1 - Host / Parasite interactions Flashcards
What is an infectious disease
Any change from a state of health in which part or all of the host’s body cannot carry on its normal functions.
Parasitism
Symbiotic interaction between two organisms in which one benefits at the expense of the other
Characteristics of parasites
- Live on or within host organism
- Dependent on host
- Parasite benefits whilst host is harmed
- Typically cause disease
Infection (process)
The parasite grows and multiplies in or on its host
Infectious disease (outcome)
Where infection detrimentally affects host function
Pathogen
Any organism causing an infectious disease
Types of pathogens
Primary pathogens and opportunistic pathogens
Primary pathogens
Cause infectious disease in healthy hosts
Opportunistic pathogens
Cause infectious disease in ‘comprised’ hosts (e.g. immunocompromised). May be a part of the normal microbiota
Pathogenicity
the ability to cause disease
Virulence
The degree or intensity of pathogenicity. Indicated by fatality rate or ability to damage host tissues
Latent state
Pathogen is present, but no symptoms are evident
Stages of the infectious disease process
- Incubation period
- Prodromal stage
- Illness period
- Convalescent period (or death)
Incubation period
Time from pathogen entry to development of signs and symptoms. Pathogen development but no clinical manifestations
Prodromal stage
Non specific clinical manifestations, may be contagious
Illness period
- Disease is most severe
- Characteristic signs and symptoms of illness
- Immune response triggered
Convalescent period (or death)
Recovery, signs and symptoms decline.
Acute non persistent
Rapid and self limiting
Chronic
Persistent or long lasting
Chronic
Persistent or long lasting
Why is knowledge of incubation periods important
Aids diagnosis and tracing spread of disease outbreaks
Parts of the infection chain
- Reservoir/source
- Transmission
- Infectious dose
- Exposure and growth
- Host susceptibility
- Exiting the host
Source
Location from which pathogen is transmitted to host
Animate source
Humans, plants, animals (e.g common cold)
Zoonosis
Disease of animals that can be transmitted to humans
Inanimate source
Soil, water, food (e.g. hepatitis A)
Reservoir
Natural environmental location where the pathogen is normally found. May or may not also be the source
Types of transmission
- Airborne
- Contact
- Vehicle
- Vector-borne
- Others: vertical (transplacental, congenital); iatrogenic (medical procedures)
Airborne transmission
- Water droplets (small particles that can remain airborne for long time and travel long distances)
- Dust (many systemic fungal infections)
Contact transmission
Person to person or animal to person
Vehicle transmission
An inanimate ‘vehicle’ is a source that has become contaminated/transmits a pathogen
Fomite
inanimate object or substance that is capable of transmitting infectious organisms
Vector tranmission
Vectors house infectious agents and transmit them one host to another, commonly
acting as a host itself (e.g. mosquito, ticks, fleas)
External transmission
- Passive carriage of pathogen on body of vector
- No growth of pathogen during transmission
Example of external transmission
Shigella dysenteriae: flies carry the bacterium on their feet from faecal sources to food
Internal transmission
Pathogen carried within vector.
Example of internal transmission
malarial parasite undergoes developmental changes in the mosquito vector
Infectious dose 50 (ID50)
Number of organisms necessary to infect 50% of an experimental group of hosts in a specified time period. Varies with pathogen
LD50
Number of pathogenic cells or amount of toxin required to kill 50% of infected individuals
Is transmission alone enough for infection to occur
No. Specific cell surface receptors and appropriate conditions for growth must be present
Extracellular pathogens
grow outside cells in blood and tissue fluids
Intracellular pathogens
grow and multiply within cells
Facultative intracellular pathogens
grow within or outside cells
Obligate intracellular pathogens
only grow when inside cells
Factors that effect host susceptibility
Genetic or constitutional factors, specific immunity and nonspecific factors
Components of host defence mechanisms
- Innate host resistance factors
- adaptive immune mechanisms
Exiting the host
Must occur if microbe is to be perpetuated. Can be active or passive escape
Active escape
- Movement of pathogen to portal of exit from host
- Relatively uncommon (burrow out through the skin)
Passive escape
- Usual method
- Excretion in faeces, urine, droplets, blood, saliva, or
in host cells shed from the body
What is virulence determined by
VIrulence factors. Can involve physical and chemical characteristics. Determine the degree to which the pathogen causes damage
4 Virulence factors
- adherence and colonisation
- invasion
- avoiding/resisting host defences
- Damage to the host
Adherence and colonisation
Help microbes to attach to host tissue (e.g. fimbriae/pili and capsules, capsid spikes of viruses
Adherences factors
Adhesins
Colonised host
Host that has microbe attached and reproduction follows
Invasion
- Spread from initial site of infection
- invasion mechanisms vary among pathogens
- Pathogens can spread by producing chemicals that disrupt the host cell surface or induce uptake by host cell
Examples of mechanisms of resisting host defences
- Bacteria can produce a sticky capsule that coats the bacterium and prevents its capture by immune cells
- Hepatitis B virus causes infected cells to produce ‘decoy’ proteins that confuse the immune system
Toxin
A microbial product or component that injures another cell or organism
Bacterial exotoxins
Secreted proteins (e.g. tetanus, cholera)
Fungal mycotoxins
Secondary metabolites (inadvertent consumption of fungus-contaminated foods)
