Lecture20%20Virulence%20evolution

Lecture20%20Virulence%20evolution - The Evolution of...

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3/31/2009 1 The Evolution of Virulence Why Bite the Hand that Feeds You? 1 Evidence for virulence evolution: Myxomatosis 2 Spread of rabbits in Australia: 1860-1920 3 Rabbits caused serious economic damage to rangeland and agricultural areas; most forms of rabbit control were ineffective 4 Isolated in S. American rabbits in 1920s Causes mild tumors in native hosts Myxoma virus: DNA virus in Poxvirus family Spread mechanically by mosquitoes and other insect vectors 5 Large lesions, facial swelling Severe case fatality rate Death within 14 days Causes severe disease in European rabbits Virulence grades for five strains (from lab trials): 6
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3/31/2009 2 Spread rapidly and caused widespread mortality Introduced to mainland in 1950 7 Initially high mortality rates followed by viral evolution Evolution of pathogen virulence Most deadly grade I gives way to grade III/IV 8 Australia 1950-81 Evolution of pathogen virulence Over time, the pathogen became less virulent, with Britain 1950-80 strains of intermediate virulence most common 9 • Today rabbit’s are much more resistant resistance increased Hosts can evolve, too resistant… resistance increased rapidly after virulence declined • The optimal virulence for a host is always ZERO 10 The evolution of parasite virulence What is a parasite? Biologically speaking: Any organism that in or on another organism, using energy and resources of that host organism, often to the detriment of its host Living beings that have to survive in their hosts and get to new ones. 11 Virulence (1) The case mortality rate of an infection (2) Pathogen-mediated reduction in host The evolution of parasite virulence fitness (3) The harm caused by pathogens to their hosts Why would a parasite cause harm to its host, on which it is so dependent? 12
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3/31/2009 3 Parasites evolve to become benign to their hosts Therefore, virulent parasites are ‘recent’ Conventional wisdom Ebola SARS HIV 13 Nature prefers that neither host nor parasite should be too hard on the other. For Nature, survival of the species is all that count Lingering misimpression 14 survival of the species is all that counts . -- Burnet and White, 1972, Natural History of Infectious
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Lecture20%20Virulence%20evolution - The Evolution of...

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