Themes of Parasitology: 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.