Principles of Communicable Diseases Epidemiology

Epidemiology

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

Epidemiologic triad

• Host

Demographic characteristics

Biological characteristics

Socioeconomic characteristics

• Agen

  Biological agents

  Physical agents

  Chemical agents

  Nutrient agents

  Mechanical agents

  Social agents

• Environment

  Physical environment

  Biological environment

  Social environment

communicable disease

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

is the entry and development or multiplication of an infectious agent in the body of man or animals.

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

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

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

Host

A person or an animal that affords subsistence or lodgment to an infectious agent under natural conditions.

Host Types include

• obligate host

• definitive (primary) host

• intermediate host

• transport host

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.

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 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

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.

Hyperendemic

 The term “hyperendemic” expresses that the disease is constantly present at high incidence and/or prevalence rate and affects all age groups equally.

Holoendemic

 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

An epidemic usually affecting a large proportion of the population, occurring 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

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….

Note

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

Attack rate

proportion of non-immune exposed individuals who become clinically ill.

Primary (index)/secondary cases

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

Zoonosis

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

Epizootic

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

Enzootic

an endemic occurring in animals,e.g. bovine TB

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 infections

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

Note

Termination of all transmission of infection by theimmune extermination 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.

Elimination

describe eradication of a disease from a large geographic region. Disease which are amenable to elimination in the meantime are polio, measles and diphther

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 and population immune proportion

Dynamics of disease Transmission (Chain of Infection)

• ource or Reservoir

• Modes of transmission

• Susceptible host

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

the person, animal, object or substance from which an infectious agent passes or is disseminated to the host (immediate source).

Types of reservoirs

Human reservoir

Animal reservoir

Non-living Reservoir

Cases

a person in the population or study group identified as having the particular disease, health disorder, or condition under investigation

Note

It occurs either due to inadequate treatment or immune response, the disease agent is not completely eliminated, leading to a carrier state.

carrier state

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:

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

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

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

(II): Modes of transmission

• Direct transmission

Direct contact

Droplet infection

Contact with soil

Inoculation into skin or mucosa

Trans-placental (vertical)

• Indirect transmission

Vehicle-borne

Vector-borne:

•Mechanical

•biological

Air-borne

Fomite-born

Unclean hands and fingers

(III): Susceptible host

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

Four stages are required for successful parasitism:

1. Portal of entry

2. Site of election inside the body

3. Portal of exit

4. Survival in external environment

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.

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

(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 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)

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

There are 4 types of transmission probabilities

• p00: tp from unvaccinated infective to unvaccinated susceptible

• p01: top from vaccinated infective to unvaccinated susceptible

• p10: top from unvaccinated infective to vaccinated susceptible

• p11: top from vaccinated infective to vaccinated susceptible

Note

• 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