"You still don't understand what you're dealing with, do you? The perfect organism. Its structural perfection is matched only by its hostility. I admire its purity. A survivor … unclouded by conscience, remorse, or delusions of morality." - Ash to Ripley (Alien, 1979)
Monday, February 17, 2014
Fight or Flight?
Two lines of
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
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.
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.
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 infected6,7.
This may be due to a trade off that lowers reproduction later on in life due to