system for a minimum of 1000 weeks (19.23 years) which allows an ample amount of time for it to be able to reproduce and pass its genes to the next generation.

References:

1) The Art of Being A Parasite by Claude Combes http://books.google.com/books?id=bEtEL2z3hXcC&pg=PA44&lpg=PA44&dq=bothriocephalus+gregarius&source=bl&ots=zEQ9I30Ou0&sig=IGbgHPXOnso-3mDBKJSu2nwbtMo&hl=en&sa=X&ei=khPvUsryCubXyAGP1oCoBA&sqi=2&ved=0CEQQ6AEwAw%23v=onepage&q=bothriocephalus%20gregarius&f=false#v=snippet&q=compatibility%20filter&f=false

2) http://www.jstor.org/stable/5390

3) http://beheco.oxfordjournals.org/content/7/2/127.short

4) http://www.sciencedirect.com/science/article/pii/0166685192900044

5) http://www.biology-online.org/dictionary/Antigen

6) http://apps.webofknowledge.com/full_record.do?product=WOS&search_mode=GeneralSearch&qid=13&SID=1FW128QdnJBfbUMaBwq&page=1&doc=4

7) http://www.cdc.gov/parasites/sleepingsickness/biology.html

8) http://www.nature.com/nrmicro/journal/v4/n6/fig_tab/nrmicro1428_F2.html

Fight or Flight?

Two lines of defense

The evolution of parasite and host interactions is a never-ending arms race: the parasite tries to get ahead of the host while the host tries to get ahead of the parasite. According to Claude Combes, two filters can represent the arms race between hosts and parasites: encounter and compatibility.  The parasite wants to open the filters where as the host wants to close them¹. Going along with this idea, there are two lines of defense that can help a host evade parasitism. The first line of defense is to avoid the parasite all together, which in turn closes the encounter filter. The second line of defense is, in the event that the encounter does occur, to fight off the infection, which closes the compatibility filter¹. Idealistically, both of these lines of defense work; however, which one do hosts utilize more frequently: flight or fight?

Flight

                 

       

Lifecycle of Paragonimus westermani

 Have you ever gotten a small scratch or bug bite that you did not notice until it started burning or itching? Most of the time we do not realize when we got the scratch or bug bite because they are so minute. The same can be applied to a host/parasite relationship. Parasites are incredibly small to the point where they are not visible to the naked eye. Hosts need to be able to detect a parasite in order to avoid it. Because they are so small, even though they give off signals for detection, the amount of signals given off by a parasite is not detectable by a host¹. Just like how we don’t realized the cut until it burns, in the same way, the host doesn’t detect the parasite until its immune system has.

Lifecycle of Dracunculus medinensis

Another reason why parasites are hard to avoid is because they are found in places that cannot be avoided, such as the water and prey². All of the parasitic helminthes (which include the roundworms, flatworms, and flukes) have a period of their life cycle in which they spend in an aquatic environment³. For example, the female Dracunculus medinensis, otherwise known as the guinea worm, causes a blister to form in the human host. The blister causes severe pain and discomfort, which can bee soothed by the water. This is when the guinea worm emerges from the skin so that it can release its eggs into the water⁴. The cycle repeats when humans ingest contaminated water. This is why avoiding parasites is not the preferred line of defense for hosts.

Fight

                  Generally, hosts tend to use the second line of defense more frequently which occurs after an encounter has been made. A host’s immune system is programmed to recognize “self” and “non-self” molecules. Every “self” molecule has a specific set of protein on the surf of the cell that allows lymphocytes to recognize them. Parasites have different proteins on their surface that alert the lymphocytes that a foreign body, or antigen, is present. The immune system goes on to produce antibodies to fight off the infection⁵. This is one way the host can fight off an infection. Another way hosts can deal with an infection is to reallocate resources¹. A host may compensate for an infection by shifting their life cycle to reproduce earlier in life. This is what the crustacean Daphnia magna tends to do when parasitized by Glugoides intestinalis6,7.  Studies show that infected Daphnia tend to produce up more offspring in their first clutch than those that were not infected^6,7. This may be due to a trade off that lowers reproduction later on in life due to the infection.

Infected Daphni vs uninfected Daphnia 8

References

  1.http://books.google.com/books?id=bEtEL2z3hXcC&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=snippet&q=line%20of%20defense&f=false

 2. http://www.cdc.gov/parasites/water.html

 3.http://parasite.org.au/para-site/contents/helminth-intoduction.html

4. http://www.cdc.gov/parasites/guineaworm/biology.html

5. http://www.cancer.gov/cancertopics/understandingcancer/immunesystem/AllPages

6. http://rspb.royalsocietypublishing.org/content/272/1562/505.full

7. http://www.ncbi.nlm.nih.gov/books/NBK2046/

8. http://evolution.unibas.ch/people/dita/old_res/organis.html

Pathogenicity and Virulence

A Reoccurring Concern

Spanish Flu, Bird Flu, Pig Flu, Ebola(diseases). Every so often it seems as if a new disease is going to be the next pandemic, with the exception of the Spanish flu that proved to be a pandemic. Somehow many of us survive to face the next one. Conspiracy theorists say the government is behind it. Though there is possibility that they are right, the correct answer lies within a deeper understanding of how pathogens function. The pathogen’s ability to infect and the magnitude of the infection play a significant role in its success. When a pathogen finds the perfect balance, it will be very successful.

http://www.invmed.info/historyofinfluenza/images/img_spanish_map.jpg [5] 

The Difference 

A pathogen is defined as a microorganism that causes disease. The pathogen has two important characteristics. The first is pathogenic character, which is defined as the pathogen’s ability to cause disease in its host. The second characteristic is virulence, which is defined as the magnitude at which the disease occurs. Pathogenicity is a quality that is either on or off. The virulence factor measures the level of pathogenicity. There are multiple virulence factors, such as attachment, destructive enzymes and toxins. Virulence factors allow for the pathogen to be more successful within a limit. To explain, if a pathogen has zero virulence then it is no longer actually a pathogen by definition because it is not causing disease. On the other hand, if the virulence factors are too high then the pathogen will be so efficient at killing its host that it may deplete its reservoir, the host it infects, and no longer reproduce.

Chestnuts No More

Chestnut trees(host) were one of the most utilized trees in America. It was used for the durability of its bark and the tastiness of its chestnuts. However, this only lasted up until the 1900.This is when the Chestnut blight took place. A fungus, Cryphonectria parasitica that originated in China and only infected dying twigs and pieces of bark, was introduced to America around 1893. In the fungus' native region there existed a different kind of chestnut tree which was resistant to the fungus. Once introduced t the American chestnut tree it only took 40 years to wipe out all the chestnut trees in America. Cryphonectria parasitica spread up to 50 miles of chestnut forestry a year. Once the chestnut trees were decimated the fungus' reproduction rate plummeted, without a host to provide the nutrients and a habitat it's reproduction was halted. ^[2]

http://www.youtube.com/watch?v=oVrGgW-bBz4 [3]

Parasite Manipulation of the Host

Mind Control Everywhere!

Being able to have mind control seem like a sophisticated power that many would like to have. In Vampire Diaries, the evil vampire, Damon, is known to use this type of techniques on others such as Jeremy. You can also find this type of behavior in cartoons such as SpongeBob Squarepants when Plankton takes over SpongeBob’s mind to try to get the secret formula for the crabby patty. Unfortunately, being able to control something or someone’s mind is very unrealistic – but not for certain parasites. This unique characteristic has given the parasites the upper hand when it comes to the evolution of parasites vs. host. Getting a host to do whatever, or go wherever is a necessity in order to get to their next life cycle. (1)

How and Why Does This Happen?

When the parasite has finally encounter the intermediate host and is able to survive the host immune system, the next step for the parasite is to somehow manage its way to get to its definitive host. It has been report by Poulin that usually the intermediate host is not the average meal that the definitive host would eat which is why this could become tricky for the parasite (2). Thanks to the product of natural selection, genetic manipulations, which is manipulation of an organism’s genome that has influenced another organism’s phenotype, is the reason of how parasites has the advantage over their host. Being capable of changing its phenotype is also one of the main reasons why parasites modify the host behavior. There are actually more other reasons to answer why specifically the parasite changes the host behavior. Another reason, probably the most obviously, is because it helps the parasite fulfill its life cycle. The last reason for why the parasite changes the host’s behavior is because it inherited this specific trait from an ancestor and it is now just embedded in the way it functions. (3)

Zombie Snails

Snails are one of the many organisms that are the victim of a parasite that is able to have mind control over them once infected. This specific parasite that is responsible is the Leucochloridium worm. This worm is able to get inside the snails eye stocks and also a result makes the eyes look more like yummy caterpillars to worms. Most importantly, the Leucochloridium worm takes over the snail’s brain. The way they are sufficient in doing so is by having the snail work its way up in plane view site so that predators, in this case birds, are quickly able to spot the infected snail. This process in the end makes for a happy parasite family that is now it able to achieve its finally life cycle in the birds stomach. The way that the cycle is able to start over again is that the snails eat the infected birds feces. Already you can quickly observe and apply the reasons to why this particular parasite wanted to have the snail behave differently. It was capable of changing the snails phenotype which in return helped it successfully complete its life cycle by being able to enter into the birds digestive tract and lastly the mind – control is inherited so it can again go through the same life cycle each time using this trait so that it can strongly complete the life cycle with little to no problem. (4)



References

(1) The Art of Being A Parasite by Claude Combes

(2) http://www.otago.ac.nz/parasitegroup/PDF%20papers/Poulin2010-ASB.pdf

(3)http://books.google.com/books?hl=en&lr=&id=dKSyEsRTk5kC&oi=fnd&pg=PA243&dq=parasite+manipulation+of+host+behavior&ots=-wyxiQvfeL&sig=oofSlPgZODIaykggdlY9k4KEY5U#v=onepage&q=parasite%20manipulation%20of%20host%20behavior&f=false

(4) http://animaldiversity.ummz.umich.edu/accounts/Leucochloridium_paradoxum/

Host Encounters

Don’t Take the Easy Way Out

Parasitism is defined as a form of symbiosis in which one organism, the parasite, benefits at the expense of another organism usually of different species, the host. The association may also lead to the injury of the host. A parasites life cycle may involve one host, or it could involve multiple hosts. For each of these evolutionary lifestyles, there are positives and negatives that affect the parasite and its overall ability to inhabit, survive, and reproduce within a definitive host. (1)

Sometimes a Challenge is Worth it

Parasites that involve more than one host into their life cycle usually go through a paratenic host. This host is one that does not usually help the maturation or development of the parasite itself, but the presence of this host may be an essential key that leads the inhabiting parasite to its definitive host. (2) The Bothriocephalus gregarius, a tapeworm found in the Turbot, goes through a copepod, and then through a paratenic host known as the Goby in order to get to its final destination in the Turbot (3). On the other hand the Bothriocephalus barbatus, another tapeworm of the fish known as Brill goes directly from the copepod to the Brill. One may believe that the shortest path is the best path, but this is not necessarily true for parasites. The B. gregarius showed a higher success rate than the B. barbatus. This could have been possible for many reasons. The usage of multiple hosts creates diversity for the parasite. Turbot rarely eat copepods after a young age, so evolution and natural selection have created a way for the B. gregarius to infiltrate the Turbot using the Goby which eats copepods, while the B. barbatus only uses the copepod to infiltrate its host fish, Brill. This gives the B. gregarius more than one option to aid in its survival and reproduction. Even though the success rate is higher for the B. gregarius because of its use of a paratenic host, the complexity also has a negative aspect (6). When transferring from host to host, the parasite will use more energy. This will create a higher amount of stress making it harder for it to survive during transfers. 

 http://wcs4.blogspot.com/2012_04_01_archive.html

Maybe Taking the Shorter Route Would Actually be Easier?

Or maybe it wouldn’t? For some parasites, a paratenic host may not be necessary for successful reproduction and life. Parasites such as entomophagous parasites, which are insects that are parasitic on other insects, only need the host to do what they need. The Ampulex compressa, also known as the jewel wasp, uses a single cockroach to lay its eggs into. The eggs hatch from inside the cockroach, grow, and repeat the process. There is no need for a paratenic host because these wasps can be completely successful without one (5). Parasites like the B. gregarius need these paratenic hosts in order to successfully occupy a host. Another example, the Toxocara leonine, a roundworm, is known to use smaller animals that become prey to bigger animals as their paratenic hosts. This once again creates more host options for the parasite (4). 

 http://www.todayifoundout.com/wp-content/uploads/2010/01/Emerald-Cockroach-Wasp.jpg

Final Deliberation

The positive aspects outweigh the negative aspects when it comes to multiple hosts for a parasite. Even though a parasite may use a little more energy to go from host to host, the idea of having options to invade is key. The options and complexity create a greater chance that a definitive host is reached. 

Links

(1) http://www.biology-online.org/dictionary/Parasitism

(2) http://www.medilexicon.com/medicaldictionary.php?t=41625

(3) http://eurekamag.com/research/038/325/importance-of-the-paratenic-host-in-the-biology-of-bothriocephalus-gregarius-cestoda-pseudophyllidea-a-parasite-of-the-turbot.php

(4) http://www.newmanveterinary.com/RW-Main.html

(5) http://thebestnursingschools.net/info/bizarre-parasitic-life-cycles

(6) http://books.google.com/books?id=bEtEL2z3hXcC&pg=PA44&lpg=PA44&dq=bothriocephalus+gregarius&source=bl&ots=zEQ9I30Ou0&sig=IGbgHPXOnso-3mDBKJSu2nwbtMo&hl=en&sa=X&ei=khPvUsryCubXyAGP1oCoBA&sqi=2&ved=0CEQQ6AEwAw#v=onepage&q=bothriocephalus%20gregarius&f=false