Monday, February 17, 2014

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?

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 host
1. 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.


                  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 the infection.
Infected Daphni vs uninfected Daphnia 8



  1. You made some good points, and I would also like to add that humans are one of the few organisms that can fight and flight. Humans posses powerful immune systems to fight off viral pathogens, and also have the knowledge to reduce parasite encounters through education. We also have the brain power to develop vaccines and antibiotics to destroy and prevent future infections.

  2. Really very interesting and very valuable information about the Fight for Flight nice work.
    Fight for Flight