Parasite Adaptation 1: The Trypanosome

Parasites justifiably get a bad rap. At best, they are gross things that burst out of the bodies of other organisms alien style. At worst, they can cause severe human disesase. One species (Ascaris lumbricoides) infects a quarter of people on the planet. This however is a relatively benign parasite compared to those of the malaria parasites – which infect up to 200 million new people a year, and may kill up to 3 million people a year. It is therefore understandable that we should feel uncomfortable about them. However, if we can think dispassionately about them, and consider them as organisms, they are really quite remarkable and have evolved some amazing solutions to survive in a hostile environment. In fact, parasitism may be the most popular lifestyle there is as most species have several parasites, and even parasites have parasites of their own.

All organisms live in an environment that presents challenges that have to be overcome – be it a lack of water, light, oxygen or predators. Parasites though live in an environment that is actively trying to kill them. Animals (and plants) have evolved elaborate immune systems that try and kill invading organisms, and in return, parasites have evolved counter measures. Some species, such as Leishmania sp. effectivly live in baths of digestive enzymes, bleach and toxic radicals called phagolysosomes. Others can also immunosuppress their hosts. An example of this would be the hook worms (Ankylostoma sp). As an aside, these are actually being tested to treat allergies. Some species (Leishmania again) can make the host produce an entirely inappropriate immune response that actually antaganises the type of response that would kill the parasite. Some species (Schistosoma sp) even cover themselves in host molecules to disguise themselves.

One group called the Trypanosoma brucei complex have a rather interesting way of evading the immune system. These are transmitted by tsetse flies and cause a wasting disease called ngana in cattle. In humans, two subspecies (T. brucei rhodesiense and T. brucei gambiensie) cause sleeping sickness. Normally, an infected individual will show peaks and troughs in the number of parasites in their blood. See Figure 1.

Figure1

This is due to the immune system clearing the parasites from the blood and new variant parasites arising that have different properties. The surface of trypanosomes is almost completely covered by a molecule called the Variant Surface Glycoprotein (Figure 2). This coat also forms a barrier that prevents the immune system seeing or reaching important membrane proteins. This is important, because if the immune system could see and reach an essential parasite surface molecule, that does not vary, that could be the end of the infection.

Up to 10% of the Trypanosome genome may be devoted to different VSG genes. The proteins they encode invoke a strong antibody response and as this response develops, it kills the trypanosomes that express that particular VSG type. Some individual trypanosomes will suddenly change the VSG molecule they express at the surface. The result is that the immune system can no longer recognise them and has to generate antibodies to the new VSG. This process repeats itself and keeps the parasite about a week ahead of the immune system. This will continue until the host dies or receives chemotherapy. The energy costs involved in producing large amounts of antibody also affects the host and results in a reduced capacity to produce more antibodies. This means that the infection ultimately gets worse.

Now the god bit. Fundies who believe in a literal garden of Eden and talking snake claim there was no death and suffering before the fall, therefore no trypanosomes (and Tyrannosaurs ate plants!) One particular one that some readers will be familiar with wrote this sick piece on the boxing day tsunami in which he wrote “He [Darwin] makes one big mistake – he assumes that the world as it is now is the world as God created it. But that is not the case. When you read Genesis One notice the repeated refrain, ‘And God say that it was good’. God did not create the world to have natural disasters, cancer and death. Something came into the world which has upset the natural order of things and polluted the whole environment. That is why, as Paul tells us in Romans 8, the whole creation ‘groans as in the pangs of childbirth’. We are faced with two choices – either the world is as it is because that is the way things are, or things are the way things are because sin came in and corrupted a good and perfect creation.”

Apparently we are to blame, and Robertson later writes “What is the explanation and solution for all this? How can we explain and more importantly cope with this bent universe and our perverse human natures?”"

This is just total bollocks (no apologies for terminology, a spade is a spade) and contradicted by fossil tumours, fossil deformities, fossils with bites, fossils with their last meal of a conspecific inside them, evidence of volcanism and earth quakes and mass extinctions all considerably pre-dating the existence of man to name but a few. It really is pathetic that people hold this view.

Despite the total absurdity of the claim, there are also internal inconsistencies in the fundie argument – although it is also applicable to the loving god argument in general. The point is that supposedly only god creates and he does so with a purpose. Now, if there was no suffering or disease, then why invent an immune system? If he is loving, then why create a trypanosome that has “built in” methods of specifically overcoming an immune system? This leaves you with the problem of loving God designing something that could have no purpose because there was no disease, then creating something to overcome this non purpose and cause suffering. It’s too mental for words.