DKSN ds agents Epi Perspective.pdf

7/30/2019 DKSN ds agents Epi Perspective.pdf

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Disease agents: Epidemiological perspectives

Specific Objectives

1.Give general description and classification regarding

agents of diseases

2.Discuss the factors contributing to disease

transmission.

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CLASSIFYING DISEASES

Acute Diseases Acute diseases are those conditions in which the

peak severity of symptoms occurs within threemonths (usually sooner), and recovery in those whosurvive is usually complete

Chronic Diseases

Chronic diseases or conditions are those in whichsymptoms continue longer than three months and in

some cases for the remainder of the persons life.Recovery is slow and sometimes incomplete.

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CLASSIFYING DISEASES (contd.)

Communicable (Infectious) Diseases

Diseases for which biological agents or their products

are the cause and which are transmissible from one

individual to another

The disease process begins when the causative agentis able to lodge and grow or reproduce within the body

The process of lodgment and growth of a

microorganism or virus in the host is termed infection :

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CLASSIFYING DISEASES (contd.)

Non-communicable (Noninfectious) Diseases/Illnessses

Those diseases or illnesses that cannot be

transmitted from an infected person to a

susceptible, healthy one

Several, or even many, factors may contribute to

the development of a given non-communicable

health condition

The contributing factors may be genetic,

environmental, or behavioral in nature

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Classification of DiseasesTypes ofDiseases_Examples

Acute Diseases Communicable

Common cold, mumps, measles, pertussis, typhoid fever, flu

Non-communicable

Appendicitis, poisoning, trauma

Chronic Diseases

Communicable

Lyme disease, tuberculosis, AIDS, syphilis,

Non-communicable

Diabetes, coronary heart disease, osteoarthritis, hypertension

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Kochs Postulates

Koch developed four criteria to demonstrate that

a specific disease is caused by a particular agent.

1. The specific agent must be associated

with every case of the disease.2. The agent must be isolated from a diseased host

and grown in culture.

3. When the culture-grown agent is introduced into

a healthy susceptible host, the agent must causethe same disease.

4. The same agent must again be isolated from theinfected experimental host.

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Infectious Disease Agents

Most infectious agents that cause disease are

microscopic in size and thus, are called microbes

or microorganisms.

Different groups of agents that cause disease are:

Bacteria

Viruses

Protozoa

Fungi Helminths (Animals)

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Infectious Diseases Throughout

History

Infectious agents have probably always caused disease in

humans.

Smallpox has been described in ancient Egyptian and Chinese

writings and may have been responsible for more deaths

than all other infectious diseases combined.There is evidence that malaria and poliomyelitis have existed

since ancient times.

In the 14th Century, the bubonic plague, or Black Death, killed

about 20 million people in Europe alone.

In the 20th Century, the 1918 influenza may have killed up to 50million people worldwide

Close to 20 million people have died of AIDS to date.

Courtesy of CDC

Recreated 1918

Influenza virions.

The 1918 Spanish

flu killed more than

500,000 people inthe United States

and up to 50 million

worldwide.

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Epidemiology : TRIADS

TRIAD of Causation

HOST ENVIRONMENT AGENT

The environment plays a key role in host agent interaction.If the environment favors the host, disease occurrence will

be prevented. If the environment favors the agent, the

disease will occur.

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TRIAD OF CAUSATION

LIVING AGENTS

Plant origin

Animal origin

Metazoa, pro tozoa, fung i,yeasts , bacteria, rickettsiae,

mycop lasma v iruses etc.

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AGENT

NON-LIVING AGENTS

(force)11



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Factors related to infectious disease agents

Pathogenicity

Ability to initiate the disease

And it is an aggressive factor of the

agent :

Includes:

Transmissibility

Infectivity

virulence 12



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Transmissibility orCommunicability

Ability to grow profusely

Ability to be shed in large numbers

Influenza virus multiply rapidly & shed

through secretions in large amount

Viability

Ability to survive adverse environmental conditions eg; smallpox virus

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Infectivity

Ability to breach the defense

mechanism of new host

M . leprae has low infectivity while

measles virus has high infectivity

Secondary attack rate is a way of

measuring infectivityThe number of secondary casesdivided by total number of susceptible persons

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Virulence

Ability to produce severe pathologic

reactions and severe clinicalmanifestations

HIV virus produce severe pathologic

reactions ultimately leading to death

The number of deaths / total number of

cases suffering from that disease

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Toxigenicity

Ability to produce exotoxin or

endotoxin

Diphtheria, tetanus and staph aureus

produce exotoxins

Salmonella and yersinia produce

endotoxins

E coli produce enterotoxins

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Invasiveness

Ability to invade and spread in tissues

Pneumococci and meningococci are

highly invasive bacteria

Bacillus cereus is not invasive that itacts through toxins

Salmonella typhi is highly invasive

while salmonella paratyphi is lessinvasive

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Tissue tropism /selectivity

Ability to live and exerts its ill effectson specific tissues of the body

Rabies virus has tropism to nerve ts

Polio virus to the anterior horn cells

Plasmodium species to the RBCs

Mycobacterium to the tissues withhigh oxygen supply

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Immunogenecity

Ability to stimulate the host andproduce specific immunity

Tubercle bacilli initiate the

production of cell mediated immunity

Salmonella stimulates specific

humoral antibodies

Polio virus produces both local and

systemic antibodies

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Host range

Ability to survive in various hostcharacteristics

Agents with Wide host range are

difficult to control than those which cansurvive in only man

Japanese B encephalitis has a very

wide host range

Syphilis occurs in man only

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Genetic Stability/mutability

Ability to change or mutate its geneticcharacteristics

Agents with high mutability are difficult

to control than those which can noteasily change their genetic make-up

Influenza virus can mutate several

times within a decade

Small pox virus is the most stable virus

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Susceptibility/ resistance

To chemotherapeutic agents

Antibiotics

disinfectants

The more susceptible, the easier to

control / kill the agents

Emergence of resistance strainshinders the success of control

measures

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Pattern of transmission

Chain of the infectious process

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The infectious disease agents may be ofexogenous or endogenous origin

Intensity of aggressiveness

Duration of exposureThe more the intensity, the lesser the duration required toproduce ill health

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DKSN ds agents Epi Perspective.pdf