Principles of Principles of Communicable Diseases Communicable Diseases

EpidemiologyEpidemiology

ObjectivesObjectives Definition of epidemiology The epidemiologic triad Definition of communicable diseases Importance of studying communicable

diseases epidemiology Terminology Dynamics of disease transmission

(chain of infection): – Human reservoir or source– Modes of transmission– Susceptible host

Definition of EpidemiologyDefinition of Epidemiology

Epidemiology is the study of the distribution and determinants of health-related states and events in populations, and the application of this study to control health problems (Last, 1983).

Epidemiologic triadEpidemiologic triad

Host

Agent Environment•Biological agents•Physical agents•Chemical agents•Nutrient agents•Mechanical agents•Social agents

•Physical environment•Biological environment•Social environment

•Demographic characteristics•Biological characteristics•Socioeconomic characteristics

Infectious Disease ModelInfectious Disease Model

HostPathogen

Environment

disease

Definition of communicable Definition of communicable diseasesdiseases

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

Importance of Studying Communicable Importance of Studying Communicable Diseases EpidemiologyDiseases Epidemiology

Changes of the pattern of infectious Changes of the pattern of infectious diseasesdiseases

Discovery of new infectionsDiscovery of new infections The possibility that some chronic The possibility that some chronic

diseases have an infective origin.diseases have an infective origin.

Terminology and DefinitionsTerminology and Definitions

InfectionInfection ContaminationContamination InfestationInfestation Contagious diseaseContagious disease Incidence and Incidence and

prevalence of prevalence of infectious diseasesinfectious diseases

EpidemicEpidemic EndemicEndemic HyperendemicHyperendemic holoendemicholoendemic PandemicPandemic

ExoticExotic Sporadic Sporadic Attack rateAttack rate Primary/secondary Primary/secondary

casescases Zoonosis, epizootic Zoonosis, epizootic

and enzooticand enzootic Nosocomial infectionNosocomial infection Opportunistic infectionOpportunistic infection EradicationEradication EliminationElimination

Terminology and Definitions (cont.)Terminology and Definitions (cont.)

VirulenceVirulence Reproductive rate of Reproductive rate of

infectioninfection HostHost Vector (source)Vector (source) Reservoir Reservoir

Incubation periodIncubation period Infectivity periodInfectivity period Serial intervalSerial interval Latent periodLatent period Transmission Transmission

Probability ratioProbability ratio

InfectionInfection

Infection is the entry and development or Infection is the entry and development or multiplication of an infectious agent in the multiplication of an infectious agent in the body of man or animals. An infection does body of man or animals. An infection does not always cause illness. not always cause illness.

There are several levels of infection There are several levels of infection (Gradients of infection):(Gradients of infection):– Colonization Colonization (S. aureus in skin and normal (S. aureus in skin and normal

nasopharynx)nasopharynx)

– Subclinical or inapparent infection Subclinical or inapparent infection (polio)(polio)

– Latent infectionLatent infection (virus of herpes simplex) (virus of herpes simplex)

– Manifest or clinical infectionManifest or clinical infection

contaminationcontamination

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

InfestationInfestation

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

Contagious diseaseContagious disease

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

HostHost

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

Vector of infectionVector of infection

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

ReservoirReservoir

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

Incidence and prevalence of Incidence and prevalence of infectious diseasesinfectious diseases

Incidence of an infectious disease: number of new Incidence of an infectious disease: number of new cases in a given time period expressed as percent cases in a given time period expressed as percent infected per year (cumulative incidence) or infected per year (cumulative incidence) or number per person time of observation (incidence number per person time of observation (incidence density).density).

Prevalence of an infectious disease: number of Prevalence of an infectious disease: number of cases at a given time expressed as a percent at a cases at a given time expressed as a percent at a given time. Prevalence is a product of incidence x given time. Prevalence is a product of incidence x duration of disease, and is of little interest if an duration of disease, and is of little interest if an infectious disease is of short duration (i.e. infectious disease is of short duration (i.e. measles), but may be of interest if an infectious measles), but may be of interest if an infectious disease is of long duration (i.e. chronic hepatitis disease is of long duration (i.e. chronic hepatitis B).B).

EpidemicEpidemic

““The unusual occurrence in a The unusual occurrence in a community of disease, specific health community of disease, specific health related behavior, or other health related behavior, or other health related events clearly related events clearly in excess of in excess of expected occurrenceexpected occurrence””

(epi= upon; demos= people)(epi= upon; demos= people) Epidemics can occur upon endemic Epidemics can occur upon endemic

states too.states too.

EndemicEndemic

It refers to the constant presence of It refers to the constant presence of a disease or infectious agent within a a disease or infectious agent within a given geographic area or population given geographic area or population group. It is group. It is the usual or expected the usual or expected frequency of diseasefrequency of disease within a within a population.population.

(En = in; demos = people)(En = in; demos = people)

Hyperendemic and holoendemicHyperendemic and holoendemic The term The term ““hyperendemichyperendemic”” expresses that expresses that

the disease is constantly present at high the disease is constantly present at high incidence and/or prevalence rate and incidence and/or prevalence rate and affects all age groups equally.affects all age groups equally.

The term The term ““holoendemicholoendemic”” expresses a high expresses a high level of infection beginning early in life level of infection beginning early in life and affecting most of the child population, and affecting most of the child population, leading to a state of equilibrium such that leading to a state of equilibrium such that the adult population shows evidence of the the adult population shows evidence of the disease much less commonly than do the disease much less commonly than do the children (e.g. malaria)children (e.g. malaria)

Pandemic and ExoticPandemic and Exotic

An epidemic usually affecting a large An epidemic usually affecting a large proportion of the population, occuring over proportion of the population, occuring over a wide geographic area such as a section a wide geographic area such as a section of a nation, the entire nation, a continent of a nation, the entire nation, a continent or the world, e.g. Influenza pandemics.or the world, e.g. Influenza pandemics.

Exotic diseases are those which are Exotic diseases are those which are imported into a country in which they do imported into a country in which they do not otherwise occur, as for example, not otherwise occur, as for example, rabies in the UK.rabies in the UK.

SporadicSporadic The word sporadic means The word sporadic means ““scattered scattered

aboutabout””. The cases occur irregularly, . The cases occur irregularly, haphazardly from time to time, and haphazardly from time to time, and generally infrequently. The cases are few generally infrequently. The cases are few and separated widely in time and place and separated widely in time and place that they show no or little connection with that they show no or little connection with each other, nor a recognizable common each other, nor a recognizable common source of infection e.g. polio, source of infection e.g. polio, meningococcal meningitis, tetanusmeningococcal meningitis, tetanus……. .

However, a sporadic disease could be the However, a sporadic disease could be the starting point of an epidemic when the starting point of an epidemic when the conditions are favorable for its spread.conditions are favorable for its spread.

Attack rates and primary/secondary Attack rates and primary/secondary casescases Attack rate: proportion of non-immune Attack rate: proportion of non-immune

exposed individuals who become clinically exposed individuals who become clinically ill.ill.

Primary (index)/secondary cases: The Primary (index)/secondary cases: The person who comes into and infects a person who comes into and infects a population is the primary case. Those who population is the primary case. Those who subsequently contract the infection are subsequently contract the infection are secondary cases. Further spread is secondary cases. Further spread is described as "waves" or "generations".described as "waves" or "generations".

Zoonosis, epizootic and enzooticZoonosis, epizootic and enzootic

Zoonosis is an infection that is Zoonosis is an infection that is transmissible under natural conditions transmissible under natural conditions from vertebrate animals to man, e.g. from vertebrate animals to man, e.g. rabies, plague, bovine tuberculosisrabies, plague, bovine tuberculosis……....

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

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

Nosocomial infectionsNosocomial infections

Nosocomial (hospital acquired) Nosocomial (hospital acquired) infection is an infection originating in infection is an infection originating in a patient while in a hospital or a patient while in a hospital or another health care facility. It has to another health care facility. It has to be a new disorder unrelated to the be a new disorder unrelated to the patientpatient’’s primary condition. s primary condition. Examples include infection of Examples include infection of surgical wounds, hepatitis B and surgical wounds, hepatitis B and urinary tract infetions.urinary tract infetions.

Opportunistic infectionOpportunistic infection

This is infection by organisms that This is infection by organisms that take the opportunity provided by a take the opportunity provided by a defect in host defense (e.g. defect in host defense (e.g. immunity) to infect the host and thus immunity) to infect the host and thus cause disease. For example, cause disease. For example, opportunistic infections are very opportunistic infections are very common in AIDS. Organisms include common in AIDS. Organisms include Herpes simplex, cytomegalovirus, Herpes simplex, cytomegalovirus,

M. tuberculosisM. tuberculosis……..

Eradication and EliminationEradication and Elimination Termination of all transmission of infection by the Termination of all transmission of infection by the

extermination of the extermination of the infectious agentinfectious agent through through surveillance and containment. Eradication is an surveillance and containment. Eradication is an absolute process, an absolute process, an ““all or noneall or none”” phenomenon, phenomenon, restricted to termination of infection from the restricted to termination of infection from the whole world.whole world.

The termThe term elimination is sometimes used to elimination is sometimes used to describe eradication of a disease from a large describe eradication of a disease from a large geographic region. Disease which are amenable geographic region. Disease which are amenable to elimination in the meantime are polio, measles to elimination in the meantime are polio, measles and diphtheria.and diphtheria.

Reproductive rate of infectionReproductive rate of infection::

Reproductive rate of infection: potential Reproductive rate of infection: potential for an infectious disease to spread. for an infectious disease to spread. Influential factors include the probability of Influential factors include the probability of transmission between an infected and a transmission between an infected and a susceptible individual; frequency of susceptible individual; frequency of population contact; duration of infection; population contact; duration of infection; virulence of the organism and population virulence of the organism and population immune proportionimmune proportion . .

Dynamics of disease Transmission Dynamics of disease Transmission (Chain of Infection)(Chain of Infection)

Source or Reservoir Modes of transmission Susceptible host

I II III

(I): Source or Reservoir(I): Source or Reservoir The starting point for the occurrence of a The starting point for the occurrence of a

communicable disease is the existence of a communicable disease is the existence of a reservoir or source of infection.reservoir or source of infection.

The source of infection is defined as The source of infection is defined as ““the person, the person, animal, object or substance from which an animal, object or substance from which an infectious agent passes or is disseminated to the infectious agent passes or is disseminated to the host host (immediate source).(immediate source). The reservoir is The reservoir is ““any any person, animal, arthropod, plant, soil, or person, animal, arthropod, plant, soil, or substance, or a combination of these, in which an substance, or a combination of these, in which an infectious agent normally lives and multiplies, on infectious agent normally lives and multiplies, on which it depends primarily for survival, and where which it depends primarily for survival, and where it reproduces itself in such a manner that it can it reproduces itself in such a manner that it can be transmitted to a susceptible host. It is the be transmitted to a susceptible host. It is the natural habitat of the infectious agent.natural habitat of the infectious agent.””

Types of reservoirsTypes of reservoirs

Reservoir

Human reservoir

Animalreservoir

Non-livingreservoir

Human reservoirHuman 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

CasesCases

A case is defined as A case is defined as ““a person in the a person in the population or study group identified population or study group identified as having the particular disease, as having the particular disease, health disorder, or condition under health disorder, or condition under investigationinvestigation””

CarriersCarriers It occurs either due to inadequate treatment or immune It occurs either due to inadequate treatment or immune

response, the disease agent is not completely eliminated, response, the disease agent is not completely eliminated, leading to a carrier state.leading to a carrier state.

It is It is ““an infected person or animal that harbors a specific an infected person or animal that harbors a specific infectious agent in the absence of discernible (visible) clinical infectious agent in the absence of discernible (visible) clinical disease and serves as a potential source of infection to others.disease and serves as a potential source of infection to others.

Three elements have to occur toThree elements have to occur to form a carrier state:form a carrier state:

1.1. The presence in the body of the disease agent.The presence in the body of the disease agent.

2.2. The absence of recognizable symptoms and signs of The absence of recognizable symptoms and signs of disease.disease.

3.3. The shedding of disease agent in the discharge or The shedding of disease agent in the discharge or excretions.excretions.

Animal reservoirsAnimal reservoirs

Zoonosis is an infection that is Zoonosis is an infection that is transmissible under natural transmissible under natural conditions from vertebrate animals conditions from vertebrate animals to man, e.g. rabies, plague, bovine to man, e.g. rabies, plague, bovine tuberculosistuberculosis……....

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

Reservoir in non-living thingsReservoir in non-living things

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

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

(II): Modes of transmission(II): Modes of transmission

Mode of transmission

Direct transmission

Indirecttransmission

Direct contactDroplet 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.

(III): Susceptible host(III): Susceptible host

An infectious agent seeks a susceptible An infectious agent seeks a susceptible host aiming host aiming ““successful parasitismsuccessful parasitism””..

Four stages are required for successful Four stages are required for successful parasitism:parasitism:

1.1. Portal of entryPortal of entry

2.2. Site of election inside the bodySite of election inside the body

3.3. Portal of exitPortal of exit

4.4. Survival in external environmentSurvival in external environment

Virulence and Case Fatality RateVirulence and Case Fatality Rate

VirulenceVirulence: is the degree of pathogenicity; the : is the degree of pathogenicity; the disease evoking power of a micro-organism in a disease evoking power of a micro-organism in a given host. Numerically expressed as the ratio of given host. Numerically expressed as the ratio of the number of cases of overt infection to the total the number of cases of overt infection to the total number infected, as determined by number infected, as determined by immunoassay. When death is the only criterion of immunoassay. When death is the only criterion of severity, this is the case fatality rate.severity, this is the case fatality rate.

Case fatality rate for infectious diseases:Case fatality rate for infectious diseases: is is the proportion of infected individuals who die of the proportion of infected individuals who die of the infection. This is a function of the severity of the infection. This is a function of the severity of the infection and is heavily influenced by how the infection and is heavily influenced by how many mild cases are not diagnosed.many mild cases are not diagnosed.

Serial interval and Infectious periodSerial interval and Infectious period

Serial interval: (the gap in time between Serial interval: (the gap in time between the onset of the primary and the the onset of the primary and the secondary cases) the interval between secondary cases) the interval between receipt of infection and maximal infectivity receipt of infection and maximal infectivity of the host (also called generation time).of the host (also called generation time).

Infectious (communicable) period: length Infectious (communicable) period: length

of time a person can transmit disease of time a person can transmit disease (sheds the infectious agent).(sheds the infectious agent).

Incubation and Latent periodsIncubation and Latent periods Incubation period: time from exposure to Incubation period: time from exposure to

development of disease. In other words, development of disease. In other words, the time interval between invasion by an the time interval between invasion by an infectious agent and the appearance of infectious agent and the appearance of the first sign or symptom of the disease in the first sign or symptom of the disease in question.question.

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

Transmission Probability Ratio Transmission Probability Ratio (TPR)(TPR)

TPR is a measure of risk transmission TPR is a measure of risk transmission from infected to susceptible individuals from infected to susceptible individuals during a contact.during a contact.

TPR of differing types of contacts, TPR of differing types of contacts, infectious agents, infection routes and infectious agents, infection routes and strains can be calculated.strains can be calculated.

There are 4 types of transmission There are 4 types of transmission

probabilities.probabilities.

TPR (cont.)TPR (cont.)

Transmission probabilities: Transmission probabilities: p00: tp from unvaccinated infective to p00: tp from unvaccinated infective to

unvaccinated susceptibleunvaccinated susceptible p01: tp from vaccinated infective to p01: tp from vaccinated infective to

unvaccinated susceptibleunvaccinated susceptible p10: tp from unvaccinated infective to p10: tp from unvaccinated infective to

vaccinated susceptiblevaccinated susceptible p11: tp from vaccinated infective to p11: tp from vaccinated infective to

vaccinated susceptiblevaccinated susceptible

TPR (cont.)TPR (cont.)

To estimate the effect of a vaccine in To estimate the effect of a vaccine in reducing susceptibility, compare the ratio reducing susceptibility, compare the ratio of p10 to p00.of p10 to p00.

To estimate the effect of a vaccine in To estimate the effect of a vaccine in reducing infectiousness, compare the ratio reducing infectiousness, compare the ratio of p01 to p00.of p01 to p00.

To estimate the combined effect of a To estimate the combined effect of a vaccine, compare the ratio of p11 to p00. vaccine, compare the ratio of p11 to p00.

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