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PATH 3610 (104)
Rob Foster (26)
Lecture 6

Lecture 6 notes

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University of Guelph
PATH 3610
Rob Foster

LECTURE 6 – VIRUS EVOLUTION Retrospective vs cohort • Understand the terminology Prospective studies (cohort, longitudinal) - Asks a specific study question (usually about how a particular exposure affects an outcome) - Recruits appropriate participants - Looks at exposures and outcomes of interest in these people over time Retrospective studies (case-control) - Relies on data/exposures and/or outcomes that have already been collected (through medical records or as a part of another study) - Data used in this way may not be as reliable as data collected prospectively as it relies on the accuracy of records made at the time and on people’s recall of events in the past, which can be inaccurate. Factors associated with the emergence of new viruses Emerging infections: two steps 1. Encountering new hosts a. Rare chance encounters of viruses with new hosts may never be detected b. Single-host infections are not transmitted among humans for many reasons 2. Expanding viral niches a. Successful encounters require access to susceptible and permissive cells b. Population density and health are important factors c. Virus populations will survive in nature only because of serial infections (a chain of transmission) Humans are constantly providing new ways to meet viruses: air travel, dams and water impoundments, irrigation, rerouting of wildlife migration patterns, wildlife parks, hot tubs, air conditioning, blood transfusion, day care centers. Human demographics, behavior, vulnerability  more people, more crowding, changing sexual habits, injection drug use, changing eating habits, more populations with weakened immune system. Economic development, land use, changing ecosystems  increasing population densities and urban poverty encourage the spread of water- and air-borne viruses; changing ecology influencing waterborne, vectorborne disease transmission; more exposure to wild animals and vectors. Technology, industry and modern agriculture  mass food production, contamination of watershed areas by cattle, use of antibiotics in food animals, new farming practices encourage human contact with rodents. International travel and commerce  persons infected with an exotic disease anywhere in the world can be in a major US city within hours; foods from other countries imported routinely into US; vectors hitchhiking on imported products. - Dramatic reductions in time to travel long distances have increased the possibility of global transport of infectious agents over short periods of time Microbial adaptation and change  jumping species from animals to humans, increased virulence, increased antibiotic resistance with increased use of antibiotics in humans and food animals Poverty, social inequality, breakdown of public health measures  lack of basic hygienic infrastructure, inadequate vaccinations, discontinued mosquito control efforts, lack of monitoring and reporting Mechanisms of viral evolution Drivers of virus evolution - Fast generation time - Large numbers of progeny o Infection of a single cell by poliovirus can yield up to 10 viral particles o When small populations of virus particles replicate, extreme fluctuations in genotype and phenotype are possible. - Large numbers of mutants o High mutation rates: genome replication is inaccurate – virus RNA and DNA polymerases are much more error prone o DNA viruses - genome replication not as error prone as RNA viruses, narrow host range, persistent infections common; most DNA viruses generate less diversity, evolve slower than RNA viruses. o Quasispecies – is a cloud of diverse non-iden
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