Infectious Disease - Mechanism of Transmission Flashcards
Koch’s postulates to determine the causality of disease (4)
1) microorganism must be found in abundance in all organisms suffering from the disease, but should not be found in healthy organisms
2) microorganism must be isolated from a diseased organism and grown in pure culture
3) cultured microorganisms should cause disease when introduced into a healthy organism
4) microorganisms must be preisolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent
-CORRECT***
Problems with the postulates
1) microorganism must be found in abundance in all organisms suffering from the disease, but should not be found in healthy organisms
-organisms like Staph, enterococcus found in both healthy and diseased organisms
2) microorganism must be isolated from a diseased organism and grown in pure culture
-not all organisms can be cultured, you can also isolate from a healthy individual
3) cultured microorganisms should cause disease when introduced into a healthy organism
-doesn’t always happen, can be asymptomatic
Disease process stages (5)
1) Incubation period
2) Prodromal period
3) Illness
4) Decline
5) Convalescence
Which stage determines how effective a pathogen is at spreading?
incubation period length
longer incubation periods = better at transmitting
short incubation period = host feels ill and take themselves out
Incubation period
interval between the introduction of an organism and the onset of the illness
asymptomatic
don’t know you’re sick
not taking steps to prevent spread
when you’re infectious
days: influenza (1 to 3), COVID (3 to 5)
weeks: varicella
years: myobacterium tuberculosis
Prodromal period
organisms actively multiplying and causing damage to host
vague, non-specific
general symptoms
fever, malaise, myalgia, loss of appetite, runny nose, conjunctivitis
infection has amplified in the host, taking over
e.g. tingling sensation before a cold sore
Illness
most severe signs and symptoms
organisms are actively multiplying in the host
clinical presentations at its peak
host is still infectious but typically less likely to transmit
feeling unwell, likely remove themselves from environment
Decline
declining signs and symptoms
Convalescence
no signs or symptoms/minimal
back to normal
period that follows active illness
patients may still be contagious (not always)
pathogen load tends to be lower than during active infection
can be fairly prolonged, with pathogen still detectable
T or F: Diseases will exhibit all 5 stages.
FALSE
Natural History of an Infectious Disease - Phases (2)
1) Pre-clinical
2) Clinical
Pre-clinical phase
A - biologic onset of disease
P - pathological evidence of
Between pre-clinical and clinical phase
S - signs and symptoms
Clinical phase
M - medical care sought
D - diagnosis
T - treatment
T or F: Diagnoses tend to be sought early on in the disease course.
FALSE
usually well into your illness when you seek care
diagnosis is late
Carrier state
someone that harbours an infectious agent for prolonged periods of time
WITOUT showing any overt clinical symptoms / signs
able to spread the pathogen
plays a major role in maintaining the chain of infection within a population
e.g. Chlamydia trachomatis, Mumps, HSV-1, HSV-2.
Famous example of a carrier
QUIZ/EXAM Q***
Mary Mallon “typhoid Mary”
food handler in the early 1900 in NY state
infected: Salmonella Typhi (typhoid), SUBCLINICAL disease
kept cooking and spread outbreaks, killing many people
typhoid caused by a specific type of salmonella
S. Typhi leads to typhoid fever but also can seed an individuals gall bladder and result in the establishment of a carrier state (sometimes life long without Tx)
form of salmonella that is extra intestinal
goes from gut to blood, can go into gallbladder
if in gallbladder - stays there and is continuously shed in urine
Following symptom onset - types of infections (3)
1) Acute
2) Chronic
3) Latent
Acute infection
rapid onset and usually lasts a short period of time
often highest pathogen burden.
Chronic infection
disease develops and established a steady state and lasts a long time
for life
e.g Mycobacterium tuberculosis, HIV, herpes family
Latent infection
typically sub-clinical
may reactivate from time to time
infection is never really eliminated
latent phenotype
3 opportunistic infections you can get with HIV
EXAM Q**
1) aspergillus
2) candida
3) salmonella
HIV stages following symptom onset
virus replicates in CD4 cells
usually use CD4 as a way of assessing immunocompromised
lower=more immunocompromised
initially - huge drop in CD4 count
recovers, but then depletes over time in a chronic fashion
CD4 count of 200 = AIDS, at risk for opportunistic infections like aspergillus
Reservoirs of Infectious Diseases (3)
1) Human
2) Animal
3) Non-living
Human reservoir types (2)
1) Symptomatic
2) Asymptomatic
-majority of spread***
Animal reservoir types (2)
1) Direct spread
2) Indirect spread
Examples of direct animal reservoirs
Rabies, Ebola
Lyme disease (Borrelia burdoferi) from Ixodes tick
Mosquitos (West Nile, Yellow Fever, Dengue, Chikungunya, Malaria)
Examples of indirect animal reservoirs
E.coli from cattle (Beef)
Salmonella from Chicken
Campylobacter from Chickens
Brucellosis from unpasteurized milk
Non-living reservoir main types (3)
1) surfaces
2) contaminated food/water
3) soil
Examples of non-living reservoirs
Phomytes = inanimate objects
door knobs - influenza virus
rusty nail - tetanus bacteria
soil - histoplasmosis/ blastomycosis
contaminated water (Vibrio cholera, Hepatitis A)
sweater - Norovirus
EXAM Q - What disease could you get from a non-living reservoir like soil?
Soil - Histoplasmosis/ blastomycosis
Factors leading to emergence of infectious diseases
human-animal contact
-87% of pathogens come from animal
building dams, cutting down forests etc.
-interacting more with our environment, greater chance to jump
e.g. bird flu
-birds can infect humans
-has mortality of 30%+
-if it can find a way to spread from human to human —> we’re fucked
plus many others
-population growth
-travel
-climate change
-aging population
-increased antibiotic use
Modes of transmission (4)
1) Direct contact
2) Indirect spread
3) Droplets
4) Air-borne
Direct contact
have to be close to someone and directly interact
shaking hands, kissing, hugging, STI’s exposure to blood and body fluids, needle sticks
e.g. Salmonella Typhi
Indirect spread
medical equipment, toys, door knobs
Droplets
respiratory droplets, close contact (> 5um)
usually < 1 meter of another person
e.g. COVID, influenza, Norovirus
distancing - 6 feet (twice what droplets can travel)
COVID mode of transmission controversy
most in medical community think its droplets and direct contact
important implications for precautions
Air-borne
airborne smaller droplets (< 5um)
> 1 meter transmission risk
travel on air currents for longer distances
e.g. TB, measles, Ebola
have to be in a negative pressure room, N95
Vehicles of Transmission (3)
1) Airborne
-REALLY BAD
2) Waterborne
-bad but manageable
3) Foodborne
Airborne vehicle of transmission
dust particles or fine droplets that can remain suspended for long periods of time
Waterborne vehicle of transmission
contaminated water
viral, bacterial and parasitic in nature
Foodborne vehicle of transmission
contaminated food
viral, bacterial and parasitic
partly because we eat everything
Interventions for the control of infectious disease dissemination - understand the… (3)
1) mode of transmission
2) properties of the pathogen
3) vector
Zika Virus overview
vector borne transmission via two types of mosquitoes
1) Asian Tiger (Aedes albopictus)
2) Egyptian Tiger (Aedes aegypti) mosquitoes
vector: mosquito
transmits parasites
originally found in Africa/Asia but has managed to travel the world, including NY
only thing that’s preventing it from coming here is our winters
Zika virus reservoirs of infection (2)
1) humans
2) non-human primates
Zika modes of transmission (2)
1) indirect
2) direct
Zika vehicles of transmission (3)
1) mosquitoes
2) semen
-ONLY arbovirus that is sexually transmitted
3) blood
What makes the aedes mosquitoes so great at transmission
1) only bites during the day
2) soft landing
3) ability to fly without making a noise
4) great anesthetic
Culex mosquito transmits….
West Nile Virus
in Ontario
Anopheles mosquito transmits…
malaria
only bites at night
easier to control with bed nets
