EpiLEC Session 6 Temporal Patterns of Disease PDF

Course Code: PHLT 2 Veterinary Epidemiology Student’s Activity sheet for Session #6 Name: _____________________________________________________________ Class number: _______ Section: ____________ Schedule: _________...

Course Code: PHLT 2 Veterinary Epidemiology Student’s Activity sheet for Session #6 Name: _____________________________________________________________ Class number: _______ Section: ____________ Schedule: ________________________________________ Date: ________________ Lesson title: Temporal Patterns of Disease in Populations Materials: - Paper and Pen Lesson Objectives: - Desktop Computer or Smart Phone with At the end of this period, the students are expected to: internet access 1. describe the epidemic curve. - pdf file copy of 2. explain the temporal patterns of disease occurrence in reference book populations. 3. appraise the current Covid-19 pandemic integrating References: knowledge on the distribution of disease in the 1. Thrusfield, Michael (1995). population. Veterinary Epidemiology. Third edition. Oxford: 1. Blackwell Science Ltd. 2. Stevenson, Mark (2005). “An Introduction to Veterinary Epidemiology.” Palmerston North: Massey University, New Zealand. Productivity Tip: If you feel unmotivated, take a break for 20 minutes. Check your plants or pet. A. LESSON PREVIEW/REVIEW 1) Introduction (2 mins) We learned in the previous session that the patterns of disease occurrence can provide useful clues as to which factors may be affecting the health of the population, by answering questions on the 'animal, place, time' concept. In this session, we will further describe the distribution of disease with respect to timing. Understanding the epidemic curve is essential in describing disease and further deciding on directed methods to disease. 2) Activity 1: What I Know Chart, part 1 (3 mins) Consider the following questions on the table below. On the first column briefly write in bullet points what you know about the question. Meanwhile, leave the third column blank until you have completed going through the whole lesson. FLM 1.0 Prepared by Daniel C. Ventura, Jr., DVM, PhD., DipVPH  Course Code: PHLT 2 Veterinary Epidemiology Student’s Activity sheet for Session #6 Name: _____________________________________________________________ Class number: _______ Section: ____________ Schedule: ________________________________________ Date: ________________ What I Know Questions: What I Learned (Activity 4) 1. Applying epidemiologic principles, what two factors can “flatten the curve” in the current Covid-19 pandemic in Central Visayas? 2. What is the difference between seasonal trend and cyclical trend of disease pattern? B.MAIN LESSON 1) Activity 2: Content Notes (20 mins) Read the full content of this lesson in the following sources: Thrusfield, Michael (1995). Veterinary Epidemiology Third Edition, p 137-146 Stevenson, Mark (2005). “An Introduction to Veterinary Epidemiology” p 8-12 Temporal Distribution and Patterns of Disease in the Population The epidemic curve o a useful means to represent the temporal distribution pattern of disease o can illustrate both: ✓ the magnitude of a problem (i.e the number of new cases occurring), and ✓ the rapidity with which the epidemic progresses. o represents in a graphical form the onset of cases of the disease, either as a histogram, a bar graph, or a frequency polygon. o An epidemic curve consists of bar charts showing time on the horizontal axis and the number of new cases on the vertical axis. The graph on the right shows the components of an epidemic curve (stylized to a symmetric shape). The horizontal dotted line indicates the average of new cases (Sinnecker, 1976). FLM 1.0 Prepared by Daniel C. Ventura, Jr., DVM, PhD., DipVPH  Course Code: PHLT 2 Veterinary Epidemiology Student’s Activity sheet for Session #6 Name: _____________________________________________________________ Class number: _______ Section: ____________ Schedule: ________________________________________ Date: ________________ Factors affecting shape of the epidemic curve ✓ The incubation period of the disease ✓ The infectivity of the agent ✓ The proportion of the susceptible animals in the population ✓ The distance between animals (i.e. animal density) A typical epidemic curve may likewise be conceived of as having 4 and occasionally 5 segments: o Slope of ascending branch of the curve can reveal something about the type of exposure or about the mode of transmission of the disease agent. o The ascending branch of the curve reflects the infectivity of the agent o The extent of the plateau and the slope of the descending branch are mainly attributed to: ✓ the availability of susceptible animals that in turn is dependent on the population-at-risk with respect to their immune status (a concept of herd immunity); ✓ some intervention (vaccination or treatment) o Contact rate among animals will also exert major impact on the rate of spread o The main peak of the curve is at times preceded by a similar peak that represent the index cases (the first case that occurred). o Secondary peak is usually due to: a) Introduction of susceptible animal into the previously epidemic area; or b) Movement of infected animals from the epidemic area and contact with susceptible animals. Threshold level and Reproduction Number FLM 1.0 Prepared by Daniel C. Ventura, Jr., DVM, PhD., DipVPH  Course Code: PHLT 2 Veterinary Epidemiology Student’s Activity sheet for Session #6 Name: _____________________________________________________________ Class number: _______ Section: ____________ Schedule: ________________________________________ Date: ________________ A considerable bulk of mathematical theory has been formulated to explain disease patterns. The threshold level refers to the minimum density of susceptible animals that theoretically is required to allow a contact‐transmitted epidemic to commence (i.e., for an infection to 'invade' a population). Kendall's Threshold Theorem mathematically defines the concept of threshold level. Application of Kendall’s Threshold Theorem to rabies in foxes: The theorem predicts that if a rabid fox infects more than one other before it dies, the disease increases exponentially; if each animal infects less than one other; the disease decreases exponentially. Numbers in the figure refers to the contact rate = the number of susceptible animals that are infected by an infected animal per unit time (a parameter related to the basic reproduction ratio) - From Macdonald and Bacon 1980) The basic reproduction number (R0), pronounced “R naught,” (also called the basic reproduction ratio or rate or the basic reproductive rate), is an epidemiologic metric used as an indicator of the contagiousness or transmissibility of infectious and parasitic agents. Simply, R0 represents the expected number of secondary infections resulting from one single infection in a completely susceptible population Three possibilities exist for the potential transmission or decline of a disease, depending on its R0 value: o R0 > 1, number of new infections will increase over time. The disease will be transmitted between susceptible animals in the population, and there may be an outbreak or epidemic. o R0 = 1, each existing infection causes one new infection. The disease will stay alive and stable, but there won’t be an outbreak or an epidemic. o R0 < 1, number of new infections will decrease over time. In this case, the disease will decline and eventually die out. FLM 1.0 Prepared by Daniel C. Ventura, Jr., DVM, PhD., DipVPH  Course Code: PHLT 2 Veterinary Epidemiology Student’s Activity sheet for Session #6 Name: _____________________________________________________________ Class number: _______ Section: ____________ Schedule: ________________________________________ Date: ________________ Importantly, a disease’s R0 value only applies when everyone in a population is completely vulnerable to the disease. This means: no one has been vaccinated no one has had the disease before there’s no way to control the spread of the disease The basic reproductive number is affected by several factors: ▪ The probability of infection being transmitted during contact ▪ The rate of contacts in the host population ▪ The duration of infectiousness. This is shown in the equation: Common Source Epidemic o subjects are exposed to a common noxious influence. o If the group is exposed over a relatively short period then disease cases will emerge over one incubation period. This is classified as a common point source epidemic. The shape of this curve rises rapidly and contains a definite peak at the top, followed by a gradual decline. o Exposure can also occur over a longer period of time, either intermittently or continuously. This creates either an intermittent common source epidemic or a continuous common source epidemic. o The shape of this curve rises rapidly (associated with the introduction of the agent). o The down slope of the curve may be very sharp if the common source is removed or gradual if the outbreak is allowed to exhaust itself. Propagated Epidemic o occurs when a case of disease serves as a source of infection for subsequent cases and those subsequent cases, in turn, serve as sources for later cases.In theory, the epidemic curve of a propagated epidemic has a successive series of peaks reflecting increasing numbers of cases in each generation. The epidemic usually wanes after a few generations, either because the number of susceptible falls below a critical level, or because intervention measures become effective. FLM 1.0 Prepared by Daniel C. Ventura, Jr., DVM, PhD., DipVPH  Course Code: PHLT 2 Veterinary Epidemiology Student’s Activity sheet for Session #6 Name: _____________________________________________________________ Class number: _______ Section: ____________ Schedule: ________________________________________ Date: ________________ The plot on the left is typical of a propagated epidemic. The curve on the right is typical of a common source epidemic. (Stevenson, 2005) Sometimes epidemic curves can show characteristics of being both common source and propagated. The graph on the right shows the weekly hazard of foot-and- mouth disease infection for cattle holdings (solid lines) and ‘other’ holdings (dashed lines) in Cumbria (Great Britain) in 2001 (Reproduced from Wilesmith et al. (2003)). The epidemic started as a common (point) source, then become propagative over time. (Stevenson, 2005) Trends in the Temporal Distribution of Disease: Long Term (Secular trends) o Occur over a period of time and represent a long term interaction between host and parasite. FLM 1.0 Prepared by Daniel C. Ventura, Jr., DVM, PhD., DipVPH  Course Code: PHLT 2 Veterinary Epidemiology Student’s Activity sheet for Session #6 Name: _____________________________________________________________ Class number: _______ Section: ____________ Schedule: ________________________________________ Date: ________________ 1. Indicates stable endemic level of disease. 2. Interaction is biased to the host then there is gradual decrease in disease 3. Interaction is biased to the parasite, then there is gradual increase in disease occurrence. Short term o Typical epidemics Cyclical Trends o Associated with regular fluctuations in the level of disease occurrence. o Associated with periodic changes in the size of the susceptible host population and/or effective contact o May produce recurrent epidemic or endemic pulsation (regular predictable cyclical fluctuations). Fluctuations may be caused by: changes in host density, management practices, survival of infectious agents, vector dynamics, Other ecological factors o Seasonal trend: fluctuations related to season 2) Activity 3: Skill-building Activities A. A disease fact sheet is uploaded in google classroom. Accomplish what is being asked and follow submission instructions B. The table below describes the interventions to rabies control and corresponding impact to factors affecting the reproductive number. Analyze the table and identify which of the tools (interventions) proves to best have an impact on the R0? State a brief explanation on your choice. Your score will depend on how you justify your response. FLM 1.0 Prepared by Daniel C. Ventura, Jr., DVM, PhD., DipVPH  Course Code: PHLT 2 Veterinary Epidemiology Student’s Activity sheet for Session #6 Name: _____________________________________________________________ Class number: _______ Section: ____________ Schedule: ________________________________________ Date: ________________ = means reduction or decrease 3) Activity 4: What I Know Chart, part 2 (2 mins) You are about to complete the lesson at this point. To refresh what you have learned, review back the questions in the What I Know Chart from Activity 1 and write your answers to the questions based on what you now know in the third column of the chart. 4) Activity 5: Check for Understanding (5 mins) Take Quiz Uploaded in Google Classroom C. LESSON WRAP-UP 1) Activity 6: Thinking about Learning (5 mins) You are done with this session! Let’s track your progress. Shade the session numbers you have completed. P1 P2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 ☐ ☐ ☐ ☐ ☐ ☐ ☐ ☐ ☐ ☐ ☐ ☐ ☐ ☐ ☐ ☐ FLM 1.0 Prepared by Daniel C. Ventura, Jr., DVM, PhD., DipVPH  Course Code: PHLT 2 Veterinary Epidemiology Student’s Activity sheet for Session #6 Name: _____________________________________________________________ Class number: _______ Section: ____________ Schedule: ________________________________________ Date: ________________ 2) My learning targets: Complete the table below. Record your scores, learning experience for the session and deliberately plan for the next session. Date Learning Target/Topic Scores Action Plan What session# did you do? What were the What were What contributed to the quality of your performance What’s the date learning targets? What activities did you your scores in today? What will you do next session to maintain today? do? the activities? your performance or improve it? FLM 1.0 Prepared by Daniel C. Ventura, Jr., DVM, PhD., DipVPH