Lecture 2 Public Health PDF

Public Health & Preventive Medicine PMBc303 2024/2025 Professor Reham Elkased  Lecture 2 Health Public health Epidemiology  Epidemiology: the science of disease It is a branch of public health that studies disease and how it spreads among hu...

Public Health & Preventive Medicine PMBc303 2024/2025 Professor Reham Elkased  Lecture 2 Health Public health Epidemiology  Epidemiology: the science of disease It is a branch of public health that studies disease and how it spreads among human and non-human populations, with the goal of controlling and limiting the spread of pathogens and other negative health problems. comes from the Greek Epi = “upon,” demos = “people,” ology = “study Epidemiologists often called “Disease Detectives," search for the cause of the disease, identify people who are at risk, and determine how to control or stop the spread or prevent it from happening again. Pioneers of Epidemiology John Snow the father of epidemiology 1854 Broad Street cholera epidemic in London observations he had made during an earlier cholera outbreak (1848–1849) proposed that: cholera was spread through a fecal-oral route of transmission and that a microbe was the infectious agent Investigated in two ways First, suspecting that contaminated water was the source of the epidemic, Snow identified the source of water for those infected. He found a high frequency of cholera cases among individuals who obtained their water from the River Thames downstream from London. This water contained the refuse and sewage from London and settlements upstream. He also noted that brewery workers did not contract cholera and on investigation found the owners provided the workers with beer to drink and stated that they likely did not drink water. Second, he mapped the incidence of cholera and found a high frequency among those individuals using a particular water pump located on Broad Street. In response to Snow’s advice, local officials removed the pump’s handle, resulting in the control of the Broad Street cholera epidemic. ✓ the first known public health response to an epidemic Epidemiology I. Epidemiologic Triangle II. Chain of infection III. Levels of disease IV. Epidemic Patterns V. Measures of Disease frequency  Epidemiologic Triangle The epidemiologic triangle (epidemiologic triad) is a tool that scientists use for addressing the three components that contribute to the spread of disease: an external agent, a susceptible host, and an environment that brings the agent and host together. Between the vertices, scientists will often describe the center of the triangle as representing time. It’s critical to fully assess each component in order to develop effective control and prevention measures. Another way to think of the triangle model is the what (the agent), who (host), where (environment), and when (time) of health issues.  Epidemiologic triangle The Agent—“What” The agent is the cause of the disease. Generally, the agent must be present for the disease to occur; however, the presence of that agent alone is not always sufficient to cause disease. A variety of factors influence whether exposure to an organism will result in disease, including the organism’s pathogenicity (ability to cause disease) and dose. The Host—“Who” The host usually refers to the human who can get the disease. A variety of factors intrinsic to the host, sometimes called risk factors, can influence an individual’s response to a causative agent. Susceptibility and response to an agent are influenced by factors such as genetic composition, and nutritional and immunologic status. The Environment—“Where” The environment is the favorable surroundings and external conditions to the host that cause or allow the disease to be transmitted. Environmental factors include physical factors such as climate, biological factors such as insects that transmit the agent, and socioeconomic factors such as crowding. Time —“When” Time may refer to: an incubation period—the time between when the host is infected and when disease symptoms occur. the duration of the illness (the amount of time a person can be sick before death or recovery occurs). the period from infection to the threshold of an epidemic for a population. 1. Infectious agents: Bacteria, fungi, parasites, viruses 2. Reservoir The habitat in which the infectious agent normally lives. Reservoirs include humans, animals, and the environment. 1- Human reservoir Carrier: a person with no clinical symptoms who is capable of transmitting the pathogen to others. Classification of carriers a- According to type Passive carrier (Healthy carrier): A person who never experiences symptoms (very dangerous – not infected - mechanical transmission). Active carrier: is an infected individual who can transmit the disease to others, may or may not exhibit signs or symptoms (asymptomatic carriers) Incubatory carrier: An active carrier who shed the M.O. during the incubation period before clinical illness begins. Convalescent carrier: An active carrier who has recovered from his illness but remains capable of transmitting it to others during the convalescent period. ✓ Incubation period: The time interval between the entry of the M.O. and the appearance of the first symptom of a disease ✓ Convalescent period: The time between the disappearance of symptoms and the disappearance of the M.O. from the host. b- According to the duration Temporary carrier: Person who shed the microorganism for a short period Chronic carrier: Person who shed the M.O. for indefinite periods c- According to the portal of exit Respiratory carrier: e.g. influenza Fecal (intestinal) carrier: e.g. typhoid, dysentery, cholera Blood carrier: e.g. hepatitis B and AIDs Urinary or sexual: e.g. gonococcus and AIDs 2- Animal reservoir: Animals or birds (Cases or carriers) e.g. rabies. An infectious disease that is transmissible under natural conditions from animals to humans is known as a zoonotic disease. 3- Environmental reservoirs: Plants, soil, and water in the environment are also reservoirs for some infectious agents, e.g. tetanus, anthrax. 3. Portal of exit is the path by which a pathogen leaves its host (reservoir). usually corresponds to the site where the pathogen is localized. (influenza viruses exit the respiratory tract). Some blood-borne agents can exit by crossing the placenta from mother to fetus (toxoplasmosis), while others exit through cuts or needles in the skin (hepatitis B) or blood-sucking arthropods (malaria).  4. Mode of Transmission of infections An infectious agent may be transmitted from its natural reservoir to a susceptible host in different ways Modes of transmission Direct Indirect Direct contact Airborne Droplet spread Vehicle borne Vector-borne Direct transmission Direct contact through skin-to-skin contact or sexually. contact with soil or vegetation harboring infectious organisms. Droplet spread by sneezing, coughing, or even talking. Source: James Gathany, CDC PHIL, 2009.  Indirect transmission Airborne when infectious agents are carried by dust or droplet nuclei suspended in the air. Vehicles may indirectly transmit an infectious agent include food, water, biological products (blood), and fomites (e.g. towels). may passively carry a pathogen or may provide an environment in which the agent grows, multiplies, or produces toxins. Parentral or blood transfusion: e.g. hepatitis B,C & AIDs Animal bite: e.g. rabies Transplacental: AIDs, hepatitis B, rubella Vectors is a living organism that transmits an infectious agent from an infected host to another host. are frequently arthropods, such as mosquitoes, ticks & fleas. Mechanical transmission: flies carrying helminthes ova on their appendages Biological transmission: bite of blood sucking insects e.g. malaria (the causative agent undergoes development and maturation in vector) 5. Portal of entry the way a pathogen enters a susceptible host. must provide access to tissues in which the pathogen can multiply or a toxin can act. 6. Susceptible Host The final link in the chain of infection. The susceptibility of a host depends on genetic or physical factors and immunity (affect an individual’s ability to resist infection).  Levels of disease Endemic refers to the constant presence and/or usual prevalence of a disease or infectious agent in a population within a geographic area. - Examples: Hepatitis A and schistosomiasis in Egypt. Epidemic refers to an increase, often sudden, in the number of cases of a disease above what is normally expected in that population in that area. - Example: severe acute respiratory syndrome (SARS) took place in 2003 Outbreak carries the same definition of epidemic, but is often used for a more limited geographic area. Pandemic refers to an epidemic that has spread over several countries or continents, usually affecting a large number of people. - Examples: Spanish Flu (1918-1920), COVID-19 (2019 - 2023) Sporadic refers to a disease that occurs infrequently and irregularly (occasionally). Endemic Epidemic Pandemic Sporadic ▪ constantly present larger than expected An epidemic that occurs Occasionally (often at a low level) number of cases worldwide without geographic ▪ within a particular in a short time concentration geographic region within a geographic region  Epidemic Patterns Epidemics can be classified according to their manner of spread through a population Epidemic Patterns Common source Propagated source Point Continuous Intermittent The epidemic patterns can be presented by the epidemic curve (or epi curve). An epidemic curve (or epi curve) is a histogram (bar chart) that shows the distribution of cases over time. x-axis: the time intervals, y-axis: case counts. visual representation of illness onset in cases associated with an epidemic. provide useful information on the size, the pattern of spread, time trend, and exposure period of the epidemic.  Common-source outbreak (a single source for all individuals infected) a group of persons is exposed to an infectious agent or a toxin from the same source. Point-source outbreak: If the group is exposed for a short time period—less than the incubation period of the pathogen. For example, a group of people gets sick after eating the same contaminated salad in a picnic. Continuous common-source outbreak: the infection occurs for an extended period of time, longer than the incubation period (over a period of days, weeks, or longer). The range of exposures and range of incubation periods tend to flatten and widen the peaks of the epidemic curve. For example, Cholera outbreak in London in 1854, by tracking the source of infection to its source, it was found to be the contaminated public water pump. Intermittent common-source outbreak: (discontinuous or sporadic) infections occur for a period, stop, and then begin again The epidemic curve often has a pattern reflecting the intermittent nature of the exposure. For example, a well that was contaminated only after large rainfalls and that cleared itself of contamination after a short period  Epidemic Patterns Common-source Propagated source Point Continuous Intermittent Propagated source outbreak Usually occurs through direct person-to-person contact Can also be indirect person-to-person contact vehicle-borne (e.g., the transmission of hepatitis B or HIV by sharing needles) or vector-borne (e.g., the transmission of yellow fever by mosquitoes). cases occur over more than one incubation period. For example, in 2014, the Ebola virus caused an epidemic that killed thousands of people, mainly in western Africa. It was a propagated epidemic as the virus spread through human contact with contaminated body fluids. As families helped sick relatives and other caregivers helped ill patients, many were exposed to the virus and became sick themselves. Propagated epidemics last longer than the common source outbreaks.  Measures of Disease frequency Incidence and prevalence rates are frequency measures that are used to characterize the occurrence of health events in a population (two aspects of morbidity). Incidence refers to the proportion or rate of persons who develop a condition during a particular time period (number of new diagnosed cases of a disease over a specified period of time). number of new cases Incidence rate = Total population Example If over the course of one year, 5 women are diagnosed with breast cancer, out of a total female study population of 200 (who do not have breast cancer at the beginning of the study period), then we would say the incidence of breast cancer in this population was 0.025. An incidence rate describes how quickly disease occurs in a population.  Prevalence refers to the proportion of persons who have a condition at or during a particular time period (the total number of cases of disease existing in a population). total number of cases Prevalence rate = total population Example So, if a measurement of cancer is taken in a population of 40,000 people and 1,200 were recently diagnosed with cancer, and 3,500 are living with cancer, then the prevalence of cancer is 0.118. So prevalence and incidence are similar, incidence includes new cases only whereas prevalence includes new and pre-existing cases. The key difference is in their numerators. The numerator of incidence = new cases that occurred during a given time period The numerator of prevalence = all cases present during a given time period