1. PBL 7 – A traveller’s tale Methods of transmission of infectious diseases

2. 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

3. 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

4. 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

5. 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.

6. Selected zoonoses occurring in Australia ORGANISMDISEASEANIMAL RESERVOIRTRANSMISSION Bacteria Bacillus anthracisAnthraxCattleContact with cattle or endospores Brucella spp.BrucellosisCattleDirect contact Borrelia spp.Lyme disease? Native animalsTick bites Chlamydia psittaciPsittacosis (ornithosis)Birds/parrotsDirect contact Coxiella burnetiiQ feverCattleDirect contact LeptospiraLeptospirosisWild mammals, cats/dogsContact with urine/water Listeria monocytogenesListeriosisDomestic animalsUnpasteurised milk Salmonella spp.SalmonellosisPoultry/Domestic livestockIngestion of contaminated food, water CampylobacterGastroenteritisPoultry/Domestic livestockIngestion of contaminated food, water Bartonella henselaeCat scratch feverCatsScratching, contact Rickettsia tsutsugamushiScrub typhusMites/ticksBites Viruses Influenza virusInfluenza (some types)Pigs, ducksDirect contact FlavivirusMurray Valley encephalitisFowl/birdsMosquitoes Orf virus (parapoxvirus)OrfSheep, goatsDirect contact AlphavirusRoss River feverNative animalsMosquitoes AlphavirusBarmah forest polyarthritisNative animalsMosquitoes Hendra virusInfluenza-like illnessFlying foxContact with infected animals (horses) Menangle virusFever/rashFlying foxContact with animals (pigs) Bat lyssavirusRabies-like illnessFlying foxBite, scratch by flying fox Protozoa Toxoplasma gondiiToxoplasmosisCats/other animalsIngestion of contaminated meat, contact with faeces Fungi TrichophytonRingworm (tinea)Domestic animalsDirect contact MicrosporumRingworm (tinea)Domestic animalsDirect contact Helminths Echinococcus granulosisHydatid cystsDogsContact with faeces Taenia saginataTapewormCattleIngestion of contaminated meat Toxocara spp.Worm infestationDogs/catsIngestion of eggs of body organs

7. 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

8. 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

9. 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

10. 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

11. Examples of how environmental changes affect the occurrence of various infectious diseases