L1 Host and parasite relationship intro

Question 1

What is Infectious Disease?

Answer 1

an infectious disease is any change from a state of health in which part or all of the host’s body cannot carry on its normal functions because of the presence of an infectious agent or its products

Question 2

Explain how an infectious disease is like a battle

Answer 2

the host provides protection, nutrients and energy
 pathogen must:
access and exploit the host
access new hosts
 the host must resist infection

Question 3

Parasitism

Answer 3

symbiotic interaction between two organisms in which one benefits at the expense of the other

Question 4

Parasites characteristics

Answer 4

•  live on or within a host organism
•  use the host to achieve metabolism (‘dependence’)
•  parasite benefits from the association
•  host is usually harmed
•  typically cause disease in the host
•  the term includes bacteria, viruses, fungi, protists

Question 5

Microbiome in human features

Answer 5

protective function, development and maturation of the immune system, regulatory function of the nervous system, metabolic and nutritional function.

Question 6

Microbiota dysbiosis

Answer 6

lead to dysregulation of bodily functions and diseases including cardiovascular diseases, cancers, respiratory diseases, etc.

Question 7

Infection

Answer 7

the parasite grows and multiplies in or on its host

Question 8

Infectious disease (an outcome)

Answer 8

where infection detrimentally affects host function

Question 9

Pathogen

Answer 9

any organism causing infectious disease

Question 10

Types of pathogens

Answer 10

Primary and opportunistic

Question 11

Primary pathogens -

Answer 11

cause infectious disease in healthy hosts

Question 12

Opportunistic pathogens -

Answer 12

cause infectious disease in ‘compromised’ hosts, e.g., immunocompromised or
wounded hosts; may be part of the normal microbiota (normal flora)

Question 13

Pathogenicity

Answer 13

the ability to cause disease

Question 14

Virulence

Answer 14

the degree or intensity of pathogenicity e.g., indicated by fatality rate or ability to damage host tissues

Question 15

What does it mean when some pathogens can enter a latent state

Answer 15

The pathogen is present, but no symptoms are evident (chicken pox/varicella)

Question 16

The infectious Disease Process

Answer 16

1. Incubation period
2. Prodromal stage
3. Illness period
4. Convalescent period (or death)

Question 17

Infection process: Incubation period

Answer 17

• Time from pathogen entry to development and the signs and symptoms (little to none)
• Pathogen development but no clinical manifestations

Question 18

Infection process: Prodromal Stage

Answer 18

• Non-specific clinical manifestations may appear like tiredness.
• Patient may be contagious

Question 19

Infection process: Illness period

Answer 19

• Disease most severe
• Characteristic signs and symptoms of the illness
• Immune response triggered

Question 20

Infection process: Convalescent period (or death)

Answer 20

This is the recovery phase and where signs and symptoms will decline

Question 21

Incubation periods

Answer 21

• Short (<1 week): localised infections - e.g., Dengue
• Medium (7-21 days): generalised infections, e.g., Measles
• Long (weeks to months) – e.g., Rabies
• Very long (years): Usually fatal – e.g., prion diseases

Question 22

List: Patterns of infection

Answer 22

• Acute
• Latent
• Chronic
• Insidious

Question 23

Patterns of infection in detail

Answer 23

• Acute non-persistent – rapid and self-limiting
• Latent – persistence of viralgenome in the host cell and periodic virus reactivation
• Chronic - persistent or otherwise long-lasting
• Insidious infections
  with fatal outcomes

Question 24

The Infection Chain links

Answer 24

1. Pathogen source/reservoir
2. Transmission
3. Infectious dose
4. Exposure & growth
5. Host susceptibility
6. Exiting the host

Question 25

Infection chain: pathogen source/reservoir

Answer 25

Source - the location from which pathogen is transmitted to host. Can be
* Animate (human,plants,animals) like a cold
* Or inanimate (soil/water/food) like water/food-bourne
Reservoir - natural environmental location where the pathogen is normally found

Question 26

Can reservoir and source be different

Answer 26

yes example: Hendra virus (reservoir = bats;
source of human infection = horses)

Question 27

Infection chain: pathogen transmission (Airborne)

Answer 27

Via water droplets
 typically, infections with respiratory involvement e.g., colds, influenza,
tuberculosis, measles
 small particles (1-4 µm diameter)
 can remain airborne for long time
 can travel long distances
 propelled from respiratory tract by sneezing, coughing, or vocalization
 also, from reservoirs - e.g., Legionnaire’s disease (air-conditioning cooling
towers)
Via dust
 many systemic fungal infections
 can be source of hospital-acquired (nosocomial) infections

Question 28

Infection chain: pathogen transmission (Contact)

Answer 28

Pathogen source and host come into contact
Person-to-person
 touching, kissing, sex (e.g., HIV, gonorrhea, clamydia)
 oral or wound secretions (e.g., herpes)
 nursing mothers (e.g., staphylococcal infections)
 Placenta (vertical transmission) (e.g., syphilis)
Animal-to-person
 animal handlers e.g., Lyssavirus (wildlife bat handlers)

Question 29

Infection chain: pathogen transmission (Vehicle)

Answer 29

An inanimate ‘vehicle’ is a source that has become contaminated/transmits a
pathogen
Common vehicles are termed fomites*
 surgical instruments, drink containers, bedding,
door handles, taps
 food and water
 cosmetics
 drugs, needles
*Fomite is an inanimate object or substance that is
capable of transmitting infectious organisms

Question 30

Infection chain: pathogen transmission (Vector)

Answer 30

Vectors house infectious agents and transmit them one host to another, commonly acting as a host itself.

External transmission
 Passive carriage of pathogen on body of vector
 No growth of pathogen during transmission
e.g., Shigella dysenteriae (shigellosis): flies carry the bacterium on their feet from faecal sources to food
Internal transmission
 Pathogen carried within vector
e.g., the plague bacterium Yersinia pestis in rat fleas during transmission from rat to human
e.g., malarial parasite undergoes developmental changes in the mosquito vector

Most vectors are artropods

Question 31

Infection chain: Infectious Dose

Answer 31

Infectious dose 50 (ID50) = number of organisms necessary to infect 50% of an experimental group of hosts in a specified time period

Question 32

Infection chain: Exposure and Growth

Answer 32

Transmission alone is not enough for infection to occur pathogen must make contact with appropriate host tissues
 specific cell surface receptors for pathogen must be present
 pathogen must find appropriate conditions (nutrients, pH,
temperature etc.) to grow
Pathogens vary in the systems they affect in hosts
respiratory, urogenital, gastrointestinal, circulatory; some affect
multiple systems
Pathogens vary in their location in hosts
 extracellular pathogens grow outside cells in blood, tissue fluids
 intracellular pathogens grow and multiply within cells

Question 33

Facultative intracellular pathogens

Answer 33

= grow within or outside cells

Question 34

Obligate intracellular pathogens

Answer 34

= only grow when inside cells

Question 35

Infection chain: Host Susceptibility

Answer 35

Genetic or constitutional factors, specific immunity and nonspecific factors that affect an individual’s ability to resist infection or to limit pathogenicity.
2 components of defence mechanism :
* innate host resistance
* adaptive immune mechanism

Question 36

Infection chain: Exiting the Host

Answer 36

For microbe perpetuation
* active
- Movement through a portal of exit from the host
- Uncommon (borrowing out)
* Passive
- Common
- Excretions in faeces, urine, blood droplets, saliva or shedding of host cells

Question 37

List Microbial virulence factors

Answer 37

1. Adherence and colonisation
2. Invasion
3. Avoiding/resisting host defences
4. Toxin formation

Question 38

MVF: Adherence and colonisation

Answer 38

Help microbes to attach to host tissue. If attached and reproduction happens then host is colonised.
 fimbriae/pili (hair-like structures) of bacteria
 capsules (sticky polysaccharide materials) of
bacteria
 capsid spikes of viruses

Question 39

Determined by pathogen’s ‘virulence factors’

Answer 39

 can involve physical and chemical characteristics
 determine the degree to which the pathogen causes damage, invasion, infectivity

Question 40

Virulence

Answer 40

The magnitude of harm cause by a microbe, determined by virulence factors.

Different stains of the same microorganism can cause difference severity of diseases.

Question 41

MVF - Invasion

Answer 41

Pathogen spread from the site of infection.
* will vary among pathogens
* Pathogen can spread by producing chemicals that disrupt host cell surface or induce a cells uptake

Question 42

Virulence - avoiding/resisting host defenses

Answer 42

• Microbes can be eliminated by the immune system
• Pathogen can evade immune system
• Bacterial pathogens can produce a sticky capsule that will coat the bacterium to prevent capture by immune cells (hepatitis B)

Question 43

Virulence - toxin formation

Answer 43

A toxin is a microbial product or component that injures another cell or organism

Bac. exotoxin secreted by proteins
Bac. endotoxin are bound to bacteria and released when the microorganism lyses or during cell division