infectious dis

Communicable Diseases

Dr. Areej Mothanna

Definition of communicable diseases

A communicable disease is an illness due to a specific infectious (biological) agent or its toxic products capable of being directly or indirectly transmitted from man to man, from animal to man, from animal to animal, or from the environment (through air, water, food, etc..) to man.

Infection

Infection is the entry and development or multiplication of an infectious agent in the body of man or animals. An infection does not always cause illness.
There are several levels of infection (Gradients of infection):
Colonization (S. aureus in skin and normal nasopharynx)
Subclinical or inapparent infection (polio)
Latent infection (virus of herpes simplex)
Manifest or clinical infection

Levels of Disease

Diseases have a range of seriousness, effect, duration, severity, and extent
Classified into 3 levels
Acute relatively severe, of short duration and often treatable
usually the patient either recovers or dies
Subacute intermediate in severity and duration, having some acute aspects to the disease but of longer duration and with a degree of severity that detracts from a complete state of health
Patient expected to eventually heal
Chronic less severe but of long and continuous duration, lasting over a long time periods, if not a lifetime
Patient may not fully recover and the disease can get worse overtime
Life not immediately threatened, but may be over long term


Contamination
The presence of an infectious agent on a body surface, on or in clothes, beddings, toys, surgical instruments or dressings, or other articles or substances including water and food

Infestation

It is the lodgment, development and reproduction of arthropods on the surface of the body or in the clothing, e.g. lice, itch mite. This term could be also used to describe the invasion of the gut by parasitic worms, e.g. ascariasis.

Contagious disease

A contagious disease is the one that is transmitted through contact. Examples include scabies, trachoma, STD and leprosy.

Host

A person or an animal that affords subsistence or lodgement to an infectious agent under natural conditions. Types include: an obligate host, definitive (primary) host, intermediate host and a transport host.

Vector of infection

An insect or any living carrier that transports an infectious agent from an infected individual or its wastes to a susceptible individual or its food or immediate surroundings. Both biological and mechanical transmissions are encountered.

Reservoir

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.

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 infetions.


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. For example, opportunistic infections are very common in AIDS. Organisms include Herpes simplex, cytomegalovirus,
M. tuberculosis….

Dynamics of disease Transmission (Chain of Infection)

Source or Reservoir
Modes of transmission
Susceptible host

I
II
III

(I): Source or Reservoir

The starting point for the occurrence of a communicable disease is the existence of a reservoir or source of infection.

The source of infection is defined as “the person, animal, object or substance from which an infectious agent passes or is disseminated to the host (immediate source). The reservoir is “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
Animal
reservoir
Non-living
reservoir

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

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”

Carriers

• It occurs either due to inadequate treatment or immune response, the disease agent is not completely eliminated, leading to a carrier state.
• It is “an infected person or animal that harbors a specific infectious agent in the absence of discernible (visible) clinical disease and serves as a potential source of infection to others.
• Three elements have to occur to form a carrier state:
• The presence in the body of the disease agent.
• The absence of recognizable symptoms and signs of disease.
• The shedding of disease agent in the discharge or excretions.


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.

Reservoir in non-living things

Soil and inanimate matter can also act as reservoir of infection.

For example, soil may harbor agents that causes tetanus, anthrax and coccidiodomycosis.

Incubation and Latent periods
Incubation period: time from exposure to development of disease. In other words, the time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease in question.

Latent period: the period between exposure and the onset of infectiousness (this may be shorter or longer than the incubation period).

(II): Modes of transmission

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

(I) Prevention

Levels of prevention

Primordial prevention

Primary prevention
Secondary prevention
Tertiary prevention


Primordial prevention
Primordial prevention consists of actions and measures that inhibit the emergence of risk factors in the form of environmental, economic, social, and behavioral conditions and cultural patterns of living etc.

Primary prevention

Primary prevention can be defined as the action taken prior to the onset of disease, which removes the possibility that the disease will ever occur.
It signifies intervention in the pre-pathogenesis phase of a disease or health problem.
Primary prevention may be accomplished by measures of “Health promotion” and “specific protection”

Primary prevention

Specific protection
Health promotion
Achieved by

Health education

Environmental modifications
Nutritional interventions
Life style and behavioral changes

Immunization and seroprophylaxis

chemoprophylaxis
Use of specific nutrients or supplementations
Protection against occupational hazards
Safety of drugs and foods
Control of environmental hazards,
e.g. air pollution


Secondary prevention
It is defined as “ action which halts the progress of a disease at its incipient stage and prevents complications.”

The specific interventions are: early diagnosis (e.g. screening tests, and case finding programs….) and adequate treatment.

Secondary prevention attempts to arrest the disease process, restore health by seeking out unrecognized disease and treating it before irreversible pathological changes take place, and reverse communicability of infectious diseases.

It thus protects others from in the community from acquiring the infection and thus provide at once secondary prevention for the infected ones and primary prevention for their potential contacts.

Tertiary prevention

It is used when the disease process has advanced beyond its early stages.
It is defined as “all the measures available to reduce or limit impairments and disabilities, and to promote the patients’ adjustment to irremediable conditions.”
Intervention that should be accomplished in the stage of tertiary prevention are disability limitation, and rehabilitation.

The End