Describe three factors that affect the outcome of a parasitic relationship
- Number of organisms inoculated
- Virulence of organism
- Host’s degree of resistance (immune compentency, lifestyle, living conditions)
What is an infection?
When a parasite is multiplying in/on a host
What is an infectious disease?
When the host cannot function normally due to presence of parasite or its products
What is a pahogen?
Parasite that causes disease
What is pathogenicity?
An organism’s ability to cause disease
What is virulence?
The degree or intensity of pathogenicity, as indicated by morbidity and mortality rates
What is a symptom?
The effects that the host feels that is not outwardly apparent
What is a sign?
An outwardly apparent symptom
What are the four phases of infection?
- Incubation period (due to initial exposure, no symptoms)
- Prodromal stage (mild symptoms)
- Illness stage (worsening symptoms until plateaus)
- Convalescent period (symptoms decrease, still contagious)
Describe the iceberg theory of infection:
The infections we see (mild or severe symptoms) are the tip of the iceberg on infections
What are three factors that can affect virulence?
- Infectivity: ability to start an infection
- Invasiveness: ability of organism to spread
- Pathogenic potential: ability of organism to cause symptoms
List four factors that affect infectivity
- Modes of transmission
- Ability to adhere to and colonize host
- Ability to grow in or on host
- Ability to initially avoid host immune system
What are the two modes of transmission?
- Direct
- Indirect (vehicles)
What are the 4 methods of direct transmission?
- Horizontal (kissing, sex)
- Airborne Transmission (respiratory droplets)
- Vertical (mother to baby)
- Vector (insect)
What are the three methods of indirect transmission?
- Contact with fomites
- Soil, H2O, food
- Airborne droplets or dust
What is a fomite?
Inanimate objects that are contaminated
What are adhesins?
Structures that help adhere to surfaces:
- Fimbriae/pili
- Capsule/glycocalyx/slime layer
- S layer
- Teichoic acid
- Viral capsids and envelopes
What is required in a host in order for colonization to take place?
- Correct pH
- Correct O2 content
- Correct temperature
What microbial products are involved in avoiding the immune system?
- IgA protease - cleaves IgA
- Leukocidins - kill lysosomes in WBC
- Capsule - inhibit phagocytosis
How are parasites able to hide within a host cell?
They survive in a phagocyte (ex. Mycobacterium tuberculosis)
How do microbes increase their invasiveness?
- Microbial products involved with invasion
- Tricking host cells into spreading them across mucous membranes and into circulation by living inside macrophages (ex. Mycobacterium tuberculosis)
What microbial products are involved with invasion?
- H2O2 and NH3(-): damage tissues
- Collagenase: breaks down collagen
- Elastase: breaks down basement membrane
- Hyaluronidase: breaks down connections between cells
- Lecithinase: breaks down plasma membranes
How do bacteria increase their pathogenic potential?
Mechanical, chemical or molecular ability to damage host:
- Infections (tissue caused by inavsiveness of microbes, or by toxins produced by microbe in teh host’s body)
- Intoxications (damage due to toxins produced by microbe outside of the host’s body entering the host): exotoxins and endotoxins
Name the four types of exotoxins:
- Neurotoxin (affect nervous system)
- Enterotoxins (affect GI tract)
- Cytotoxins (affect cellular function)
- Superantigens (over stimulate immune system and can cause similar response to endotoxin)
