Principles of Communicable Diseases Epidemiology

Principles of Communicable Diseases EpidemiologyAss. Prof. Dr. Wijdan AkramCommunity Medicine Department

Objectives

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 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.


EpidemiologyDeals with one populationRisk  caseIdentifies causes Infectious disease epidemiology Two or more populations A case is a risk factor The cause often known
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What is infectious disease epidemiology?


Two or more populationsHumansInfectious agentsHelminths, bacteria, fungi, protozoa, viruses, prionsVectors Mosquito (protozoa-malaria), snails (helminths-schistosomiasis)Blackfly (microfilaria-onchocerciasis) – bacteria?AnimalsDogs and sheep/goats – EchinococcusMice and ticks – Borrelia What is infectious disease epidemiology?
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A case is a risk factor …Infection in one person can be transmitted to others (www)
What is infectious disease epidemiology?

Routes of transmission

Direct Skin-skin Herpes type 1 Mucous-mucous STI Across placenta toxoplasmosis Through breast milk HIV Sneeze-cough Influenza
Indirect Food-borne Salmonella Water-borne Hepatitis A Vector-borne Malaria Air-borne Chickenpox
Exposure A relevant contact – depends on the agent Skin, sexual intercourse, water contact, etc (www)

Some Pathogens that Cross the Placenta

Modes of Disease Transmission

No infection Clinical Sub-clinical Carrier

Death Carrier Immunity No immunity
Outcome
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Exposure to Infectious Agents

Infection

Susceptible
Susceptible
Dynamics of infectiousness
Dynamics of disease
Incubation period
Symptomatic period
Non-diseased
Latent period
Infectious period
Non-infectious
Infection
Time
Time
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Timeline for Infection


Cases Index – the first case identified Primary – the case that brings the infection into a population Secondary – infected by a primary case Tertiary – infected by a secondary case P
S
S
T
Susceptible
Immune
Sub-clinical
Clinical
S
T
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Transmission

Data from Dr. Simpson’s studies in England (1952) Measles

Chickenpox
Rubella
Children exposed Children ill attack rate
251 201 0.80
238 172 0.72
218 82 0.38
Attack rate =
ill exposed
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Person-to-Person Transmission


Infectivity (ability to infect) (number infected / number susceptible) x 100 Pathogenicity (ability to cause disease) (number with clinical disease / number infected) x 100 Virulence (ability to cause death) (number of deaths / number with disease) x 100 All are dependent on host factors
Epidemiologic Triad-Related Concepts

Horton & Parker: Informed Infection Control Practice (www)

Chain of Infection

Infectious Disease Epidemiology: Major Differences

A case can also be an exposure Subclinical infections influence epidemiology Contact patterns play major role Immunity There is sometimes a need for urgency
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Epidemiologic 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 Model

Host
Pathogen
Environment
disease

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.

Importance of Studying Communicable Diseases Epidemiology

Changes of the pattern of infectious diseases Discovery of new infections The possibility that some chronic diseases have an infective origin.

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

latent infection: is a situation in which a virus is present in the body, but it remains dormant, not causing any overt symptoms. The patient is still infected with the virus, and he or she can pass the virus on to others when they are exposed to the dormant virus. Latent infections can also be activated, causing symptoms and illness to emerge again. A classic example of a latent infection is herpes simplex, which periodically flares up to cause cold sores before going dormant again.

Some infections *can never be fully flushed from the body, becoming latent with the use of medications and other measures to control the virus and inhibit replication. In these cases, the latent infection may periodically flare up in response to environmental cues. Latent infections can also be caused when* a virus mutates, becoming impossible to eradicate, or when a course of treatment is not completed, allowing a virus to remain dormant in the body.

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.

Definitive host: The definitive host is the one in which a parasite reaches sexual maturity and undergoes reproduction. It is mostly a vertebrate. Intermediate host development of the parasite occurs but it does not reach sexual maturity, is referred as intermediate host and is usually invertebrate :

Paratenic or transport host: Sometimes the parasite enters a host in which it does not undergo any development but remains alive till it gains entry in the definitive host or intermediate host. Such a host is termed as paratenic or transport host or a carrier host. These hosts are important for the completion of the life cycle of certain parasites as they are believed to bridge the ecological gap between the intermediate and the definitivehost

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.

Incidence and prevalence of infectious diseases

Incidence of an infectious disease: number of new cases in a given time period expressed as percent infected per year (cumulative incidence) or number per person time of observation (incidence density). Prevalence of an infectious disease: number of cases at a given time expressed as a percent at a given time. Prevalence is a product of incidence x duration of disease, and is of little interest if an infectious disease is of short duration (i.e. measles), but may be of interest if an infectious disease is of long duration (i.e. chronic hepatitis B).


Epidemic
“The unusual occurrence in a community of disease, specific health related behavior, or other health related events clearly in excess of expected occurrence”(epi= upon; demos= people)Epidemics can occur upon endemic states too.

Endemic

It refers to the constant presence of a disease or infectious agent within a given geographic area or population group. It is the usual or expected frequency of disease within a population. (En = in; demos = people)

Hyperendemic and holoendemic

The term “hyperendemic” expresses that the disease is constantly present at high incidence and/or prevalence rate and affects all age groups equally.The term “holoendemic” expresses a high level of infection beginning early in life and affecting most of the child population, leading to a state of equilibrium such that the adult population shows evidence of the disease much less commonly than do the children (e.g. malaria)

Pandemic and Exotic

An epidemic usually affecting a large proportion of the population, occuring over a wide geographic area such as a section of a nation, the entire nation, a continent or the world, e.g. Influenza pandemics. Exotic diseases are those which are imported into a country in which they do not otherwise occur, as for example, rabies in the UK.

Sporadic

The word sporadic means “scattered about”. The cases occur irregularly, haphazardly from time to time, and generally infrequently. The cases are few and separated widely in time and place that they show no or little connection with each other, nor a recognizable common source of infection e.g. polio, meningococcal meningitis, tetanus…. However, a sporadic disease could be the starting point of an epidemic when the conditions are favorable for its spread.

Zoonosis, epizootic and enzootic

Zoonosis is an infection that is transmissible under natural conditions from vertebrate animals to man, e.g. rabies, plague, bovine tuberculosis…..An epizootic is an outbreak (epidemic) of disease in an animal population, e.g. rift valley fever.An Enzootic is an endemic occurring in animals, e.g. bovine TB.

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….

Control

Concept of control: The term disease control describes ongoing operations aimed at reducing: The incidence of disease The duration of disease and consequently the risk of transmission The effects of infection, including both the physical and psychosocial complications The financial burden to the community.


Control activities focus on primary prevention or secondary prevention, but most programs combine both.
control
elimination
eradication

Eradication and Elimination

Termination of all transmission of infection by the exterminationابادة of the infectious agent through surveillance and containment. Eradication is an absolute process, an “all or none” phenomenon, restricted to termination of infection from the whole world. The term elimination is sometimes used to describe eradication of a disease from a large geographic region. Disease which are amenable to elimination in the meantime are polio, measles and diphtheria.

Monitoring

Monitoring is "the performance and analysis of routine measurements aimed at detecting changes in the environment or health status of population" (Thus we have monitoring of air pollution, water quality, growth and nutritional status, etc). It also refers to on -going measurement of performance of a health service or a health professional, or of the extent to which patients comply with or adhere to advice from health professionals.

Surveillance

surveillance means to watch over with great attention, authority and often with suspicion According to another, surveillance is defined as "the continuous scrutiny (inspection) of the factors that determine the occurrence and distribution of disease and other conditions of ill-health"


Objectives of Surveillance
The main objectives of surveillance are: (a) to provide information about new and changing trends in the health status of a population, e.g., morbidity, mortality, nutritional status or other indicators and environmental hazards, health practices and other factors that may affect health (b) to provide feed-back which may be expected to modify the policy and the system itself and lead to redefinition of objectives, and (c) provide timely warning of public health disasters so that interventions can be mobilized.

Control of infectious diseases (the 4 “C”s Control

Cases
Contacts
Carriers
Community
Diagnosis
notification
isolation
disinfection
treatment
follow up
release
observation
detection
Epidemiological Investigation & containment
standard
strict
protective

Reproductive rate of infection:

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

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.

(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
propagative
Cyclo-prop.
Cyclo-develop.

(III): Susceptible host

An infectious agent seeks a susceptible host aiming “successful parasitism”.Four stages are required for successful parasitism:Portal of entrySite of electionانتخاب inside the bodyPortal of exitSurvival in external environment

Virulence and Case Fatality Rate

Virulence: is the degree of pathogenicity; the disease evoking power of a micro-organism in a given host. Numerically expressed as the ratio of the number of cases of overt infection to the total number infected, as determined by immunoassay. When death is the only criterion of severity, this is the case fatality rate. Case fatality rate for infectious diseases: is the proportion of infected individuals who die of the infection. This is a function of the severity of the infection and is heavily influenced by how many mild cases are not diagnosed.

Serial interval and Infectious period

Serial interval: (the gap in time between the onset of the primary and the secondary cases) the interval between receipt of infection and maximal infectivity of the host (also called generation time). Infectious (communicable) period: length of time a person can transmit disease (sheds the infectious agent).

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).

Transmission Probability Ratio (TPR)

TPR is a measure of risk transmission from infected to susceptible individuals during a contact. TPR of differing types of contacts, infectious agents, infection routes and strains can be calculated. There are 4 types of transmission probabilities.

TPR (cont.)

Transmission probabilities: p00: tp from unvaccinated infective to unvaccinated susceptible p01: tp from vaccinated infective to unvaccinated susceptible p10: tp from unvaccinated infective to vaccinated susceptible p11: tp from vaccinated infective to vaccinated susceptible

TPR (cont.)

To estimate the effect of a vaccine in reducing susceptibility, compare the ratio of p10 to p00. To estimate the effect of a vaccine in reducing infectiousness, compare the ratio of p01 to p00. To estimate the combined effect of a vaccine, compare the ratio of p11 to p00.



Community Immunity ("Herd Immunity")Vaccines can prevent outbreaks of disease and save lives.When a critical portion of a community is immunized against a contagious disease, most members of the community are protected against that disease because there is little opportunity for an outbreak. Even those who are not eligible for certain vaccines—such as infants, pregnant women, or immunocompromised individuals—get some protection because the spread of contagious disease is contained. "

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