epidemiology of communicable diseases dr. zaher nazzal md, abcm

EPIDEMIOLOGY OF COMMUNICABLE DISEASES DR. Zaher NazzalMD, ABCM

Introduction• Vaccination and control of communicable diseases are

two of the great public health achievements of the last century

• Infectious diseases are now the world's biggest killer of children and young adults; both are vital age groups.

• They account for more than 13 million deaths a year - one in two deaths in developing countries

04/18/23 Epidemiology of Communicable Diseases 2

Introduction• About 32 percent of all deaths worldwide are caused

by communicable diseases, maternal and perinatal conditions and nutritional deficiencies.

• Almost 90 percent of these deaths are caused by pneumonia, tuberculosis (TB), diarrheal diseases, malaria, measles, and the HIV.

• It is a fundamental part of the whole of Epidemiology.

Epidemiology• “The study of the distribution and determinants of a

health-related event and application of this study to control of health problems” (John Last, 1988).

• That branch of medical science which treats epidemic (Parkin,1873)

• Epidemiology is the study of epidemics and their prevention (Kuller LH)


Definitions• Infection: The entry and development or multiplication of an

infectious agent in a human or an animal with the response of the body to defend it self against that invader.

An infection doesn’t always cause illness.

There are several level of infectionColonization: (e.g., S. aureus in skin and normal nasopharynx)Subclinical or in apparent infection: (e.g., polio)Latent infection: (e.g., herpes simplex)Manifest or clinical infection:


Definitions……cont’d

• Infectious disease A clinically manifest disease of man or animals

resulting from an infection.• Infectious agent An organism (virus, bacteria, rickettsia, fungus, protozoan, or helminthes) that is capable of producing infection or infectious disease .



• Communicable disease An illness due to a specific infectious agent or its

toxic products that arises through transmission of that agent or its products from an infected person, animal or inanimate reservoir to a susceptible host; either directly or indirectly.

• Contagious disease A disease that is transmitted through direct contact



• Reservoir Any person, animal, arthropod, plant, soil, or

substance ; in which an infectious agent normally lives and multiplies, on which it depends primarily for survival, and where it reproduces itself in such manner that it can be transmitted to a susceptible host.

• Host A person or animal ( including birds and

arthropods) that afford subsistence or lodgment to an infectious agent under natural conditions.



• Susceptible A person or animal not possessing sufficient resistance

against a particular pathogenic agent to prevent contracting infection or disease when exposed to the agent.

•Carrier A person or animal that harbors a specific infectious

agent without apparent clinical disease and serves as potential source of infection.



• Transmission of infectious agents Any mechanism by which an infectious agent is spread

from a source or reservoir to a person.



• Infectivity:

Capacity of agent to enter and multiply in a susceptible host (hence produce infection/disease) (polio and measles have high infectivity)

• Pathogenicity:

Capacity of agent to cause clinical disease in the infected host (measles has high pathogenicity)



• Virulence The degree of pathogenicity of an infectious agent.

i.e. the ability of the agent to invade and damage tissues of the host causing severe manifestations or death.

• Toxigenicity:

The ability of a microbe to produce bio chemicals, known as toxins that disrupt the normal functions of cells or are generally destructive to human cells and tissues



• Resistance:

Ability of agent to survive adverse environmental conditions (hepatitis agents generally very resistant whereas influenza viruses are typically fragile).

Note: “resistance” is also applied to the host.

• Antigenicity:

Ability of agent to induce antibody production in the host (e.g. re-infection with measles virus is very rare).


Endemic, Epidemic and Pandemic

• Endemic The constant presence of a disease or infectious

agent within a given geographic area or population group, without importing from outside.

• Epidemic The unusual occurrence of a disease (in excess of the

expected) in a community or region.


• Outbreak:The unusual occurrence of a disease(in excess of the

expected) in a localized small area.

• Pandemic: It is an epidemic that spreads over several countries or

continents, affecting a large number of people.


Nosocomial infections• Nosocomial (hospital acquired) infection is an

infection originating in a patient while in a hospital or another health care facility.

• It has to be a new disorder unrelated to the patient’s primary condition.

• Examples include infection of surgical wounds, hepatitis B and urinary tract infections.


Opportunistic infection

• This is infection by organisms that take the opportunity provided by a defect in host defense (e.g. immunity) to infect the host and thus cause disease.

• Opportunistic infections are very common in AIDS. (Herpes simplex, cytomegalovirus, M. tuberculosis….etc)


Zoonosis, epizootic and enzootic• Zoonosis is an infection that is transmissible under

natural conditions from vertebrate animals to man, e.g. rabies, plague, bovine tuberculosis…..

• An Epizotic is an outbreak (epidemic) of disease in an animal population, e.g. rift valley fever.

• An Enzotic is an endemic occurring in animals, e.g. bovine TB.


Eradication and Elimination• Eradication: termination of all transmission of infection by

the extermination of the infectious agent through surveillance and containment.

• Eradication is an absolute process, an “all or none” phenomenon, restricted to termination of infection from the whole world.

• Example: smallpox, polio

• Elimination: it is sometimes used to describe eradication of a disease from a large geographic region. Disease which is amenable to elimination in the meantime is measles.


DYNAMIC OF DISEASES TRANSMISSIONDr. Zaher NazzalMD, ABCM

Chain of Infection

A process that begins when an agent leaves its

reservoir or host through a portal of exit, and is

transported by some mode of transmission, then enters

through an appropriate portal of entry to infect a

susceptible host.


Chain (Cycle) of infection

Susceptible HostSusceptible Host

AgentAgent

ReservoirReservoir

Mode of transmissionMode of transmission

Portal of ExitPortal of ExitPortal of InletPortal of Inlet

IP

PC


The requisites (essentials) for the perpetuation of communicable diseases:

The elements of the cycle of infection:

1.Presence of microbiological agent.

2.Presence of reservoir.

3.Portal of exit.

4.Mode of transmission.

5.Portal of entry (inlet).

6.Presence of susceptible host.


Incubation Period

• The period from exposure to infection to the onset of symptoms or signs of infectious disease.

The length of incubation period depends on:o The portal of entry.o The rate of growth of the organism in the host.o The dosage of the infectious agent.o The host resistance.


Period of Communicability

• The time during which an infectious agent my be

transmitted directly or indirectly from an infected

person to a susceptible person or animal.

• Its length varies from one disease to another


AGENT


1. Agent

• Microorganisms are responsible for disease production (viruses, bacteria, protozoa, parasites, fungi,..

• Agent factors that affect disease transmission: o Infectivityo Pathogenicityo Virulenceo Aantigenicity


Infectivity: The ability of an agent to invade and multiply (produce infection) in a susceptible host.How to measure (Infectivity); ease & spread of infection?Secondary Attack Rate•The proportion of exposed susceptible persons who become infected.

•Examples: High infectivity: Measles, Chickenpox Low infectivity: Leprosy

100sec

xessusceptiblofNumber

casesondaryofNumberrateattackSecondary


Pathogenicity: Is the ability of the organisms to produce specific clinical

reaction after infectionIt refers to the proportion of infected persons who develop

clinical disease.Examples: • High pathogenicity: Measles, Chickenpox• Low pathogenicity: Polio, Tuberculosis, Hepatitis A,

Meningitis, AIDS It can be measured by:Ratio of clinical to sub-clinical case=

caseslSubclinica

casesClinical


Virulence: It refers the ability of organisms to produce severe

pathological reaction.

It is the proportion of persons with clinical disease who become severely ill or die.

Examples: Rabies, Hemorrhagic fevers caused by Ebola and Murberg viruses.

Case fatality rate

100disease that of cases ofnumber Total

disease a from deaths ofnumber Total ratefatality Case x


Mechanisms of disease production (pathogenesis):

• Invasiveness:(Pneumococcosis, measles).

•Toxicity: (Tetanus, Botulism).

•Hypersensitivity: (Tuberculosis).

•Others: Immune suppression: (AIDS).


SOURCE OR RESERVOIR


2. Source or Reservoir• The reservoir of an agent is the habitat in which an

infectious agent normally lives, grows, and multiplies.

• “any person, animal, arthropod, plant, soil, or substance, or a combination of these, in which an infectious agent normally lives and multiplies, on which it depends primarily for survival, and where it reproduces itself in such a manner that it can be transmitted to a susceptible host. It is the natural habitat of the infectious agent.”


Types of reservoirs

Reservoir

Human reservoir

Animalreservoir

Non-livingreservoir


Path

ogen

Res

ervo

irsHumans are

the most important

reservoir of human

infectious disease.


Human reservoir

Human reservoir

Cases Carriers


Cases

A case is defined as “a person in the population or

study group identified as having the particular disease,

health disorder, or condition under investigation”


Cases

Cases are classified as•Primary case•Index case•Secondary casesAccording to spectrum of disease:•Clinical cases: (mild/severe-typical/atypical)

•Sub-clinical cases•Latent infection cases


Carriers

• A person or animal without apparent disease who harbors a specific infectious agent and is capable of transmitting the agent to others.

• It occurs either due to inadequate treatment or immune response, or the disease agent is not completely eliminated, leading to a carrier state.


Carriers are dangerous because:

1.They do not show any clinical manifestation so they carry normal life.

2.The carrier and his contacts are not aware of their condition so, they take no precautions.

3.It is difficult to discover them.

4.It is not always possible to deal with them.

5.They can remain infectious for a long time leading to repeated introduction of the disease to contacts.


Types of Carriers:1. Asymptomatic (In-apparent) carrier:

The carrier state that may occur in an individual with an infection that is in-apparent throughout its course

Examples: Polio virus, meningococcus, hepatitis A virus

2. Incubatory, Convalescent, Post-Convalescent carriers:

The carrier state may occur during the incubation period, convalescence, and post convalescence of an individual with a clinically recognizable disease.

Examples of Incubatory carrier: Measles, chickenpox


Examples of convalescent carriers: Salmonella typhi, Diphtheria, hepatitis B virus

3.Healthy (chronic) carriers: They continue to harbour an agent for an extended time (months or years) following the initial infection.Examples: Hepatitis B virus, S. typhi


Human reservoir

Human reservoir

Cases Carriers

According to spectrum of disease:•Clinical cases (mild/severe-typical/atypical)•Sub-clinical cases•Latent infection cases

•Primary case•Index case•Secondary cases

Type:•Incubatory•Convalescent•Healthy

Duration:•Temporary•Chronic

Portal of exit:•Urinary•Intestinal•Respiratory•others


Animal reservoirs• Zoonosis is an infection that is transmissible under natural

conditions from vertebrate animals to man, e.g. rabies, plague, bovine tuberculosis…..

• There are over a 100 zoonotic diseases that can be conveyed from animal to man.

brucellosis (cows and pigs),

anthrax (sheep),

plague (rodents),

rabies (bats, dogs, and other mammals).


Zoonoses

Zoonoses are Human

Diseases with Animal

Reservoirs.


Toxoplasmosis Zoonoses


Reservoir in non-living things• Water, Soil and inanimate matter can also act as reservoir

of infection.

For example• Soil may harbor agents that causes tetanus, anthrax.

• Pools of water are the primary reservoir of Legionnaires’

bacillus.


3. Portal of exit

• Portal of exit is the path by which an agent leaves

the source host.

• Examples:

• Respiratory tract

• GIT

• Skin and mucous membrane


MODES OF TRANSMISSION


4. Modes of transmission• Many different ways• As a rule only one route for each pathogen

o Typhoid fever ----- vehicle transmissiono Common cold---------direct contact

• But there are others that may be transmitted by several route

o AIDs, Salmonellosis, Hepatitis B, brucellosis, Q fever,…..etc.


Mode of transmission

Direct transmission Indirect transmission

Direct contact

Droplet infection

Contact with soil

Inoculation into skin or mucosa

Trans-placental (vertical)

Vehicle-borne

•Vector-borne:•Mechanical•biological

Air-borne

Fomite-born

Unclean hands and fingers

propagative

Cyclo-prop.

Cyclo-develop.


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Path

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Direct transmissionDirect Contact

•There is essentially immediate transfer of the agent from a reservoir to a susceptible host by direct contact or droplet spread.• Direct contact occurs through: Skin-to-skin contact, kissing, and sexual intercourse.• Direct contact refers also to contact with soil or vegetation harbouring infectious organisms.•STDs, AIDs, leprosy, Skin and Eye infections


Contact Transmission

Rhinovirus?


Direct transmissionDroplet Infection

•Direct projection of a spray of droplets of saliva and naso-pharyngeal secretions •Produced by sneezing, coughing, or even talking.•Particles <5mmm can penetrate deeply and reach the alveoli.•Usually limited to a distance of max 1 meter (30-60cm).

o Close proximity o Overcrowding o Lack of ventilation

•Common cold, TB, Meningococcal Meningitis, Whooping cough.


Droplet Transmission

Less thanone meter

Measles?


Direct transmissionContact with soil

•Direct exposure of the susceptible tissues to the disease agent in soil•E.g. Tetanus, Hookworm.

Inoculation into skin or mucosa•The agent is directly inoculated into skin or mucosa.•E.g. rabies, hepatitis B,

Trans-placental• TORCH, Varicella, Hepatitis B, AIDs


Indirect TransmissionFrom a reservoir to a susceptible host by: •Vehicle borne: inanimate vehicle•Vector borne: animate vector•Airborne transmission: suspended air particles

The infectious agent must be capable of surviving outside the human host in the external environment.


Vehicle borne:• An infectious agent is carried from a reservoir to a

susceptible host by an inanimate intermediary. • Vehicles include:1. Contaminated food and water: typhoid, paratyphoid, food

poisoning, dysentery and cholera.

2. Biologic products (blood): Hepatitis B, AIDS, Syphilis.

3. Fomites (inanimate objects such as: door knobs, toys, handkerchiefs, bedding, or surgical instruments). Influenza.


Food

born

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Hepatitis A


Wat

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Cryptosporidium parvum?


Com

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Out

brea

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Street Pump.


Indirect-Contact Transmission

Beddings are an example of a Fomite, an inanimate object that can transmit

pathogens between people.

Methicillin-resistant

Staphylococcus aurius

(MRSA)?


Indirect-Contact Transmission

Door knobs are another good example of a

fomite.

Influenza virus?


Vector-borne• Vector is an arthropod or any living carrier (e.g. snail) that

transport an infectious agent to a susceptible individual.• Arthropod include flies and mosquitoes, fleas,

cockroaches, ticks and mites, sucking lice, eyc.

• Mechanical transmission: the agent does not multiply or undergo physiologic changes in the vector.

For example, flies carry Shigella on appendages.• Biologic transmission: When the agent undergoes

changes and/or multiplication within the vector before it is transmitted.


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AirborneAirborne transmission occurs by particles that are suspended in air. There are two types of these particles:

- dust and - droplet nuclei.

1.Dust particles:-result from re-suspension of particles that have settled on floor or bedding,- infectious particles blown from the soil by the wind. Example: Fungal spores.


2. Droplet nuclei• They represent the dried residue of droplets that have been

coughed or sneezed into the air.• They are very tiny particles less than 5 µ (microns) in size and

may remain suspended in the air for long periods.Examples: • Tuberculosis is transmitted more often indirectly, through

droplet nuclei, than directly, through direct droplet spread. • Legionnaires’ disease and histoplasmosis also spread

through airborne transmission.


Airborne Transmission

More than

one meter

Mycobacterium tuberculosis?


5. Portal of entry

An agent enters a susceptible host through a portal of

entry.

-The portal of entry must provide access to

tissues in which the agent can multiply or a toxin can

act.

- Often, organisms use the same portal to enter a

new host that they use to exit the source host.


THANK YOUTHANK YOU


SUSCEPTIBLE HOST


6. Susceptible hostA susceptible host is the final link in the chain of infection.The host is a person or other living organism that can be

infected by an infectious agent under normal conditions.An infectious agent seeks a susceptible host aiming “successful

parasitism”.Four stages are required for successful parasitism:

1. Portal of entry2. Site of election inside the body3. Portal of exit4. Survival in external environment


Susceptibility of a host depends on

1. Genetic factors

2. General factors

3. Host defense (Specific acquired immunity)


General factors which defend against infection:

• The skin,

• Mucous membranes,

• Gastric acidity,

• Cilia in the respiratory tract,

• The cough reflex, and

• Nonspecific immune response.


General factors that may increase susceptibility

• Malnutrition,

• Alcoholism, and

• Disease or therapy which impairs the immune

response (Cortisone, cytotoxic drugs, ...


Host Defence• Host defence against infection is at onceoLocal and systematicoSpecific and no-specificoHumoral (B cells) and cellular (T cells)• Most of infectious agents stimulate multiple host defence

mechanisms.


Specific acquired immunity:

It refers to protective antibodies that are directed against a specific agent.

It is of two types:

1.Active immunity: Resistance developed in response to stimulus by an antigen either;•Naturally by infecting agent or •Artificially by vaccine and usually characterized by the presence of antibody produced by the host.


2. Passive immunity:

Immunity conferred by an antibody produced in

another host and may be acquired;

• Naturally by an infant from its mother (3- 6 mths)

• Artificially by administration of an antibody

containing preparation (antiserum or immune

globulin).


Herd Immunity

It is the state of immunity of a group or a community.

Also it is;

“The resistance of a group or a community to an

infectious agent, based on the immunity of a high

proportion of individual members of the group”.


Herd Immunity

If the herd immunity is sufficiently high, the occurrence

of an epidemic is highly unlikely

High level of immunity (by high vaccination coverage)

------- makes elimination of a diseases possible.

It was crucial in polio and diphtheria

Herd immunity may be determined by serologic survey


epidemiology of communicable diseases dr. zaher nazzal md, abcm

Documents

century infectious diseases

definitions infection

specific infectious

diarrheal diseases

clinical infection

apparent infection

abcm slide

contracting infection