Paul Ewald, an evolutionary biologist, believes that humans can coax high-virulence pathogens (such as malaria and cholera) to evolve so that they are more dependent on the human host to come into physical contact with his surroundings to spread. This, in essence, would cause these pathogens to evolve so that they have less virulence, and thus have less of an impact on their hosts (see pages 118-123). Ewald's idea connects primarily to Big Idea 1, the idea that the process of evolution drives the diversity and unity of life.
First, explain how the idea of a pathogen's virulence connects to Ewald's unique idea. Next, describe how this coaxing process would work from an evolutionary standpoint. Explain the mechanisms involved in the evolutionary process and be specific to Ewald's idea of forcing evolution. Use terminology from the evolution unit and connect back to Big Idea 1. Now, prove that you understand this process by finding a high-virulence disease (other than cholera or malaria) and providing a way to cause (over long periods of time) evolution in that disease so that it has much less of an impact on its human hosts. Use pages 119-122 for examples of how forced evolution in pathogens can be achieved, and page 118 for a list of high-virulence diseases. Be creative!
Use your Survival of the Sickest book, course content, and any outside resources to answer this question.
Jeeho Lee (jelee4@students.d125.org)
Jeeho Lee (jelee4@students.d125.org)
All organisms have a common goal: survive and reproduce. Similarly, any infectious pathogen has the same goal, but the level of virulence differs. Virulence is defined as, “the degree to which an organism destroys its host” (Moalem 118). Paul Ewald believes that the level of virulence of an infectious agent is determined by how a given parasite gets from host to host.
ReplyDeleteOn pg 118, Moalem looks at the three basic ways a microbe can move from host to host. Diseases with hosts in close proximity can be transmitted through the air or physical contact. These diseases have evolutionary pressures against virulence, because the microbes need to have their hosts to carry them and introduce them to other hosts for infection. This means the human hosts needs to be healthy enough to be mobile. The common cold, for example, evolved to a level of virulence that enables mobility of the host and the survival of the disease. A disease can also move from host to host via an intermediate organism like a mosquito, flea, or fly. In this situation, the host does not to be mobile in order to guarantee the survival of the disease. Malaria has evolved to keep its host vulnerable to attack by blood sucking mosquitoes. By rendering the host to the brink of death, there are more parasites in the blood of the host for transmission to be more successful in the next host. Finally, a microbe can attack another host by traveling through contaminated food or water. Cholera, again, doesn’t need its host to be mobile. The disease spreads easily through contaminated water sources. (continued...)
Weilly Tong (wtong4@students.d125.org)
Looking at the three modes transmission, the first way, which has evolutionary pressures against virulence, is the least deadly to the human host. Based on Ewald’s idea, transmission through physical contact must have low virulence because the parasite gets from host to host without incredibly harming the original host. Rather than having a, “kind of arms race in which we’re using one antibiotic weapon against the organism, and the organism evolves a defensive weapon against that antibiotic” (Moalem 122), Ewald’s theory relies on forcing evolution on the microbe in order to create mild version to an extent in which most people wouldn’t even know they’re infected. In order to make the high virulence diseases of malaria and cholera mild, we have to coax the disease to evolve into a milder form in which the host isn’t severely sickened. The basic idea lies in shutting down the modes of transmission. For example, by implementing mosquito nets for every malaria patient, mosquitoes would not be able to spread the malaria pathogen. Connecting back to Big Idea 1 and the evolution unit, that current strain of malaria would be selected against as there would be no way for it to survive and reproduce since it depends on transmission via mosquitoes. Natural selection would take its toll and microbes that possessed a favorable trait that allowed transmission to another host without an intermediate organism would have a selective advantage. Over time, some form of the advantageous trait would appear in the entire population (microevolution). Over a lot of time and successive events of natural selection, the population of microbes would possess the trait that allows it to spread to other hosts without the use of mosquitoes. In the end, it is a win-win situation. The disease would render humans uncomfortable, but it would not be detrimental to their health as the disease requires the host to be mobile for direct contact. Similarly, the disease would be able to survive and reproduce.
ReplyDeleteAnother highly virulent disease transmitted by contaminated supplies is Typhoid Fever. According to the Centers for Disease Control and Prevention, “Typhoid fever is a life-threatening illness caused by the bacterium Salmonella Typhi. Persons with typhoid fever usually have a sustained fever as high as 103° to 104° F (39° to 40° C). They may also feel weak, or have stomach pains, headache, or loss of appetite. In some cases, patients have a rash of flat, rose-colored spots”. Typhoid fever spreads through contact with food and drinks handled by a person with Salmonella Typhi and through drinking or coming into contact with contaminated water supplies. Transmission needs to occur through contaminated supplies as Typhoid Fever renders the victim immobile and weak. By looking at Ewalds theory to force evolution upon the virulent strain, implementing strict health regulations and cleaning water supplies will force the disease to evolve away from virulence. By protecting water supplies and eliminating contamination, transmission of the disease will be ineffective. Natural selection will take its toll and the strain of Typhoid fever with the trait that allows it to infect a host without transmission via contaminated supplies will be selected for. Over a large period of time, Typhoid Fever will have evolved to keep the host mobile in order for the disease to spread.
Weilly Tong (wtong4@students.d125.org)