DISEASE TRANSMISSION
Dr. Alamin Alabid
Phone: 09123353580
RESERVOIR(Portal of Exit)
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MODE OF TRANSMISSION
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(Portal of Entry)
SUSCEPTIBLE HOST
Chain of Transmission
Agent (e.g. mico-organism)
Reservoir of infection
Portal of Exit
Suitable mode of transmission
Portal of entry into susceptible host
Susceptible host
Agent
Susceptible Host Reservoir
Mode of transmission
Cycle of infection
Portal of ExitPortal of Inlet
Chain of Transmission
Portal of Portal of
Exit Entry
Transmission
Reservoir Host
Micro-Organism(Agent)
RESERVOIR
Definition:
- it is the normal habitat in which
the disease causing agent lives,
multiplies & grows
- without a reservoir, the agent
cannot perpetuate itself in nature
RESERVOIR
Definition: habitats where infxs agent can survive & (multiply). E.g. microbes causing nosocomial (hosp. acquired) infxns can be found in patients, hosp. Staff, equipment
Strategies for breaking link:
Sterilize equipments
Isolate infxs patients
Infx hosp staff relieved from clinical duties
TYPES OF RESERVOIRS 1. Symptomatic cases e.g. influenza, measles,
chickenpox
2. Carriers – people who carry infectious agent
but show no S&S of disease
- inapparent carriers
(free of S&S of disease thru’out course of
infxn but can shed infxs agent e.g. polio,
Hepatitis A)
- incubatory carriers
(transmit agent prior onset of disease e.g. AIDS,
Hepatitis B)
- convalescent carriers chronic carriers
(e.g. typhoid – “Typhoid Mary”)
TYPES OF RESERVOIRS
(contd)
3. Animal Carriers (Zoonoses)
e.g. rabies, plague, Ebola, Nipah virus
etc.
4. Inanimate objects, water, food, soil,
air & other inert substances
CONTROL MEASURES DIRECTED
TO RESERVOIR OF INFECTION
1. Active case finding & prompt Rx
2. Detection & Rx of subclinical cases & contacts
3. Detection of carriers & eliminate carrier state e.g. typhoid
4. Health education (public)
5. Eliminate animal reservoir & vector e.g. rabies
6. Environmental sanitation (proper excreta &
waste disposal, safe water supply, proper
& adequate housing, food quality control,
vector control)
PORTALS OF ENTRY & EXIT
Infectious agent leaves host (EXIT) enters new susceptible host (ENTRY)
Entry & Exit Sites PORTALS
1. Respiratory tract
2. Conjuctiva
3. Urogenital tract
4. GIT
5. Skin
6. Placenta
PORTAL OF EXIT
Definition: pathway by which infxs agent exits the reservoir (e.g. resp. tract, GIT, Genito-UT, skin, blood & body secretions)
Strategies for breaking link:
Cover mouth & nose when coughing
Avoid indiscriminate spitting
Use gloves
Screen blood donors
Portals of Exit
1. Respiratory tract
2. Gastero-intestinal tract
3. Salivary gland (e.g. mumps***)
4. Skin
5. Urogenital tract
6. Blood (bleeding, insect bites,
needles, blood transfusions)
TRANSMISSION OF
INFECTIOUS DISEASE
Definition:
- Any mechanism by which an infectious
agent is spread to another host
Routes/Modes of Transmission
Transmission = spread
For disease to spread the pathogen has to be transmitted from a reservoir or source to a susceptible host
Thus, to prevent spread of disease, we need to know possible routes of transmission
Mode of Transmission
Definition: mechanism thru w. infxs agent move from reservoir to susceptible host
Transmission:
-Direct - contact (person-to-person)
- sexual contact
-Intrauterine transmission
-Transdermal
-Indirect - contaminated vehicles (food, water),
- inanimate objects ( beddings, dressings
- Airborne (dust, droplet nuclei)
- Vector borne (biological, mechanical)
Routes/Modes of Transmission
1. Person-to-person
- direct contact e.g. touching
- indirect contact e.g. fomites*
- droplet transmission
2. Common vehicle***
- waterborne
- airborne
- foodborne
3. Vectorborne ( esp. insects)
- mechanical (feet of a fly)
- biological (organism multiply within the
vector e.g. malaria, dengue)
FOMITES*
Definition:
An inanimate object that can carry
micro-organisms on its surface
E.g. beddings contaminted with
vomitus/excretions of a cholera patient
Strategies to break Chain of
Transmission
Disrupt direct contact – gloves, handwashing, disifection
Interrupt indirect contact – proper disposal of contaminated materials
Prevent contamination of food & water
Eliminate vectors thru public health measures
Proper housing; good ventilation; avoid overcrowding
Proper disposal of excreta & waste
Portal of Entry
Definition: pathway by which infxs agent enter susceptible host
E.g. resp. tract, GIT, Genitourinary tract, broken skin, placenta
Strategies for breaking link:
Use masks, gloves
Isolate cases
Disposable needles
Portals of Entry
1. Mucous membranes of the:
respiratory tract
gastero-intestinal tract
urogenital tract
2. Skin
3. Conjunctiva
4. Parenteral route (intravenous, injxns)
“Preferred” portals of entry
Most pathogens/micro-organisms have a “preferred” or specific portal of entry.
E.g.
Salmonella typhi GIT typhoid
Influenza virus respiratory tract influenza (flu)
Mycobacterium tuberculosis tuberculosis (TB)
Yersinia pestis flea bites Man’s skin plague
BLOCKING THE AGENT’S PORTAL
WILL EFFECTIVELY STOP
TRANSMISSION
Some agents have “preferred” portals of entry
& exit
Examples: TB, influenza (resp. tract)
Schistosomiasis (enter thru’ skin; exit thru’
urine or feces)
Gonorrhea (urogenital tract)
HIV (multiple routes: urogenital, rectal
mucosa, skin (injxn), blood, placenta)
SUSCEPTIBLE HOST
Definition: Person who lacks sufficient
resistance to a specific pathogenic organism.
Factors influencing susceptibility:
Age, sex, race, occupation, nutritional status,
lifestyle practices, genetics, concomitant
disease & current medications, immunity
status, immunocompromised state
SUSCEPTIBLE HOST
Strategies for breaking link:
Vaccinate hi risk gp (e.g. Hepatitis B
vaccination for health workers, soldiers)
Screen health care workers
Health intervention – vector control,
clean & safe water supply
Why do disease outbreaks
(epidemics) occur?
Interaction: agent/host/environment
Chain of transmission
Modes of spread
Patterns of Occurrence of
Communicable Diseases
1. Cluster
2. Endemic
3. Epidemic
4. Pandemic
5. Sporadic
Cluster
An aggregation of cases in a given
area over a particular period without
regard to whether the number of
cases is more than expected
Endemic
The constant presence of a disease
or infectious agent amongst humans
within a a defined population or
region or country
Epidemics and Outbreak
Epidemic
Occurrence of clearly more cases of a specific disease in a community or region in excess than what is normally expected in a region or community
Outbreak : a localised epidemic
Pandemics : epidemics affecting several countries & continents (e.g. cholera, flu)
Outbreak
A localised epidemic occurring over a
short period
The occurrence of more cases of a
disease than expected in a given area
or among a specific group of people
over a particular period of time