PBL 7 – A traveller’s tale
Methods of transmission of
infectious diseases
Communicable diseases
• Infectious diseases that can spread from one host to
another
– Those that spread more easily than others are called
‘contagious’
• Can be transmitted in various ways:
– By contact with aerosols from infected host
– Via body fluids such as blood, sputum, urine, semen and
faeces
– By infected skin cells that have been shed into the
environment
– From open lesions or wounds
– By contact with contaminated surfaces
Non-communicable diseases
• Those infectious diseases that would not
normally be spread from an infected host to a
healthy person during the course of usual contact
and normal daily activities
• Include those diseases that are acquired by
mechanical transfer of a pathogen from the
environment into the host tissue or of normal
body flora from one site to another, eg.
– Tetanus acquired through a puncture wound
– Cystitis
Reservoirs of infection
• A habitat where microorganisms can persist for
long periods
• It may be either:
– a favourable environment where the organisms are
able to survive or even increase in numbers or
– An unfavourable environment where the organism
survives in a resistant form such as cysts for
protozoans or endospores for bacteria
• The host may be human, animal, or a non-living
environment such as soil or water
• A source of infection is the object from which the
infectious agent is actually acquired
– May be also be the reservoir if opportunity exists for
the microorganisms to pass directly from the reservoir
to the susceptible host
• eg. Humans are both reservoir and source for disease in
measles and syphilis
• A fomite is an inanimate object that can carry
microorganisms on its surface
– Eg. In hospitals, fomites include medical equipment
and instruments, computer keyboards, door handles,
ID badges, soiled linen and dressings, keys etc.
Selected zoonoses occurring in Australia
ORGANISM
DISEASE
ANIMAL RESERVOIR
TRANSMISSION
Bacteria
Bacillus anthracis
Anthrax
Cattle
Contact with cattle or endospores
Brucella spp.
Brucellosis
Cattle
Direct contact
Borrelia spp.
Lyme disease
? Native animals
Tick bites
Chlamydia psittaci
Psittacosis (ornithosis)
Birds/parrots
Direct contact
Coxiella burnetii
Q fever
Cattle
Direct contact
Leptospira
Leptospirosis
Wild mammals, cats/dogs
Contact with urine/water
Listeria monocytogenes
Listeriosis
Domestic animals
Unpasteurised milk
Salmonella spp.
Salmonellosis
Poultry/Domestic livestock
Ingestion of contaminated food, water
Campylobacter
Gastroenteritis
Poultry/Domestic livestock
Ingestion of contaminated food, water
Bartonella henselae
Cat scratch fever
Cats
Scratching, contact
Rickettsia tsutsugamushi
Scrub typhus
Mites/ticks
Bites
Influenza virus
Influenza (some types)
Pigs, ducks
Direct contact
Flavivirus
Murray Valley encephalitis
Fowl/birds
Mosquitoes
Orf virus (parapoxvirus)
Orf
Sheep, goats
Direct contact
Alphavirus
Ross River fever
Native animals
Mosquitoes
Alphavirus
Barmah forest polyarthritis
Native animals
Mosquitoes
Hendra virus
Influenza-like illness
Flying fox
Contact with infected animals (horses)
Menangle virus
Fever/rash
Flying fox
Contact with animals (pigs)
Bat lyssavirus
Rabies-like illness
Flying fox
Bite, scratch by flying fox
Toxoplasmosis
Cats/other animals
Ingestion of contaminated meat, contact with faeces
Trichophyton
Ringworm (tinea)
Domestic animals
Direct contact
Microsporum
Ringworm (tinea)
Domestic animals
Direct contact
Echinococcus granulosis
Hydatid cysts
Dogs
Contact with faeces
Taenia saginata
Tapeworm
Cattle
Ingestion of contaminated meat
Toxocara spp.
Worm infestation
Dogs/cats
Ingestion of eggs of body organs
Viruses
Protozoa
Toxoplasma gondii
Fungi
Helminths
Transmission of microorganisms
Types of transmission:
• Horizontal transmission
– From person to person
• Direct contact
– with reservoir or source
• Indirect contact
– with reservoir or source by
means of either:
• Common vehicle
• Mechanical vector
• Biological vector
• Vertical transmission
– From mother to foetus across
placenta
Contact transmission
Many pathogens are too fragile to exist for any length of time outside their
host.
• Direct contact
– Refers to close or intimate contact between the infected person and a susceptible
individual
– Zoonoses can be transmitted to humans by direct contact with an infected animal
• Indirect contact
– Occurs when microorganisms from infected host or reservoir are deposited on
inanimate object (fomite) and transmitted to susceptible host
• Faecal-oral transmission
– Pathogens are shed in the faeces and transmitted by either direct or indirect contact
– Usually the pathogen is carried on the hands and then contaminate food which is
ingested
• Droplet transmission
– Microorganisms contained in body secretions (usually from URT) expelled from the body
by coughing, sneezing or talking
– Pathogens are carried in droplets over a distance of less than one metre and inhaled
directly by the new host
Common vehicle transmission
• Airborne transmission
– Occurs when disease-causing microorganisms are carried on air currents over distances
greater than one metre
– These pathogens must be able to survive outside the host and tolerate dry conditions
• Includes fungal spores, bacterial endospores or eggs of parasites
– Pathogens may also be present in fine water aerosol sprays that are too light to settle
• Legionella pneumophila (Legionnaire’s disease)
• Water-borne transmission
– Faecal contamination of water supply for drinking or bathing
• Examples include hepatitis A and E, typhoid, cholera and polio
• Protozoa such as Giardia, Entamoeba and bacterium Legionella pneumophila live and multiply
in water reservoirs
• Food-borne transmission
– Ingestion of food containing pathogens
• Eg. Meat infected with tapeworms or Toxoplasma
• Salmonella on raw or improperly handled/cooked chicken
Transmission by vectors
•
•
Vectors are living agents, usually insects that transmit an infectious agent from one host to
another
Major vectors:
–
–
–
–
–
–
•
Mosquitoes
Fleas
Ticks
Lice
Sandflies
Other biting flies and bugs
Two forms of transmission by vectors exist:
–
Mechanical transmission
•
Passive transport of microorganisms on outside of insect’s body from source to susceptible host
–
–
Biological transmission
•
•
•
Occurs when an insect bites an infected host and ingests blood or body fluid containing the pathogen
The pathogen then multiplies or goes through a stage in its life cycle in the vector (resulting in increased
number of organisms being present in vector)
When the insect bites the next host, the pathogen may be present in regurgitated blood or in faeces that is
deposited at the site of the bite, and introduced into the body when the bite is scratched
–
•
eg. Fly taking pathogen from rotten food or faeces and then landing on open wound or eye in susceptible host
In case of malaria, protozoa are present in the saliva of the mosquito and are injected into the bloodstream of the
new host when the mosquito feeds
The best method of control of vector-borne diseases is to interrupt the transmission by
destroying the vectors or their habitats
Examples of how environmental changes
affect the occurrence of various infectious
diseases