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

Viruses

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

Protozoa

Toxoplasma gondii Toxoplasmosis Cats/other animals Ingestion of contaminated meat, contact with faeces

Fungi

Trichophyton Ringworm (tinea) Domestic animals Direct contact

Microsporum Ringworm (tinea) Domestic animals Direct contact

Helminths

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

Transmission of microorganismsTypes 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 transmissionMany 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– eg. Fly taking pathogen from rotten food or faeces and then landing on open wound or eye in 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– 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