Download Adaptive Virulence - Honors 490 - Professor Penner

Samantha Johnson and Olivia Lunardo
Professor Penner
September 26, 2013
Group Notes # 6
Ebersole Lecture #2
Reading Highlights
Chapter 5: “When Water Moves like a Mosquito”
 Transmission by a cultural vector = transmission from immobile host to susceptible when
one characteristic is present in human culture
 Pathogen from infected to a susceptible
 Infected individual to susceptible
 Susceptible to the infected
 Virulent strains of diseases has greater time to survive on surfaces/vectors and cause greater
excretion of the pathogen from the host
 Areas where waterborne transmission can occur  V. Cholera are more virulent due to great
potential in invading and multiplying in humans
 Areas were waterborne transmission is less likely  less virulent V. Cholera
 More cost efficient and more accessible to take away mode of transmission (aka. modify
environment) to stop virulent pathogens from spreading than to create/give vaccines out
Chapter 6: “Attendant-Borne Transmission (Or How Are Doctors and Nurses like
Mosquitoes, Machetes, and Moving Water?)”
 Those who take care of others in need can become vectors
 Carry pathogens from patient to patient without becoming infected
 Direct contact or indirect through medical equipment, favor rapidly replicating and high
virulent pathogens
 Neonatal unit, diarrheal pathogen can infect infants with low defenses, on hands of
nurses, not easily washed off
 Provide pathogen with high fitness benefits, large number of contacts, must attend
infected babies often, causing increased dermal colonization and increased exposure
to pathogen
 Pathogen favors increase in virulence in attendant-borne, pathogens evolve in scale of
months and years in hospital environments
 Escherichia coli usually little negative effect on hosts, but certain strains can be more
dangerous
 Cultural vector hypothesis: virulent outbreaks typical in hospitals
 Due to increase in virulence there is a increase in severity of infections due to increase in
institutional increase
 Hospital pathogens more dangerous than those outside of institutions
 Hygiene not at the cause of transmission, instead isolation procedures and antibiotics
necessary
 Staphylococcus aureaus found in attendant-borne nurseries
 Does not often cause dangerous symptoms outside of hospital, led to infecting 500
patients in one hospital
 Becomes resistant to antibiotics due to constant exposure in hospital setting
 Handwashing increase and guidelines led to end of spread
 Infections due to direct introduction into bloodstream also very dangerous
 Genetic differences may be at the source for lethality and severity of institution based
pathogens compared to community based ones
 Indicator of severity of infection is if it is found in the bloodstream, most likely to happen
if caught in a hospital
 Attendant-borne transmission may cause evolutionary increase in virulence, virulence
falls due to procedures to block attendant-borne transmission
 Mid-nineteenth century, attendant-borne transmission still culprit
 Ignaz Semmelqweis noticed high infection rate in women attended by physician in child
birth compared to by midwives, thought it was due to them examining dead women
before aiding in labor
 Strict hand washing routine caused reduction in mortality, idea of post mortem
transmission
 3-pronged approach to reduction
 Stringent hygiene standards to stop transmission
 Increase staff-to-patient ratios
 use appropriate rates of hand washing with gloves
 Breast feeding and skin-to-skin contact with healthy mothers and babies for benign
protective bacteria
 Benign organisms from mother during breast feeding better than inoculation
 Room baby with mother to promote breast feeding and less attendant contact
 Antibiotics used selectively
 Only use with dangerous strains, not mild strains
 Lower evolution rate of mild strains
 Promising results due to interventions, reduces virulence of disease
 Can be applied to elderly homes, kennels, and agricultural situations
 Pathogens able to be transmitted in multiple attendant situations
Class Notes
 4 Horsemen of the Apocalypse: War, Death, Famine, Pestilence
 Characters will vary slightly depending on the time period
 Pestilence first shown as conquest in the bible
 First view that people see about disease  demonstrates how we personify
 4 Conceptual Views of Disease
 1. Undefeatable Foe (ex. Horsemen, Greek Gods, God’s Will)
 Disease personified, disease set on Earth to afflict us
 2. Evolutionary Thinking
 Resulting from biological evolution by Darwin
 Microbes as disease discovered by Pastor

1940s-50s pathogens became resistant in hospitals
 Cleanliness neglected, infestation, and resistance due to antibiotics due to constant
exposure to them
 Push on cleanliness in maternity wards
 Evolutionary arms race – humans vs. pathogens
 3. Direction of Evolution of Disease
 60s-70s, main pressure for evolution was to become benign, even beneficial to our
bodies through natural selection
 Thought that due to natural selection they do not want to kill us so strong selection for
microbes to become benign
 Virulent maladaptive, gradually disappear and replaced by less harmful bacteria
 Pathogens: organisms that generate harm
 4. Adaptive Virulence
 Disease does not always evolve to be benign
 Adaptive Virulence: most organisms are as virulent as they are due to
equilibrium
 Ewald’s theory
 1. Realized nature of virulence: not just waste products of microbe or due to
location, but by multiplication rate
 Big population=More severe sickness
 Virulence= dn/dt
 2. Nature of Fitness in Parasites
 Fitness: relative capacity to produce offspring
 Different for parasites, need to infect new hosts not just reproduce, host will
eventually expire and there is a constant need for a new one to survive
 Virulence dependent on a cost benefit relationship, virulence never 0, optimal
virulence where the costs and benefits are the furthest apart when graphs
 Optimal virulence individual for each pathogen
 3. Mode of Parasitic Transmission had big affect of cost/benefit curve
 Vector versus contact disease
 Contact to contact = most disease, host necessary
 Vector = middle agent used to carry (fly, mosquito, etc) and have different
fitness needs, host not as important
 Adaptive virulence: vector disease is more virulent than contact disease
 Examples: malaria, sleeping sickness, lime disease, yellow fever
 Some disease can be both: Bubonic plague
o-observed
c-control
Contact
Vector
Total diseases
High Virulance
Low Virulance
Total
5 o/ 10.7 c
40 o/ 34.3 c
45
10 o/ 4.3 c
8 o/ 13.7 c
18
15
48
63
Reactions:
It’s interesting to think about how we perceived diseases and how they are portrayed
from early civilizations to now. As we discussed with Professor Ebersole the timeline of how
humans came to understand disease has been ever developing and is filled with major steps
forward and even further steps back. The timeline can be seen even in other discussions that we
have had. One example would be a discussion with Professor Carlon on the Iatros reading.
People from that time period believed it was the Greek Gods who caused the disease because
they would get upset. And even in today’s society we make references to the different
perspectives that people have in regards to the disease. Sometimes one of the conceptual views is
part of a storyline of a TV episode (such as the 4 horsemen in Charmed) or a conceptual view is
taught in class. In Olivia’s pharmacology class, the idea of an evolutionary arms race by creating
new drugs against pathogens is quite present.
Despite the changing views on pathogens, one thing that seems to have followed them is
their personification. From their personification in the 4 Horseman, to their ability to move
amongst hosts, it seems that pathogens have always had certain characteristics that have been
attributed to humans, such as conquest. It almost seems fitting that pestilence was first conquest,
since disease is the constant attempt to conquest the host. From the undefeatable foe to the
current focus on evolutionary direction, pathogens seem to have a connection to our own
evolution.
Their evolution is connected directly to ours, which may be attributed to why is continues
to be personified. With changes in our own hygiene and hospital standards, pathogens have had
to evolve in certain ways to infect new hosts and continue their survival. As we have culturally
evolved, they have evolved as well. It is interesting to think that their evolution is partly depend
on our own.
Other reactions brought up in class included a student pointing out that humans can be
like diseases as we try to adapt to new environment. Professor Ebersole responds that as long as
humans can manage their environment, they can live on and on unlike a disease where it
constantly has to infect a new host. Another student asks what is a successful parasite, the benign
or virulent one? Ebersole answers that both are as long as it is producing over time.