Introduction to Epidemiology. Definition, History of Epidemiology Purpose/Use of Epidemiology

Faculty of Medicine Introduction to Community Medicine Course (31505201) Unit 4 Epidemiology Introduction to Epidemiology. Definition, History of Epidemiology Purpose/Use of Epidemiology Concepts in the infectious diseases By Hatim Jaber MD MPH JBCM PhD 20+23-10-2016

Presentation outline Time Introduction to unit 4 Epidemiology 12:00 to 12:10 Public Health & Epidemiology 12:10 to 12:20 Key Words 12:20 to 12:30 Main components 12:30 to 12:40 Concepts in the infectious diseases

Introduction to unit 4 Epidemiology Definition, History of Epidemiology Purpose/Use of Epidemiology Disease Causation Levels of prevention Concepts in the infectious diseases Measurements of Morbidity and Mortality Sources of Data and methods of data collection Epidemic Investigation and Management Epidemiological Surveillance

Epidemiology Epi = upon Demos = population Logos = study of Definition The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems Last, 1988

Epidemiological Principles Diseases (or other health events) don t occur at random Diseases (or other health events) have causal and preventive factors which can be identified Diseases and health have a distribution Epidemiology focuses on populations rather than individual persons, tissues or organs.

«The art of epidemiological thinking is to draw conclusions from imperfect data» George W. Comstock

Epidemiology and Preventive Medicine Epidemiology is the central science of public health, and preventive medicine is a clinical approach to public health practice. Epidemiology can provide the preventive medicine practitioner with the following: - Information on the state of the health of the population. - Methods for identifying possible determinants of health and disease within individuals. - Appropriate population groups for interventions. - Understanding the origins of public health recommendations.

Why have chronic diseases increased in importance? Epidemiologists investigate both infectious and chronic (noncommunicable) diseases Favorable demographic changes and public health successes during the 1900 s (quality and availability of food, water, housing, sanitation; communicable disease control) Epidemic transition (i.e. change) Lower overall death rate Greater life expectancy Shift in major causes of death (from infectious to noncommunicable) Change in lifestyles (access & economy)

Defining characteristics of chronic diseases Prolonged course of illness Multiple risk factors Long latency period Non-contagious origin (non-communicable) Uncertain etiology (causation) Functional impairment or disability Incurability

Key Words: The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems. Distribution : Time, place, person

The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems. Population = Public health

The study of the distribution and determinants of health related states or events in specified human populations and its application to the control of health problems. Application = Information for action

Major components of the definition 1. Population. The main focus of epidemiology is on the effect of disease on the population rather than individuals. For example malaria affects many people in Ethiopia but lung cancer is rare. If an individual develops lung cancer, it is more likely that he/she will die. Even though lung cancer is more killer, epidemiology gives more emphasis to malaria since it affects many people.

Major components of the definition 2. Frequency. This shows that epidemiology is mainly a quantitative science. Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions. Frequency of diseases is measured by morbidity and mortality rates. 3. Health related conditions. Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health. Health related conditions are conditions which directly or indirectly affect or influence health. These may be injuries, births, health related behaviors like smoking, unemployment, poverty etc. 4. Distribution. Distribution refers to the geographical distribution of diseases, the distribution in time, and distribution by type of persons affected. (where, When and who)

Major components of the definition 5. Determinants. Determinants are factors which determine whether or not a person will get a disease. 6. Application of the studies to the promotion of health and to the prevention and control of health problems. This means the whole aim in studying the frequency, distribution, and determinants of disease is to identify effective disease prevention and control strategies.

Natural history of disease The natural history of disease refers to the progression of disease process in an individual over time, in the absence of intervention. There are four stages in the natural history of a disease. These are: 1. Stage of susceptibility 2. Stage of pre-symptomatic (sub-clinical) disease 3. Stage of clinical disease 4. Stage of recovery, disability or death

Four stages in the natural history 1. Stage of susceptibility In this stage, disease has not yet developed, but the groundwork has been laid by the presence of factors that favor its occurrence. Example: unvaccinated child is susceptible to measles. 2. Stage of Pre-symptomatic (sub-clinical) disease In this stage there are no manifestations of the disease but pathologic changes (damages) have started to occur in the body. The disease can only be detected through special tests since the signs and symptoms of the disease are not present. Examples: Detection of antibodies against HIV in an apparently healthy person. Ova of intestinal parasite in the stool of apparently healthy children. The pre-symptomatic (sub-clinical) stage may lead to the clinical stage, or may sometimes end in recovery without development of any signs or symptoms.

3. The Clinical stage At this stage the person has developed signs and symptoms of the disease. The clinical stage of different diseases differs in duration, severity and outcome. The outcomes of this stage may be recovery, disability or death. Examples: Common cold has a short and mild clinical stage and almost everyone recovers quickly. Polio has a severe clinical stage and many patients develop paralysis becoming disabled for the rest of their lives. Rabies has a relatively short but severe clinical stage and almost always results in death. Diabetes Mellitus has a relatively longer clinical stage and eventually results in death if the patient is not properly treated.

4. Stage of recovery, disability or death Some diseases run their course and then resolve completely either spontaneously or by treatment. In others the disease may result in a residual defect, leaving the person disabled for a short or longer duration. Still, other diseases will end in death. Disability is limitation of a person's activities including his role as a parent, wage earner, etc Examples: Trachoma may cause blindness Meningitis may result in blindness or deafness. Meningitis may also result in death.

Aims of Epidemiology 1. Describe the health status of population 2. Explain the etiology of disease 3. Predict the frequency and distribution of disease 4. Control diseases in populations

Uses of Epidemiology Determine the magnitude and trends Identify the etiology or cause of disease Determine the mode of transmission Identify risk factors or susceptibility Determine the role of the environment Evaluate the impact of the control measures

Uses of Epidemiology To make a community diagnosis. Epidemiology helps to identify and describe health problems in a community (for example, the prevalence of anemia, or the nutrition status of children). To monitor continuously over a period of time the change of health in a community. (for example, the effect of a vaccination programme, health education, nutritional supplementation). To practice surveillance for a specific disease in order to be able to act quickly and so cut short any outbreak (example cholera). To investigate an outbreak of a communicable disease, analyze the reasons for it, plan a feasible remedy and carry it out, and monitor the effects of the remedy on the outbreak. To plan effective health services. Effective services, interventions and remedies all depend on accurate community data.

Epidemiologist Core Functions Public health surveillance Outbreak investigations Data analysis Evaluation of disease control programs Communication Management and teamwork

Epidemiologic triad Host Demographic characteristics Biological characteristics Socioeconomic characteristics Agent Biological agents Physical agents Chemical agents Nutrient agents Mechanical agents Social agents Environment Physical environment Biological environment Social environment

Host, Agent, Environment

Terminology and Definitions Infection Contamination Infestation Contagious disease Incidence and prevalence of infectious diseases Epidemic Endemic Hyperendemic holoendemic Pandemic Exotic Sporadic Attack rate Primary/secondary cases Zoonosis, epizootic and enzootic Nosocomial infection Opportunistic infection Eradication Elimination

Terminology and Definitions (cont.) Virulence Reproductive rate of infection Host Vector (source) Reservoir Incubation period Infectivity period Serial interval Latent period Transmission Probability ratio

Infectious Diseases Epidemiology Definition Communicable disease (infectious disease) is an illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person, animal, or reservoir to a susceptible host, either directly or indirectly through an intermediate plant or animal host, vector, or the inanimate environment.

Components of the infectious process The infectious process of a specific disease can be described by the following components, which constitute of the chain of disease transmission. 1. The Agent 2. Its reservoirs 3. Its portal of exits 4. Its mode of transmission 5. Its portals of entry 6. The human host

Components of the infectious process I. The Agents The agents in the infectious process range from viral particles to complex multi-cellular organisms II. Reservoirs A reservoir is an organism or habitat, in which an infectious agent normally lives, transforms, develops and/or multiplies. Reservoirs for infectious agents may be humans, animals, plants or other inanimate objects.

Types of reservoirs Reservoir Human reservoir Animal reservoir Non-living reservoir

Human reservoir Human reservoir cases Primary case Index case Secondary cases Type: Incubatory Convalescent healthy carriers According to spectrum of disease: Clinical cases (mild/severe-typical/atypical) Sub-clinical cases Latent infection cases Duration: Temporary Chronic Portal of exit: Urinary Intestinal Respiratory others

Reservoir Habitat in which the disease normally lives and multiplies People -Symptomatic - Smallpox -Asymptomatic - HIV Animals (Zoonoses) -Brucellosis -Plague Environmental -Histoplasmosis -Legionnaires bacillus

Components of the infectious process Some diseases with human reservoirs are: Most bacterial and viral respiratory diseases HIV/AIDS/Sexually Transmitted Infections (STIs), measles, typhoid etc. All infected humans, whether showing signs and symptoms of the disease or not, are potential sources of infection to others. A person who does not have apparent clinical disease, but is a potential source of infection to other people is called a Carrier. An example of carrier is a person infected with HIV. A person infected with HIV might not have the signs and symptoms but he/she is capable of transmitting the infection to others Some diseases are transmitted to human beings from animals. These diseases are called zoonoses. Examples: Rabies, anthrax, etc.

Components of the infectious process III. Portal of Exit Portal of exit is the way the infectious agent leaves the reservoir. Possible portals of exit include all body secretions and discharges: Mucus, saliva, tears, breast milk, vaginal and cervical discharges, excretions (feces and urine), blood, and tissues. For example feces is the portal of exit for the eggs of hook worm.

IV. Mode of Transmission

IV. Mode of Transmission Modes of transmission include the various mechanisms by which agents are conveyed to other susceptible hosts. Transmission may be direct or indirect. 1. Direct Transmission 1.1 Direct contact: Occurs when there is contact of skin, mucosa, or conjunctiva with infectious agents directly from person or vertebrate animal, via touching, kissing, biting, passage through the birth canal, or during sexual intercourse. Example: HIV/AIDS/STIs, rabies 1.2 Direct Projection: is transmission by projection of saliva droplets during coughing, sneezing, singing, spitting or talking. Example: common cold 1.3 Transplacental: is transmission from mother to fetus through the placenta. Example: syphilis, HIV/AIDS

Indirect transmission 2. Indirect transmission The following are the different types of indirect transmission. 2.1 Vehicle-borne: Transmission occurs through indirect contact with inanimate objects fomites: bed sheets, towels, toys, or surgical instruments; as well as through contaminated food, water, IV fluids etc. 2.2 Vector-borne: The infectious agent is conveyed by an arthropod to a host. Vectors may be biological or mechanical. -Biological vector: A vector is called biological vector if the agent multiplies in the vector before transmission. Example: anopheles mosquito is a biological vector for malaria. -Mechanical vector: A vector is called mechanical vector if the agent is directly infective to other hosts, without having to go through a period of multiplication or development in the vector. The vector simply carries the agent by its body parts( leg, proboscis etc) to convey it to susceptible hosts. Example: Flies are mechanical vectors for the transmission of trachoma.

Indirect transmission Airborne: which may occur by dust or droplet nuclei (dried residue of aerosols) Example: Tuberculosis. When pulmonary tuberculosis patients cough, they emit many aerosols which consists the agents of tuberculosis. When these aerosols dry droplet nuclei will be formed. These droplet nuclei will remain suspended in the air for some time. When another healthy susceptible individual breaths he/she will inhale the droplet nuclei and become infected with tuberculosis. Examples: -Nasal mucosa is portal of entry for common cold -Conjunctiva is the portal of entry for trachoma -Injury site is portal of entry for tetanus

VI. Susceptible human host: The susceptible human host is the final link in the infectious process. Host susceptibility or resistance can be seen at the individual and at the community level. Host resistance at the community (population) level is called herd immunity. Herd immunity can be defined as the resistance of a population to the introduction and spread of an infectious agent, based on the immunity of a high proportion of individual members of the population, thereby lessening the likelihood of a person with a disease coming into contact with b susceptible. Example - If 90 % of the children are vaccinated for measles, the remaining 10 % of the children who are not vaccinated might not become infected with measles because most of the children (90 %) are vaccinated. That means transmission from infected person to other susceptible children will not be easier.

Basic Epidemiological Terms Disease Incidence: Percentage of population that contracts a disease in a given time period Disease Prevalence: Percentage of population that has the disease during a given time period

Levels of disease

Basic Epidemiological Terms Sporadic Disease: occurs only occasionally (i.e. Polio in US) Endemic Disease: constantly present in population (i.e. common cold or ear infection) Epidemic Disease: Many people acquiring a disease in a short time period (i.e. Influenza, Gonorrhea, AIDS) Pandemic Disease: Worldwide Epidemics (i.e. Influenza and AIDS)

Epidemic The unusual occurrence 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.

Endemic It refers to 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)

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)

Pandemic and Exotic An epidemic usually affecting a large proportion of the population, occurring over a wide geographic area such as a section of a nation, the entire nation, a continent or the world, e.g. Influenza pandemics. Exotic diseases are those which are imported into a country in which they do not otherwise occur, as for example, rabies in the UK.

Sporadic The word sporadic means scattered about. The cases occur irregularly, haphazardly from time to time, and generally infrequently. The cases are few and separated widely in time and place that they show no or little connection with each other, nor a recognizable common source of infection e.g. polio, meningococcal meningitis, tetanus. However, a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread.

Attack rates and primary/secondary cases Attack rate: proportion of non-immune exposed individuals who become clinically ill. Primary (index)/secondary cases: The person who comes into and infects a population is the primary case. Those who subsequently contract the infection are secondary cases. Further spread is described as "waves" or "generations".

Zoonosis, epizootic and enzootic Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man, e.g. rabies, plague, bovine tuberculosis.. An epizotic is an outbreak (epidemic) of disease in an animal population, e.g. rift valley fever. An Enzotic is an endemic occurring in animals, e.g. bovine TB.

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. Examples include infection of surgical wounds, hepatitis B and urinary tract infections.

Opportunistic infection This is 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. For example, opportunistic infections are very common in AIDS. Organisms include Herpes simplex, cytomegalovirus, M. tuberculosis.

Basic Epidemiological Terms Virulence: the severity of disease that the agent causes to the host The Host: is the organism that is susceptible to the effect of the agent. The status of the host is very important and is generally classifiable as: susceptible, immune or infected. The host s response can vary from showing no effect to manifesting subclinical disease, atypical symptoms, straight forward illness or severe illness.

Basic Epidemiological Terms The Portal of Exit: is a pathway by which the agent can leave the source. This pathway is usually related to the pathway where the agent is localized.

Basic Epidemiological Terms A Portal of Entry: a pathway into the host that gives the agent an access to the tissue where it can multiply or act. Often the agent enters the host the same way it left the source.

Basic Epidemiological Terms The Environment : is the conditions or influences that are not part of either the agent or the host, but that influence their interaction. A wide variety of factors including physical, climatologic, biologic, social and economic factors can come into play.

Carriers

CARRIERS A Carrier is defined as an infected person or animal that harbors a specific infectious agent in the absence of discernible clinical disease and serves as a potential source of infection for others

Carriers It occurs either due to inadequate treatment or immune response, the disease agent is not completely eliminated, leading to a carrier state. FEATURES OF CARRIER Three elements have to occur to form a carrier state: 1. The presence in the body of the disease agent. 2. The absence of recognizable symptoms and signs of disease. 3. The shedding of disease agent in the discharge or excretions. 4. As a source of infection to others

TYPE A) Incubatory B) Convalescent C) Healthy DURATION A)Temporary B)Chronic CARRIERS

Cases A case is defined as a person in the population or study group identified as having the particular disease, health disorder, or condition under investigation

Virulence and Case Fatality Rate Virulence: is the degree of pathogenicity; the disease evoking power of a micro-organism in a given host. Numerically expressed as the ratio of the number of cases of overt infection to the total number infected, as determined by immunoassay. When death is the only criterion of severity, this is the case fatality rate. Case fatality rate for infectious diseases: is the proportion of infected individuals who die of the infection. This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed.

Serial interval and Infectious period Serial interval: (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time). Infectious (communicable) period: length of time a person can transmit disease (sheds the infectious agent).

Incubation and Latent periods 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).

Transmission Probability Ratio (TPR) TPR is a measure of risk transmission from infected to susceptible individuals during a contact. TPR of differing types of contacts, infectious agents, infection routes and strains can be calculated. Transmission probabilities: p00: tp from unvaccinated infective to unvaccinated susceptible p01: tp from vaccinated infective to unvaccinated susceptible p10: tp from unvaccinated infective to vaccinated susceptible p11: tp from vaccinated infective to vaccinated susceptible

TPR (cont.) To estimate the effect of a vaccine in reducing susceptibility, compare the ratio of p10 to p00. To estimate the effect of a vaccine in reducing infectiousness, compare the ratio of p01 to p00. To estimate the combined effect of a vaccine, compare the ratio of p11 to p00.

Eradication and Elimination Termination of all 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. The term elimination 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 and diphtheria.

And Enjoy