Transmission of infection (13) Flashcards
What are the 6 links in the chain of infection?
1) Pathogenic microorganism
2) Reservoir
3) Means of escape
4) Mode of transmission
5) Means of entry
6) Host susceptibility
What is the first link in the chain of infection?
> The pathogen
Can be viruses, bacteria, fungi, protozoa and prions (prions are the cause of rare progressive neurodegenerative disorders such as mad cow disease)
How well the pathogen is able to thrive depends on its pathogenicity (ability to produce disease), its degree of virulence (its severity/harmfulness) and its invasiveness (its tendency to spread)
What is the second link in the chain of infection?
> The reservoir
The principal habitat in which a pathogen lives, flourishes and is able to multiply
Common reservoirs include humans, animals, insects and the environment
In humans there are 2 forms of reservoir- 1) Acute clinical cases (someone is infected and is displaying signs and symptoms of the disease) 2) Carriers (where someone has been colonised with an infectious agent but is not unwell
Acute clinical cases are more likely to be diagnosed and treated meaning that contacts will be traced and activities restricted
Carriers present more of a risk to those around them as they do not display any signs/symptoms of the disease
What are the 4 main types of carriers?
1) Incubatory carriers- people who are infectious before their own symptoms start (prodromal period)
2) Asymptomatic carriers- an individual who is able to transmit an infection to others, without ever developing the infection themselves
3) Convalescent carriers- people who are in the recovery phase of their illness, but who continue to be infectious
4) Chronic carriers- someone who has recovered but continues to be a carrier of the infection
What are animal and insect reservoirs?
> Any infectious disease transmitted under natural conditions from animal to human is referred to as zoonosis
Examples include lyme disease (ticks), rabies (dogs, cats, foxes and bats), salmonella (poultry, cattle) and malaria (mosquitoes)
What are environmental reservoirs?
> Environment contains large number of reservoirs of infection including
1) Soil- Acts as a reservoir for Clostridium tetani which causes tetanus
2) Water- acts as a reservoir for Legionella pneumophila which causes Leginnaire’s disease
What is the third link in the chain of infection?
> The portal of exit
Any route which enables a pathogen to leave the reservoir or host
In humans the main portals of exit are:
1) Gastro intestinal- via vomiting, diarrhoea or biting- Hepatitis B
2) Genitourinary- via sexual transmission- syphilis, gonorrhoea
3) Respiratory- through coughing, sneezing, spitting- COVID 19
4) Skin- via skin lesions- Herpes Zoster
5) Trans-placental- transmission from Mother to foetus- AIDS
What is the fourth link in the chain of infection?
> 2 main modes of transmission- direct and indirect
What is direct transmission
> Tends to be instantaneous
Occurs when there is direct contact with the infectious agent
Examples include tetanus, glandular fever, colds, flu and sexually transmitted diseases
What is indirect transmission?
> Can occur through animate mechanisms such as fleas, ticks, flies, or mosquitoes
Can also occur via inanimate mechanisms such as food, water or biological products or surgical instruments
Can also be airborne, when tiny particles of an infectious agent are carried by dust or droplets in the air
What is the fifth link in the chain of infection?
> The means by which an infection is able to enter a susceptible host
Portals of entry include
1) Inhalation- via respiratory tract
2) Absorption- via mucous membranes such as the eye
3) Ingestion- via gastrointestinal tract
4) Inoculation- Result of an inoculation injury such as a stabbing
5) Introduction- via the insertion of medical devices
What is the sixth link in the chain of infection?
> The susceptible host
Represented by absent/weakened defence mechanisms
Age- in particular very young or very old
Presence of malnutrition/dehydration
Any underlying chronic disease
If the host is immobile/bedbound
If taking any medication which could disrupt/supress their immune response
