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)

Chain of infection The host: the 3rd link

Portals of exit

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

Chain of infection The host: the 3rd link

Portals of entry

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

Thanks