Epidemiology Of Communicable Diseases

FCM (Dra. Collao) Epidemiology of Infectious Diseases 13 November 08

Objectives: At the end of the session, the student must be able to: 1. Define the following: • sporadic, endemic, epidemic, pandemic, • herd immunity, • attack rates 2. Differentiate: • Communicable from non-communicable disease • Isolation from quarantine • Mechanical from biological vector • Incubation from generation period 3. Describe the intrinsic properties of disease agents 4. Discuss the different host agent interaction. 5. Discuss the different pathogenetic mechanism 6. Discuss the different mechanism of transmission.

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Endemic:

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Sporadic: occurrence of disease, on and off, cases seemingly unrelated to each other, low incidence level Pandemic: a marked increase in disease incidence of world-wide or continental distribution, usually of sudden occurrence Herd immunity: resistance of a group to invasion and spread of an infectious agent Secondary attack rate: rate thru which spread of disease is measured number of cases of a disease developing during a stated time period among those members of a closed group who are at risk = # of new cases in a group – initial case(s)_ # of susceptible persons in the grp –initial cases

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Infectious Diseases • Also known as communicable diseases.

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Transmitted directly to man from man (anthropozoonoses) or from animals (zoonoses) to man. Characterized by cyclic periodicities and seasonal predilections Genetics only an indirect role = predispositions

The EPIDEMIOLOGIC TRIANGLE Man

Environment

Disease agent

Factors intrinsic to the environment

The study of these factors and their dynamics in disease transmission is EPIDEMIOLOGY The methodology for this study consists of: • Data generation • Data analysis • Evidence-based intervention planning The main tool used for data generation is DISEASE SURVEILLANCE Sources of Data • field health service information system • program-based reports

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illness reports

Terms:

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Occurrence of a disease over and above its expected levels of occurrence. Should be qualified by the presence of clustering :chronological and geographic

Establishng Levels of Occurrence • Taking the averages (over a period of 3-4 years) • Consider highs and lows • Use of incidence and prevalence

Disease Transmission is Affected By: • Factors intrinsic to man • Factors intrinsic to the disease • Role of vectors

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EPIDEMICS • Also called outbreaks

EPIDEMICS • Are calamities in themselves • May result from the occurrence of natural or physical calamities • Occurs as a result of environmental sanitation breakdown, displacement of populations and more conducive environmental conditions for disease prevalence and transmission Necessary… • early detection and management of cases • surveillance : monitor trends • identify cause

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plan preventive measures

Origin/ Cause of Epidemics: • New Disease o Introduction of disease to the community for the first time o Dse of lower animals affecting man for the first time o Dse discovered for the first time •

Old Disease o Introduction of new strain of the disease o Immigration of a large number of susceptible individuals

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FCM – Epidemiology of Infectious Diseases by Dra. Collao

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Decrease resistance of population due to catastrophe such as famine, earthquakes, floods, etc Increased virulence/ pathogenicity

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Increased agent population

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Termination of Epidemic • Exhaustion of susceptibles • Elimination of the agent • Closure of secondary transmission Differentiate: • Communicable vs non-communicable disease • Isolation vs quarantine • Mechanical vs biological reservoir

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Incubation period vs generation period

Communicable disease: due to a specific infectious agent or its toxic products, arising through transmission of that agent or its products from reservoir to susceptible host, either directly as from an infected person or animal or indirectly by the agency of an intermediate plant or animal host, a vector, or the inanimate environment; also includes infestation Incubation period: from the time the agent enters the host to the time the disease becomes manifest. Generation period: period between the receipt of infection by a host and maximal communicability of the host • • • •

Describe the intrinsic properties of disease agents Discuss the different host-agent interaction. Discuss the different pathogenetic mechanism Discuss the different mechanism of transmission.

The Ecologic Model of Disease Causation

1. 1.The lever or Balance Host

Agent Environment

Host

Agent

Ecologic Model Host

Agent

Agent any element, substance, or force whether living or nonliving, the presence of which can initiate or perpetuate a disease process Types: o Living vs non-living o Physical , mechanical o Chemical, nutrients Characteristics: • Inherent: physical features, biologic requirements, chemical make-up, viability, resistance • Those directly related to man: infectivity, pathogenicity, virulence, antigenicity • Those related to the environment: reservoirs and sources of infection and mode of transmission

Those directly related to man • Infectivity: ability to gain access and adapt to the human host to the extent of finding lodgment and multiplication • Pathogenicity: measures the ability of the agent to cause a specific reaction • Virulence: severity of the reaction, usually measured in terms of fatality • Antigenicity: ability to stimulate a response Different pathogenetic mechanisms • Direct tissue invasion • Production of a toxin • Immunologic enhancement or allergic reaction leading to damage to the host • Persistent or latent infection • Enhancement of host susceptibility to drugs of otherwise minimal toxicity • Immune suppression Different mechanisms of transmission • Direct transmission • Indirect transmission: o Vehicle-borne o Vector-borne:  Mechanical  biological o Airborne: 1-5 um  Droplet nuclei  Dust Disease Causation 1. Agent increase in virulence or taken in massive doses 2. Agent remains the same but there is a change in the characteristic of the hosta. Decrease in host resistance b. Change in the activity of the host – increase in contact with the diseased person

Evironment 2.

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Environment

Isolation & Quarantine • Isolation – separation during the period of communicability of infected persons or animals from others .

FCM – Epidemiology of Infectious Diseases by Dra. Collao •

To prevent the spread of the disease to those who are susceptible. a. Hands must be washed after contact with the patient or potentially contaminated articles and before taking care of another patient. b. Articles contaminated with infectious materials should be appropriately discarded or bagged & labeled before being sent for decontamination & reprocessing.

Categories of Isolation : 7 categories 1. Strict Isolation – to prevent highly contagious and virulent infections that may spread by both air and contact . • Specification: • Private room • Use of mask , gloves & gown for all persons entering the room. • Special ventilation requirements with the room at negative pressure to surrounding areas is desirable

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3. 4. 5. 6.

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Contact Isolation – • For less highly transmissible or serious infections • Disease or conditions which are spread primarily by close or direct contact. Respiratory Isolation – diseases spread through the air Tuberculosis IsolationEnteric Precaution Drainage Secretion precaution Blood & Body fluid precaution

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