BIOL131 Lecture 12: Epidemiology PDF

BIOL131 Lecture 12: Epidemiology Epidemiology The study of factors, implicated in disease progression, that determine its frequency, distribution and severity in cohorts of individuals epi-demos-ology (greek) among-the people-study of World Health Organisation (WHO) – global epidemiological servic...

BIOL131 Lecture 12: Epidemiology Epidemiology The study of factors, implicated in disease progression, that determine its frequency, distribution and severity in cohorts of individuals epi-demos-ology (greek) among-the people-study of World Health Organisation (WHO) – global epidemiological service http://www.who.int/en/ ‘Epidemiological triangle’ of causal factors Consists of: • external agent • a susceptible host • an environment that brings the host and agent together • “disease results from the interaction between the agent and the susceptible host in an environment that supports transmission of the agent to that host” -Centers for Disease Control and Prevention https://www.cdc.gov/csels/dsepd/ss1978/lesson1/section1.html Factors in disease causation Agents: Host: • Bacteria • Viruses • Pathogens • Genetic susceptibility (e.g. BRCA1/2 & breast cancer) Environment: • Products of carbon combustion • Pesticides • Radiation • Smoking • Diet (malnutrition, mutagens) Campbell and Reece 10th Ed p338-334, p444, p982 Infectious agents ‘Spontaneous generation’ disproved in 1859 by Louis Pastor and John Tyndall Disease was thought to be contracted from ‘bad smells’ Yersina pestis – bacterium transmitted by flea (arthropod) vector = plague ‘Golden age of microbiology’ 1874-1917 - Hygiene, disinfection - Vaccination - Antibiotics Rate of disease spread • Endemic – diseases that occur at a constant rate within a given population • Epidemic – incidence of disease above the endemic rate Influenza virus • Pandemic (worldwide epidemic – new strains, little immunity) Campbell and Reece 10th Ed p622-24 History’s deadliest events Deaths (millions) • AIDS pandemic (1981-2016) HIV infections: >80 million • Influenza pandemic (1918-1919) Influenza virus • Black Death (1348-1350) Yersinia pestis (bacterium) >40 20-40 20-25 • World War II (1937-1945) 15.9 • World War I (1914-1918) 9.2 Influenza pandemic • 2009 Influenza A strain; subtype H1N1 (swine ‘flu) ~18,000 deaths, 4% of annual ‘flu deaths • Influenza A virus responsible for three previous pandemics during the 20th century: Spanish flu in 1918-19 (20-100x106 fatalities), Asian flu in 1957-58 (2x106) Hong Kong flu in 1968–69 (1x106). • Fatality rate is normally ‹0.1%; Spanish flu ›2.5% Influenza • Influenza A undergoes major genetic changes very rapidly, therefore immunity becomes redundant. • 2009 pandemic was a re-assortment of genes between human, avian and pig ‘flu strains. Antigenic Drift Antigenic Shift Case study video – Luverne, Minnesota https://www.youtube.com/watch?v=bKpLls0gjwE ‘Causal pies’ model of epidemiology Individual factors that contribute to cause disease are shown as pieces of a pie. Disease occurs once all the pieces of the pie are present. The factors that contribute to disease may include intrinsic host factors as well as the agent and the environmental factors of the agent-hostenvironment triangle. This is a more useful model to study non-communicable diseases like cancer where there is more than one causative factor. Genetic susceptibility Ethnic background has significant impact on disease susceptibility and resistance. Examples include: Sickle-cell anaemia which is more common in people of African or Mediterranean heritage Tay-Sachs disease (infant neurodegeneration) which is more likely to occur among people of Ashkenazi (eastern and central European) Jewish or French Canadian ancestry. Campbell and Reece 10th Ed p338-334 In the developed world… Over the course of the last century a transition in the major causes of death from infectious to chronic diseases has occurred. WHO warns of growing epidemic of premature death from ‘non-communicable diseases’ • Cancer • Coronary Heart Disease • Type II Diabetes Factors that influence disease distribution Person Location Age Geographic Gender Social Ethnic background Occurrence Education Episodic Occupation Cyclical Secular Sources of factors implicated in disease progression Genetics • • Diet • mutations • variation • carcinogens nutrition microflora Psychogenic Lifestyle • mental health • sedentary Malnutrition • developing world Environment • pollution Obesity • developed world Nature and nurture interact: impossible to specify quantitatively the contribution of either www.els.net Gene-Environment interactions: Sharon Kardia, 2007. Epidemiology of Cancer • Cancer is a leading cause of death worldwide and accounted for 7.6 million deaths (around 13% of all deaths) in 2008 – WHO, 2011 • Tobacco use is a major risk factor for cancer. Harmful alcohol use, poor diet and physical inactivity are other main risk factors. • More than 30% of cancer deaths can be prevented. • Deaths from cancer worldwide are projected to continue to rise to over 11 million in 2030. http://www.who.int/mediacentre/factsheets/fs297/en/ Is cancer just back luck? Cancers with environmental or genetic risk factors Tomosetti and Vogelstein Science 347: 78-81 2015 Carcinogenic effects of tobacco • In Britain, males born around 1900, took up smoking during First World War, by 1955 rate of lung cancer in men younger than 55 were highest in the world • Association of smoking with lung cancer is the most important discovery in the history of cancer epidemiology • Young at greatest risk • Carcinogenic effects also in pancreas, bladder and kidney Global variation in effect of smoking on disease Peto J. (2001) Cancer epidemiology in the last century and the next decade. Nature 411: 390-395 Other cancers where carcinogens are implicated • Colon • Asbestos • Stomach • Benzene • Bladder • Viruses (HPV, RSV) • Breast • Prostate Hazard symbol for carcinogenic chemicals • Aflatoxin B • Radiation • Pancreas Campbell and Reece 9th Ed. p373-377 www.els.net - Environmental Carcinogens and Mutagens: Edward Loechler, 2001. Effect of migration on cancer rate % people who developed prostate, colon, stomach and breast cancer in populations with similar genetic background • grey bar = population of Japanese people who moved to Hawaii • red & green bars = endogenous Japanese population • Acquire cancer incidence of host population Peto J. (2001) Nature 411: 390-395 • Best evidence that carcinogens of dietary and/or environmental origin play the most significant role in cancer causation, rather than genetic background (with the exception of specific mutations). Shift in disease type • Shift to lifestyle and age-associated afflictions, rather than infectious disease and malnutrition. • Find causative factors of chronic diseases through epidemiological studies Epidemiology: Mims Microbiology 4th Ed. p445-458 ‘Epidemiological transition’ The twentieth century, in spite of global wars and turmoil, has seen an unprecedented rise in human life expectancy. This substantial social achievement has led to concern over implications for public spending on old-age support and healthcare.