Understanding mechanisms of transmission Flashcards
what is included in the cycle of infection
disease organism (agent), reserviour, mode of escape, mode of transfer, mode of entry and susceptible host
what happens in the cycle of infecction
pathogen movement between reservoir, source and host
what is a source
immediate location of pathogen prior to host infection.
Pathogen reproduction may or may not happen. (a source and reservoir can be one the same)
what is a reservoir
natural habitat of the pathogen
Where normal pathogen reproduction happens
( source and reservoir can be one the same)
what is a host
organism damaged as a result of microbial growth and/or metabolic activities and/or toxin production
what are the 2 reservoirs of infectious disease
inanimate and animate
what is inanimate
Soil (tetanus)
Clostridium tetani
Decaying vegetable matter (listeriosis)
Listeria monocytogenes
flu-like illness
abortion in pregnant women
what can Clostridium tetani spread to
it comes from the soil and pathogen movement occurs between cows and horses
what is animate
Animals= zoonotic diseases (tuberculosis)
Mycobacterium bovis
Plants ???
Humans= acute infection ( incubatory, active, convalescent carriers, influenza or influenza virus)
or healthy/subclinical carrier
(typhoid fever or Salmonella typhi)
what is the Cycle of infection for Bovine Tuberculosis
Mycobacterium bovis spreads from cows to the host or from cows milk to the host it is a zoonotic disease
what are the most important reservoirs of human infection
humans
what are the phases of an acute infection
Incubatory phase
Active phase
Convalescent phase
Recovery
(pathogens excreted during all phases until recovery)
what are the pathogens like in subclinical infections
Pathogens excreted almost continually and recovery MAY not happen.
who was typhoid Mary
Mary Mallon 1869 –1938
was a Cook and a Subclinical carrier of Salmonella typhi
No symptoms
Bacteria present in gut
Passed on via fecal oral route into food
Forced into quarantine as would not accept that she caused disease and she continued to work as a cook.
what are the mechanisms of transmission of infection
- Airborne
- Direct Contact
- Vehicles
- Vectors
Pathogens are normally specialised for transmission mainly via one mechanism but can have occasional, limited spread by other mechanisms.
what are the majority of airborne diseases
respiratory infections
what are pathogens normally carried on
dust particles or vapour drops (aerosols).
what is the size of particals dependant on
on number of organisms originally present, the larger the number of organisms then the larger the size of the particles.
what can large particles do
settle out quickly, short carriage distance.
what can small particles do
remain in suspension for a long time, long carriage distance.
what are droplet nuceli
evaporation of water from exhaled particles and composed of mucus and microorganisms from the respiratory tract.
-1-4 µm in diameter
-suspended in the air for a long time
-coughing, singing, loud talking all produce copious quantities of droplet nuclei.
what diseases are spread by airborne transmission
TB, Diphtheria and whooping cough
What is TB
Tuberculosis- Mycobacterium tuberculosis= acid fast rod
-chronic lung infection
-Indirect damage
-3+ million deaths/yr
-incidence rising and antibiotic resistance rising
What is diphtheria
Diphtheria= Corynebacterium diptheriae- gram positive rod
-acute infection of upper respiratory tract
-controlled by vaccination
-epidemics in Russia after change from USSR
-health system breakdown
large population movements
what is whooping cough
Whooping cough= Bordtella pertussis-gram negative, short rod
-acute infection of upper respiratory tract
-controlled by vaccination
-recent epidemics in UK and USA
Why is it on the increase?- vaccines effects wear off after 2 /3 years
what is direct contat
physical interaction between the host and the source/reservoir.
Person-to-person contact is the most important route
-Sex
-surface contacts
-nursing mothers
-transplacental transfer
what are the pathogens transmitted by contact transmission like
fragile and do not survive for long periods outside of the body of a host
what is indircet contact
Transmission from the reservoir to the host is via an intermediary, usually an inanimate object (fomites)
-Bedding
-Eating utensils
-Medical instruments= endoscopes etc.
what diseases spread by direct contact
rabies and gonorrhoea
what is rabies
Rabies virus- Rhabdovirus- Negative strand RNA
viral infection of CNS, respiration affected
spread by bite from infected rodent/dog/etc.
what is gonorrhoea
Neisseria gonorrhoeae
gram negative diplococcus
infection of genital mucus membranes
spread by sexual contact
Antibiotic resistance rising
what is vehicle transmission
-Vehicles are normally inanimate
-Vehicles can transmit disease to large number of individuals at the same time.
-Most important vehicles: Food and Water- Possibility of growth of pathogen in contaminated food
small contamination can lead to large numbers.
how is the vast majority of disease spread
by ingestion of contaminated food/water result in gastrointestinal infection.
what diseases are spread by water or food
cholera or polio
what is cholera
-Vibrio cholerae, gram negative comma-shaped bacterium.
-spread by faecal pollution of water
-Follows major societal uphevals e.g war, earthquake etc.
-adheres to epithelial layer of small intestine
-produces toxin which disrupts osmotic balance, promotes diarrhoea and kills cells.
what is polio
-Polio virus
-Short lived gastrointestinal infection
-complications can result in CNS damage
-paralysis
what is vector transmission
-Pathogen is transmitted via a live intermediate.
-Most common vectors are Arthropods (ticks, fleas, mites etc).
-Pathogens may be carried in a number of different manners:
External
Internal
what is external in vector tansmission
pathogen is simply on the surface of the arthropod and is passively carried to the host. (bacilliary dysentery, Shigella sp.)
what is internal in vector transmission
pathogen is carried inside the arthropod.
There are two consequences for this type of transmission
the pathogen is unchanged (Plague), or the pathogen is fundamentally changed in the vector, normally essential steps of the life cycle undertaken, biological transmission
- Sleeping sickness, caused by the flagellated protist Trypanosoma brucei
what are 2 examples of vector transmission
sleeping sickness or plague
how do we prevent infection (2 words)
vaccination and quarantine
what are the controls against animals
eliminate infected individuals- relatively easy with domesticated animals (tuberculosis) in UK and ROI
More difficult with wild animals (rabies)- Western Europe
what are the controls for humans
-remove infected individuals=(quarantine) and limit freedom of movement of infected individuals
-limit freedom of movement of individuals from regions where infection occurring
what are the controls for inanimate -soil
difficult to implement anything useful
what are the controls directed against host susceptibility
-Immunisation= Reduction of pathogens being released to new hosts
-Social programmes to improve nutrition, housing, education etc.
what are the controls against transmission
Vector Spread
-eliminate breeding grounds= Drain swamps
-interrupt life-cycle= Remove one host in multihost systems
-prevent successful mating= Sterile males
-Novel ways to kill the insects eg the fungus= Metarhizium anisopliae
- provision of clean water
- control of food production and processing
-adequate storage methods
-education
how do we limit direct contact spread
social pressure to change sexual habits and education
how do we limit airborne spread
little can be done if infected individuals are present in a population
What can many resiviours be
Asymptomatic- major problem and better diagnosis
