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Unformatted text preview: 1/17/2009 1 Population biology of Population biology of microparasites microparasites. I . I Catastrophic seal mortality in the Catastrophic seal mortality in the North Sea, 1988 North Sea, 1988 Harbor seal ( Phoca vitulina ) April1988: First record on Denmark Started in the Wadden sea Spread through Baltic & North sea Reached UK by late summer Epidemic subsided by winter Phocine distemper virus (PDV) Phocine distemper virus (PDV) Caused by a morbillivirus..like measles and canine distemper Spread through respiratory aerosol Harbor seals develop severe Harbor seals develop severe pneumonia Seals with damaged lungs cannot dive Many starve or die of secondary infections Recovered individuals become effectively immune Spread through Northern Europe Spread through Northern Europe Thousands of dead seals washed ashore Killed ~ 60% of each colony affected Seals haul out during summer months to breed Aggregations may help trigger outbreaks Seals move around a lot Seals & Seals & Phocine Phocine Distemper Virus Distemper Virus Counts of Dead Seals in The Wash (estuary on East coast of England) 250 300 350 400 ead Total fade out Never seen again. Until Summer 2002 50 100 150 200 Number found De Time Summer 2002 Summer 2002 After 1988, 14 years passed with no mortality due to PDV May 2002: large numbers of dead seals in Danish colonies By fall 2002: up to 30,000 seals were dead (60% of total population) Pattern consistent with re-introduction event 1/17/2009 2 Where did PDV come from? Where did PDV come from? Originated.. Most likely from Harp Seals 30% harp seals positive in Greenland Harp seals had carried virus even before 1988 Virus has little effect on harp seals Harp seal ( Phoca groenlandica ) Harp seals migrated from Arctic regions to Danish waters during 1988 in response to exploitation of Atlantic fisheries The questions The questions Why are there epidemics? When can an infectious disease establish? When can it persist? When can it persist? How can epidemics be prevented? Can we eradicate a pathogen? Under what conditions can pathogens regulate their hosts? Quantifying infections at the Quantifying infections at the population level population level Epidemiologists: measure and model disease changes over time and space Prevalence proportion of hosts infected or expressing disease (# Infected / total N) Incidence change in prevalence per unit time (number of new cases) What is an epidemic? What is an epidemic?...
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