EpiLEC Session 8 Overview of Determinants of Disease PDF

Course Code: PHLT 2 Veterinary Epidemiology Student’s Activity sheet for Session #8 Name: _____________________________________________________________ Class number: _______ Section: ____________ Schedule: ____...

Course Code: PHLT 2 Veterinary Epidemiology Student’s Activity sheet for Session #8 Name: _____________________________________________________________ Class number: _______ Section: ____________ Schedule: ________________________________________ Date: ________________ Lesson title: Overview of the Determinants of Disease 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. Explain the distribution and patterns of disease - pdf file copy of occurrence in space. reference book 2. Apply epidemiologic principles in relating disease spread in a geographical area Reference: 1. Thrusfield, Michael (1995). Veterinary Epidemiology. 1. Third edition. Oxford: Blackwell Science Ltd. Productivity Tip: Determine distractions in focusing with your studies and manage them. A. LESSON PREVIEW/REVIEW 1) Introduction (2 mins) A determinant is any factor, or variable that can affect the frequency with which a disease occurs in a population (Putt, et.al. (1987)). Meanwhile, Thrusfield (1995) defines a determinant as any characteristic that affects the health of a population. Considering these definitions, determinants cover all probable factors contributing to the cause of a disease. Etiology is defined as the determination of a cause of disease or pathology. It is independent and not to be used interchangeably with epidemiology. Etiology is not only disease specific but also specific to an individual animal. While a particular cause may lead to a disease in an animal, a similar set of factors could lead to a different illness being manifested in another animal. Knowledge of determinants identifies factors to manage in disease occurrence and further facilitates identification of categories of animal that are at particular risk of developing disease. It therefore is a prerequisite for disease prevention and is an aid to differential diagnosis. FLM 1.0 Prepared by Daniel C. Ventura, Jr., DVM, PhD., DipVPH  Course Code: PHLT 2 Veterinary Epidemiology Student’s Activity sheet for Session #8 Name: _____________________________________________________________ Class number: _______ Section: ____________ Schedule: ________________________________________ Date: ________________ 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. What I Know Questions: What I Learned (Activity 4) 1. How do infection differ from disease? 2. How does etiology differ from a determinant? 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 75-97  Putt, S.N.H., et.al. (1987). Veterinary Epidemiology and Economics in Africa, A Manual for Use in the Design and Appraisal of Livestock Health Policy. International Livestock Centre for Africa (ILCA) Manual No. 3, p 6-13 Classification of Determinants Determinants can be classified in three ways, as: 1. primary and secondary; 2. intrinsic and extrinsic; 3. associated with host, agent or environment. Primary and secondary determinants - Primary determinants are factors whose variations exert a major effect in inducing disease. Frequently, primary determinants are necessary causes. The primary determinants may include other genetically determined factors. FLM 1.0 Prepared by Daniel C. Ventura, Jr., DVM, PhD., DipVPH  Course Code: PHLT 2 Veterinary Epidemiology Student’s Activity sheet for Session #8 Name: _____________________________________________________________ Class number: _______ Section: ____________ Schedule: ________________________________________ Date: ________________ - Secondary determinants correspond to predisposing, enabling and reinforcing factors. Meanwhile, Putt, et.al. (1987) broadly classified determinants as being either intrinsic or extrinsic in nature.  Intrinsic determinants are physical or physiological characteristics of the host or disease agent (or intermediate host or vector, if present) which are generally determined genetically.  Extrinsic determinants are normally associated with some form of environmental influence to the host or disease agent (or intermediate host or vector, if present). They may also include interventions made by man into the disease process by the use of drugs, vaccines, dips, movement control and quarantines. Thrusfield (1995) acknowledged that some determinants (both primary and secondary) are internal to the host. These determinants are intrinsic, also termed endogenous (Greek: endon = within). In contrast, some determinants are external to the host. Such determinants are extrinsic, also termed exogenous (Greek: exo = outside). Intrinsic Determinants Disease agents as determinants of disease Characteristic of the infecting agent (Infectivity, Virulence and Pathogenecity) Host/agent relationships Methods of transmitting infectious agents Host determinants Extrinsic Determinants Climate Soils Man INTRINSIC DETERMINANTS A. Disease agents as determinants of disease Agents associated with disease can be categorized into two broad groups: “Living” agents, such as viruses, bacteria, rickettsia, protozoa, helminthes, arthropods, etc. “Non-living) agents, such as heat and cold, water, nutrients, toxic substances, etc. FLM 1.0 Prepared by Daniel C. Ventura, Jr., DVM, PhD., DipVPH  Course Code: PHLT 2 Veterinary Epidemiology Student’s Activity sheet for Session #8 Name: _____________________________________________________________ Class number: _______ Section: ____________ Schedule: ________________________________________ Date: ________________ Disease cannot occur in the absence, but conversely, disease need not always result from the presence of the agent. A common misconception is that infection is synonymous with disease. There's a difference between infection and disease. Infection is defined as the invasion of a living organism (the host), by another living organism (the agent). Disease is defined as a derangement in the function of the whole body of the host or any of its parts. An infection occurs when bacteria, viruses or other microbes that cause disease enter the animal body and begin to multiply (often the first step to disease). Disease occurs when the cells in the infected body are damaged — as a result of the infection — and signs and symptoms of an illness appear. B. Characteristics of the Infecting Agent Infectivity o the measure of the ability of a disease agent to establish itself in the host. o Attempts to quantify infectivity normally involve the use of a statistics known as ID50 (the individual dose or numbers of the agent required to infect 50% of a specified population of susceptible animals under controlled environmental conditions) Virulence o a measure of the severity of a disease caused by a specified agent. In its strict sense, virulence is a laboratory term and is used to measure the varying ability of disease agents to produce disease under controlled conditions. o often quantified by a statistic known as LD50 which refers to the individual dose or numbers of the agent which will kill 50% of a specified population of susceptible animals under controlled environmental conditions. Pathogenicity o the ability of a particular disease agent of known virulence to produce disease in a range of hosts under a range of environmental conditions. C. Host/Agent Relationships The two mechanisms whose consequences are of particular importance in the field of livestock disease control are the carrier state and antigenic variation. Creation of the Carrier State FLM 1.0 Prepared by Daniel C. Ventura, Jr., DVM, PhD., DipVPH  Course Code: PHLT 2 Veterinary Epidemiology Student’s Activity sheet for Session #8 Name: _____________________________________________________________ Class number: _______ Section: ____________ Schedule: ________________________________________ Date: ________________ The term "carrier" is used to describe an individual that is infected by a disease agent and is capable of disseminating that disease agent but shows no sign of clinical disease. Three types of carrier state are o True carrier - an infected individual capable of disseminating the infectious agent but which never exhibits clinical signs of disease. o Incubatory carrier - an infected individual capable of disseminating the infectious agent while the disease is still in the incubatory stage. o Convalescent carrier - an individual that continues to disseminate the infectious agent after the clinical signs of the disease have disappeared Antigenic Variation Some species of disease agent seek to evade the hosts' defense mechanisms by altering their antigenic characteristics. Technically, antigenic variation is the multiphasic expression of antigenically different, but functionally conserved, molecules within a clonal population. Example, new strains of influenza virus continually emerge which have mutated so they are no longer destroyed by the antibodies and T-cells that we developed in previous infections. There are two main types of antigenic variation: o Antigenic Drift involves only minor changes in antigenicity, so that hosts previously infected with the agent retain a certain degree of immunity to the drifted strain. o Antigenic shift involves a major change in antigenicity, so that previously infected individuals possess to the shifted agent. Other terms used to further define host/agent relationships include: o Incubation Period, which is the period of time that elapses from the infection of the host by the agent to the appearance of clinical symptoms. o Prepatent Period, which is the period between the infection of the host by the agent and the detection of the agent in the tissues or secretions of the host. o Period of Communicability, which is the period of time during which an infected host remains capable of transmitting the infective agent. D. Methods of Transmitting Infectious Agents Ascertaining the means by which disease agents are transmitted is a major objective in epidemiological studies, since once the mechanisms by which a particular disease is transmitted are understood, it may become possible to introduce measures to prevent transmission from taking place. FLM 1.0 Prepared by Daniel C. Ventura, Jr., DVM, PhD., DipVPH  Course Code: PHLT 2 Veterinary Epidemiology Student’s Activity sheet for Session #8 Name: _____________________________________________________________ Class number: _______ Section: ____________ Schedule: ________________________________________ Date: ________________ There are three main ways by which disease agents are transmitted from infected to susceptible hosts. These methods of transmission are not mutually exclusive; the same disease agent may be transmitted by more than one of the following ways: Contact Transmission Direct contact – the agent is conveyed between hosts through direct physical contact. Indirect contact – the agent is normally contained in the excretions, secretions, or exhalations of the infected host i.e. in the feces, urine, milk, saliva, placenta and placental fluids, or as aerosols or droplets in the breath. Contact transmissions can be further distinguished according to whether they occur Horizontally between individuals of the same generation or Vertically between individuals of different generations. In vertical transmissions the infectious agent is usually passed from dam to offspring either in the uterus or through the colostrum. The main factors determining whether or not transmission takes place in contact- transmitted diseases are: o The ability of the agent to survive in the environment. o The extent of the contact that occurs between infected and susceptible individuals of the host populations and their mobility within these populations. Vehicular Transmission In vehicular transmission the agent is transferred between infected and susceptible hosts by means of an inanimate substance or object (sometimes called ‘fomite’), such as water, foodstuffs, bedding materials, veterinary equipment and pharmaceuticals or on the skin, hair or mouthparts of animals. In contrast to indirect transmission, the survival time of the agent in or on the vehicle is usually prolonged. This means in effect, that vehicular transmission can take place over greater distances and over longer time periods. Hygiene, disinfection and control over the distribution of likely vehicles of transmission are important factors in the control of vehically transmitted diseases. Vectors and Intermediate Hosts A Definitive Host is a host in which the agent undergoes a sexual phase of its FLM 1.0 Prepared by Daniel C. Ventura, Jr., DVM, PhD., DipVPH  Course Code: PHLT 2 Veterinary Epidemiology Student’s Activity sheet for Session #8 Name: _____________________________________________________________ Class number: _______ Section: ____________ Schedule: ________________________________________ Date: ________________ development. It is usually a vertebrate. An Intermediate Host is a host in which the agent undergoes an asexual phase of its development. Intermediate hosts can either be vertebrates or invertebrates. A Vector is an invertebrate animal that actively transmits an infectious agent between infected and susceptible vertebrates. Essentially, vectors can transmit infectious agents in two ways:  Mechanical Transmission – the vector serve as a vehicle whereby the infectious agent is conveyed from one host to another without undergoing a stage of development or multiplication.  Biological Transmission – the infectious agent can undergo some stage of development or multiplication in the vector. In this case the vector is serving either as an intermediate or definitive host, depending on which stage of the development cycle of the agent takes place within it. In addition, vectors may be able to pass the agent on to their own offspring transovarially. Transovarial Transmission enables an infectious agent to be maintained in a vector population through many generations without that population having to be reinfected, and, as such, the vector population remains a continuous source of risk. If transovarial transmission does not occur, at least one stage in each generation of the vector must become infected before transmission of the agent can take place. Arthropod vectors that undergo metamorphosis have the capacity to pass an agent from one developmental stage to the next. This is known as Transtadial Transmission. Usually in transtadial transmission, one developmental stage becomes infected with the disease agent and the following stage transmits it. If different developmental stages feed on different host species, transtadial transmission can provide a mechanism for an inter- species transmission of disease agents. E. Host Determinants The main intrinsic determinant in the host which can influence the frequency of occurrence of infection and disease are species, breed, age, sex. Species Susceptibilities and Natural Reservoirs Most disease agents are capable of infecting a range of animal species, both vertebrate FLM 1.0 Prepared by Daniel C. Ventura, Jr., DVM, PhD., DipVPH  Course Code: PHLT 2 Veterinary Epidemiology Student’s Activity sheet for Session #8 Name: _____________________________________________________________ Class number: _______ Section: ____________ Schedule: ________________________________________ Date: ________________ and invertebrate. The severity of the disease resulting from such infections may, however, vary between the species concerned. The multi-species susceptibility to disease agents is particularly important if the species concerned are able to maintain the disease agent within their populations i.e. to function as natural reservoirs of infection. The failure of programmes aimed at controlling a certain disease in one species has often been blamed on the presence of natural reservoir species, because they can reintroduce the infectious agent. When investigating the potential of a certain species to act as a natural reservoir of a particular disease agent, and the implications this would have on disease control policy, the following considerations need to be borne in mind: Infection with the Disease Agent. Although it may be possible to infect a certain host species with a disease agent under laboratory conditions, this may only be achievable by using a method of transmission that does not occur naturally (e.g. intracerebral inoculation). If this is the case, that particular host species is unlikely to play a significant role in the epidemiology of the disease. Ability of a Host Species to Maintain a Disease Agent. It may prove possible to demonstrate that a particular host species can be infected by a certain disease agent and that the infection can be accomplished by a natural means of transmission. Transmission from the Natural Reservoir. Even if a species can function as a natural reservoir for a particular disease agent, transmission from that reservoir to domestic livestock may only occur rarely and in certain, clearly defined circumstances. If this is the case, the reservoir species is unlikely to cause a major problem in the initial control of disease in question. However, when the frequency of occurrence of the disease becomes a possibility, the implications of the presence of reservoir host species for the success of the proposed eradication programme may have to be re-assessed. Breed Susceptibilities Within a host species, wide ranges of susceptibility to a particular disease are often observed between different breeds. It is important, however, to distinguish between the differences in susceptibility that are genuinely related to breed or species and the differences that may arise as a result of previous exposure to infection. Within breeds too, differences in susceptibility to the same disease agent have been noted between strains or families. This has led, in recent years, to the development of breeding programmes designated to select for disease resistance. Selective breeding FLM 1.0 Prepared by Daniel C. Ventura, Jr., DVM, PhD., DipVPH  Course Code: PHLT 2 Veterinary Epidemiology Student’s Activity sheet for Session #8 Name: _____________________________________________________________ Class number: _______ Section: ____________ Schedule: ________________________________________ Date: ________________ has been pioneered in the field of poultry industry where a large number of different “line” of poultry have been developed that are resistant to such diseases as Marek’s disease, salmonellosis and even vitamin D and manganese deficiencies. Age Susceptibilities Differences in susceptibility to disease are often seen between different age groups. There is however, often a problem in distinguishing between true age resistance in young animals and passive resistance occasioned by the transfer of maternal antibodies via the placenta or in the colostrums. A false impression of age susceptibility may also be created when a highly infectious disease occurs frequently in a population. It may, for instance, appear that only young individuals affected by the disease in question. This may not be due to a difference in age susceptibility but simply because the older individuals, who had been infected previously, represent a surviving and immune population. Sex Associations in Disease In these associations the clinical signs of disease are associated with sexual attributes, as in the case of diseases of the reproductive tract, rather than with the fact that males may be more susceptible than females or vice versa. Sometimes too, one particular sex may be regarded by farmers as being of greater value than the other and will therefore receive a correspondingly greater amount of care and attention when sick. EXTRINSIC DETERMINANTS OF DISEASE Extrinsic determinants of disease are important in epidemiology in that they can have effects on the host, on the agent, and on the interactions between the host and the agent. They can also affect intermediate hosts or vectors involved in the transmission of a disease, and thus determine the type and extent of a disease transmission taking place. There are three major extrinsic determinants. The first two are climate and soils, which, by interacting in a variety of ways, affect the environment of the host, the agent, and the intermediate host or vector, if they are present. The third major factor is man, who, uniquely among animals, has the ability to modify both the environment in which he lives and the environment in which he keeps his livestock. The Three Major Extrinsic Determinants: FLM 1.0 Prepared by Daniel C. Ventura, Jr., DVM, PhD., DipVPH  Course Code: PHLT 2 Veterinary Epidemiology Student’s Activity sheet for Session #8 Name: _____________________________________________________________ Class number: _______ Section: ____________ Schedule: ________________________________________ Date: ________________ Climate When considering climate as a determinant of disease, a distinction is normally made between the macroclimate or weather, and the microclimate. The term microclimate refers to the actual climatic conditions prevailing in the specific, restricted environment where the host, agent, vector or intermediate host actually live. Macroclimates. A large number of different factors combine to make up the macroclimate. Some of these factors (heat, cold, rainfall, wind, humidity, etc) can act as disease agents in their own right, either individually or in combinations. As such they can cause disease in young and newborn animals which are particularly sensitive to heat, cold and dehydration. In older animals they tend to act more as indirect determinants of disease in that they can produce – either alone or in combinations with other managemental and nutritional determinant – “stress” conditions in the host, which may lower its resistance both to infection and, if infection takes place, to disease. Microclimates. Regions where existing macroclimatic conditions might be thought unsuitable for the transmission of a disease may, in fact, contain limited areas where the microclimatic conditions are suitable for the survival of the disease agent and its vector or intermediate host. Soils By interacting with climate, soil determine vegetation and the environment in which the livestock are kept. The main effect of vegetation is on nutrition. Soils therefore act indirectly as determinants of disease by causing starvation, if there is little or no vegetation, or nutritional imbalances such as protein, energy, vitamin or mineral deficiencies. Malnutrition can be direct cause of disease, or it can stress the host and thus increase its susceptibility to infection and disease from other sources. Soils can also have an effe3ct on the ability of the agent to survive in the environment, through such factors as water logging, pH, etc. Man Man is often able to create favorable, artificial microclimates for livestock rearing by providing such inputs as housing, water supplies, irrigation etc. Unfortunately, this often results in the creation of conditions favorable for the survival of disease agents and their intermediate hosts or vectors. This means that, by altering the environment, man can alter the determinants of the diseases present in that environment. The changes in determinants will favor some diseases and be detrimental to others. Thus, changes in systems and methods of production will result in changes in the relative importance of the disease present, with perhaps some new diseases being introduced and others disappearing. FLM 1.0 Prepared by Daniel C. Ventura, Jr., DVM, PhD., DipVPH  Course Code: PHLT 2 Veterinary Epidemiology Student’s Activity sheet for Session #8 Name: _____________________________________________________________ Class number: _______ Section: ____________ Schedule: ________________________________________ Date: ________________ Man is also able to interfere directly in the disease process through the use of drugs, vaccines, movement controls, quarantines etc. Among the main tasks of the epidemiologist is the investigation of the efficacy of such measures, as well as to design ways in which they can be used most efficiently and to monitor the effects of their introduction on disease incidence. 2) Activity 3: Skill-building Activities A disease narrative will be uploaded in google classroom. Your task is to identify and classify the determinants of the disease. 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) Link to short quiz is 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 ☐ ☐ ☐ ☐ ☐ ☐ ☐ ☐ ☐ ☐ ☐ ☐ ☐ ☐ ☐ ☐ 2) My learning targets: Complete the table below. Record your scores, learning experience for the session and deliberately plan for the next session. FLM 1.0 Prepared by Daniel C. Ventura, Jr., DVM, PhD., DipVPH  Course Code: PHLT 2 Veterinary Epidemiology Student’s Activity sheet for Session #8 Name: _____________________________________________________________ Class number: _______ Section: ____________ Schedule: ________________________________________ Date: ________________ 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