BIO220H1 Lecture 7: Intro to Evolutionary Medicine and the evolution of virulence

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29 Jan 2019
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Introduction to Evolutionary Medicine and the evolution of virulence
• What is evolutionary medicine?
Three reasons for our vulnerability to disease
Virulence evolution
Questions in Medicine
What is causing damage or disease?
What functions or pathways are being disrupted?
How do we interfere with the organisms or pathways that are
causing disease or damage?
How do we repair any damage?
How can we protect against the causative agents of disease?
Evolutionary Medicine
Definition: Application of evolutionary principles to the problems of health
Approach: Asks why (ultimate) rather than how (proximate) questions
Utility?
Better understanding
= better prevention, treatment
Evolutionary Medicine
Diseases need both proximate explanations of underlying mechanisms as well as
ultimate explanations of why any vulnerabilities exist
Questions in Evolutionary Medicine
Why are we vulnerable to disease?
Why do we age?
Which symptoms of disease are harmful? Which are helpful?
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Is this response (e.g., fever, resistance, virulence) adaptive, and for whom?
When do we expect drug resistance to evolve?
Will vaccines lead to the evolution of pathogens?
When do we expect the evolution of extreme virulence?
Three reasons why we are vulnerable to disease
1. Tradeoffs. Structures and systems must balance conflicting demands
e.g DNA repair
pro: longevity
con: fecundity (cost, fewer resources)
e.g Immune defense
pro: Targets pathogens
con: Might target itself
2. Mismatch with current environments. Our environment has changed at a rate that
exceeds our rate of evolution.
3. Pathogen evolution. Pathogens evolve faster than we do.
(variation for selection to add on)
Virulence Evolution
Examples – the problem
Conventional Wisdom
Pathogen Adaptation
The tradeoff hypothesis
Note on terminology: parasite = pathogen
Ebola virus
Causes Ebola hemorrhagic fever, a severe and often fatal disease
Spread by direct contact with contaminated material (including from dead bodies)
High fever, headache, stomach and chest pain,
vomiting, and severe internal bleeding
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In 1976, the first Ebola outbreak in humans occurred in Zaire (now DRC)
and Sudan and then disappeared
~315 cases, of those 280 died (88% of cases)
Since then, there have been ~25 outbreaks in Africa.
Usually there are a small number of deaths, but also a
small number of cases (case fatality rates tend to be > 50%).
Ebola is extremely virulent
What explains variation in severity?
Ebola virus vs. Rhinovirus
Virulence
The additional mortality rate that a pathogen imposes on an infected
individual (the host)
(high virulencehigh mortality)
Conventional Wisdom
Over time the coevolution of pathogens and
their host will lead to a mutualistic association
Pathogens that harm their hosts thereby harm themselves
Instances of highly virulent pathogens are cases
where the host‐pathogen relationship is recent
H5N1, “Bird flu”
1997, deadly outbreak of bird flu reported in Hong Kong
Subtyping: H5N1 (first isolated from chickens in Scotland in 1950)
As of December 13, 2018: 860 human cases confirmed, 454 deaths
Virtually every case involves close contact with birds
Estimated case fatality rate > 50%
(Transmition of the virus is difficult but extremely lethal when getin human)
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