Parasites are small organisms, often viruses or bacteria, that lives off of another organism, its host. From pages 118-123, various types of parasites are described in how they transport themselves from one host to another, to continue to survive and reproduce. Dr. Moalem gives the examples of cholera and malaria as two diseases that come from parasitic infection and explains how both diseases can decrease virulence in the pathogens. In relation to Big Idea 4, which is “biological systems interact, and these systems and their interactions possess complex properties”, research another type of disease that comes from a parasitic pathogen, describe how the pathogen manages to transport itself from host to host, explain to what extent this pathogen is virulent (define the word virulent in your answer), and describe a possible solution for making this disease less virulent.
Jenny S. Li - jesli4@students.d125.org
In addition to cholera and malaria, typhus is another disease that comes from a parasitic pathogen. Typhus is a bacterial disease spread by lice or fleas. Typhus is caused by one of two types of bacteria: Rickettsia typhi or Rickettsia prowazekii. The bacterium rickettsia typhi causes murine or endemic typhus – this bacterium is often spread by rats to fleas to humans. On the other hand, the bacterium Rickettsia prowazekii causes epidemic typhus – this bacterium is often spread by lice.
ReplyDeleteThe two kinds of bacteria (Rickettsia typhi or Rickettsia prowazekii) that result in symptoms of typhus when coming in contact with the host move from one host to another by hitching a ride on an intermediate organism. In this disease, the intermediate organism that the bacteria uses to transport itself from host to host is a flea. Because the bacteria involved in causing typhus do not rely on their hosts to carry them around and introduce them to new hosts, the damage done to a host as a result of these bacteria feeding on the host is not a concern. In fact, there is an evolutionary advantage for the typhus bacteria to harm their host as much as possible.
Dr. Moalem defines virulence as “the degree to which an organism destroys its host” (118). Because the bacteria that are responsible for causing typhus do not rely on their host to come in contact with another host, they perform great damage to the host and move on to infect their next host via the transportation of fleas. Thus, the bacteria involved in typhus are considered to be highly virulent due to their lack of reliance on their hosts to transport them to a new host. Paul Ewald, one of the pioneers of evolutionary biology, believes that “the key factor that determines virulence is how a given parasite gets from host to host” (118). Since the bacteria moves from one host to another with the help of fleas (and not humans, which are the hosts), the bacteria can kill its hosts along the way and proceed to do the same to another host by travelling on fleas. If it could be made possible so that the bacteria Rickettsia typhi or Rickettsia prowazekii depended on their hosts for transportation to new hosts, then the pathogen would be less virulent and harm the host at a much lesser degree.
(Tina Moazezi – tmoazez4@students.d125.org)
Lyme disease is a disease that involves a parasitic pathogen. According to webmd, Lyme disease is a bacterial infection transmitted by a tick. Blacklegged ticks, also known as Deer Ticks, infected with the bacterium Borrelia burgdorfen are the organisms responsible for the transmission of Lyme disease. These ticks usually feed on and mate on deer during part of their life cycle. The increasing deer population in the northeast and increasing suburban development has contributed the increasing number of Lyme disease cases.
ReplyDeleteDeer ticks that are infected with the bacterium B. burgdorfen are not highly virulent seeing as they require a host for transportation to other hosts. According the Dr. Moalem, virulence is “the degree to which an organism destroys its host” (Moalem 118). Originally, deer ticks move from one deer host to another. Since deer ticks are extremely tiny (adult ticks are the size of sesame seeds), their movement is greatly limited. In order to combat this obstacle, deer ticks simply move from one host to another. For the deer to be able to move around and come into close contact with another deer, the deer must, "be relatively healthy- certainly healthy enough to be mobile” (Moalem 118). Since deer ticks infected with B. burgodorfen require an intermediate organism for survival and reproduction, the pathogen is considered to have a low level of virulence. Even though the deer will be sickened and fatigued due to the bacterial infection, it will be healthy enough to act as a mode of transportation for the deer tick. The deer ticks have developed this selective advantage in order for it to survive and reproduce.
Once humans are infected with Lyme disease, the story is a little different. Since humans are relatively more active and intelligent, ticks can be easily removed. This inhibits the survival and reproduction of the deer tick. Similarly, “bulls eye” rashes, loss of facial muscle tone, fatigue, and dizziness are common symptoms of Lyme disease. These easily identifiable physical symptoms allow humans to identify Lyme disease and find and remove the tick. I agree with the comment above that pathogens would be less virulent if the host is harmed to a lesser degree. In this case, the pathogen would be more likely to survive and reproduce if the deer tick was less susceptible to discovery. If the deer tick were to evolve to an even smaller size or infect its host without the development of unique symptoms, the deer tick would be less prone to discovery. The pathogen would be less virulent as the host would not feel as sickened and irritated, and the pathogen would be able to spread more effectively.
http://www.cdc.gov/lyme/signs_symptoms/index.html
http://www.webmd.com/rheumatoid-arthritis/arthritis-lyme-disease
Weilly Tong (wtong4@students.d125.org)