BIOEE 1780 Lecture Notes - Lecture 33: Coevolution, Reassortment, Horizontal Gene Transfer
7 May 2018
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Virulence: the ability of a pathogen to cause disease; the harm inflicted by a pathogen, which is a
byproduct of growth and reproduction of the pathogen; within host selection
• Virulence is the highest when there is the highest mortality.
• Selection for between-host transmission is the highest when the population density is the lowest.
• Ex.) Plasmodium (causes malaria) evades the human immune system by adhering red blood cells to
arteries (prevents clearing in the spleen).
• There is a point at which pathogens become too virulent.
o If a host dies before the pathogen can be transmitted to a new host, the pathogen will die as
well.
o Loses its ability to become transmissible
• Less virulent pathogens have a greater chance for transmission.
• The virulence of a pathogen is influenced by 2 opposing agents of selection:
o Selection for within host replication
o Selection for between host transmission
• Evolution of Lower Virulence
o Improving hygiene
o Providing clean drinking water
o Wearing masks
o Getting vaccinations
• Evolution of High Virulence
o Overcrowding in prisons and refugee camps
o Breakdown of hygiene after natural disasters
o Rapid trans-global air travel
• Example: Myxoma virus and rabbit control in Australia
o The virus was introduced to control the rabbit population.
o Mortality:
• Year 1 - killed 99.8% of rabbits
• Year 2 - killed 90% of rabbits
• Year 3 - killed 40% of rabbits
• In just 3 years, the pathogen was evolving.
o RHDV was then introduced.
• Waterborne pathogens have increased mortality (highly virulent).
o In regions with the cleanest water, these pathogens will have lower virulence. Pathogen isn't
using water systems to be transmitted to new hosts.
o In regions with dirtier water, these pathogens will have higher virulence.
There's coevolution of virulence and antibiotic resistance.
• Increased virulence = more people seeking antibiotics → increased antibiotic resistance → causes
increased virulence
Resistance: the capacity of pathogens to defend against antibiotics or other drugs
• When antibiotics work properly, it is hard for resistance to evolve because all of the infecting
bacteria die.
• Sometimes patients fail to finish the full course of antibiotics, and resistant bacteria survive and can
continue to evolve as they move from person to person.
• Antibiotic resistance evolves very quickly - can be tracked within a single patient.
o Bacteria have evolved resistance to every antibiotic, often within a few years.
o Many bacteria evolve resistance to multiple drugs and show extreme resistance.
Document Summary
Selection for between-host transmission is the highest when the population density is the lowest: ex. ) Plasmodium (causes malaria) evades the human immune system by adhering red blood cells to arteries (prevents clearing in the spleen). There is a point at which pathogens become too virulent. If a host dies before the pathogen can be transmitted to a new host, the pathogen will die as well: loses its ability to become transmissible. Less virulent pathogens have a greater chance for transmission. The virulence of a pathogen is influenced by 2 opposing agents of selection: selection for within host replication, selection for between host transmission, evolution of lower virulence. In just 3 years, the pathogen was evolving: rhdv was then introduced, waterborne pathogens have increased mortality (highly virulent). In regions with the cleanest water, these pathogens will have lower virulence. Pathogen isn"t using water systems to be transmitted to new hosts. In regions with dirtier water, these pathogens will have higher virulence.
