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Epidemiology of Infectious Diseases M. Tevfik DORAK (www)www.

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Presentation on theme: "Epidemiology of Infectious Diseases M. Tevfik DORAK (www)www."— Presentation transcript:

1 Epidemiology of Infectious Diseases M. Tevfik DORAK (www)www

2 Infectious Disease Epidemiology: Major Differences A case can also be an exposure Subclinical infections influence epidemiology Contact patterns play major role Immunity There is sometimes a need for urgency (www)www

3 Epidemiology Deals with one population Risk case Identifies causes Infectious disease epidemiology Two or more populations A case is a risk factor The cause often known (www)www What is infectious disease epidemiology?

4 Two or more populations Humans Infectious agents Helminths, bacteria, fungi, protozoa, viruses, prions Vectors Mosquito (protozoa-malaria), snails (helminths-schistosomiasis) Blackfly (microfilaria-onchocerciasis) – bacteria? Animals Dogs and sheep/goats – Echinococcus Mice and ticks – Borrelia What is infectious disease epidemiology? (www)www

5 A case is a risk factor … Infection in one person can be transmitted to others (www)www What is infectious disease epidemiology?

6 The cause often known An infectious agent is a necessary cause What is infectious disease epidemiology used for? Identification of causes of new, emerging infections, e.g. HIV, vCJD, SARS Surveillence of infectious disease Identification of source of outbreaks Studies of routes of transmission and natural history of infections Identification of new interventions (www)www What is infectious disease epidemiology?

7 Concepts Specific to Infectious Disease Epidemiology Attack rate, immunity, vector, transmission, carrier, subclinical disease, serial interval, index case, source, exposure, reservoir, incubation period, colonization, generations, susceptible, non-specific immunity, clone, resistance, repeat episodes …


9 Definitions Infectious diseases Caused by an infectious agent Communicable diseases Transmission – directly or indirectly – from an infected person Transmissible diseases Transmission – through unnatural routes – from an infected person Note Infections are often subclinical – infections vs infectious diseases! Antonyms not well-defined Non-communicable diseases – virus involved in pathogenesis of diabetes? Chronic diseases – HIV? TetanusMeasles vCJD Infectious Disease (www)www

10 Routes of transmission Direct Skin-skin Herpes type 1 Mucous-mucous STI Across placenta toxoplasmosis Through breast milk HIV Sneeze-cough Influenza Indirect Food-borne Salmonella Water-borne Hepatitis A Vector-borne Malaria Air-borne Chickenpox Exposure A relevant contact – depends on the agent Skin, sexual intercourse, water contact, etc (www)www

11 (www)www

12 Some Pathogens that Cross the Placenta

13 Modes of Disease Transmission

14 No infection Clinical Sub-clinical Carrier Death Carrier Immunity No immunity Outcome (www)www Exposure to Infectious Agents

15 Infection Susceptible Dynamics of infectiousness Dynamics of disease Incubation period Symptomatic period Non-diseased Latent period Infectious period Non-infectious Infection Time (www)www Timeline for Infection

16 Cases Index – the first case identified Primary – the case that brings the infection into a population Secondary – infected by a primary case Tertiary – infected by a secondary case P S S T Susceptible Immune Sub-clinical Clinical S T (www)www Transmission

17 Data from Dr. Simpsons studies in England (1952) MeaslesChickenpoxRubella Children exposed Children ill attack rate 251 201 0.80 238 172 0.72 218 82 0.38 Attack rate = ill exposed (www)www Person-to-Person Transmission

18 Disease is the result of forces within a dynamic system consisting of: agent of infection host environment Epidemiologic Triad

19 Agent Host Environment Age Sex Genotype Behaviour Nutritional status Health status Infectivity Pathogenicity Virulence Immunogenicity Antigenic stability Survival Weather Housing Geography Occupational setting Air quality Food (www)www Factors Influencing Disease Transmission

20 Infectivity (ability to infect) (number infected / number susceptible) x 100 Pathogenicity (ability to cause disease) (number with clinical disease / number infected) x 100 Virulence (ability to cause death) (number of deaths / number with disease) x 100 All are dependent on host factors Epidemiologic Triad-Related Concepts

21 Predisposition to Infections (Host Factors) Gender Genetics Climate and Weather Nutrition, Stress, Sleep Smoking Stomach Acidity Hygiene


23 Horton & Parker: Informed Infection Control Practice (www)www Chain of Infection

24 Peters: Tropical Medicine and Parasitology, 2001 (www)www Infection Cycle of Schistosomiasis

25 Lipoldova & Demand, 2006 (www)www Infection Cycle of Leishmaniasis

26 Iceberg Concept of Infection

27 Bacteria Viruses Fungi Protoctists / Protozoa Helminths Algae Prions (proteinaceous infectious agents) Infectious Agents

28 Phylogenetic Classification of Bacteria Oxford Textbook of Medicine

29 Phylogenetic Classification of Viruses Oxford Textbook of Medicine

30 Mabbott & MacPherson, Nat Rev Microbiol 2006 (www)www Prions (PRI-ons: proteinaceous infectious agents)

31 A host that carries a pathogen without injury to itself and serves as a source of infection for other host organisms (asymptomatic infective carriers) (www)www Reservoirs

32 Humans {hepatitis} Other Vertebrates {zoonoses} Birds & Bats {histoplasmosis} NOT vectors Reservoirs

33 Vectors A host that carries a pathogen without injury to itself and spreads the pathogen to susceptible organisms (asymptomatic carriers of pathogens)

34 Vectors Other parasites have life cycles that involve intermediate organisms, or vectors, which carry disease-causing microorganisms from one host to another. The protozoan blood parasite that causes sleeping sickness, or trypanosomiasis, infects humans, cattle, and other animals. It uses the tsetse fly as a vector to carry it from one host to the next. When a tsetse fly bites an infected animal, it picks up the parasite when it sucks blood. When an infected fly bites another animal, the parasite enters the bloodstream and begins to reproduce in the new host.

35 Arthropod Vectors Pathogen - Vector Viruses (Arbovirus) - Mosquitoes Bacteria (Yersinia) - Fleas Bacteria (Borrelia) - Ticks Rickettsias (R. prowazeki) - Lice, ticks Protozoa (Plasmodium) - Mosquitoes Protozoa (Trypanozoma) -Tsetse flies Helminths (Onchocerca) - Simulium flies

36 The same organism is present in every case It is isolated or grown in pure culture The disease can be reproduced in healthy animals after infection with pure culture The identical pathogen is reisolated from the experimental animals Kochs Postulates


38 Ecological Factors in Infections Altered environment {Air conditioning} Changes in food production & handling {intensive husbandry with antibiotic protection; deep- freeze; fast food industry} Climate changes {Global warming} Deforestation Ownership of (exotic) pets Air travel & Exotic journeys / Global movements Increased use of immunosuppressives/ antibiotics American Museum of Natural History Exhibition: Epidemic! The World of Infectious Disease (www)www

39 Infectious Disease Process Direct tissue invasion Toxins Persistent or latent infection Altered susceptibility to drugs Immune suppression Immune activation (cytokine storm)

40 Mathematical Modelling in Infectious Disease Epidemiology

41 A measure of the potential for transmission The basic reproductive number, R 0, the mean number of individuals directly infected by an infectious case through the total infectious period, when introduced to a susceptible population R 0 = p c d contacts per unit time probability of transmission per contact duration of infectiousness Infection will ….. disappear, if R < 1 become endemic, if R = 1 become epidemic, ifR > 1 (www)www Reproductive Number, Reproductive Number, R 0

42 Useful summary statistic Definition: the average number of secondary cases a typical infectious individual will cause in a completely susceptible population Measure of the intrinsic potential for an infectious agent to spread (www)www Reproductive Number, Reproductive Number, R 0

43 If R 0 < 1 then infection cannot invade a population – implications: infection control mechanisms unnecessary (therefore not cost-effective) If R 0 > 1 then (on average) the pathogen will invade that population – implications: control measure necessary to prevent (delay) an epidemic Reproductive Number, Reproductive Number, R 0

44 p condoms, acyclovir, zidovudine c health education, negotiating skills D case ascertainment (screening, partner notification), treatment, compliance, health seeking behaviour, accessibility of services R 0 = p c d (www)www Reproductive Number, Reproductive Number, R 0 Use in STI Control

45 p, transmission probability per exposure – depends on the infection HIV, p(hand shake)=0, p(transfusion)=1, p(sex)=0.001 interventions often aim at reducing p use gloves, screene blood, condoms c, number of contacts per time unit – relevant contact depends on infection same room, within sneezing distance, skin contact, interventions often aim at reducing c Isolation, sexual abstinence d, duration of infectious period may be reduced by medical interventions (TB, but not salmonella) (www)www What determines What determines R 0 ?

46 Immunity – herd immunity If R 0 is the mean number of secondary cases in a susceptible population, then R is the mean number of secondary cases in a population where a proportion, p, are immune R = R 0 – (p R 0 ) What proportion needs to be immune to prevent epidemics? If R 0 is 2, then R 0.50 If R 0 is 4, then R 0.75 If the mean number of secondary cases should be < 1, then R 0 – (p R 0 ) < 1 p > (R 0 – 1)/ R 0 = 1 – 1/ R 0 If R 0 =15, how large will p need to be to avoid an epidemic? p > 1-1/15 = 0.94 The higher R 0, the higher proportion of immune required for herd immunity (www)www

47 Endemic - Epidemic - Pandemic Endemic Transmission occur, but the number of cases remains constant Epidemic The number of cases increases Pandemic When epidemics occur at several continents – global epidemic Time Cases R = 1 R > 1 R < 1 (www)www

48 Endemic Epidemic Number of Cases of a Disease Time Endemic vs Epidemic

49 Levels of Disease Occurrence Sporadic level: occasional cases occurring at irregular intervals Endemic level: persistent occurrence with a low to moderate level Hyperendemic level: persistently high level of occurrence Epidemic or outbreak: occurrence clearly in excess of the expected level for a given time period Pandemic: epidemic spread over several countries or continents, affecting a large number of people (www)www


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