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Infectious Disease Epidemiology Tunisia, 27 October 2014 Prof. Dr. Alena Petrakova Acknowledgments: MediPIET Consortium FIIAPP-ISCIII and ECDC Learning Objectives • Define Infectious/Communicable Disease (ID) • Become familiar with concepts and basic ID terminology • List the ID transmission routes • Understand the ID occurrence in population and ID dynamics • Describe the tools to control ID 2 John SNOW John Snow (15 March 1813 – 16 June 1858) was an English physician and a leader in the adoption of anaesthesia and medical hygiene. He is considered one of the fathers of modern epidemiology, in part because of his work in tracing the source of a cholera outbreak in Soho, London, in 1854. His findings inspired fundamental changes in the water and waste systems of London, which led to similar changes in other cities, and a significant improvement in general public health around the world. 3 The risks affecting a population shift over the time Globalisation and its side-effects Demographic changes: Population growth, urbanization, increasing life expectancy at birth, etc. Changes in healthcare provision: (+): Better laboratory tests, better diagnostics, better treatment, etc. (-): Increased use of antibiotics - antimicrobial resistance, etc. Globalization of the food supply: Global transport of food and animals Changes in human behaviour: International travel, changes in dietary habits, intravenous drug use, unsafe sex, etc. Changes in environment and climate: Floods, hurricanes, droughts / increased human contact with reservoirs, insects Concepts Epidemiology (Lecture of Prof. Dr. Nissaf Bouafif) Infectious disease / Communicable disease Infectious agent Reservoir Susceptible host Source of infection Case Incubation period Epidemiology The study of the distribution and determinants of healthrelated states or events in specified human populations, and the application of this study to the control of health problems John Last, Dictionary of Epidemiology (1983, 1988, 1995, 2001) There are many definitions of epidemiology, but every epidemiologist will know exactly what it is that he or she does. Defining epidemiology is difficult primarily because it does not represent a body of knowledge, as does, for example, anatomy, nor does it target a specific organ system, as does cardiology. Epidemiology represents a method of studying a health problem and can be applied to a wide range of problems, from transmission of an infectious disease agent to the design of a new strategy for health care delivery. Infectious / Communicable disease A clinically manifested disease of humans or animals resulting from an infection „Communicable disease” means an illness caused by an infectious agent or its toxins that occurs through the direct or indirect transmission of the infectious agent or its products from an infected individual or via an animal, vector or the inanimate environment to a susceptible animal or human host. (CDC U.S.) Infectious agent An organism (virus, rickettsia, bacteria, fungus, protozoan or helminth) that is capable of producing infection or infectious disease Reservoir Any person, animal, arthropod, plant, soil or substance in which an infectious agent normally lives and multiplies, on which it depends primarily for survival, or where it reproduces itself in such manner that it can be transmitted to a susceptible host Susceptible host A person or animal nor possessing sufficient resistance to a particular infectious agent to prevent contracting infection or disease when exposed to that agent Source of infection The person, animal, object or substance from which an infectious agent passes to a host Case A person with an infectious disease of interest Primary case: Person who brings a disease into a population Secondary case: Persons who are infected by the primary case Index case: The 1st case to be discovered during an outbreak investigation Incubation period The time interval between initial contact with an infectious agent and the first appearance of symptoms associated with the infection Some special features A case may also be an exposure Person with infection can also be source of infection People may be immune Having had an infection or disease could result to resistance to an infection (immunity) A case may be a source without being recognized as a case Asymptomatic/sub-clinical infections There is sometimes a need for urgency Epidemics may spread fast and require control measures Transmission dynamics important for prevention and control Transmission routes Direct transmission Indirect transmission Mucous to mucous membrane Waterborne Across placenta Airborne Transplants, blood, etc. Foodborne Skin to skin Vectorborne Sneezes, cough Objects Hello Incubation period: 4-7 days (3-14 days) Dengue fever Flavivirus - Dengue virus 1-4 Human without immunity Human Mosquito bite Mosquito bite Vector borne - Aedes Hello Typhoid fever Incubation period: 8-14 days (3-60 days) Salmonella typhi Humans Humans Ingestion of water or food contaminated Faeces and urine Faecal-oral route Hello Incubation period: 10 days (2-30 days) Leptospirosis Leptospira interrogans Humans. Serovar Specific immunity Wild and domestic animals: rats, cattle, swine, etc Urine of infected animals Skin/mucous/tissues Contact /ingestion of urine or contaminated water Dynamics of disease and infectiousness Latent period Infectious period Incubation period Infection Clinical disease Onset of symptoms Non-infectious period Recovery Resolution of symptoms Time Dynamics of disease and infectiousness (ii) Second patient Transmission Latent period Incubation period First patient Clinical disease Transmission Latent period Incubation period Infection Infectious period Infectious period Clinical disease Serial interval or generation time Time Disease occurrence in populations Sporadic: occasional cases occurring at irregular intervals Endemic: continuous occurrence at an expected frequency over a certain period of time and in a certain geographical location Epidemic or outbreak: occurrence in a community or region of cases of an illness with a frequency clearly in excess of normal expectancy Pandemic: epidemic involves several countries or continents, affecting a large population Factors influencing disease transmission Agent Environment • Infectivity • Weather • Pathogenicity • Housing • Virulence • Geography • Immunogenicity • Occupational setting • Antigenic stability • Air quality • Survival • Food Host • Age • Sex • Genotype • Behaviour • Nutritional status • Health status Basic reproductive rate (R0) • Average number of individuals directly infected by an infectious case during her or his entire infectious period, when she or he enters a totally susceptible population – R0 < 1 - the disease will disappear – R0 = 1 - the disease will become endemic – R0 > 1 - there will be an epidemic 800 number of cases 700 600 500 400 300 200 100 0 1 2 3 4 generation 5 6 7 800 number of cases 700 600 500 400 300 200 100 0 1 2 3 4 generation 5 6 7 Herd immunity or Herd effect also called Community immunity • General immunity to a pathogen in a population based on the acquired immunity to it by a high proportion of members over time (even if some transmission may still occur) • Presence of immune individuals protects those who are not themselves immune Herd immunity threshold Minimum proportion (p) of population that needs to be immunized in order to obtain herd immunity p > 1 - 1/R0 e.g. if R0 = 3, immunity threshold = 67% if R0 = 16, immunity threshold = 94% Important concept for immunization programs and eradication of an infectious disease Vaccination coverage for elimination Critical proportion, Pc Pc = 1-1/Ro 100% 80% 60% 40% rubella measles 20% 0% 0 2 4 6 8 10 12 14 Basic reproduction number, Ro 16 18 20 Tools for Control of Communicable Diseases Isolation Treatment Vaccination Prophylaxis Disinfection Quarantine Surveillance Surveillance High quality national surveillance is the corner-stone of infectious disease prevention and control. - World Health Organisation, 2004 MediPIET: WHY? – WHEN? - HOW? April 2012: Technical Consultation Conference May 2013: Preparatory Technical Meeting (Kick-off-meeting) June 2013 – September 2014: Working together on deliverables September 2014: Final Project Meeting…Starting Implementation MediPIET 2013 - 2017 Useful References: • Heymann D: Control of Communicable Diseases Manual, 19th ed. 2008 • M´ikanatha NM, Lynfield R, Van Beneden ChA, de Valk H: Infectious Disease Surveillance, First ed.2007 • Anderson RM & May RM, Infectious Diseases of Humans: Dynamics and Control, 11th ed. 2006 • Barreto ML, Teixeira MG, Carmo EH: Infectious diseases epidemiology. J Epidemiol Community Health 2006; 60; 192-195 • Giesecke J: Modern Infectious Disease Epidemiology, 2nd ed. 2002 • McNeill, WH: Plagues and Peoples, 3rd ed. 1998 • Beaglehole R, Bonita R: Basic Epidemiology, WHO Geneva, 1993, reprinted in 2005 Thank you! Wishing you all full success! Prof. Dr. Alena Petrakova [email protected]