- The air or physical contact. An example is the common cold.
- An intermediate organism. An example is malaria
- Contaminated food or water. An example is cholera.
Microbes that move through the first method rely on hosts to carry them an introduce them to new hosts. They need the host to be relatively healthy, so experts believe microbes such as the one that causes the cold will never evolve to kill us. Microbes that move through the second method, such as malaria, does not need our help to meet new hosts, but rather wants to incapacitate us so we can be attacked by mosquitos that do the transporting. It is an evolutionary advantage for the microbe to push us to the brink of death. Microbes that move through the third method does not need us to move around to find new hosts, but in the case of cholera, if it does not have a good delivery system such as an open water source it would want us to keep living so that it could keep living.
Relating this back to Big Idea 1: The process of evolution drives the diversity and unity of life, research 3 different diseases caused by microbes (one to fit in each category of transport). Explain how each microbe is transported, and the degree of virulence. If the disease benefits from us being incapacitated explain how we can change that (like in the case of cholera).
(Posted by Sriya Potham, spotham4@students.d125.org)
One disease that spreads through air of through physical contact is tuberculosis, caused by a strain of mycobacteria. According to a study by Kaye and Frieden in 1996, it is a highly virulent disease, with 60% of infected patients who do not receive modern medical attention dying. Furthermore, they estimated that about two million die each year from contracting TB every year, most of them in the Third World. This is an interesting counter-example to the generalization that Dr. Moalem makes on page 118. Even though the bacteria require the host to spread the virus, tuberculosis ultimately has a rather high mortality rate (three out of five infected patients without treatment die). The evolutionary reason for this is most likely the fact that the bacterium can be airborne, and can travel from infected patient to someone who is caring for t; in this way, the infected does not need to be mobile for the disease to spread – others will come to the one sick. The main way to prevent the spread is to wear protective clothing: face masks and gloves decrease the spread.
ReplyDeleteA disease that spreads through an intermediate organism is Lyme disease. It is caused by bacteria in the genus Borrelia and is spread by tics. This disease’s symptoms range from a rash (erythema migrans – the bull’s eye pattern) to eventual facial palsy, memory loss, to paraperisis (los of lower motor function). Lyme disease is rarely fatal. According to Kugeler, et. al, there was only one record consistent clinical manifestations of Lyme disease as a cause of death from 1999-2003. It virulence, therefore, is very low. It is treated with antibiotics. The most likely evolutionary reason for this is the fact that humans are not required for the bacterium to continue its life cycle.
Typhoid fever is a disease spread through water or food contaminated with an infected person’s fecal matter. It is caused by the bacterium Salmonella typhi. Symptoms include headakes, decrease in white blood cells, delirium, diarrhea, internal hemorrhages, and encephalitis. According to Butler et. al, with treatment, the fatality rate is about 3-6%, depending on gender. According to the New Zealand Communicable Disease Manual, the mortality rate without treatment is about 12-30%. This is a virulent disease, but not as much as tuberculosis. Such a lower mortality rate makes sense as fecal matter from the infected is necessary for the spread of the disease. As long as the patient is infected, the person can potentially spread the disease every time they need to excrete waste. As in the case of cholera, modern sanitation decreases the chance of getting the disease. Typhoid is most prevalent in India, as there is a large population of people, and not all have access to modern plumbing. This was the case in the example by Dr. Moalem on page 121: Ecuador, with poor water sanitation had a higher death rate than neighboring Peru, which had good water sanitation.
Kaye and Frieden, 1996:
http://epirev.oxfordjournals.org/content/18/1/52.full.pdf+html?ijkey=8943299fdfd10b41911cb25632d0ceed25fcc4a6&keytype2=tf_ipsecsha
Kugeler, et. al
http://cid.oxfordjournals.org/content/early/2010/12/27/cid.ciq157.abstract
New Zeland
https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=7&ved=0CFUQFjAG&url=http%3A%2F%2Fwww.health.govt.nz%2Fsystem%2Ffiles%2Fdocuments%2Fpublications%2Fcd-manual-typhoid-may2012.pdf&ei=7fhcUcTpK-qb2QWGj4HABw&usg=AFQjCNGl_w8siXHr5QVGoyk9SBmA4IpzzQ&bvm=bv.44770516,d.b2I&cad=rja
(Gabriel Antoniak gantoni4@students.d125.org)