BIO220-Slides-Lecture9-W2012

BIO220-Slides-Lecture9-W2012 - The Evolution of Virulence...

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1 The Evolution of Virulence II An overview of the evolution of virulence • The evolution of the influenza • Polio
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2 Three Stages in the Evolution of Virulence Stage 1: Accidental Infection Many pathogens can cause infections in novel hosts. • some may fail to cause secondary infections (e.g rabies, West Nile) • some may cause a short chain of infections and then quickly die out (Ebola, SARS) • Accidental infections can be particularly virulent, but are not necessarily so. Low virulence infections are probably underestimated. • No evolutionary prediction about their virulence is possible. • Accidental infections are rare.
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3 Three Stages in the Evolution of Virulence Stage 2: Virulence evolution soon after invasion • Successful invasion requires a chain of host to host transmission. • Initially this will be an epidemic (a rapid increase in the number of infected hosts). • The invading pathogen will be poorly adapted to its host and therefore there will be rapid evolution of the pathogen, including its virulence. • The rabbit – Myxoma story is a good example of this process.
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4 Three Stages in the Evolution of Virulence Stage 3: Evolution of optimal virulence • Evolution of the pathogen will slow as adaptation occurs • Near equilibrium, a “trade-off boundary” will be reached • The trade-off is between replication rate and life of infection • Total transmissions of the infection are expected to be maximized.
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5 Evolution of Influenza
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16 Marked annual cycle in seasonal flu outbreaks (Oz) year monthy mortality
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7 The death toll World Health Organization estimates respiratory illness killed 4 million people in 1999. ~ 1/4 - 1/2 million from flu Centre for Disease Control estimates 36,000/year in USA. Health Canada estimates 4-8000 people die in Canada of the flu each year. Death is usually due to associated illness
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8 Immunity and the flu most individuals recover from the flu and are thought to maintain lifelong immunity to that particular strain Evidence : 1977 re-emergence of a genetically identical variant of strain H1N1, which was last observed in 1957. Majority of illness occurred in people under the age of 20. Older individuals retained immunity from the 50’s.
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19 Rapid evolution at several sites NA and HA : surface proteins. Escape humoral response. These proteins determine the antigenic shift NA neuraminidase : allows escape from host cell and spread throughout body. HA haemagluttinin
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BIO220-Slides-Lecture9-W2012 - The Evolution of Virulence...

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