The doctors informed Robert of the diagnosis. After they explained the cause of his illness, Robert asked “Will I be ok? Do you have a medication to kill Trypanosoma brucei?”“There is medication to treat this disease, Mr. Bragg,” said the doctor. “It’s called suramin. It is very effective at killing Trypanosoma brucei when given early enough in the disease process, but it can also cause severe side effects, including joint pain, severe weakness, light sensitivity and even loss of consciousness. We need to start your treatment at once despite these side effects because the disease has a high fatality rate if left untreated. Fortunately, you are not exhibiting signs of severe damage to your central nervous system, such as violent behavior, convulsions, or coma, so I think that we have caught the disease at an early enough stage for treatment to be successful. However, we will first examine your central nervous system (CNS) fluid for the presence of parasites to confirm that the disease has not progressed.”
“All right, doctor. Do what you have to… but is there any chance that I can recover from this parasite on my own, without risking the side effects of that medication?” A second doctor interjected: “Actually, the human immune system is somewhat capable of killing Trypanosoma brucei and lowering the number of parasites in the blood; however, the parasite has adapted a way to continually evade the immune system so that it can continue replicating.” “If we were to count the number of parasites in your blood every day,” explained the doctor, “we would likely notice that the parasite level would steadily increase for a period of time, perhaps one week, then the parasite level would fall drastically over one or two days as large numbers of parasites were killed by your immune system, only to rise again the following week. This trend would continue until you were given medication to clear the parasites, and would look like this if graphed.”
“I don’t understand,” said Robert. “If my immune system is capable of killing the parasites, why would the number of parasites in my blood repeatedly rebound in that way?” The doctor explained that in order for African trypanosomes to become successful parasites and survive in the bloodstream of their human hosts, they had evolved a mechanism to evade the host’s immune response. “African trypanosomes are covered by a protective coat containing proteins called variant surface glycoprotein (VSG). Although VSG helps protect the parasite, it’s also an antigen, which means it triggers the immune system to respond by making antibodies against it, which can lead to the destruction of the parasite. The genome of African trypanosomes contains many variations, or alleles, of the gene that encodes VSG. Only one allele is expressed at a time, but the parasite can vary which allele is expressed, allowing it to change its VSG coating as soon as the host’s immune system becomes effective at recognizing one particular variant of VSG.” The doctor continued: “Every spike in parasite levels in the graph represents a switch in VSG expression. It takes time for the immune system to adapt to each new VSG. Once it does, parasites are rapidly killed and parasite levels drop sharply, only to increase again after another round of VSG switching.”
What would happen if T. brucei suddenly loss the ability to undergo antigenic variation?
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