MLAB 2434 – CLINICAL MICROBIOLOGY SUMMER, 2005 CECILE SANDERS & KERI BROPHY

1. MLAB 2434 – CLINICAL MICROBIOLOGYSUMMER, 2005CECILE SANDERS & KERI BROPHY Chapter 6 – Host-Parasite Interaction

2. Chapter 6 – Host-Parasite Interaction (cont’d) • Pathogen – microbe that can cause disease in a susceptible host • Opportunistic Pathogen – microbe that can cause disease only if a significant change occurs in host resistance or within the organism itself

3. Chapter 6 – Host-Parasite Interaction (cont’d) • Opportunistic Infections - infections caused by opportunistic pathogens • Iatrogenic Infections – infections resulting from medical treatment or procedures

4. Chapter 6 – Host-Parasite Interaction (cont’d) • Virulence – relative ability of a microorganism to cause disease, or the degree of pathogenicity • Virulence factors – factors such as capsules, toxins, enzymes, cell wall receptors, pili, etc. that allow pathogens to evade or overcome host defenses

5. Chapter 6 – Host-Parasite Interaction (cont’d) • Host Resistance Factors • Physical barriers – skin • Cleansing mechanisms • Desquamation • Fluids of the eye (IgA and lysozyme) • Respiratory, digestive, urinary, and genital tracts have fluids and movements to cleanse the surfaces

6. Chapter 6 – Host-Parasite Interaction (cont’d) • Antimicrobial Substances • Lysozymes • Antibodies • β-lysins • Interferon • Indigenous Microbial Flora • Phagocytosis • Chemotaxis • Inflammation • Immune Responses

7. Chapter 6 – Host-Parasite Interaction (cont’d) • Infectious Agent Factors • Adherence – most infectious agents must attach to host cells before infection occurs • Proliferation – pathogens must be able to replicate after attachment to host cells (overcome host resistance factors) • Tissue Damage – makes the infection visible; results from toxins or from host inflammatory substances

8. Chapter 6 – Host-Parasite Interaction (cont’d) • Exotoxins • Most have two units, one for attachment and the other is the toxin • Commonly coded by phages or plasmids • Endotoxins • G- bacteria • Toxicity caused by the lipid A portion of the lipopolysaccharide in the cell wall • Cause dramatic host responses, including inflammation, fever, hypotension, septic shock, death

9. Chapter 6 – Host-Parasite Interaction (cont’d) • Invasion – all pathogens have the ability to penetrate and grow in tissues • Dissemination • Spread of organisms to distant sites • Some pathogens stay at site (C. diphtheriae); others spread (Salmonella ssp.)

10. Chapter 6 – Host-Parasite Interaction (cont’d) • Routes of Transmission • Airborne • Coughing, sneezing, talking • Droplet nuclei • Airoborne pathogens must be resistant to drying and inactivation by ultraviolet light • Examples: Strep throat, otitis media, diphtheria, rhinoviruses (colds)

11. Chapter 6 – Host-Parasite Interaction (cont’d) • Transmission by Food and Water • Ingestion of contaminated food or water • Sometimes oral-fecal route • Pathogens must be able to survive stomach conditions and compete with normal flora of the gut • Preformed toxins (Clostridium botulinum, S. aureus) vs. toxins produced after infection (C. difficile, V. cholerae)

12. Chapter 6 – Host-Parasite Interaction (cont’d) • Close Contact • Passage of organisms by salivary, skin, and genital contact • Examples: Infectious mononucleosis, STDs • Cuts and Bites • Arthropods • Zoonoses – diseases of animals accidentally transmitted to humans; examples: plague, rabies, tularemia

13. Chapter 6 – Host-Parasite Interaction (cont’d) • Epidemiology – study of the occurrence, distribution and causes of disease and injury • Definitions • Carrier – person or animal who harbors and spreads microorganisms that cause disease but does not become ill; examples: N. meningitidis, S. pyogenes

14. Chapter 6 – Host-Parasite Interaction (cont’d) • Endemic – Organism or disease is constantly present in a population; examples: Cholera is endemic in third world countries • Epidemic – Disease affects a significantly large number of people at the same time in a geographic area; examples: Influenza • Pandemic – Worldwide epidemic

15. Chapter 6 – Host-Parasite Interaction (cont’d) • Incidence Rate – number of times a new event occurs in a given period; usually given as cases per 1000 or 100,000 population • Incubation Period – time between exposure to a pathogen and the onset of symptoms • Index Case – first case of a disease which serves as source of infection

16. Chapter 6 – Host-Parasite Interaction (cont’d) • Morbidity Rate – rate at which an illness occurs • Mortality Rate – number of deaths caused by a disease in a population • Nosocomial Infection – infection acquired during hospitalization • Reservoir – source of infection, such as a person, animal or something in environment

17. Chapter 6 – Host-Parasite Interaction (cont’d) • Surveillance – collection of data pertaining to disease occurrence • Surveillance and Reporting • Certain diseases are required by law to be reported to public health authorities • http://www.tdh.state.tx.us/ideas/report/conditions/

18. Chapter 6 – Host-Parasite Interaction (cont’d) • Diseases that have significant effect on the population (STD) or have potential for grave consequences (anthrax, plague) • Centers for Disease Control and Prevention (CDC) • World Health Organization (WHO)