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Epidemiology of Infectious Diseases

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Presentation on theme: "Epidemiology of Infectious Diseases"— Presentation transcript:

1 Epidemiology of Infectious Diseases
M. Tevfik DORAK (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)

3 What is infectious disease 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)

4 What is infectious disease epidemiology?
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 (www)

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

6 What is infectious disease epidemiology?
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)

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 Infectious Disease Definitions Note 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? Tetanus Measles vCJD (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)

11 (www)

12 Some Pathogens that Cross the Placenta

13 Modes of Disease Transmission

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

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

16 Transmission 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 T Susceptible Immune Sub-clinical Clinical (www)

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

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

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

20 Epidemiologic Triad-Related Concepts
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

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


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

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

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

26 Iceberg Concept of Infection

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

28 Phylogenetic Classification of Bacteria
Oxford Textbook of Medicine

29 Phylogenetic Classification of Viruses
Oxford Textbook of Medicine

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

31 (asymptomatic infective carriers)
Reservoirs 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)

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

33 (asymptomatic carriers of pathogens)
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 Koch’s Postulates 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

37 Koch’s 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)

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 Reproductive Number, R0 R0 = p • c • d
A measure of the potential for transmission The basic reproductive number, R0, the mean number of individuals directly infected by an infectious case through the total infectious period, when introduced to a susceptible population R0 = p • c • d probability of transmission per contact duration of infectiousness contacts per unit time Infection will … disappear, if R < 1 become endemic, if R = 1 become epidemic, if R > 1 (www)

42 Reproductive Number, R0 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)

43 Reproductive Number, R0 If R0 < 1 then infection cannot invade a population implications: infection control mechanisms unnecessary (therefore not cost-effective) If R0 > 1 then (on average) the pathogen will invade that population implications: control measure necessary to prevent (delay) an epidemic

44 Reproductive Number, R0 R0 = p • c • d
Use in STI Control R0 = p • c • d p condoms, acyclovir, zidovudine c health education, negotiating skills D case ascertainment (screening, partner notification), treatment, compliance, health seeking behaviour, accessibility of services (www) 8

45 What determines R0 ? 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)

46 Immunity – herd immunity
If R0 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 = R0 – (p • R0) What proportion needs to be immune to prevent epidemics? If R0 is 2, then R < 1 if the proportion of immune, p, is > 0.50 If R0 is 4, then R < 1 if the proportion of immune, p, is > 0.75 If the mean number of secondary cases should be < 1, then R0 – (p • R0) < 1 p > (R0 – 1)/ R0 = 1 – 1/ R0 If R0 =15, how large will p need to be to avoid an epidemic? p > 1-1/15 = 0.94 The higher R0, the higher proportion of immune required for herd immunity (www)

47 Endemic - Epidemic - Pandemic
Time Cases R > 1 R = 1 R < 1 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 (www)

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

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)


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