9/10/2010Dr. Salwa Tayel19/10/2010Dr. Salwa Tayel1

2 2 Associate Professor Family and Community Medicine Department King Saud University By Infectious Disease Epidemiology

Dr. Salwa Tayel 39/10/ Learning Objectives At the end of this lecture you (will) be able to: Explain common definitions and basic concepts used in epidemiology. Describe natural history and spectrum of infectious diseases and their implications for public health.

Dr. Salwa Tayel 4 4 Infection: It is the entry, development and multiplication of an infectious agent in the body of man or animal. Outcome of infection varies. Infectious disease: A clinically manifest disease of man or animal resulting from infection. In-apparent infection: The infection does not become manifest at any stage.

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69/10/ An illness due to a specific infectious agent or its toxic products that arises through transmission of that agent from reservoir to susceptible host. Communicable Disease (typhoid, influenza,.. Non- Communicable Disease (DM, cancer,.. Communicable Disease

Dr. Salwa Tayel 79/10/ Natural history Natural history of disease refers to the progress of a disease process in an individual over time, in the absence of intervention. The natural history of a disease describes the course of the disease in an individual starting from the moment of exposure to the causal agents till one of the possible outcomes occurs.

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99/10/ Spectrum of disease The idea that an exposure can lead to varying signs, symptoms and severity of the same disease in the population is the spectrum of disease. Why do we have varying degrees of severity? Prognosis? The outcome will depend on the interactions of host, agent and environmental factors.

Dr. Salwa Tayel 109/10/2010 Dr. Salwa Tayel 10 Classification of diseases according to clinical severity (spectrum)

Dr. Salwa Tayel 119/10/2010 Dr. Salwa Tayel 11 Examples: Tuberculosis, Polio, Hepatitis A, Meningitis, AIDS Class A: Inapparent infection

Dr. Salwa Tayel 129/10/2010 Dr. Salwa Tayel 12 Examples: Measles, Chickenpox Class B: Classic cases

Dr. Salwa Tayel 139/10/2010 Dr. Salwa Tayel 13 Examples: Rabies, Hemorrhagic fevers caused by Ebola and Murberg viruses. Class C: Severe or Fatal infections

Dr. Salwa Tayel 149/10/2010 Dr. Salwa Tayel 14 Implications for public health

Dr. Salwa Tayel 159/10/2010 Dr. Salwa Tayel 15 The relation of severity of illness to disease statistics.

Dr. Salwa Tayel 169/10/2010 Dr. Salwa Tayel 16 The pyramid and iceberg of disease 1Diseased, diagnosed & controlled 2Diagnosed, uncontrolled 3Undiagnosed or wrongly diagnosed disease 4Risk factors for disease 5Free of risk factors Diagnosed disease Undiagnosed or wrongly diagnosed disease

Dr. Salwa Tayel 179/10/2010 Dr. Salwa Tayel 17 Cases of illness correctly diagnosed by clinicians in the community often represent only the “tip of the iceberg.” Many additional cases may be too early to diagnose or may remain asymptomatic. Examples: Tuberculosis, meningitis, polio, hepatitis A, AIDS. The risk is that persons with in-apparent or undiagnosed infections may be able to transmit infection to others. Iceberg phenomenon

Dr. Salwa Tayel 189/10/2010 Dr. Salwa Tayel 18 Applications of the concepts of natural history and spectrum of disease Persons with in-apparent or undiagnosed infections can transmit infections to others. Control measures must be directed toward all infections capable of being transmitted to others; –both clinically apparent cases and –those with in-apparent or undiagnosed infections.

Dr. Salwa Tayel 1923/10/2010 Dr. Salwa Tayel 19 Chain of infection

Dr. Salwa Tayel 2023/10/2010 Dr. Salwa Tayel 20 Chain of infection A process that begins when an agent leaves its reservoir or host through a portal of exit, and is conveyed by some mode of transmission, then enters through an appropriate portal of entry to infect a susceptible host.

Dr. Salwa Tayel 2123/10/2010 Dr. Salwa Tayel 21 Agent Susceptible Host Reservoir Mode of transmission Cycle of infection Portal of Exit Portal of Inlet IP PC

Dr. Salwa Tayel 2223/10/2010 Dr. Salwa Tayel 22 The requisites (essentials) for the perpetuation of communicable diseases: The elements of the cycle of infection: 1. Presence of microbiological agent. 2. Presence of reservoir. 3. Portal of exit. 4. Mode of transmission. 5. Portal of entry (inlet). 6. Presence of susceptible host.

Dr. Salwa Tayel 2323/10/2010 Dr. Salwa Tayel Agent: Microorganisms are responsible for disease production (viruses, bacteria, protozoa, parasites, fungi,..

Dr. Salwa Tayel 2423/10/2010 Dr. Salwa Tayel 24 Mechanisms of disease production (pathogenesis): Invasiveness: (Pneumococcosis, measles). Toxicity: (Tetanus, Botulism). Hypersensitivity: (Tuberculosis). Others: (Immune suppression; AIDS).

Dr. Salwa Tayel 2523/10/2010 Dr. Salwa Tayel 25 Agent factors that affect disease transmission: Infectivity, Pathogenicity, Virulence, Antigenicity,…

Dr. Salwa Tayel 2623/10/2010 Dr. Salwa Tayel 26 Infectivity The ability of an agent to invade and multiply (produce infection) in a susceptible host. How to measure ( Infectivity); ease & spread of infection? Secondary Attack Rate The proportion of exposed susceptible persons who become infected. Examples: High infectivity: Measles, Chickenpox Low infectivity: Leprosy Low infectivity: Leprosy

Dr. Salwa Tayel 2723/10/2010 Dr. Salwa Tayel 27 Pathogenicity Is the ability of the organisms to produce specific clinical reaction after infection It refers to the proportion of infected persons who develop clinical disease. Examples: High pathogenicity: Measles, Chickenpox (Class B) Low pathogenicity: Polio, Tuberculosis, Hepatitis A, Meningitis, AIDS (Class A) It can be measured by: Ratio of clinical to sub-clinical case=

Dr. Salwa Tayel 2823/10/2010 Dr. Salwa Tayel 28 Virulence Virulence It refers the ability of organisms to produce severe pathological reaction. It is the proportion of persons with clinical disease who become severely ill or die. Examples: Rabies, Hemorrhagic fevers caused by Ebola and Murberg viruses. (Class C) It can be measured by: Case fatality rate

Dr. Salwa Tayel 2923/10/2010 Dr. Salwa Tayel 29 Antigenicity (Immunogenicity) The ability of the organism to produce specific immunity (antibodies or antitoxin). It can be measured by: Second attack frequency: measles, mumps and chickenpox. Second attacks are rare in measles, mumps and chickenpox. common cold, syphilis and gonorrhea. Re-infection occurs as in case of common cold, syphilis and gonorrhea.

Dr. Salwa Tayel 3023/10/2010 Dr. Salwa Tayel 30 Human reservoirs 2. Reservoir of infection The reservoir of an agent is the habitat in which an infectious agent normally lives, grows, and multiplies. Types of reservoirs: Humans, animals, and the environment. Two types of human reservoir exist: Cases: persons with symptomatic illness Cases: persons with symptomatic illness Carriers Carriers

Dr. Salwa Tayel 3123/10/2010 Dr. Salwa Tayel 31 Pathogen Reservoirs Humans are the most important reservoir of human infectious disease.

Dr. Salwa Tayel 3223/10/2010 Dr. Salwa Tayel 32 Carrier A person or animal without apparent disease who harbors a specific infectious agent and is capable of transmitting the agent to others.

Dr. Salwa Tayel 3323/10/2010 Dr. Salwa Tayel 33 Carriers are dangerous because: 1.They do not show any clinical manifestation so they carry normal life. 2.The carrier and his contacts are not aware of their condition so, they take no precautions. 3.It is difficult to discover them. 4.It is not always possible to deal with them. 5.Chronic carriers can remain infectious for a long time leading to repeated introduction of the disease to contacts.

Dr. Salwa Tayel 3423/10/2010 Dr. Salwa Tayel 34 Types of Carriers: 1.Asymptomatic (In-apparent) carrier: The carrier state that may occur in an individual with an infection that is in-apparent throughout its course Examples: Polio virus, meningococcus, hepatitis A virus 2.Incubatory, Convalescent, Post-Convalescent carriers: The carrier state may occur during the incubation period, convalescence, and post convalescence of an individual with a clinically recognizable disease. Examples of Incubatory carrier: Measles, chickenpox

Dr. Salwa Tayel 3523/10/2010 Dr. Salwa Tayel 35 Convalescent carriers: Examples: Diphtheria, hepatitis B viruses and Salmonella species **According to duration of carriage: The carrier state may be (transient carrier or chronic carrier). Chronic carriers: They continue to harbour an agent for an extended time (months or years) following the initial infection. They continue to harbour an agent for an extended time (months or years) following the initial infection. Examples: Hepatitis B virus and Salmonella typhi

Dr. Salwa Tayel 3623/10/2010 Dr. Salwa Tayel 36 Animal reservoirs Zoonoses: vertebrate Zoonoses: Infectious diseases that are transmissible under normal conditions from vertebrate animals to humans. (with humans as incidental hosts) Zoonotic diseases include:  brucellosis (cows and pigs),  anthrax (sheep),  plague (rodents),  rabies (bats, dogs, and other mammals).

Dr. Salwa Tayel 3723/10/2010 Dr. Salwa Tayel 37 Zoonoses Zoonoses are Human Diseases with Animal Reservoirs.

Dr. Salwa Tayel 3823/10/2010 Dr. Salwa Tayel 38 Zoonoses Toxoplasmosis

Dr. Salwa Tayel 3923/10/2010 Dr. Salwa Tayel 39 Environmental reservoirs: Soil, and water Soil: Agents live and multiply in the soil. Soil: Agents live and multiply in the soil.Examples: -Tetanus spores and - Fungal agents; (those causing histoplasmosis) Pools of water are the primary reservoir of Pools of water are the primary reservoir of Legionnaires’ bacillus.

Dr. Salwa Tayel 4023/10/2010 Dr. Salwa Tayel 40 Thank you Bibliotheca Alexandrina Website