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Disease Transmission During Epidemics

• An epidemic involves the rapid spread of an infectious disease within a specific area or population during a defined time.
• The Ebola virus disease epidemic in West Africa (2014-2016) spread through direct contact with blood or body fluids of infected individuals (alive or deceased).
• Transmission also occurred via objects contaminated with these fluids and potentially through infected fruit bats or non-human primates.
• Ebola virus disease is NOT transmitted through food or airborne droplets

Modes of Transmission of Infectious Diseases

• Direct contact involves physical interaction between a host and an infected organism.

Horizontal Transmission

• Transmission between individuals of the same generation
• Examples include touching, sexual contact, kissing, contact with secretions, biting, bodily fluids, prenatal, perinatal, and wound contact.

Vertical Transmission

• Transmission between parent and offspring.

Indirect Contact

• Disease transmission without direct contact, often involving fomites (contaminated objects).
• Airborne transmission and infection from reservoirs outside the host, such as contaminated surfaces, also fall under this category.
• Examples include airborne transmission (coughing/sneezing), fomites, contaminated food or surgical instruments, and vectors.

Vector Transmission

• A live agent (vector) carries and transmits an infectious pathogen to another organism, commonly through arthropods.
• Mosquitoes transmit pathogens during blood-sucking.
• Infected plants and fungi can also act as vectors.

Koch's Postulates

• Robert Koch developed the agar-plate technique for culturing microorganisms.
• He identified and cultured anthrax bacillus and demonstrated that injecting these cultures into healthy sheep caused anthrax.
• Koch's postulates identify specific microorganisms:
• The microorganism must be present in every diseased host.
• It must be isolated and cultured in a laboratory.
• Inoculation of a healthy host with the pure culture must produce the same symptoms.
• The microorganism must be re-isolated from the secondary host and identified as the original microorganism.

Limitations of Koch's Postulates

• Asymptomatic infections can make the third postulate untrue.
• Viruses, protozoa, and prions cannot be studied using these postulates.

Pasteur's Experiments on Microbial Contamination

• Louis Pasteur is credited with establishing the science of microbiology.
• He found that bacteria cause wine, beer, and vinegar spoilage, leading to pasteurization.
• His swan-necked flask experiments disproved spontaneous generation by showing that boiled broth remained sterile unless exposed to air.
• Pasteur investigated anthrax, contributed to the germ theory of disease, and developed vaccines for chicken cholera, anthrax, and rabies.
• He established the principle of immunity as a way to prevent infectious disease.

Adaptations of Pathogens for Entry and Transmission

Bacteria

• Adhesion factors: pili, fimbriae, adhesions, and biofilms.
• Invasion factors: bacterial proteins, enzymes, capsules, IgA protease, and toxins.

Viruses

• Surface proteins for adhering to host cell receptors.
• Receptor-mediated endocytosis or pore formation for cell entry.
• Some viruses enter as an envelope and inject their nucleic material for replication.

Fungi

• Adhesion factors: wall or capsule molecules.
• Thermotolerance for adapting to host temperatures.
• Dimorphism: conversion from mycelium to parasitic yeast.
• Cell wall, capsules, and hydrolytic enzymes for defense and nutrient acquisition.
• Evasion mechanisms: capsule production, suppression of cytokines.

Protozoa

• Microtube protrusion for host cell entry (e.g., Toxoplasma gondii).
• Receptor-mediated attachment (e.g., Trypanosoma cruzi).
• Production of enzymes and toxins.
• Pathogen enters vacuole and deactivates lysosomal enzymes

Macroparasites

• Hookworms: secrete immunomodulatory proteins.
• Ticks: specialized mouthparts and attachment cement, secrete molecules to prevent vasoconstriction and clotting.
• Endo- secrete proteins inhibit immune response
• Ecto- Specialised mouth parts + enzymes that prevent inflammation

Prions

• Host B lymphocytes facilitate prion invasion of dendritic cells in lymph tissue.
• Prions invade nervous tissue via autonomic nerves.
• They may "piggyback" on proteins like ferritin to enter the gut.
• Prions significantly impact nerve signaling due to protein misfolding.

Animal Responses to Pathogens

First Line of Defense: Barriers

• Physical and chemical changes occur in animal cells and tissues in response to pathogens.
• These are typically non-specific responses.

Physical Barriers

• Skin: keratin coating and symbiotic bacteria.
• Mucous membranes: mucus production and reflex actions.
• Cilia: trap and secrete pathogens.

Chemical Barriers

• Stomach acid: lethal environment for pathogens.
• Saliva and tears: lysozymes and immunoglobulin A (IgA).
• Urine: antimicrobial peptides.
• Sebum: waterproofs and lubricates skin.
• Sweat: lactic acid and lysosomes.

Second Line of Defense: Programmed Responses

• Programmed physical and chemical responses are non-specific.

Physical Responses

• Fever: creates an uninhabitable environment for pathogens.
• Cell death: seals off infected areas (granuloma formation).
• Vomiting and diarrhea: expel harmful substances.
• Increased urination: flushes out pathogens.
• Wound healing: vasoconstriction, platelet plug, fibrin clot.
• Peristalsis: moves food through the alimentary canal.
• Sphincters: maintain constriction of body passages.

Chemical Responses

• Histamine production (inflammation): aids wound repair and pathogen destruction.
• Histamines dilate capillaries, increasing blood flow and WBC movement.
• Endogenous pyrogens: increase body temperature.

Phagocytosis

• Carried out by specialized white blood cells (leukocytes).
• Neutrophils: release toxins to kill pathogens and recruit immune cells.
• Monocytes: differentiate into dendritic cells and macrophages.
• Macrophages: antigen-presenting cells.
• Dendritic cells: trigger the adaptive immune response.
• Natural killer cells: kill tumor and virus-infected cells.

Cytokines

• Chemical messengers produced during infection.
• Promote the development and differentiation of T and B lymphocytes.
• Interferons: signal uninfected cells to destroy RNA and reduce protein synthesis and infected cells to undergo apoptosis.

Lymph System

• Transports lymph with lymphocytes (WBC, T-cells, B-cells).

Human Immune System Response

Innate and Adaptive Immune Systems

• The innate immune system includes the first and second lines of defense.

Innate Immune System Response

• Inflammation: histamines dilate capillaries to increase blood flow and WBC movement.
• Phagocytosis: engulf and digest bacteria.
• Complement System: A group of twenty soluble proteins assisting other defense mechanisms in destroying pathogens, stimulating the activation of phagocytes, attracting them to the site of infection or destroying membranes of pathogens. Complement proteins are made in liver cells and macrophages. In the third line of defence, these proteins are attracted to the pathogen-antibody complex, acting as a signal for phagocytes and other lymphocytes called B cells to destroy the pathogen (opsonisation).
• Cell Death: infected cells die (apoptosis).

Blood Cell Differentiation

• Bone marrow is the main site of blood cell development
• Multipotent Hematopoietic Stem Cells (HSCs) develop into either myeloid of lymphoid cells
• These further differentiate into different types of blood cells that are used in the innate and acquired immune system.

Adaptive Immune System

• Has two areas → humoral response (B-cells) and cell-mediated response (T-cells).
• Macrophages engulf microbes, express antigens on MHCII, and become antigen-presenting cells.
• Helper T-cells bind to the foreign protein/peptide complex and release cytokines.
• B-cells differentiate into plasma cells (secrete antibodies) and memory B-cells (Humoral response).
• Cytotoxic T-cells bind to infected cells using MHC1, lyse them, and release interferons (Cell-mediated response).
• Suppressor T-cells stop the immune response.

Major Histocompatibility Complex (MHC)

• MHC1: present in all nucleated cells to distinguish "self" from "non-self."
• MHC2: only on APCs (macrophages, dendritic cells, B-cells) to present antigens to helper T-cells.

The specific Response:

• T-cell receptors are surface proteins but their structure is unlike antibodies → Bound to cell membrane.
• Clonal selection theory states that all the B and T cells for all the possible antigens are already present in very small amounts in the immune system from birth.

5 Ways to ensure a pathogen is stopped from harming the host:

• Neutralisation → Antibodies bind to and coat pathogens, blocking their activity; preventing it from entering other cells.
• Agglutination → Neutralised pathogens clump together and are surrounded by thousands of antibodies → Form antigen-antibody complexes that are attractive to phagocytes.
• Precipitation → Of dissolved antigens
• Activation → of the complement system leading to lysis of infected cells
• Opsonization → Enhance phagocytosis by natural killer cells.

Controlling the Spread of Infectious Diseases

Factors in Disease Spread

• Local: sanitation, overcrowding, poor communication, animal husbandry, cultural beliefs.
• Regional: geography (mountains, deserts, coastlines).
• Global: increased travel, migration, antibiotic misuse.

Ebola Outbreak in West Africa

• Local: contact with wild animals, burial practices, movement to cities, distrust of medical workers.
• Regional: population movement, political instability, poor medical infrastructure.
• Global: international response, quarantine measures, WHO mobilization.

Disease Prevention Procedures

• Public health campaigns: government and health organization resolution, information collection, coordination of efforts, education.
• Strict guidelines must be followed in hospitals when sterilising equipment and when health workers move from patient to patient
• Vaccine mandates in certain job areas -The law requires that certain diseases are reported if detected, so appropriate strategies can be formed to limit its spread.

Pesticides

• Used to kill vectors (insects, fungi, weeds, rodents).
• Insecticides, fungicides, and herbicides.
• Overuse leads to resistance and environmental damage.

Data Interpretation

• Mobility of individuals and the portion that are immune or immunised

Incidence

• The Number of new cases occurring during a specified time; like an infection rate or a probability risk expressed as a %:
• An incidence rate describes how quickly disease occurs in a population. It is based on person-time, so it can accommodate persons coming into and leaving the study.

Prevalence

• The Proportion of the population that has the disease at a particular point in time.
• It is, therefore, a way of assessing the overall burden of disease in the population, so it is a useful measure for administrators when assessing the need for services or treatment facilities.
• Incidence is only new cases while prevalence is all cases → old and new cases

Mobility

• Humans act as carriers for pathogens and may spread the disease to new locations when they move

Herd immunity

• reduces the change for non-immunised individuals to come into contact and thus contract the disease-causing microbe

Diseases in South East Asia

Malaria

• Malaria is an acute febrile illness caused by Plasmodium parasites, which are spread to people through the bites of infected female Anopheles mosquitoes
• The latest World Malaria report shows 247 million cases of Malaria in 2021
• There is a vaccine for recommended to children who are most at risk

Dengue Fever

• Dengue Fever is a mosquito-borne viral disease that has rapidly spread over the past years, being transmitted by female mosquitoes mainly of the species Aedes aegypti.
• Brought by a mosquito vector Aedes Aegypti
• Vaccine in place but only for secondary infection prevention