Unit 4- Disease Mechanism PDF

Unit 4: Disease Mechanism Presented by: Ms Muhongo  Learning objectives By the end of this unit, the students should be able to : ❖Understand fundamentals of infectious diseases process ❖✓ Understand transmission of a disease ❖✓ Outline the basic principles and importanc...

Unit 4: Disease Mechanism Presented by: Ms Muhongo  Learning objectives By the end of this unit, the students should be able to : ❖Understand fundamentals of infectious diseases process ❖✓ Understand transmission of a disease ❖✓ Outline the basic principles and importance of epidemiology ❖✓ Explain principles of disease ❖✓ Describe communicable and contagious diseases, emerging and re- emerging diseases. ❖✓ Explain periods, types, pattern of infection  Concepts Pathogenesis ✓ This can be defined as the development of a disease and the chain of events leading to that disease. Infection ✓ This refers to an invasion of the body by harmful microorganisms. Infectious diseases ✓ These are disorders caused by pathogenic microorganisms. ✓ Its also an illness due to a pathogen/ its toxic product transmitted from an infected person/animal/contaminated inanimate object to a susceptible host.  Introduction ❖To understand infectious diseases, we must consider four key elements: the disease-causing organism (causative agent), how it spreads (mode of transmission) , the person or animal it affects (susceptible host), and the surrounding conditions (environment). ❖For a disease to occur, there must be a specific pathogen that can be transmitted from one living being to another. However, the impact of the disease will be influenced by external factors (environment).  4 key elements in infectious diseases 1. Agent: The microorganism causing the disease (e.g., bacteria, virus, fungus, parasite). 2. Host: The organism (usually a human or animal) that is infected by the agent. 3. Environment: The external conditions that influence the spread and impact of the disease (e.g., climate, sanitation, population density). 4. Mode of Transmission: The way the agent moves from one host to another (e.g., airborne, contact, waterborne, vector-borne). The agent: mechanisms in causing a disease Infectious agents use various strategies to cause disease. These include: ❖Adherence and colonization: Attaching to host cells and establishing a presence. ❖Invasion: Penetrating host tissues and spreading within the body. ❖Damage to host cells: Directly harming cells or releasing toxins. ❖Immune evasion: Avoiding the host's immune response. ❖Persistence: Remaining in the host for an extended period.  Infection Once a microorganism enters the body, it can result in either infection or colonization, and it's important to differentiate between the two terms. ✓Infection occurs when a pathogenic organism not only enters and establishes itself but also multiplies and destruct the host’s normal anatomical barriers, leading to disease. ✓Colonization, on the other hand, happens when the pathogen enters and multiplies but does not invade the host's tissues or cause disease, nor does it trigger a specific immune response.  Infection based on the origin There are two types based on the origin: ❖Endogenous infections occur when pathogens from the host's own microbiota or internal sources become harmful, often due to factors like weakened immunity or disruption of normal microbial balance. Examples include urinary tract infections and certain post-surgical infections. ❖Exogenous infections are caused by pathogens from external sources, such as other people, animals, or contaminated environments. They are transmitted through direct contact, airborne particles, vectors, or contaminated food and water. Examples include the common cold, influenza, and foodborne illnesses. Infection Based on the clinical manifestation produced ❖Acute Infections: Sudden onset with intense symptoms that resolve quickly, such as influenza or streptococcal pharyngitis. ❖Chronic Infections: Persistent over a long period with ongoing or recurrent symptoms, like tuberculosis or chronic hepatitis B. ❖Latent Infections: Pathogens remain dormant in the body and can reactivate later, such as herpes simplex virus or latent tuberculosis. ❖Localized Infections: Confined to a specific area, causing localized symptoms like a skin abscess or otitis media. ❖Systemic Infections: Affect the entire body, often involving multiple organs and causing widespread symptoms, such as sepsis or septicemia. ❖Primary infection: Initial infection in a healthy individual ❖Secondary infection: Occurs due to weakened immune system after primary infection (e.g., pneumonia after viral respiratory infection) ❖Focal infection: Starts in one area, spreads to another (often via bloodstream) (e.g., dental infection to heart)  Reasons why infection does not always occur ❖Microbe may land at an anatomic site where it unable to multiply. ❖Many pathogens must attach to a specific receptor site ❖Antibacterial factors that destroy/inhibit the growth of bacteria may be present ❖Indigenous microflora of that site may inhibit growth of the microbe ❖Indigenous microflora may produce antibacterial factors ❖Individual’s nutritional and health status  THE CHAIN OF INFECTION There are six components in the infectious disease process (also known as the chain of infection): 1. Infectious agent (pathogen) 2. Reservoir 3. Portal of exit 4. Mode of transmission 5. Portal of entry 6. Susceptible host Components in Chain of infection Pathogen agent (causative agent) ❖The infectious agent is the first link in the chain of infection. It is the microorganism that causes disease. This can include bacteria, viruses, fungi, parasites, or prions. Reservoir ❖The reservoir is the second link in the chain of infection. Is the place where an infectious agent normally lives and multiplies. It's essentially the habitat for the pathogen. This could be a person, animal, plant, soil, or substance. Portal of exit ❖ It’s the third link in the chain of infection. Its how the pathogens escapes/ leaves the reservoir. ❖ Example through coughing, sneezing, insect biting, faeces etc. Components in Chain of infection Method/ mode of transmission It’s the fourth link in the chain of infection. Its how pathogens spread from reservoir to host or from one host to another. E.g. through contact. Portal of entry It’s the fifth link in the chain of infection Its how the pathogens enters a susceptible host. ❖ Example through mouth, blood, nose etc. Susceptible host It’s the sixth link in the chain of infection. A person at risk of picking up a pathogen or develop an infection 10/3/2024 WU 13  Reservoir of pathogens ✓ There are 4 types of reservoir: ✓ Human reservoirs ✓ Animal réservoirs ✓ Insect reservoirs ✓ Non-living reservoirs  Human Reservoirs There are two forms of human reservoirs: ❑ Acute clinical cases: These are individuals who are actively sick and showing symptoms of the disease. They are often easily identifiable as a potential source of infection. ❑ Carriers: These individuals harbor the infectious agent but show no symptoms or only mild ones. They are often unaware of their infection and can unknowingly spread the disease. There are several types of carriers  Types of human carrier Carriers can be further subdivided into four main types: 1. Incubatory carriers: People who are infectious even before their own symptoms start. 2. Asymptomatic Carriers: Individuals who never develop any symptoms but can still transmit the infection. 3. Convalescent carriers: People who have recovered from the illness but continue to be infectious for a period. 4. Chronic carriers: Individuals who recover from an acute illness but continue to harbor the pathogen for a long time  Animal reservoirs Human can be infected by diseases that have animal reservoirs. Many of these diseases (zoonoses/ zoonotic diseases) are transmitted from animal to animal but human incidental get infected. (zoonoses/ zoonotic diseases) refers to infectious diseases that is transmissible under natural conditions from animal to human. Zoonosis is in singular form  Insect reservoirs -ARTHROPODS ❖This group includes mosquitoes, biting flies, lice, fleas) ❖Arthropods are also referred to as vectors ❖They take blood from an infected person and transfer the pathogen to the healthy person.  Non-living reservoirs Non-living, includes water, food, air, fomites and soil. Water- Poor sanitation and hygiene in underdeveloped countries lead to water contamination with feces. This causes diseases like typhoid fever and cholera, spread through the fecal-oral route. Food- contamination is due to spoilage that occurs naturally Fomites are objects or surfaces that can carry germs, such as patient gowns, bedding, utensils, and medical equipment. These items can transmit infections in healthcare settings. Air can be contaminated by dust or respiratory secretions expelled into air by sneezing, talking and coughing. Soil- habitants to most pathogenic microbes and they find a way to pass the physical barriers that protect the body if they are to cause disease  MODES OF TRANSMISSION OF INFECTION Infectious diseases spread through direct and indirect contact. ❑Direct transmission occurs when an infectious agent is transferred directly/ physically from an infected person or animal to a susceptible host ❑Indirect transmission occurs when an infectious agent is transferred to a susceptible host via an intermediate object or surface,  Principal Direct Modes Of Transmission 1. Person-to-Person Contact: Involves direct physical contact between an infected individual and a susceptible host, such as touching, kissing, or sexual intercourse. 2. Droplet Transmission: Occurs when respiratory droplets containing pathogens are expelled during sneezing, coughing, or talking, typically affecting individuals within a short distance (less than 1 meter). 3. Skin-to-Skin Contact: Pathogens can spread through direct contact with the skin, as seen in infections like chickenpox or head lice. 4. Body Fluids: Transmission can occur through contact with infected body fluids (e.g., blood, saliva), leading to diseases like HIV and hepatitis. 5. Vertical Transmission: Pathogens may be transmitted from mother to child during pregnancy, childbirth, or breastfeeding.  Principal Indirect Modes Of Transmission 1. Airborne Transmission: Infectious agents can remain suspended in the air for extended periods, allowing them to be inhaled by individuals who are not in close proximity to the infected person (e.g., measles, tuberculosis). 2. Vehicle-borne Transmission: Pathogens can be transmitted via contaminated inanimate objects (fomites) or substances, such as food, water, or medical equipment. 3. Vector-borne Transmission: Involves transmission through vectors like mosquitoes or ticks, which can carry pathogens from one host to another (e.g., malaria, Lyme disease). Strategies For Breaking The Chain Of Infection To prevent the spread of infectious diseases, it is crucial to break the chain of infection at one or more links. The strategies can be done through: ❖ Practicing Broad goals in order to break the chain of infection ❖Carrying out various Methods in order to break the chain of infection (accomplishing the broad goals strategies)  Broad goals in breaking chain of infection ❖Eliminate or contain the reservoirs of pathogens. ❖Prevent contact with infectious substances from exit pathways (portal of exit) ❖Eliminate means of transmission. ❖Block exposure to entry pathways (portal of entry) ❖Reduce or eliminate the susceptibility of potential hosts. Methods for breaking the chain of infection ❖Ensure proper decontamination of surfaces and medical instruments. ❖Use gloves, gowns, masks, respirators and other personal protective equipment/appliances whenever appropriate to do so. ❖Use needle safety devices during blood collection ❖Practicing effective hand hygiene procedures. ❖Maintaining good nutrition and adequate rest and reduced stress. ❖Obtaining immunizations against common pathogens. ❖Practicing insect and rodent control. ❖Practicing proper patient isolation procedures. 10/3/2024 WU 26 INTERACTIONS BETWEEN PATHOGENS, HOSTS AND ENVIRONMENT Occurrence or non-occurrence of infectious diseases depends on the following factors: 1. Pathogen Factors 2. Host Factors 3. Environment Factors PATHOGEN FACTORS ❖The effect of the agent on the host is determined by its virulence, toxicity and dose. - Virulence- The ability or power of the pathogen to cause disease. - Toxicity- This is the ability of the pathogen’s toxins to produce a disease. - Dose- The number or load of the pathogen to cause disease. THE HOST FACTORS ❖The person’s health status ❖The person’s nutritional status ❖Age ❖Lifestyle ❖Socio-economic level ❖Occupation ❖Travel ❖Hygiene ❖Substance abuse ❖Immune status  THE ENVIRONMENT FACTORS ❖Physical factors such as geographical location, climate, heat, cold, humidity, and season of the year. ❖Availability of appropriate reservoirs ❖Sanitary and housing conditions ❖Adequate waste disposal ❖Adequate health care ❖Availability of safe, clean water especially for drinking.  Epidemiology ✓ Can be defined as the study of a disease in a population. ✓ Involves the study of the frequency and distribution of disease and other health-related factors in defined human populations. The epidemiologist : ✓ Collects clues on the causative agent ✓ Pathology and sources ✓ Modes of transmission ✓ Tracks the numbers and distribution of cases of disease in the community ✓ The outcome of these studies helps public health departments develop prevention and treatment programs and establish a basis for predictions.  Epidemiological Statistics Epidemiological statistics applies statistical methods to study the distribution and causes of health and disease in populations, offering tools to collect, analyze, and interpret health data. Key concepts and measures Incidence- Measures the number of new cases of a disease that occur in a population over a specific period of time. Prevalence- Measures the total number of cases of a disease present in a population at a specific point in time. Mortality Measures the number of deaths from a disease in a population Morbidity Measures the burden of illness, including both the incidence and prevalence of disease.  Geographic Distribution of diseases When epidemiological studies are used to examine parameters such as particular geographic areas and the degree of harm caused by a disease, they classify diseases as: ▪ Sporadic- occur in a random and occasionally and pose no threat to public heath. ▪ Endemic- diseases that are constantly in the population but in low numbers to be a public health problem e.g. common cold flu. ▪ Epidemic- when the incidence of a disease suddenly becomes higher than the normally expected number of individuals affected by the disease. ▪ Pandemic- when the incidence of a disease suddenly becomes higher throughout the world. Terms for disease occurrence  The etiology of disease ▪ Any negative change in a person's health is considered to be a disease. ▪ This change has a cause, which we refer to as the disease's etiology. ▪ Its crucial to understand presence of normal microbial flora before understanding what causes diseases. ▪ Microbes in the human body have beneficial relationships. ▪ Although most of these microorganisms are harmless, some of them are pathogenic, and if these bodily parts are damaged, opportunistic microorganisms can find a way into the body and cause infections.  Stages/periods in the course of an infectious disease: ❖Incubation period ❖Prodromal period ❖Illness period ❖Period of decline ❖Convalescent period 10/3/2024 WU 36  1. Incubation period ❖The incubation period refers to the time between exposure to a pathogenic organism and the onset of symptoms of a disease. ❖It is the period during which the pathogen multiplies and spreads within the host, before causing noticeable illness. ❖The length of the incubation period can vary significantly depending on several factors, including:: Health, immune and nutritional status of the host, the virulence of pathogen, number of pathogen enter the body 10/3/2024 WU 37  2. Prodromal period ❖This is the phase of a disease during which symptoms begin to appear, but they are often vague and nonspecific. ❖Patients may feel "out of sorts" or experience mild symptoms like fatigue, headache, or muscle aches. ❖It's important to recognize the prodromal period as it can be a sign that an illness is developing, and early intervention may be beneficial. 10/3/2024 WU 38  3. Illness period ❖ Is the phase when a patient experiences the full symptoms of a disease, such as fever, cough, or diarrhea. ❖During this period, communicable diseases are most easily transmitted from one person to another.  4. Period of decline ❖The period of decline is the phase of a disease when the symptoms begin to lessen/ subsiding and the patient's condition starts to improve. ❖This is typically followed by the convalescent period, where the body continues to heal and regain strength.  5. Convalescent period ❖ Is the phase of a disease after the acute symptoms have subsided, but the body is still recovering. ❖ During this time, the patient's strength and energy gradually return, and their overall health improves. ❖ However, some people may continue to experience lingering symptoms, such as fatigue or weakness, even after the acute illness has passed. 10/3/2024 WU 41  Duration of a Disease ✓ Acute - Symptoms develop rapidly ✓ Chronic - Disease develops slowly ✓ Subacute Symptoms between acute and chronic ✓ Latent- Disease with a period of no symptoms when the patient is inactive  Infectious Diseases ✓ Symptom: A change in body function that is felt by a patient, such as pain, fatigue, or nausea. ✓ Sign: A change in a body function that can be measured or observed by someone else, such as a fever, rash, or abnormal blood test result. ✓ Syndrome: A specific group of signs and symptoms that accompany a particular disease or disorder.  Epidemiology of infectious Diseases Classification of infectious diseases in the community: 1. Communicable disease- a disease that is spread from one host to another. NB: Communicable diseases are said to be contagious.. WHY? ✓ Because they can easily spread from one host to another 2. Noncommunicable disease- a disease that is not transmitted from one host to another 3. Zoonotic diseases-Infectious diseases that humans acquire from animal sources Communicable Diseases Examples  Epidemiology of infectious Diseases Hospital epidemiology: Nosocomial Infections/ hospital acquired infections ✓ Infectious diseases acquired by patients during their hospital stay Factors that results in these diseases are:  Types of nosocomial infectious diseases ✓ Exogenous- infectious diseases acquired from the health care environment ✓ Endogenous- infectious diseases from normal microbiota due to factors within the health care setting ✓ Latrogenic- infectious diseases from modern medical procedures Nosocomial infections examples  Control and prevention of nosocomial infections ✓ The establishment of an effective infection control organization ✓ Decontamination and sterilization of equipment ✓ Aseptic techniques should be enforced ✓ Cleaning and disinfection of the hospital environment ✓ Educating of antibiotic resistance ✓ Formulation of disinfection documentation policy ✓ Disinfection and antiseptic should be adhered to ✓ Wearing of protective clothing  Causes of emerging diseases ✓ Changes in human demographics and behaviour ✓ Ecological changes such as dams deforestation and climate changes ✓ Increased international travel ✓ Increased exposure to exotic animals ✓ Misuse of antibiotics and other antimicrobial agents ✓ Breakdown of public health measures  Emerging Infectious Diseases List of some of the infectious diseases that have emerged in the last three decades ✓ Avian influenza (bird flu) ✓ Ebola haemorrhagic fever ✓ HIV infection ✓ Sever acute respiratory syndrome (SARS) ✓ West Nile encephalitis  Causes of re-emerging infectious diseases ✓ Pathogen mutations and genetic recombination's ✓ Acquired drug resistance ✓ Decreased compliance with vaccination policies ✓ Breakdown in public health measures ✓ Population shifts, war and civil conflicts, famine, floods, droughts, and bioterrorism  Re-emerging infectious diseases Some of infectious diseases that have re-emerged in new geographic areas in resent years includes; ✓ Cholera, ✓ Dengue Fever, ✓ Diphtheria, ✓ Malaria, ✓ Rift Valley Fever, ✓ TB, ✓ Yellow Fever Types of infection  Pattern of infection This is how infection establish on the host’s body Thank you