L1 Host and parasite relationship intro Flashcards
What is Infectious Disease?
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
Explain how an infectious disease is like a battle
the host provides protection, nutrients and energy
pathogen must:
access and exploit the host
access new hosts
the host must resist infection
Parasitism
symbiotic interaction between two organisms in which one benefits at the expense of the other
Parasites characteristics
- 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
Microbiome in human features
protective function, development and maturation of the immune system, regulatory function of the nervous system, metabolic and nutritional function.
Microbiota dysbiosis
lead to dysregulation of bodily functions and diseases including cardiovascular diseases, cancers, respiratory diseases, etc.
Infection
the parasite grows and multiplies in or on its host
Infectious disease (an outcome)
where infection detrimentally affects host function
Pathogen
any organism causing infectious disease
Types of pathogens
Primary and opportunistic
Primary pathogens -
cause infectious disease in healthy hosts
Opportunistic pathogens -
cause infectious disease in ‘compromised’ hosts, e.g., immunocompromised or
wounded hosts; may be part of the normal microbiota (normal flora)
Pathogenicity
the ability to cause disease
Virulence
the degree or intensity of pathogenicity e.g., indicated by fatality rate or ability to damage host tissues
What does it mean when some pathogens can enter a latent state
The pathogen is present, but no symptoms are evident (chicken pox/varicella)
The infectious Disease Process
- Incubation period
- Prodromal stage
- Illness period
- Convalescent period (or death)
Infection process: Incubation period
- Time from pathogen entry to development and the signs and symptoms (little to none)
- Pathogen development but no clinical manifestations
Infection process: Prodromal Stage
- Non-specific clinical manifestations may appear like tiredness.
- Patient may be contagious
Infection process: Illness period
- Disease most severe
- Characteristic signs and symptoms of the illness
- Immune response triggered
Infection process: Convalescent period (or death)
This is the recovery phase and where signs and symptoms will decline
Incubation periods
- 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
List: Patterns of infection
- Acute
- Latent
- Chronic
- Insidious
Patterns of infection in detail
- 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
The Infection Chain links
- Pathogen source/reservoir
- Transmission
- Infectious dose
- Exposure & growth
- Host susceptibility
- Exiting the host
Infection chain: pathogen source/reservoir
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
Can reservoir and source be different
yes example: Hendra virus (reservoir = bats;
source of human infection = horses)
Infection chain: pathogen transmission (Airborne)
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
Infection chain: pathogen transmission (Contact)
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)
Infection chain: pathogen transmission (Vehicle)
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
Infection chain: pathogen transmission (Vector)
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
Infection chain: Infectious Dose
Infectious dose 50 (ID50) = number of organisms necessary to infect 50% of an experimental group of hosts in a specified time period
Infection chain: Exposure and Growth
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
Facultative intracellular pathogens
= grow within or outside cells
Obligate intracellular pathogens
= only grow when inside cells
Infection chain: Host Susceptibility
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
Infection chain: Exiting the Host
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
List Microbial virulence factors
- Adherence and colonisation
- Invasion
- Avoiding/resisting host defences
- Toxin formation
MVF: Adherence and colonisation
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
Determined by pathogen’s ‘virulence factors’
can involve physical and chemical characteristics
determine the degree to which the pathogen causes damage, invasion, infectivity
Virulence
The magnitude of harm cause by a microbe, determined by virulence factors.
Different stains of the same microorganism can cause difference severity of diseases.
MVF - Invasion
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
Virulence - avoiding/resisting host defenses
- 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)
Virulence - toxin formation
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
