Epidemic ProcessEpidemic Process

Prof. Dr. N. Ribarova, MDProf. Dr. N. Ribarova, MD

EpidemiologyEpidemiology is a separate branch is a separate branch of medical science which studies the of medical science which studies the causes for the emergence, causes for the emergence, development and dissemination of development and dissemination of infectious diseases in human society, infectious diseases in human society, and applies the gathered knowledge and applies the gathered knowledge to fight, prevent and eventually fully to fight, prevent and eventually fully eradicate these diseases.eradicate these diseases.

Infectious processInfectious process – a process – a process of interaction between the of interaction between the macroorganism and the macroorganism and the microorganism which has microorganism which has penetrated and is reproduced in penetrated and is reproduced in the macroorganism, at the macroorganism, at environmental conditions.environmental conditions.

Epidemic process – a complex Epidemic process – a complex specific process of disseminating specific process of disseminating the causative agents of infectious the causative agents of infectious diseases in human society (human diseases in human society (human population) that is manifested population) that is manifested externally as a continuous chain of externally as a continuous chain of sequential interrelated infectious sequential interrelated infectious processes on a particular territory.processes on a particular territory.

This is the main form of existence of This is the main form of existence of pathogenic microorganisms and their pathogenic microorganisms and their survival as a biological species.survival as a biological species.

The The continuitycontinuity of the epidemic of the epidemic chain is the most characteristic chain is the most characteristic feature of the feature of the epidemic processepidemic process and also a determinative factor and also a determinative factor for its interruption and for its interruption and liquidation.liquidation.

Structure of the epidemic Structure of the epidemic processprocess

Several conditions are required for Several conditions are required for the emergence of an epidemic the emergence of an epidemic process: the presence of its process: the presence of its constituent parts or links:constituent parts or links:

source of infection;source of infection;mechanism of transmission of the mechanism of transmission of the

infection;infection;susceptible population.susceptible population.

The three links which are The three links which are functionally interrelated are the functionally interrelated are the “elementary cell”“elementary cell” containing the containing the major features of a particular major features of a particular epidemic process. It emerges epidemic process. It emerges among people within a particular among people within a particular territory or in the territory or in the epidemic epidemic focusfocus..

Epidemic focus:Epidemic focus:The inhabited territory of the The inhabited territory of the

source of infection and the source of infection and the environment within which it is environment within which it is capable, at definite conditions, to capable, at definite conditions, to transmit the infection to the transmit the infection to the susceptible individualssusceptible individuals

Active forces of the epidemic processActive forces of the epidemic processThe The main active forcesmain active forces of the epidemic of the epidemic

process are an aggregate of the process are an aggregate of the interaction of its interaction of its three linksthree links through the through the impact of impact of social conditionssocial conditions and the and the factors of the factors of the natural environmentnatural environment..

The leading role for the dissemination of The leading role for the dissemination of the infectious diseases in the society as the infectious diseases in the society as well as for their prevention and well as for their prevention and liquidation belongs to liquidation belongs to social conditionssocial conditions..

Forms of intensity of the epidemic Forms of intensity of the epidemic processprocess

SporademiaSporademia – a form of the epidemic – a form of the epidemic process in which the number of infectious process in which the number of infectious diseases does not exceed the diseases does not exceed the usual levelusual level and and there is no established epidemiological there is no established epidemiological connection in relation to time, place and connection in relation to time, place and route of transmission of the infection route of transmission of the infection between the epidemic foci (sporadicox = between the epidemic foci (sporadicox = interrupted, demos = people).interrupted, demos = people).

Epidemic outbreakEpidemic outbreak – a series of – a series of interrelated infectious diseases of the same interrelated infectious diseases of the same etiology in a confined organized group etiology in a confined organized group (crèches, kindergartens, schools, etc.)(crèches, kindergartens, schools, etc.)

EpidemicEpidemic – a form of the epidemic process in – a form of the epidemic process in which the incidence significantly which the incidence significantly exceeds the exceeds the usual incidenceusual incidence for a particular region and there for a particular region and there exists an epidemiological connection in time, place exists an epidemiological connection in time, place and route of transmission among the epidemic foci and route of transmission among the epidemic foci (epi = upon, demos = people).(epi = upon, demos = people).

PandemicPandemic – a form of the epidemic process where – a form of the epidemic process where there is a unusual high incidence due to the there is a unusual high incidence due to the epidemiological connection in time, place and route epidemiological connection in time, place and route of transmission (pan = whole, demos = people).of transmission (pan = whole, demos = people).

EndemicEndemic – a form of the epidemic process in which – a form of the epidemic process in which the incidence significantly exceeds the usual the incidence significantly exceeds the usual incidence and there exists an epidemiological incidence and there exists an epidemiological connection in time, place and route of transmission connection in time, place and route of transmission among the epidemic foci, at specific for the among the epidemic foci, at specific for the particular territory natural (social) conditions particular territory natural (social) conditions (natural endemicity, demographic endemicity).(natural endemicity, demographic endemicity).

An exact quantitative threshold An exact quantitative threshold for the transition of one form of for the transition of one form of infectious diseases into another infectious diseases into another could hardly be defined. For their could hardly be defined. For their differentiation, the “usual” level of differentiation, the “usual” level of infectious incidence for a specific infectious incidence for a specific nosological entity within a nosological entity within a particular territory is applied as a particular territory is applied as a criterion for a relative quantitative criterion for a relative quantitative estimate.estimate.

Periodicity of the epidemic Periodicity of the epidemic processprocess

Seasonal variations: seasonal Seasonal variations: seasonal fluctuations of the epidemic process fluctuations of the epidemic process within one year (seasonal increase within one year (seasonal increase and decrease of incidence);and decrease of incidence);

Cyclic periodical fluctuations in the Cyclic periodical fluctuations in the course of the epidemic process with course of the epidemic process with regular alternation of increase and regular alternation of increase and decrease of incidence systematically decrease of incidence systematically recurrent within a definite time recurrent within a definite time interval exceeding one year.interval exceeding one year.

Source of infectionSource of infection – an – an object, a living organism serving object, a living organism serving as a place of natural and as a place of natural and sustainable existence, sustainable existence, development and reproduction of development and reproduction of pathogenic microorganisms from pathogenic microorganisms from which these could separate and which these could separate and cause a new disease.cause a new disease.

AnthroponosesAnthroponoses – diseases – diseases where the only source of where the only source of infection is infected man infection is infected man (diseased or carrier of (diseased or carrier of infection)infection)

ZoonosesZoonoses – diseases which are – diseases which are common for man and animals, with a common for man and animals, with a common source of infection.common source of infection.

In zoonoses, the main form of existence In zoonoses, the main form of existence of pathogenic microorganisms is the of pathogenic microorganisms is the epizootic processepizootic process (their dissemination (their dissemination among the animal population). The among the animal population). The epidemic processepidemic process in thee diseases in thee diseases depends on the epizootic process – people depends on the epizootic process – people are infected by animals which have are infected by animals which have diseased or are carriers of the infection.diseased or are carriers of the infection.

Sapronoses:Sapronoses:Diseases cause by conditionally Diseases cause by conditionally

pathogenic microorganisms, at pathogenic microorganisms, at reduced defensive strength of the reduced defensive strength of the organisms and penetration of the organisms and penetration of the microorganisms through unusual microorganisms through unusual (additional) portals of entry.(additional) portals of entry.

The epidemiological The epidemiological importance of the source of importance of the source of infection is determined by the infection is determined by the periodperiod of progression of the of progression of the disease and its clinical disease and its clinical formform..

Periods of progression of the infectious Periods of progression of the infectious diseasedisease

incubation;incubation; initial, which can be preceded by a initial, which can be preceded by a

prodromal stage;prodromal stage; peak, or period of full development of the peak, or period of full development of the

disease;disease; reconvalescent;reconvalescent; residual – in certain cases. The residual – in certain cases. The

epidemiological importance of the separate epidemiological importance of the separate periods depends on the infectious disease periods depends on the infectious disease group – respiratory, intestinal, blood, group – respiratory, intestinal, blood, mucocutaneous infection.mucocutaneous infection.

Clinical forms of the course of Clinical forms of the course of an infectious disease of certain an infectious disease of certain epidemiological significanceepidemiological significance

I. Acute clinical formsI. Acute clinical forms1. Typical course1. Typical course * mild* mild * moderate* moderate * severe* severe

2. Atypical course2. Atypical course* * severesevere – fulminant, foudroyant, of limited – fulminant, foudroyant, of limited

epidemiological significance;epidemiological significance;* * mildmild – of great epidemiological significance – – of great epidemiological significance –

undiagnosed, efficacious antiepidemic undiagnosed, efficacious antiepidemic measures are nor performed in due time;measures are nor performed in due time;

* * abortiveabortive – beginning with typical signs but – beginning with typical signs but are suddenly terminated by prompt recovery;are suddenly terminated by prompt recovery;

* * ambulatoryambulatory – mild, atypical, the disease is – mild, atypical, the disease is endured “on foot”;endured “on foot”;

* * subclinicalsubclinical – “not obvious” or inapparent – – “not obvious” or inapparent – with less manifected clinical signs;with less manifected clinical signs;

* * asymptomaticasymptomatic – there are data for infection, – there are data for infection, without apparent clinical signs, but without apparent clinical signs, but paraclinical and microbiological data prove paraclinical and microbiological data prove the presence of an infectious process.the presence of an infectious process.

II. Chronic formsII. Chronic forms – continuous or periodical – continuous or periodical but prolonged elimination of the causative but prolonged elimination of the causative agent from the macroorganism.agent from the macroorganism.

III. Latent formsIII. Latent forms – continuous presence of – continuous presence of the causative agent in the macroorganism the causative agent in the macroorganism but with its periodic activation at certain but with its periodic activation at certain conditions and prerequisites. Such conditions and prerequisites. Such patients ensure a sustained maintenance patients ensure a sustained maintenance of the epidemic process.of the epidemic process.

IV. Carrier stateIV. Carrier state – a form of the infectious – a form of the infectious process in which the health state of the process in which the health state of the macroorganism is not apparently affected macroorganism is not apparently affected but there is a possibility of eliminating the but there is a possibility of eliminating the pathogenic microorganism from within the pathogenic microorganism from within the macroorganism.macroorganism.

Types of carriers of infectionacute (up to 3 months)

healthy (passive) chronic (over 3 months)

carriers of infection temporarily

active convalescent (active)

chronic

continuous periodic

Reservoirs of infectionReservoirs of infectionAnimals which maintain the infection in nature. A Animals which maintain the infection in nature. A

man can become infected from these animals in man can become infected from these animals in various circumstances:various circumstances:

- everyday contact with animals;- everyday contact with animals;- breeding animals;- breeding animals;- use of foodstuffs of animal origin or water - use of foodstuffs of animal origin or water

contaminated by animals;contaminated by animals;- injured skin while hunting, butchering, flaying, - injured skin while hunting, butchering, flaying,

processing meat, meat products, poultry, fish;processing meat, meat products, poultry, fish;- processing products of animal origin – fur, wool, - processing products of animal origin – fur, wool,

bristle, hooves;bristle, hooves;- animal biting;- animal biting;- through arthropods – live carriers of infectious - through arthropods – live carriers of infectious

diseases.diseases.

Mechanisms of transmitting Mechanisms of transmitting the infection:the infection: a process though a process though which the elimination of the which the elimination of the pathogenic microorganisms from pathogenic microorganisms from the recipient organism, their the recipient organism, their passing through the environment passing through the environment and penetration into another and penetration into another susceptible organism is done.susceptible organism is done.

Phases of the process:Phases of the process: elimination of the causative elimination of the causative

agent from the infected agent from the infected organism;organism;

passing of the causative agent passing of the causative agent through the environment;through the environment;

penetration of the causative penetration of the causative agent into another susceptible agent into another susceptible organism.organism.

Diagram of the mechanism of Diagram of the mechanism of transmission of the infectiontransmission of the infection

source of infection susceptible population

environment

I phase

II phase

III phase

Portal of exit of the infectionPortal of exit of the infection – a – a definite site (organ, system) through definite site (organ, system) through which the causative agent of the infectious which the causative agent of the infectious disease is eliminated from the infected disease is eliminated from the infected macroorganism.macroorganism.

through the digestive system;through the digestive system;through the respiratory system;through the respiratory system;through the circulatory system;through the circulatory system;through the outer coverings – skin, through the outer coverings – skin,

mucous membranes.mucous membranes.Additional (minor) possibilities – Additional (minor) possibilities –

through urine, milk, sexual secretions.through urine, milk, sexual secretions.

Portal of entry of the Portal of entry of the infectioninfection – the site of penetration – the site of penetration of the microorganisms into the of the microorganisms into the macroorganism.macroorganism.

Primary localization of the causative agentPrimary localization of the causative agent – the site (organ, tissue) where microorganisms – the site (organ, tissue) where microorganisms have been localized in the macroorganism for have been localized in the macroorganism for the first time, following their penetration the first time, following their penetration through a “portal of entry”.through a “portal of entry”.

Secondary localizationSecondary localization – the subsequent – the subsequent passing of the causative agent into various passing of the causative agent into various organs and tissues of the macroorganism.organs and tissues of the macroorganism.

Specific (main) localizationSpecific (main) localization – localization of – localization of the microorganisms in the organs and systems the microorganisms in the organs and systems ensuring not only favourable conditions for their ensuring not only favourable conditions for their reproduction but also their elimination from the reproduction but also their elimination from the infected organism into the environment in view infected organism into the environment in view of subsequent new infection.of subsequent new infection.This is the localization that determines the This is the localization that determines the mechanism of transmission of the infection.mechanism of transmission of the infection.

There is a mutually conditioned There is a mutually conditioned relation between the specific relation between the specific localization and the mechanism localization and the mechanism of transmission of the infection of transmission of the infection which guarantees the continuity which guarantees the continuity of the epidemic process of of the epidemic process of infectious diseases and their infectious diseases and their survival as a microbial species.survival as a microbial species.

Types of mechanisms of Types of mechanisms of transmitting the infectiontransmitting the infection

1. Fecal oral;1. Fecal oral;2. Airborne;2. Airborne;3. Transmissive (blood-borne);3. Transmissive (blood-borne);4. Contact.4. Contact.The so-called The so-called “vertical mechanism”“vertical mechanism”

of transmitting the infection has of transmitting the infection has definite significance – transplacental, definite significance – transplacental, transovarial, transphasic infection.transovarial, transphasic infection.

Factors of the transmission of the Factors of the transmission of the infectioninfection

- elements of the environment (objects, - elements of the environment (objects, water, foodstuffs, insects, arthropods, water, foodstuffs, insects, arthropods, etc.) which take part in the single-etc.) which take part in the single-point or sequential transmission of the point or sequential transmission of the pathogenic microorganisms from the pathogenic microorganisms from the source of infection to the susceptible source of infection to the susceptible organism. Types of factors:organism. Types of factors:

* biotic and abiotic;* biotic and abiotic;* intermediate and final;* intermediate and final;* primary (major) and secondary.* primary (major) and secondary.

Route of transmission of the Route of transmission of the infectioninfection – a set of factors which – a set of factors which take part in the dissemination of the take part in the dissemination of the infectious disease within a particular infectious disease within a particular territory for a specified point of time.territory for a specified point of time.

Types of routes of transmission Types of routes of transmission of the infection:of the infection: water-borne, water-borne, alimentary, milk-borne, airborne, alimentary, milk-borne, airborne, dust-borne, contact, vector (through dust-borne, contact, vector (through live carriers).live carriers).

Classification of infectious diseasesClassification of infectious diseasesThe attempts at systematizing the The attempts at systematizing the

infectious diseases have been infectious diseases have been commenced already in the antiquity.commenced already in the antiquity.

Contagious and miasmaticContagious and miasmatic – – during the Renaissance;during the Renaissance;

Classification according to a Classification according to a biological principle – biological principle – anthroponosesanthroponoses and and zoonoses;zoonoses;

Clinical anatomicClinical anatomic classification classification (acute, chronic, neuroinfections, (acute, chronic, neuroinfections, venereal, rash, etc.) – the beginning venereal, rash, etc.) – the beginning of XIX century;of XIX century;

Etiological classificationEtiological classification of the infectious of the infectious diseases (coccal, bacterial, spirochetal, diseases (coccal, bacterial, spirochetal, protozoal, actinomycoses) – the end of XIX protozoal, actinomycoses) – the end of XIX century;century;

Classification according ti the ecological Classification according ti the ecological epidemiological principle – in 1941 by L. V. epidemiological principle – in 1941 by L. V. Gromashevsky. It is based on the way of Gromashevsky. It is based on the way of circulation of the causative agent in nature circulation of the causative agent in nature as a biological species with a unified as a biological species with a unified epidemiological principle – the specific epidemiological principle – the specific localization of the causative agent in the localization of the causative agent in the organism and the mechanism of organism and the mechanism of transmitting the iunfection conditioned by transmitting the iunfection conditioned by this specific localization.this specific localization.

Classification of infectious diseasesClassification of infectious diseasesSubgroupsGroups of infections

Anthroponoses Zooanthroponoses

Intestinal infections

abdominal typhus, paratyphus A and B, shigellosIs, cholera, viral hepatitis A and E, poliomyelitis, etc.

salmonellosis, brucellosis, leptospirosis, botulism, ornitosis, hemorrhagic fever with renal syndrome, etc.

Respiratory infections

diphtheria, scarlet fever, pertussis, measles, varicella, rubella, meningococcal meningitis, influenza, epidemic parotitis, acute respiratory infections, pulmonary tuberculosis, etc.

respiratory mycoplasmosis

Blood-borne infections (transmissive)

epidemic typhus fever, recurrent typhus, malaria, etc.

plague, tularemia, rickettsioses, yellow fever, seasonal encephalitis, Crime-Congo hemorrhagic fever, etc.

Contact infections

viral hepatitis B, C and D, wound infections, erysipel, AIDS, gonorrhea, lues, etc.

anthrax, gas anaerobic infection, tetanus, rabies, etc.

Susceptible populationSusceptible population – the third – the third link of the epidemic chainlink of the epidemic chain

Obligatory prerequisite for the Obligatory prerequisite for the emergence of the epidemic process is the emergence of the epidemic process is the presence and interaction of its three links.presence and interaction of its three links.

The The susceptibilitysusceptibility of the organism is of the organism is determined by its ability to respond to the determined by its ability to respond to the penetration of a pathogenic agent with an penetration of a pathogenic agent with an infectious process.infectious process.

Susceptible populationSusceptible population – of critical – of critical importance for the emergence and importance for the emergence and dissemination of the epidemic process.dissemination of the epidemic process.

SusceptibilitySusceptibility – determined by – determined by the ability of the organism to the ability of the organism to serve as a site of dwelling and serve as a site of dwelling and development of the causative development of the causative parasitic agent.parasitic agent.

An An infectious processinfectious process as a as a response to the invasion of a response to the invasion of a pathogenic agent could develop pathogenic agent could develop only in a susceptible organism.only in a susceptible organism.

ResistanceResistance (non-susceptibility) – (non-susceptibility) – notion reflecting the opposite side notion reflecting the opposite side of susceptibility. In fact, of susceptibility. In fact, resistance resistance = natural= natural non-susceptibility non-susceptibility (congenitally determined, of (congenitally determined, of genotypic character).genotypic character).

The macroorganism has a perfect immune The macroorganism has a perfect immune system against pathogenic microorganisms.system against pathogenic microorganisms.

The immune system may be considered as The immune system may be considered as a universal mechanism for elimination of a universal mechanism for elimination of everything alien – bacteria, viruses, cancer everything alien – bacteria, viruses, cancer cells, transplanted tissue or a proper cell cells, transplanted tissue or a proper cell which has become alien to the organism.which has become alien to the organism.

ImmunityImmunity – a complex of defensive – a complex of defensive physiological reactions of the whole physiological reactions of the whole organism or a method of defending the organism or a method of defending the organism from protein bodies bearing organism from protein bodies bearing genetically alien information.genetically alien information.