Introduction to Infectious Diseases: Terms and Concepts

Course Administration and PID Personnel * Course Details: Ross University School of Veterinary Medicine (RUSVM). Principles of Infectious Diseases (PID). * Course Team: * Dr. Ricardo Guti rrez, Lic. Microb., MSc., Ph.D.: Assistant Professor of Veterinary Bacteriology and Course Coordinator. * Dr. George Nadas: Immunology (locum). * Dr. Pithua: Epidemiology. * Dr. Ghosh: Virology. * Dr. Yao: Parasitology. * Instructional Approach: Different professors utilize different teaching styles to provide enrichment. Specific learning objectives are provided per lecture or topic to serve as guidelines. * Academic Support: Students are encouraged to keep up with information, ask questions, and collaborate with peers and Teaching Assistants (TAs). Assistance includes one-on-one or group meetings and email contact. * Assessment Weighting: * Block 1 and Block 2 * Block 3: Includes current material plus $10\text{--}25\%$ from previous blocks. * Block 4: Includes current material plus $10\text{--}25\%$ from previous blocks. * Block 5: Includes current material plus $10\text{--}25\%$ from previous blocks. * Block 6: Includes current material plus $10\text{--}25\%$ from previous blocks. # Literature and Educational Resources * Primary Textbook: Veterinary Microbiology and Microbial Disease, Second Edition (Wiley-Blackwell). * Authors: P.J. Quinn, B.K. Markey, F.C. Leonard, E.S. FitzPatrick, S. Fanning, and P.J. Hartigan. * Curriculum Structure (Section I): * 1. Microbiology, microbial pathogens, and infectious disease. * 2. Infection and immunity. * 3. Subdivisions, classification, and morphological characterization of infectious agents. * 4. Immunodeficiency diseases. * 5. Vaccines and vaccination. * 6. Molecular diagnostic methods. * Curriculum Structure (Section II: Introductory Bacteriology): * 7. The structure of bacterial cells. * 8. Cultivation, preservation, and inactivation of bacteria. * 9. Bacterial genetics, mechanisms of genetic variation, and gene databases. * 10. Laboratory diagnosis of bacterial disease. * 11. Antibacterial agents. * 12. Antibacterial resistance. * 13. Bacterial colonization, tissue invasion, and clinical disease. # Historical Foundations of Microbiology * Antonie van Leeuwenhoek (1670s): First observed microorganisms (microbes), defined as any organism invisible to the naked eye. * Agostino Bassi (1835): Demonstrated that the silkworm disease known as muscardine was contagious and caused by the microscopic fungus Beauveria bassiana. * Louis Pasteur (1850s–1880s): * Studied the role of microbes in fermentation and disease. * Developed the process of pasteurization. * Developed vaccines against rabies and anthrax. * Robert Koch (1860s–1880s): * Discovered the bacteria responsible for anthrax, tuberculosis, and cholera. * Formulated Koch’s postulates to determine the etiological agent of a disease. # Classification and Size of Infectious Agents * Size Hierarchy (Smallest to Largest): * 1. Prions. * 2. Viruses. * 3. Bacteria. * 4. Fungi. * 5. Protozoa. * 6. Multicellular parasites (the largest, up to approximately $1\,\text{cm}$). * Ecological Classification: Organisms are classified by their ecological niche and capacity to multiply inside or outside hosts (animals, humans, arthropods). * Opportunistic/Facultative Agents: Can live independently or as pathogens depending on conditions. * Obligate Agents: Must reside within a host to complete their life cycle. # Key Definitions and Infectious Disease Concepts * Microbiome: A microbiota consisting of a diverse collection of fungi, bacteria, archaea, viruses, and mites covering animal tissues (e.g., skin, gastrointestinal tract, respiratory tract). * Infectious Disease: Disorders caused by organisms such as bacteria, viruses, fungi, or parasites. * Communicable Disease: Any infectious disease that spreads from individual to individual. This may require specific modes of transmission (vector, sexual, etc.) via direct or indirect routes. * Contagious Disease: A subset of infectious diseases that spread easily from one individual to another via air or physical touch. * Comparison Summary: * Infectious diseases are not always transmissible (e.g., tetanus). * All communicable and contagious diseases are infectious. * All contagious diseases are communicable. * Not all communicable diseases are contagious (some do not spread easily). * Zoonotic Pathogen: A microorganism that is a commensal or pathogen in animals but can be transmitted to humans to cause disease. Transmission occurs through direct contact, bioproducts, or vectors (e.g., ticks). # Public Health and Regulatory Terms * Notifiable (Reportable) Disease: Diseases legally mandated to be reported to public health authorities to control or prevent spread. * Term usage: "Notifiable" is typically used for national-level jurisdiction, while "reportable" varies by local, state, or national levels. * Exclusions: Not all notifiable conditions are infectious (e.g., cancer, lead poisoning, congenital disorders). # The Infectious Disease Triad * Pathogen Factors: * Pathogenicity. * Dispersal efficiency. * Survival efficiency. * Inoculation dose. * Environment Factors: * Abiotic and biotic factors. * Housing management. * Population density. * Sanitation. * Nutrition. * Prevention measures. * Susceptible Host Factors: * Breed, age, sex, and genotype. * Physiology and susceptibility. * Immune response. # Outcomes and Stages of Exposure * Exposure Pathways: * 1. Exposure with no infection (no colonization). * 2. Infection leading to disease, then recovery (with immunity or non-immunity). * 3. Infection leading to sub-clinical disease (carrier) or commensalism. * 4. Infection leading to disease followed by death or permanent disability. * 5. Infection leading to disease followed by persistence (carriers). * Carrier Types: * Active Carriers: Currently shedding or exhibiting the agent. * Latent Carriers: The agent is present but dormant or subclinical. * Stages of Disease Progression: * 1. Exposure. * 2. Incubation period. * 3. Prodromal period (mild signs). * 4. Illness. * 5. Outcome: Death, recovery, or end of symptoms (could become a latent carrier). # Koch’s Postulates and Limitations * The Postulates (1884): * 1. The suspected pathogen must be found in every case of disease and absent from healthy individuals. * 2. The suspected pathogen can be isolated from the host and grown in pure culture. * 3. A healthy, susceptible individual subject to infection with the cultured pathogen must develop the same signs and symptoms of the disease. * 4. The same pathogen must be re-isolated from the newly diseased host. * Limitations to Koch’s Postulates: * Unculturable bacteria (cannot be grown in pure culture). * Subclinical infections (pathogen present without disease). * Co-infections (multiple agents involved). * Tumor-related diseases. * Opportunistic and commensal pathogens. * Distant disease (effects occurring far from the site of infection). * Toxin-related diseases (symptoms caused by toxins, not just the presence of the organism). # Questions & Discussion * Audience Interaction: The lecture concludes by asking the students if they have any questions regarding the introductory terms and concepts.