Intro to Infectious Diseases - PID Lecture I Summer 2024 PDF

PRINCIPLES OF INFECTIOUS DISEASES SUMMER 2024 PEDRO BITTENCOURT, DVM, MSC, PHD Welcome to Principles of Infectious Diseases Susceptible host Breed, age, sex, genotype Physiology and susceptibility Immune response Infectious disease Pathogen Pathogenicity Dispersal efficiency Survival efficiency Inoc...

PRINCIPLES OF INFECTIOUS DISEASES SUMMER 2024 PEDRO BITTENCOURT, DVM, MSC, PHD Welcome to Principles of Infectious Diseases Susceptible host Breed, age, sex, genotype Physiology and susceptibility Immune response Infectious disease Pathogen Pathogenicity Dispersal efficiency Survival efficiency Inoculation dose Environment Abiotic and biotic factors Housing management Population density Sanitation Nutrition Prevention Learning Objectives Define infection, disease, and infectious diseases. Define the different types of host-microbe interactions Describe the potential outcomes of the host’s exposure to infectious microbes Understand the importance of Koch’s postulates for the identification of pathogens causing diseases and its limitations Describe the stages of infectious diseases Define One Health and describe its importance Terminology Infection = the invasion and multiplication of pathogens in an individual or population Disease = a disorder of structure or function in the host that adversely impacts the host and is not simply a result of physical injury Infectious Diseases = diseases caused by pathogenic microorganisms (e.g. bacteria, viruses, parasites, fungi or prions). Some diseases are communicable = can be spread directly or indirectly, from one animal to another. Etiology = study of the cause of disease Microorganism or Microbe: any organism that is invisible to the naked eye First observed by Antonie van Leeuwenhoek in the 1670s HISTORY Agostino Bassi - 1835 Demonstrated that a silkworm disease, known as muscardine, was contagious and was caused by a microscopic fungus: Beauveria bassiana Louis Pasteur - 1850s-1880s Studied the role of microbes in fermentation and disease, developed the pasteurization process and developed vaccines against rabies and anthrax Robert Koch - 1860s-1880s Discovered the bacteria responsible for anthrax, tuberculosis and cholera; developed the Koch’s postulates. RO S S U N I V E RS I T Y S C H O O L O F V E T E RI N A R Y M E DI CI NE Smallest Prions Viruses Bacteria Fungi Protozoa Largest Multicellular parasites RO S S U N I V E RS I T Y S C H O O L O F V E T E RI N A R Y M E DI CI NE Microbe-Host relationship Free-living Commensal A microorganism that is a normal inhabitant of the animal body In commensal relationships, either the microbe or host (or both) derives benefit; neither is harmed under normal circumstances Pathogen A microorganism capable of causing disease Includes commensals and non-commensals RO S S U N I V E RS I T Y S C H O O L O F V E T E RI N A R Y M E DI CI NE Microbe-Host relationship Obligate pathogen A microorganism that must produce disease* to transmit and thereby survive evolutionarily. Obligate pathogens are not commensals, but they can produce asymptomatic infections *In this context: damage to the organism at a molecular level, with or without clinical signs Facultative pathogen Microorganism which can infect and multiply in hosts, occasionally causing disease, but also capable of multiplying in the environment RO S S U N I V E RS I T Y S C H O O L O F V E T E RI N A R Y M E DI CI NE Microbe-Host relationship Commensal pathogen A microorganism that is commonly found within the indigenous microbiota that can cause disease in normal hosts with some regularity. Example: Staphylococcus aureus, Streptococcus pneumoniae, Candida albicans Not all manifest as pathogens with the same frequency, some may require significant impairment of the host’s immunity to cause disease - Opportunistic pathogens RO S S U N I V E RS I T Y S C H O O L O F V E T E RI N A R Y M E DI CI NE Microbe-Host relationship Environmental pathogen A microorganism capable of causing disease that is transmitted to the host from an environmental source such as water or soil. Examples: Clostridium tetani, Clostridium botulinum Zoonotic pathogen A microorganism that is a coloniser or pathogen on animals and that can be transmitted to humans either through vectors (i.e. ticks) or direct contact with the animal or its products. Examples: Brucella abortus, Rabies RO S S U N I V E RS I T Y S C H O O L O F V E T E RI N A R Y M E DI CI NE Attributes of Pathogens Enter host Can occur through the skin, mucosa or body’s orifices Commensal pathogens may skip this step, they’re “already there” Establish a unique habitat within the host Usually requires crossing physical barriers and/or breaching the host’s defenses Multiply within host Typically what causes disease Exit to infect new host Essential trait for obligate pathogens OUTCOMES OF EXPOSURE TO INFECTIOUS AGENTS OUTCOMES OF EXPOSURE TO INFECTIOUS AGENTS EXPOSURE ≠ INFECTION ≠ DISEASE KOCH’S POSTULATES 1. The suspected pathogen must be found in every case of disease and not be found in healthy individuals. 2. The suspected pathogen can be isolated and grown in pure culture. 3. A healthy test subject infected with the suspected pathogen must develop the same signs and symptoms of disease 4. The pathogen must be re-isolated from the new host. KOCH’S POSTULATES KOCH’S POSTULATES LIMITATIONS Inability to isolate the organism in pure culture Inability to isolate the organism in cell-free culture Organisms for which a single species acts as host Subclinical infection Organisms that cause ‘distant’ infection Disease caused by toxins Diseases that require coinfection Organisms associated with tumors Five stages of an infectious disease 1. Incubation - Pathogen multiplies without clinical signs 2. Prodromal - General and unspecific signs 3. Illness - Signs and symptoms are more severe and specific 4. Decline - Number of pathogens and clinical signs begin to decline 5. Convalescence - Return to normalcy* Five stages of an infectious disease