disease transmission

• pathogens are disease causing agents that include:

  • prions
  • viroids
  • viruses
  • bacteria
  • fungi
  • protazoa
  • metazoa
• protozoa = single-celled eukaryotes e.g. plasmodia cause malaria

  metazoa = multicellular eukaryotes e.g. parasitic flatworms

  prions - misfolded proteins that can transmit misfolded shape to other proteins e.g. Mad Cow Disease
• pathogens need a mode of transmission to ensure survival of the species

  • knowledge of mode of transmission helps shape responses and preventative measures
• infectivity = the ability of an organism to enter, survive and multiply in a host
• infectiousness = indicates comparative ease with which the organism is transmitted to other hosts
• contagious = infectious agents passed from person to person or person to animal
• infectious disease = disease spread by infectious agents but not from contact with an infected person
• different types of disease transmission:

  • air-borne transmission
  • contact transmission
  • vehicle transmission
  • vector transmission
  • trans-placental transmission
• air borne transmission:

  • infected droplets are sprayed out when an infected person coughs, sneezes or breathes and can be carried by air to healthy people
  • dust particles can also be infected

  e.g. cold, flu, measles, small pox, chicken pox
• contact transmission:

  (i) direct

  • personal contact (direct physical contact)

  e.g. Polio Myelitis
• contact transmission:

  (ii) indirect

  • contact with contaminated surface / substance
  • common in hospitals, communities, sports
  • also includes vehicle borne transmission

  e.g. ringworm, small pox, scarlett fever
• vehicle transmission:

  • transmission of disease through contaminated water, ice, milk, food or other biological products (passively carry pathogen)

  e.g. cholera, typhoid fever, viral hepatitis, E. Coli
• vector = organism that doesn't cause disease itself but that transmits infection by conveying pathogens from one host to another

  e.g. arthropods, mosquitos
• vector transmission:

  (i) mechanical

  • vector picks up infectious agent on outside of body (passive)
  • flies would be an example of a mechanical vector

  e.g. dysentery, enteric fever, scarlet fever, tuberculosis
• vector transmission:

  (ii) biological

  • usually arthropods i.e. mosquitos, lice, ticks

  1. propagative - disease agent multiples and increases in vector e.g. plague bacilli in rat fleas
  2. cycle-propagative - parasite undergoes cycle of development in vector with multiplication e.g. malaria in mosquitos
  3. cycle-developmental - parasite undergoes cycle of development without multiplication e.g. guinea worm in cyclops
• trans-placental transmission:

  • passed from infected mother to foetus in the uterus, during childbirth or during postnatal contact
  • e.g. syphilis, rubella, measles
• communicable disease = infectious disease readily transmitted from person to person
• endemic = small number of cases continually present in population in geographical area

  e.g. malaria
• epidemic = rapid spread of disease to large number of hosts in a given population in a small amount of time

  e.g. cholera
• pandemic = disease spread across large region or worldwide

  e.g. COVID-19
• endemics can flare up to epidemics due to:

  • reduced immunity in a population
  • local conditions of transmission change
• a communicable disease may be transmitted directly or indirectly, but in order to perpetuate itself, there must be continuous transmission between individuals in a population

  • to eradicate a disease, the chain of transmission must be broken
• to eradicate / effecively control a disease, we must know:

  • the cause of disease
  • its mode of transmission

  many diseases are controlled using environmental factors
• in addition to medical and drug intervention, we must include:

  • education e.g. water quality and safety
  • environmental engineering e.g. sanitation facilities
  • food (hygiene and quality) e.g. malnutrition
  • climate e.g. movement of vectors [migration]
• in low to middle income countries, 80% of all illnesses are caused by water-borne diseases

  • diarrhoea is the leading cause of childhood death in LMICs
• water-related diseases include:

  • those due to micro-organisms and chemicals in the water
  • diseases with part of their life cycle in water (e.g. schistomiasis)
  • diseases with water-related vectors (e.g. malaria)
  • diseases carried by aerosols containing microorganisms (e.g. legionellosis)
• there are 1.7 billion cases of diarrhoea per year and approximately 525,000 deaths

  • diarrhoea is a largely preventable disease, usually caused by unsafe drinking water, sanitation and hand hygiene
• the disability- adjusted life year (DALY) is a measure of overall disease burden, expressed as the number of years lost due to ill health, disability or early death

  i.e. one DALY = one healthy year of life lost

  • global DALY for diarrhoea is 99.6 million
• waterborne disease:

  • involves ingestion of contaminant
  • can be biological or chemical

  1. biological - water contaminated by human/animal excrement e.g. cholera, typhoid
  2. chemical - water contaminated by chemicals e.g. arsenic, nitrates from fertiliser
• water washed disease:

  • diseases cause by poor personal hygiene and skin and eye contact with contaminated water

  e.g. scabies, trachoma, typhus
• water based disease:

  • diseases caused by parasites found in intermediate organisms living in contaminated water

  e.g. schistomiasis, guinea worm
• water related diseases:

  • diseases caused by insect vectors that breed and feed near contaminated water

  e.g. dengue, yellow fever, malaria
• faeces is contaminated with bacteria, viruses and possibly other parasites

  • it is spread on fields, can leak through ground to the water supply, flies can land on it and remnants can be found on fingers
  • this can then be passed directly to a host or be passed onto food
• economic consequences of waterborne disease:

  • cost of treatment
  • cost of transport (to treatment)
  • loss of income (taking care of ill person)
  • educational impact (missing school due to illness)
  • stunted growth and development
• WASH programmes are programmes designed to reduce incidence of waterborne disease in the developing world that adopt a combined approach to educate and introduce interventions that address problems associated with water, sanitation and hygiene
• sanitation is the hygienic means of promoting health through preventing exposure to hazardous substances, as well as teh treatment and proper disposal of sewage
• slit trench

  • relies on the antimicrobial fauna in soil to inactivate pathogens in faeces

  
• latrine: a toilet, especially a communal one in a camp or barracks.

  • partially covered long pit that needs to be regularly emptied

  
• Ventilated Improved Pit (VIP) latrine: A pit latrine with a ventilated chamber above the pit

  • brick lined permanent toilet that doesn't need to be emptied
  • mortar only placed between layers so contents of latrine can seep through cracks

  A) pit collar

  B) pit

  
• improved water sources include:

  • piped water
  • public tap
  • tubewell / borehole
  • protected dug well