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Infectious Disease Epidemiology
This document provides an overview of infectious disease epidemiology. It begins with a brief history of some major infectious disease outbreaks and their impacts. It then discusses concepts and definitions relevant to infectious disease epidemiology, including reservoirs, modes of transmission, epidemiological triad, and terminology. The document outlines the importance of studying infectious disease epidemiology and highlights current challenges like antimicrobial resistance and emerging/re-emerging pathogens. It also summarizes successes in disease eradication/elimination and the ongoing global burden of infectious diseases.
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Infectious Disease Epidemiology
- 1. Infectious Disease Epidemiology LtCol A S Kushwaha
- 2. • Infectious diseases- History • Why study Infectious diseases • What is infectious disease epidemiology • Concepts / definitions – IDE
- 3. HISTORY • WORST SCOURGESOF MANKIND • THREAT TO HUMAN SURVIVAL • KILLED MORE MEN THAN ALL THE WARS • CAHNGED THE COURSE OF HISTORY 10/15/2013 3
- 4. • 14th century •1819• 1831 • 1854-56 • 1899-1902 - - Europe - plague kills 20-45 % of the world’s population - 50 million deaths due to H1N1 spanish Flu - Cairo – 13 % of population succumbs to cholera - Crimean war – deaths due to dysentery were 10 times higher than deaths due to casualties Boer War – deaths due to dysentery were 5 times higher than deaths due to casualties
- 6. INFECTIONS • Pre 19thCentury – Era of Microbes (we didn’t know much about them) • 20th Century – Man – upper hand Vs Microbes (antibiotics, vaccines, safe water, sanitation)
- 7. Because infectious diseaseshave been largely controlled in the United States, we can now close the book on infectious diseases.” William Stewart, MD U.S. Surgeon General, 1967
- 8. Microbes are smarter Microbesand vectors swim in the evolutionary stream, and they swim faster than we do. Bacteria reproduce every 30 minutes. For them, a millennium is compressed into a fortnight. They are fleet afoot, and the pace of our research must keep up with them, or they will overtake us. Microbes were here on earth 2 billion years before humans arrived, learning every trick for survival, and it is likely that they will be here 2 billion years after we depart (Krause 1998).
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- 11. 21st Century • Microbesare back in news • Resistance • Newer pathogens • Changing environment – warming • Bioterrorism
- 12. SUCCESSES • Eradication ofSmallpox in 1977 • Elimination of Poliomyelitis from the Western Hemisphere in 1994 • Potential elimination of global poliomyelitis in the next 5 to 10 years • Potential elimination of measles in the next 10 to 20 years • Vaccines in development for prevention of diarrheal diseases, cervical cancer (HPV)
- 13. Mortality due toinfectious diseases
- 15. CHALLENGES • More pathogenshave been identified than the drugs developed • Many pathogens no longer respond to drugs • Human activity has accelerated this imbalance • HIV
- 16. Wonder drugs toSuper bugs
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- 19. Emerging & Re-emerginginfections
- 21. Global Burden ofinfectious diseases • One death in three of the 54 million deaths worldwide is from an infectious cause • Virtually all of these deaths are in developing areas of the world – mainly India and sub-Saharan Africa • Disproportionately affect children • Many of the developing world deaths are due to preventable causes – – – – Pneumonia and Diarrhea – account for 40% of these deaths Tuberculosis Measles Malaria
- 22. • Of theseven biggest killers worldwide, TB, malaria, hepatitis, and, in particular, HIV/AIDS continue to surge – HIV/AIDS and TB likely to account for the overwhelming majority of deaths from infectious diseases in developing countries by 2020 – Acute lower respiratory infections, diarrheal diseases and measles appear to have peaked at high incidence levels
- 23. Emerging AIDS RelatedInfectious Diseases 1. Pneumocystis carinii pneumonia 2. Tuberculosis 3. Mycobacterium-avium complex 4. Kaposi’s sarcoma (HHV-8) 5. HSV-2 6. Cryptosporidium 7. Microsporidium 8. Cryptococcus neoformans 9. Penicillium marneffei 10. Disseminated salmonella 11. Bacillary angiomatosis (Bartonella henselae) 12. HPV
- 24. Some Emerging Non-AIDS related Infectious Diseases 1. SARS 2. West Nile disease 3. Variant CJD disease 4. Monkey pox 5. Ebola and Marburg viruses 6. Dengue 7. Influenza H5/N1 (?) 8. Hanta virus 9. E. Coli O157 :H7 10. Antibiotic-resistant – Pneumococci – Staph-aureus – Gonococci – Salmonella 11. Cryptosporidium 12. Anthrax
- 25. Millions of deaths,worldwide, all ages,
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- 28. 10/15/2013 28 Charrel et al.2007. N Engl J Med 356;8
- 29. Factors Leading toEmergence of Infectious Diseases • • • • • • AIDS Population growth Speed and ease of travel Dam building Global climate change Increased antibiotic use for humans and animals • Encroachment of human populations on forest • Industrial commercial agriculture • War and social disruption • Relocation of animals • Growth of daycare • Aging of the population • Human-animal contact
- 30. Emerging Infectious DiseasesRelated to Animal Contact Disease 1. HIV-1, LHIV-2 2. Influenza 3. Ebola 4. Marburg 5. Hanta virus 6. Arena virus 7. Variant CJD 8. Cryptosporidia 9. Hendra virus 10. Nipah virus 11. SARS 12. Monkey pox Animal Primates (Africa) Water fowl, pigs, chickens (N5/N1) Primates (“bush meat”) Primate (“bush meat”) Deer mouse Various rodents Cattle Cattle Fruit bats Pigs, fruit bats Civet cats Prairie dogs
- 31. Epidemiology • Study ofdistribution & determinants of disease and health related events and its application in control and prevention.
- 32. What is infectiousdisease epidemiology? • • • • 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)
- 33. Importance of StudyingCommunicable Diseases Epidemiology • Changes of the pattern of infectious diseases • Discovery of new infections • The possibility that some chronic diseases have an infective origin.
- 34. What is infectiousdisease 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)
- 35. What is infectiousdisease epidemiology? A case is a risk factor … Infection in one person can be transmitted to others (www)
- 36. Concepts Specific toInfectious Disease Epidemiology Attack rate, immunity, vector, transmission, carrier, subclinical disease, serial interval, index case, source, exposure, reservoir, incubation period, colonization, generations, susceptible, nonspecific immunity, clone, resistance, repeat episodes …
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- 39. Objectives • The epidemiologictriad • Definition of communicable diseases • Importance of studying communicable diseases epidemiology • Terminology • Dynamics of disease transmission (chain of infection): – Human reservoir or source – Modes of transmission – Susceptible host
- 40. Epidemiologic triad •Demographic characteristics •Biologicalcharacteristics •Socioeconomic characteristics Host Agent •Biological agents •Physical agents •Chemical agents •Nutrient agents •Mechanical agents •Social agents Environment •Physical environment •Biological environment •Social environment
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- 42. Definition of communicablediseases • A communicable disease is an illness due to a specific infectious (biological) agent or its toxic products capable of being directly or indirectly transmitted from man to man, from animal to man, from animal to animal, or from the environment (through air, water, food, etc..) to man.
- 43. Importance of StudyingCommunicable Diseases Epidemiology • Changes of the pattern of infectious diseases • Discovery of new infections • The possibility that some chronic diseases have an infective origin.
- 44. Terminology and Definitions • • • • • • • • • • Infection Contamination Infestation Host Contagiousdisease Epidemic Endemic Hyperendemic Holoendemic Pandemic • Exotic • Sporadic • Zoonosis, epizootic and enzootic • Nosocomial infection • Opportunistic infection • Eradication • Elimination
- 45. Infection • Infection isthe entry and development or multiplication of an infectious agent in the body of man or animals. • An infection does not always cause illness. • There are several levels of infection (Gradients of infection): – – – – Colonization (S. aureus in skin and normal nasopharynx) Subclinical or inapparent infection (polio) Latent infection (virus of herpes simplex) Manifest or clinical infection
- 46. Contamination • The presenceof an infectious agent on a body surface, on or in clothes, beddings, toys, surgical instruments or dressings, or other articles or substances including water and food.
- 47. Infestation • It isthe lodgment, development and reproduction of arthropods on the surface of the body or in the clothing, e.g. lice, itch mite. This term could be also used to describe the invasion of the gut by parasitic worms, e.g. ascariasis.
- 48. Host • A personor an animal that affords subsistence or lodgement to an infectious agent under natural conditions. • Types include: – an obligate host- the only host – definitive (primary) host-attains maturity or passes sexual stages – intermediate host-passes asexual stage – transport host-does not undergo development
- 49. Contagious disease • Acontagious disease is the one that is transmitted through contact. Examples - scabies, trachoma, STD.
- 50. Epidemic • “The unusualoccurrence in a community of disease, specific health related behavior, or other health related events clearly in excess of expected occurrence” • (epi= upon; demos= people) • Epidemics can occur upon endemic states too.
- 51. Endemic • It refersto the constant presence of a disease or infectious agent within a given geographic area or population group. It is the usual or expected frequency of disease within a population. • (En = in; demos = people)
- 52. Endemic - Epidemic- Pandemic R>1 R=1 R<1 Time 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)
- 53. Number of Casesof a Disease Endemic vs Epidemic Endemic Time Epidemic
- 54. Hyperendemic and holoendemic •The term “hyperendemic” expresses that the disease is constantly present at high incidence and/or prevalence rate and affects all age groups equally. • The term “holoendemic” expresses a high level of infection beginning early in life and affecting most of the child population, leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (e.g. malaria)
- 55. Sporadic • The wordsporadic means “scattered about”. • Cases - irregularly, haphazardly and generally infrequently. • Cases - few and separated widely in time and place e.g. polio, meningococcal meningitis, tetanus…. • May be starting point of an epidemic
- 56. Pandemic and Exotic •An epidemic usually affecting a large proportion of the population, occuring over a wide geographic area such as a section of a nation, the entire nation, a continent or the world, e.g. Influenza pandemics(1918,1957 & 2009). • Exotic diseases are those which are imported into a country in which they do not otherwise occur, as for e.g., rabies in the UK, Yellow fever in India, CCHF
- 57. Zoonosis, Epizootic andEnzootic • Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man, e.g. rabies, plague, bovine tuberculosis – Anthropozoonoses e.g., Rabies – Zooanthroponoses e.g., Human TB in cattle – Amphixenosis e.g., T.cruzi • An Epizootic is an outbreak (epidemic) of disease in an animal population, e.g. Rift valley fever, Anthrax. • An Enzootic is an endemic occurring in animals, e.g. Bovine TB.
- 58. Nosocomial infections • Nosocomial(hospital acquired) infection is an infection originating in a patient while in a hospital or another health care facility. It has to be a new disorder unrelated to the patient’s primary condition. E.g., infection of surgical wounds, hepatitis B and urinary tract infections.
- 59. Opportunistic infection • Thisis infection by organisms that take the opportunity provided by a defect in host defense (e.g. immunity) to infect the host and thus cause disease. • E.g., opportunistic infections are very common in AIDS. Organisms include Herpes simplex, cytomegalovirus, M. tuberculosis etc.
- 60. Iatrogenic (Physician induced)Disease • Any untoward or adverse consequence of a preventive, diagnostic or therapeutic regimen or procedure that causes impairment, handicap, disability or death resulting from a physician’s professional activity or from professional activity of other health professionals. • E.g., reaction to penicillin, hepatitis B infection following blood transfusion.
- 61. Eradication • Termination ofall transmission of infection by the extermination of the infectious agent through surveillance and containment. Eradication is an absolute process, an “all or none” phenomenon, restricted to termination of infection from the whole world.
- 62. Elimination • The termelimination is sometimes used to describe eradication of a disease from a large geographic region. Disease which are amenable to elimination in the meantime are polio, measles, leprosy and diphtheria.
- 63. Dynamics of diseaseTransmission (Chain of Infection) I Source or Reservoir II Modes of transmission III Susceptible host
- 64. (I): Source orReservoir • The source of infection is defined as “the person, animal, object or substance from which an infectious agent passes or is disseminated to the host. • The reservoir is “any person, animal, arthropod, plant, soil, or substance, or a combination of these, in which an infectious agent normally lives and multiplies, on which it depends primarily for survival, and where it reproduces itself in such a manner that it can be transmitted to a susceptible host. • It is the natural habitat of the infectious agent.”
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- 66. Human Reservoir 1. AIDS(HIV infection) 2. Syphilis 3. Gonorrhea 4. Shigellosis 5. Typhoid fever 6. Hepatitis-B virus 7. Herpes simplex virus
- 67. Animal Reservoir (Zoonoses) 1.Nontyphoidal salmonellosis 2. Brucellosis 3. Anthrax 4. Listeriosis 5. Viral encephalitis (SLE,WEE, CEE) 6. Rabies 7. Plague
- 68. Soil Reservoir 1. Histoplasmosis 2.Coccidioidomycosis 3. Blastomycosis 4. Tetanus 5. Botulism
- 69. Water Reservoir 1. Pseudomonasinfections Sepsis, UTI, “hot tub” folliculitis 2. Legionnaires’ disease 3. Melioidosis
- 70. Human Reservoir Human reservoir Cases •Primarycase •Index case •Secondary cases According to spectrum of disease: •Clinical cases (mild/severe-typical/atypical) •Sub-clinical cases •Latent infection cases Type: •Incubatory •Convalescent •Healthy Duration: •Temporary •Chronic Carriers Portal of exit: •Urinary •Intestinal •Respiratory •others
- 71. Exposure to InfectiousAgents No infection Death Clinical Carrier Sub-clinical Immunity Outcome Carrier No immunity
- 72. Cases • A caseis defined as “a person in the population or study group identified as having the particular disease, health disorder, or condition under investigation”
- 73. Carriers • • • It occurs eitherdue to inadequate treatment or immune response, the disease agent is not completely eliminated, leading to a carrier state. It is “an infected person or animal that harbors a specific infectious agent in the absence of discernible (visible) clinical disease and serves as a potential source of infection to others. Three elements have to occur to form a carrier state: 1. 2. 3. The presence of the disease agent in the body. The absence of recognizable symptoms and signs of disease. The shedding of disease agent in the discharge or excretions.
- 74. Animal reservoirs • Zoonosisis an infection that is transmissible under natural conditions from vertebrate animals to man, e.g. rabies, plague, bovine tuberculosis. • There are over a 100 zoonotic diseases that can be conveyed from animal to man.
- 75. (II): Modes oftransmission Mode of transmission Direct transmission Direct contact Droplet infection Contact with soil Inoculation into skin or mucosa Trans-placental (vertical) Indirect transmission Vehicle-borne Vector-borne: Mechanical Biological Air-borne Propagative Cyclo-propagative Cyclo-developmental Fomite-borne Unclean hands & fingers
- 76. Routes of transmission Direct Indirect Skin-skin Herpes type 1 Mucous-mucous STI Across placenta toxoplasmosis Through breast milk HIV Sneeze-cough Influenza Food-borne Salmonella Water-borne Hepatitis A Vector-borne Malaria Air-borne Chickenpox Ting-borne Scarlatina Exposure A relevant contact – depends on the agent Skin, sexual intercourse, water contact, etc (www)
- 77. Modes of DiseaseTransmission
- 78. • Propagative transmission(e.g.viruses, YF, WNV, EEE, etc.) • Cyclo-developmental (e.g. Wuchereria bancrofti-Bancroftian filariasis) • Cyclo-propagative transmission-. (e.g. malaria, Chagas) 10/15/2013 78
- 79. (III): Susceptible host • • Aninfectious agent seeks a susceptible host aiming “successful parasitism”. Four stages are required for successful parasitism: 1. 2. 3. 4. Portal of entry Site of election inside the body Portal of exit Survival in external environment
- 80. Incubation and Latentperiods • Incubation period: time from exposure to development of disease. In other words, the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question. • Latent period: the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period).
- 81. Serial interval, Generationtime and Infectious period • Serial interval: Gap in time between the onset of the primary and the secondary cases • Generation time : Interval between receipt of infection and maximal infectivity of the host • Infectious (communicable) period: length of time an infectious agent can be transmitted directly or indirectly from an infected person to another person, from an infected animal to man or from an infected person to animal.
- 82. • Index Case –Person that comes to the attention of public health authorities • Primary Case – Person who acquires the disease from an exposure • Secondary Case – Person who acquires the disease from an exposure to the primary case – Secondary attack rate
- 83. Secondary attack rate •The number of exposed persons developing the disease within the range of the incubation period, following exposure to the primary case. • SAR = No. of exposed persons developing the disease within the range of incubation period Total no. of exposed / susceptible contacts X 100
- 84. Virulence and CaseFatality Rate • Virulence – Degree of pathogenicity; the disease evoking power of a microorganism in a given host. – Numerically expressed as the ratio of the number of cases of overt infection to the total number infected. – When death is the only criterion of severity, this is the case fatality rate. • Case fatality rate – Proportion of infected individuals who die of the infection. This is a function of the severity of the infection.
- 85. Case Fatality Rate Casefatality rate (%) = Number of deaths due to disease x 100 Number of cases of disease
- 86. Host defences • Local •Systemic Active Immunity •Humoral •Cellular •Combination • Specific Passive Immunity • Non Specific •Normal human Ig •Specific Human Ig •Animal Antitoxins or Antisera
- 87. Herd Immunity • Thelevel of resistance of a community or group of people to a particular disease. • Provides an immunological barrier to spread of disease in the human herd. • If herd immunity sufficiently high, the occurrence of epidemic is unlikely. • If high level of immunity is achieved and maintained to a point where the susceptible persons are reduced to a small proportion, it may even lead to elimination of a disease e.g., Polio. • Herd immunity does not protect against Tetanus.
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- 92. Infectious Disease Process Directtissue invasion Toxins Persistent or latent infection Altered susceptibility to drugs Immune suppression Immune activation (cytokine storm)
- 93. • Microbial pathogenesis-process of causing disease • Colonization - presence of microbes at site of body – Does not imply tissue damage or disease symptoms – Does imply invasion of site and multiplication
- 94. Timeline for Infection Dynamicsof infectiousness Latent period Infectious period Non-infectious Susceptible Time Dynamics of disease Incubation period Symptomatic period Non-diseased Susceptible Time (www)
- 95. 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 T S P Susceptible Immune Sub-clinical S S T Clinical (www)
- 96. After invasion: theeffective reproduction number, R(t) • As pathogen invades, the number of susceptibles declines through recovery (or death) • Eventually, insufficient susceptibles to maintain chains of transmission • On average each infectious person infects < 1 other, epidemic dies out Initial invasion, R(t) = R0 Peak of epidemic R(t) = 1
- 97. Changes to R(t),over an epidemic 1200 number 1000 800 Susceptible Incident cases Recovered R=1 600 R<1 R>1 400 R=R0 200 0 0 0.05 0.1 time 0.15 0.2
- 98. 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)
- 99. Reproductive Number, R0 Ameasure 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 probability of transmission per contact R0 = p • c • d duration of infectiousness contacts per unit time (www)
- 100. Infection will ….. if R< 1 --------- disappear, if R = 1 --------- become endemic, if R > 1 --------- become epidemic,
- 101. 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
- 102. Reproductive Number, R0 Usein STI Control R0 = p • c • d p c condoms, acyclovir, zidovudine health education, negotiating skills D case ascertainment (screening, partner notification), treatment, compliance, health seeking behaviour, accessibility of services (www)
- 103. 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)
- 104. Immunity – herdimmunity 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)
- 105. Vaccination coverage required forelimination Critical proportion, Pc 100% Pc = 1-1/Ro 80% 60% 40% 20% rubella 0% 0 2 4 6 measles 8 10 12 14 16 18 20 Basic reproduction number, Ro
Editor's Notes
- #79 Propagative transmission-organism undergoes a change in its numbers, i.e. amplification only in the body of the vector. (e.g. viruses, YF, WNV, EEE, etc.)Cyclo-developmental transmission-organism undergoes cyclical change but does not multiply. (e.g. Wuchereriabancrofti-Bancroftianfilariasis)Cyclo-propagative transmission-organism undergoes a cyclical change and multiplies.(e.g. malaria, Chagas)