Mechanisms of infectious disease, with examples What is the disease-causing mechanism? What are its hosts? Why does it i

1. Mechanisms of infectious disease, with examples What is the disease-causing mechanism? What are its hosts? Why does it infect this particular tissue? what are the unique features of this tissue? How can it be prevented? How can it be treated/cured?

2. What are the pathogenic factors? Establishment of infection adhesion, colonization Invasion of tissues Evading immune system Damage to host toxins (exo and endotoxins) cytotoxicity Damaging effects of immune response Natural or opportunistic pathogen?

3. Examples • Skin • Respiratory system • Gastrointestinal system • Genitourinary system

4. Features of the skin • Dry • Salty • Acidic • Sebum (oil) around hair follicles • Hair follicles are anaerobic • Much normal flora • Opportunistic

5. Skin is an effective barrier (but is subject to damage) Has substantial nervous and immune function Major normal flora: diphtheroids and staphylo- cocci; also fungi Most staphylococci are harmless to healthy people Many virulence factors identified

6. Acne • Propionibacterium acnes is found on most people • Hair follicle damage can interfere with sebum secretion • Bacteria become trapped in sebum; multiply • Inflammatory response actually damages tissue

7. S.aureus is a major pathogen

8. S. Aureus as a pathogen • Attaches to hair follicles and moves inward • Many degradative enzymes; toxins; can resist immune system • Almost everyone is a carrier at some point • Any boil, etc., can shed many bacteria • Many resistant strains • MRSA-positive patients are isolated in hospitals

9. Streptococcus can cause skin and systemic infections

10. Skin diseases spread by vectors • Ticks, mice, lice, mosquitoes • Often zoonoses (infect other animals; humans by accident) • Can cause serious systemic symptoms • Early treatment may be effective

11. Note that humans do not infect each other Life cycle of the black-legged tick (Lyme disease)

12. Exanthems: skin rashes

13. What causes rashes? • Mostly viral, e.g., measles, mumps, rubella (sometimes bacterial or allergic) • Some localized (herpes, VSV) • Caused by toxins, direct skin damage, immune reaction • Often accompanied by fever, malaise, headache, joint pain, etc.

14. Rashes, continued • Exanthems historically were used for diagnosis • Treatment usually not necessary for viral rashes • Viral rashes especially common in children • Symptoms mild in young children, but increase in severity with aging • http://www.dermnetnz.org/viral/viral-exanthem.html

15. “Childhood” viruses are usually mild but can cause serious side effects can be teratogenic can have neurological complications Chickenpox, measles, rubella have only human reservoirs inhaled promising targets for vaccination

16. Fungal infections (mycoses) • Dermatophytes • Named according to sites infected • Some strains are more invasive than others • Dampness is the critical factor • Treatments will be topical

17. Summary • Skin is complex organism with normal flora • Bacteria cause skin infections through toxic processes • Some infectious agents are transmitted by vectors; have serious side effects • Viral infections cause distinctive rashes • Some fungi can invade skin and cause disease