Infectious Disease Epidemiology BMTRY 713 January 11, 2016 Lectures 2 Continuation of Introduction & Overview

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1 Infectious Disease Epidemiology BMTRY 713 Lectures 2 Continuation of Introduction & Overview Learning Objectives 1. Explain the magnitude of infectious diseases 2. Describe major principles in infectious disease epidemiology 3. Describe immunity and threshold of infection in a population 4. Understand the main terminologies and temporal patterns in infectious disease epidemiology Infectious Disease (ID) Epidemiology ID transmissible illness from an infectious agent or its toxic products Establishes relationship between agent, host, and the environment Purpose prevention of ID by breaking the chain of transmission Find the weakest link Design rational measures of prevention Selassie, AW (DPHS, MUSC) 1

Fundamental Principles of ID Prevention: The Epidemiologic Triad Haddon Matrix: A search for the weakest link in ID transmission Level/Stage of Prevention Pre-event (Primary Prevention) During the

modification of the parasite Female Anopheles mosquito Entomological surveillance to assess vector competence Genetic changes in mosquitoes to disable asexual cycle of parasite Healthy susceptible

2 Fundamental Principles of ID Prevention: The Epidemiologic Triad Haddon Matrix: A search for the weakest link in ID transmission Level/Stage of Prevention Pre-event (Primary Prevention) During the event (Secondary) Post-event (Tertiary) Intermediate Final Agent Host Physical (Vector/Vehicle) Host Environment Plasmodium vivax in Sick person with malaria Hemoparasite density surveillance Genetic modification of the parasite Female Anopheles mosquito Entomological surveillance to assess vector competence Genetic changes in mosquitoes to disable asexual cycle of parasite Healthy susceptible host Treatment with Antimalarial therapy Prophylactic treatment ; Vaccine development Physical/Social Environment Swampy poor neighborhood; Pesticide spray; Drainage of swamps; Social mobilization Environmental modification - Drainage of swamps, etc. Selassie, AW (DPHS, MUSC) 2

Epidemiology Major causes of human suffering A third of all deaths around the world 43% of the deaths in developing world Lack of access to safe water is main cause Changing

3 Epidemiology Major causes of human suffering A third of all deaths around the world 43% of the deaths in developing world Lack of access to safe water is main cause Changing picture of ID 40 new pathogens since 1970 Changes in virulence (natural, manmade) Changes in population at risk (aging) Changes in the environment (warming) Selassie, AW (DPHS, MUSC) 3

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5 Selassie, AW (DPHS, MUSC) 5

6 Case Description Selassie, AW (DPHS, MUSC) 6

7 Classification of infectious disease Clinicians use clinical manifestations or organ systems (Viral hepatitis, BSE) Microbiologists by agent (Neisserial infections, Meningococcal infections) Epidemiologists 2 common methods Reservoir of organism zoonoses Means of transmission STD, food-borne Classification by reservoir Human (Anthroponoses) Animal (Zoonoses) Soil (Sapronoses) Water (Sapronoses, Waterborne diseases) Selassie, AW (DPHS, MUSC) 7

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10 Infections transmitted by more than one means Anthrax Contact with infected animals During butchering -- cutaneous disease Foodborne Consumption of meat from an infected animal GI anthrax, higher mortality rate Inhalation Inhaling spores pulmonary anthrax, usually fatal Selassie, AW (DPHS, MUSC) 10

11 Perinatal transmission Major concern earlier in pregnancy Rubella results in mild disease for mom Congenital anomalies (ocular, cardiac) in infant Herpes Simplex Virus In utero infection rare, generally causes abortion Infection at birth exposure to maternal lesion Biologic characteristics of the organism Infectivity Pathogenicity Virulence Immunogenicity Inapparent infections Carrier states Selassie, AW (DPHS, MUSC) 11

12 Infectivity Ability of the agent to cause infection in susceptible host (Attack Rate) Two measures Minimum number of infectious particles required to establish infection Proportion of susceptible individuals who develop infection after exposure Pathogenicity Ability of a microbial agent to induce disease Helicobacter pylori with vac A and cag A allelles are much more pathogenic than helicobacter pylori infections without these characteristics Proportion of cases who develop disease Selassie, AW (DPHS, MUSC) 12

13 Virulence Severity of the disease after infection Best measured by Case-fatality rate Proportion of clinical cases who develop severe disease (Apoptosis) Selassie, AW (DPHS, MUSC) 13

14 Host Defense Nonimmunologic Integument (Skin, Membranes) Secretions (Tears, Mucus, Gastric juice) Peristalsis Immunologic Cell media Humoral (Antibody) Selassie, AW (DPHS, MUSC) 14

Platelets WB RB A scanning electron microscope

a neutrophil, and many small disc-shaped

15 Platelets WB RB A scanning electron microscope image from normal circulating human blood. One can see red blood cells, several white blood cells including lymphocytes, a monocyte, a neutrophil, and many small disc-shaped platelets 5.3% 2.3% 0.4% 30% 62% Selassie, AW (DPHS, MUSC) 15

16 Immunogenicity The property that endows a substance with the capacity to provoke an immune response Selassie, AW (DPHS, MUSC) 16

17 Types of immunogenicity High, life-long immunity (Active) Measles, polio Weak, reinfection is common (Passive) Rhinovirus Infection (Common cold) Plasmodium falciparum (Malaria) Nonprotective, deleterious to the host Streptococci infection resulting in glomerulonephritis or rheumatic fever Inapparent infections Infection that can be documented by the isolation of the organism by culture, identification by PCR, or through a specific immune response but the person remains asymptomatic Measure of low pathogenicity, e.g. polio Rare in some diseases, e.g. measles, smallpox, hanta virus Selassie, AW (DPHS, MUSC) 17

18 Carrier state An individual who is asymptomatic but capable of transmitting disease to others HBV infection is acquired perinatally Typhoid Mary (Ms. Mary Mallon) HIV has a long carrier state, average approximately 10 years Selassie, AW (DPHS, MUSC) 18

19 Selassie, AW (DPHS, MUSC) 19

20 Examples of Herd Immunity & Herd Protection Selassie, AW (DPHS, MUSC) 20

R is the reproductive number that indicate the number of secondary cases expected to be caused by a single, typical infected individual in a population with some level of susceptibility.

21 R is the reproductive number that indicate the number of secondary cases expected to be caused by a single, typical infected individual in a population with some level of susceptibility. E.g. R z, where z=(0>r<3) Herd Immunity Index Case 1 st Generation 2 nd Generation Basic reproductive number R 0 = 4 Basic reproductive number R 0 = 1 R 0 =1 Selassie, AW (DPHS, MUSC) 21

22 for the disease to die out. P=1 [R 0 ] -1 1 [R 0 ] -1 is sometimes referred as Vaccination Threshold (V) Herd Immunity Threshold of selected IDs Selassie, AW (DPHS, MUSC) 22

23 Disease measures Rate Incidence new cases in at risk pop. Prevalence Incidence x Duration Case Fatality Persons dying of infected Attack Persons with the disease of exposed Risk Relative Risk Odds Ratio Hazard Ratio Attack Rate & Risk of ID Selassie, AW (DPHS, MUSC) 23

24 Temporal trends in infectious disease Seasonal variation Vector-transmitted diseases Annual variation Dependent upon the number of susceptible individuals in the community Variation over decades Decrease in incidence and mortality in some Large number of new infections occurring Selassie, AW (DPHS, MUSC) 24

Health care workers and infectious diseases

Introduction Health care workers and infectious diseases Objectives 1. What is an infectious disease?? 2. What is an infection and disease?? 3. Causes of re-emerging of the problem of the infectious diseases