BIO220H1 Lecture 7: Intro to Evolutionary Medicine and the evolution of virulence
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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?
• 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
• 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 virulence→high 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)