On page 120 Dr. Moalem talks about the
adaptations that parasites and bacteria have made to spread more
effectively and the concept of virulence. He says that certain
bacteria and parasites have evolved to bring the host to a point of
mobility to spread the the bacteria but keep the host in a relatively
healthy state. Other Bactria incapacitate the host and use the
symptoms of their presence to spread themselves like cholera. The
bacteria and parasites that incapacitate use other organisms to help
spread them as well like malaria and mosquito ( Big Idea 4 –
Biological systems interact, and these systems and their interactions
have complex properties.).
Describe how these relationships could
have developed over time and how they may have effected the evolution
of the bacteria and the organism. Also, using your book, explain why
the common cold may never evolve to incapacitate the host and what
disadvantages and advantages incapacitation may possess.
PART 1
ReplyDeleteOn page 118, Dr. Moalem describes three different ways a pathogen can survive and reproduce. One of the methods is that the bacteria/parasite moves from host to host through the close proximity in the air or direct contact. With this method, generally the pathogen doesn’t have a dangerous virulence; the bacteria/parasites want to keep their hosts alive and well enough so that they can transport them to the next host to survive and reproduce. Over time, this relationship develops from bacteria or parasites that are easily transported from one host to another. The bacteria or parasites that evolve to have low virulence and keep their hosts alive enough to allow for them to spread in number have developed this method because of how their hosts behave. For example, an STD like chlamydia survives and spreads very easily because of the behaviors of its hosts. The symptoms of chlamydia are sometimes not seen, and people don’t get treatment if they don’t know if they have chlamydia or not. The evolution of the bacteria that causes the disease of chlamydia has evolved to not have a high degree of virulence because the STD is so easily spread by human hosts as they continue to have unprotected sexual intercourse. The bacteria doesn’t need to kill its host to survive and reproduce because as the host is alive, he/she is helping the bacteria spread by continuing to have unprotected sexual intercourse as he/she doesn’t believe there is anything wrong with their body. This is just one example of a bacteria/parasite that didn’t need to kill its host to survive and reproduce, but how it evolved to do so applies to most bacteria/parasites that do not have a high degree of virulence.
http://www.cdc.gov/std/chlamydia/stdfact-chlamydia.htm
The second method of surviving and reproducing that pathogens use is through incapacitating the hosts. One example used is cholera, which is a disease that is spread through unprotected water supplies. This type of disease, which has a high virulence, has an easier mode of transportation (in the case of cholera, it’s the unprotected water supplies) and doesn’t need the host to be mobile and transport it around. Over time, diseases like cholera, which have an easier method of transporting itself, surviving, and finding new hosts, evolve to be more virulent towards its hosts. The bacteria/parasites that tend to destroy their hosts on their pathway to survive and reproduce evolve to not be dependent on the host as transportation. However, if the mode of transportation of the disease is no longer there (like if there were cleaner water supplies in areas with cholera), then the bacteria/parasite that causes the disease would no longer survive and would either die out or evolve to find a different way to spread.
http://www.netdoctor.co.uk/travel/diseases/cholera.htm
(Jenny S. Li - jesli4@students.d125.org)
PART 2
ReplyDeleteThe third method of transmitting bacteria/parasites from host to host is through the use of intermediate organisms to spread the disease. One disease that utilizes this method to spread is malaria, which uses mosquitoes. The reason why malaria has evolved to use mosquitoes as its method of transporting the disease is because mosquitoes bite anyone and can spread the disease to a variety of different people in areas with Anopheles mosquitoes (mainly in sub-Saharan Africa and some places in Asia and Latin America). Malaria is a disease that doesn’t need its host to be fully functional because mosquitoes do all the transporting of the disease. Without the mosquitoes, however, malaria would have no mode of transmission and would be forced to evolve in a different manner to survive and reproduce. With the use of mosquitoes (or any intermediate organism for this mode of transmission), malaria (or any other disease similar to malaria) has evolved to not need the host to survive as it has other means of transporting the parasites to other hosts. On page 119, it states that it’s better for malaria to kill its victims because more parasites in the blood of the hosts lead to mosquitoes sucking up more parasites and more infection for other hosts.
www.who.int/mediacentre/factsheets/fs094/en/index.html
The common cold uses the first method of transportation of its virus; it doesn’t kill its host to survive and reproduce. The reason why the cold has evolved to not kill its hosts is because the cold virus triggers various symptoms that have a definite purpose. On p. 110, it states that sneezes are triggered by the cold virus to spread in the air and contaminate other people. The cold still needs us to be alive for us to sneeze or touch things to spread the virus. It will probably never evolve to kill its hosts because its mode of transmission is successful enough so that death of the host is unnecessary. The advantages of killing the host may be that the virus could evolve and begin spread through some sort of intermediate organism as a mode of transportation or through contaminated food/water so that keeping the host alive would be superfluous. The disadvantage of killing the host is that then the host would be dead and couldn’t spread the virus through sneezing and coughing.
(Jenny S. Li - jesli4@students.d125.org)