Dynamics Of Disease Transmission PDF

BSc Medical Sciences: Epidemiology Assistant Prof. Dr. Soza Th. Baban  Key aspects of Epidemiology It concerns with: Transmissibility This easily spreads - refers how...

BSc Medical Sciences: Epidemiology Assistant Prof. Dr. Soza Th. Baban  Key aspects of Epidemiology It concerns with: Transmissibility This easily spreads - refers how from one a disease individual to another- Virulence of pathogens by disease/Ebola > - Describes caused how severe pathogen can be > cause severe - symptom Even death. showing clear symptoms visible symptoms (But theperson is infected The course of clinical and sub-clinical infections still o & Duration of protective immunity immunity After recovering from against a disease , or individual being vaccinated may develop Human contact pattern Example /- crowded areas transmission of increase airborne disease. > know - how interactions influence outbreaks. Epidemiology looks at how disease spread, How bad they are, how long people stay protected, after recovery or vaccination, and how human behaviors affect disease spread. Learning objectives 1. To learn modes of disease transmission by using epidemiologic understanding involves spread how disease model. agent > - host - environment 2. To define important terms related to the occurrence of disease ↳ Epidemic in a population. ↳Endenia - Reservoir 3. To calculate an attack rate and illustrate how it may be used to is proportion of people who become ill measure person-to-person transmission of a disease. 4. To describe the steps in an outbreak investigation and introduce how cross-tabulation may be used to identify the source.  I keep six honest serving-men (They taught me all I knew); Their names are What and Why and When And How and Where and Who. —Rudyard Kipling (1865–1936) Introduction to Disease Transmission Understanding how diseases spread within populations is crucial for public health. In this lecture, we'll explore the factors influencing the spread of infectious diseases, how we can identify => their origins, and what can be done to prevent their spread. - Key examples we'll look at include COVID-19 and malaria.  Explains how diseases are transmitted > through three interconnected comp The epidemiologic triad of a disease. - - Infectious disease is a disease caused by infectious agent or its toxic products. This agent can be transmitted by an infected person, an animal or a reservoir directly or indirectly through a vector. the cause of the disease The epidemiologic Person/Animal who triad is the relationship between↑an infectious gets infected disease spreads agent where the and its host, its mode of transmission and the environment in which the transmission takes place or promotes the exposure. - => Human susceptibility factors– genetic, behavioural, nutritional, immunity Humans affected animal infectious External factor organism that influence disease transmission  The epidemiologic triad of a disease The Epidemiological Triad explains how diseases are transmitted. Example: In malaria, the agent is the Plasmodium parasite, the host is humans, and the environment is areas where mosquitoes thrive. This slide talks about Where The place where germs from ve of Infection germs come and grow Ex/ Cholera-editry water Sources and Reservoirs Rabies Animal (Deg) > - Infectious diseases can originate from different sources: - Human Reservoirs (People with the infection (symptomatic or carriers) Animal Reservoirs (zoonoses) Diseases or anthrax like come rabies from infected animal Environmental Reservoirs. Contaminated water or soil Rabies (animal reservoir) Causative agent: Rebies virus Cholera (water as and environmental reservoir) Causative agent: Vibrio cholerae The epidemiologic triad of a disease Human susceptibility is determined by a variety of factors: genetic background and behavioral, nutritional, immunologic characteristics The immune status of an individual is determined by many factors including prior experience both with natural infection and with immunization. The factors that can cause human disease: biologic Physical chemical nutritional stress Behavioral  Modes of Transmission of Disease Direct transmission: Direct person-to-person contact Indirect transmission: Common vehicle Vector Different organisms spread in different ways, and the potential of a given organism for spreading and producing outbreaks depends on the, such as its - characteristics of the organismate of growth, the route by which it is transmitted from one person to another, and the number of susceptible persons in the community.  Explains how germs move from one Mode of Transmission& of Respiratory Viruses - Respiratory infections are spread by virus-containing droplets and I aerosols exhaled by infected individuals during breathing, speaking, coughing, and sneezing. Droplets are categorized into two types: Respiratory droplets (>5–10 𝜇m in diameter) Aerosols ( - Symptoms are Virulence of the organism (how efficient the organism is at producing disease) The ability of organism > - to cause the disease Site in the body where the organism multiplies. Wheretheorganisa a Host characteristics as the immune F response Ex Emunty age genetic predispositive  Epidemiologic understanding of disease distribution Endemic, Epidemic, and Pandemic Always present in one area Endemic is defined as the habitual presence of a I disease within a given geographic area. A disease that's always around in one place Example: 1. Crimean-Congo haemorrhagic fever is endemic in Pakistan Human infections caused by the virus have been occurring throughout the year and in wide geographic areas of the country. The WHO authorized to contain the seasonal spike and prevent its spread from endemic foci to other non-endemic areas. Epidemiologic understanding of disease distribution Endemic, Epidemic, and Pandemic Epidemiologic understanding of disease distribution Endemic, Epidemic, and Pandemic When lots of people in town or area get sick suddenly Epidemic is defined as the occurrence in a community or region of a group of illnesses of similar nature, clearly in excess of normal expectancy and derived from a common or a propagated source. 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. Sudden increase in cases in one place  2 Epidemiologic understanding of disease distribution Endemic, Epidemic, and Pandemic Pandemic refers to a worldwide epidemic. The disease’s growth is exponential.  Epidemiologic understanding of disease distribution Disease Outbreaks When a sickness spreads all over the world An outbreak is defined as an increase in incidence of a disease above expected levels in a particular location or population in a given time period. Example: A food becomes contaminated with a microorganism. If an outbreak occurs in the group of people who have eaten the food → common-vehicle exposure If the food served only once →single exposure to the people who ate it. If the food served more than once →multiple exposures to people who eat it more than once. Epidemiologic understanding of disease distribution Disease Outbreaks - waterd waste When a water supply is contaminated with sewage because of leaky pipes: The contamination - can be eithere periodic, causing multiple exposures as a result of changing pressures in the water supply system, which may cause intermittent contamination, or continuous, in which case a constant leak leads to persistent contamination.  one time exposure to the source What are the characteristics of single-exposure, common-vehicle outbreak? Shared source like food water , First, such outbreaks are generally explosive—that is, there is a sudden and rapid increase in the number of - cases of the disease or condition in a population. Example: Single-exposure common-vehicle epidemics of noncommunicable diseases, such as the epidemic of leukemia following the explosion of an atomic bomb in Hiroshima and Nagasaki What are the characteristics of single-exposure, common-vehicle outbreak? Second, the cases are limited to people who share the common exposure. This is self-evident, because in the first wave of cases we would not expect the disease to develop in people who were not exposed unless there was another independent source of the disease in the community. Third, in a food-borne outbreak, cases rarely occur in persons who did not eat the food—that is, those who acquire the disease from a primary case who ate the food. people sick Immunity and Susceptibility - The amount of disease in a population depends on a balance between the number of people in that population who are susceptible and therefore at risk for the disease and the number of people who are not susceptible or immune and therefore not at risk. They may be immune because they have had the disease previously (and have antibodies) or because they have been immunized. Clearly if the entire population is immune, no epidemic can develop.  Immunity and Susceptibility Immunity plays a crucial role in preventing and controlling the spread of diseases: Innate Immunity: The body's immediate defense against pathogens. Adaptive Immunity: The body’s ability to develop specific defenses after exposure to a pathogen. Herd Immunity: When a large part of the population becomes immune, reducing disease spread. A * when most people are immune , (Protecting everyone Vaccines are an important tool for achieving herd immunity (e.g., the role of COVID-19 vaccines). A way to train your body to fight germs.  Herd Immunity Herd immunity is defined as the resistance of a group of people to an attack by a disease to which a large proportion of the members of the group are immune. If a large percentage of the population is immune, the entire population is likely to be protected, not just those who are immune.  Herd Immunity Why does herd immunity occur? It happens because disease spreads from one person to another in any community. Once a certain proportion of people in the community are immune, the likelihood is small that an infected person will encounter a susceptible person to whom he can transmit the infection; more of his encounters will be with people who are immune.  Why does herd immunity occur? When we carry out immunization programs, it may not be necessary to achieve 100% immunization rates to immunize the population successfully. We can achieve highly effective protection by immunizing a large part of the population; the remaining part will be protected because of herd immunity * Occurs when enough people in a population are immune making it difficult for the disease spread to  What percentage of a population must be immune for herd immunity to operate? This percentage varies from disease to disease. For example, Measles, which is highly communicable, it has been estimated that 94% of the population must be immune before the chain of transmission is interrupted.  Incubation period The incubation period is defined as the interval from receipt of infection to the time of onset of clinical illness (the onset of recognizable symptoms). If you become infected today, the disease with which you are infected may not develop for a number of days or weeks. During this time, the incubation period, you feel completely well and show no signs of the disease.  Why does disease not develop immediately at the time of infection? the time needed for the organism to replicate sufficiently until it reaches the critical mass needed for clinical disease to result. the site in the body at which the organism replicates—whether it replicates superficially, near the skin surface, or deeper in the body (e.g., in the gut). The dose of the infectious agent received at the time of infection may also influence the length of the incubation period. Middle East Respiratory Syndrome Coronavirus (MERS-CoV) outbreak In September 2012, health officials in Saudi Arabia first reported a severe acute respiratory illness with symptoms of fever, cough, and shortness of breath. The causative organism was shown to be the Middle East Respiratory Syndrome Coronavirus (MERS-CoV), which has an incubation period of about 5 or 6 days. MERS-CoV likely came from infected camels in the Arabian Peninsula and spread through person-to-person close contact, with health care personnel at higher risk of infection if universal precautions were not adhered to. All MERS-CoV cases that have been identified had a positive history of someone living in or traveling to countries in or near the Arabian Peninsula. Middle East Respiratory Syndrome Coronavirus (MERS-CoV) outbreak Another outbreak of MERS-CoV occurred in the Republic of Korea in 2015 and was also linked to a returning traveler from the Arabian Peninsula. As of May 2017, WHO has reported that there had been 1952 laboratory- confirmed cases of infection with MERS-CoV from 27 countries, of whom 693 (36%) had died. An outbreak of MERSCoV in the Republic of Korea was seen in 2015 but was rather contained, whereas the epidemic remains active in Saudi Arabia. A major contributor to the Korean control of the epidemic was probably the strong infection control measures implemented early on for diagnosing and isolating probable MERS-CoV cases and for reducing interpersonal contacts of travelers with a history of travel to highly affected areas.  Incubation periods of viral diseases Different diseases have different incubation periods.  Outbreak Investigation The three critical variables in investigating an outbreak or epidemic are as follows: 1. When did the exposure take place? 2. When did the disease begin? 3. What was the incubation period for the disease? If we know any two of these, we can calculate the third.  Exploring Occurrence of Disease When a disease appears to have occurred at more than an endemic (usual) level and we wish to investigate its occurrence, we ask: Who was attacked by the disease? Risk factors such as sex, age, and race and behavioral (smoking) When did the disease occur? Certain diseases occur with a certain periodicity. Diarrheal disease is most common during the summer months. Respiratory disease is most common during the winter months. Where did the cases arise? Disease is not randomly distributed in time or place.