Virulence is known as the degree to which an organism destroys its host. On page 118, Paul Ewald is stated as being one of the pioneers of evolutionary biology, especially the evolution of infectious diseases and how pathogens select for/against traits that harm their hosts. Virulence relates to Big Idea 4 (biological systems interact, and these systems and their interactions possess complex properties) because pathogens are highly complex organisms that are in constant interaction with virtually every organism in the world. What does Ewald believe to be the key factor that determines virulence?
Pinworms and Ebola (page 118) represent the two extremes of infectious diseases. Research these two diseases and explain how they have evolved over time. Also, state how each disease travels from host to host, any epidemics that have occurred throughout history, symptoms, treatments, and mortality rate.
(Partha Ryali pryali4@students.d125.org)
On page 118, Dr. Moalem writes Paul Ewald’s observation on how virulence seems to be determined. According to Ewald, virulence is determined by how a parasite (or virus or bacterium) moves from host to host, or in other words, how it spreads itself. On the same page, we are given two examples of pathogens: pinworms, which has very low virulence and is nearly harmless to humans, and Ebola, which is one of the most aggressive, deadliest diseases known to man. They have clearly different mechanisms of evolution, virulence, and symptoms.
ReplyDeletePinworms are thought to have originated not in the Americas, but near Siberia at least 10000 years ago; they then likely migrated across a land bridge called the Beringia around 18000 years ago. This means that they were originally adapted to very cold conditions. However, as they migrated south, they evolved so that they could successfully enter the infective stage at much, much warmer temperatures, allowing them to infect humans in the warmer continents (in the manner that those with a mutation favoring survival in warmer conditions were more easily able to reproduce than those that did not have this mutation). Furthermore, there exists evidence suggesting that pinworms actually used human hosts as a means of traveling between continents (source: http://www.stanford.edu/class/humbio103/ParaSites2006/Enterobius/history.htm). As a side effect, this would mean that pinworms would naturally evolve methods of living off their human hosts to survive by adapting to conditions and resources within the human body, which could possibly explain the pinworm’s virulence as of today (they relied on the human’s ability to travel, which is why pinworm has very low virulence). Pinworms travel from host to host in a very simple manner. Humans can be infected if they ingest the eggs of the pinworm, which may occur if a person comes into physical contact with bed sheets of an infected person and then bites his/her fingernails without washing hands (sources: http://www.cdc.gov/healthywater/hygiene/disease/pinworms.html and http://www.dpd.cdc.gov/DPDx/html/Enterobiasis.htm). Although the condition is extremely common, there have been no documented cases of severe pinworm epidemics in human history. The most common symptom of pinworm infection is itchy anal region, but insomnia due to disturbed sleep, abdominal pain, and appendicitis may occur as well (source: http://www.cdc.gov/parasites/pinworm/disease.html). The disease may be treated with three steps of medication: mebendazole, pyrantel pamoate, and albendazole. The drugs are meant to eliminate the pinworms and prevent re-infection (source: http://www.cdc.gov/parasites/pinworm/treatment.html). Mortality rate in adults is near 0%.