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Complete Summary Communicable Diseases (VU Master Health Sciences IPA)

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Containment Strategies

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Vrije Universiteit Amsterdam (VU)

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Communicable Diseases

This is a complete summary of Communicable Diseases, a course in the master Health Sciences (VU), specialisation International Public Health (IPA). This summary contains all the readings from the book 'Communicable Diseases' by Webber, all of the lectures and all of the 35 diseases that have to be ...

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Summary Containment Strategies
Lecture 1: Introduction to the course and assignment theory

: (EIDM)‘linking the problem to the solution’
HEALTH INTERVENTIONS
5 steps in health intervention:
1. What is the problem (is it a priority health problem)?
2. What factors cause the problem?
3. How can these factors be changed?
4. What overall intervention strategies are most appropriate and cost effective (including what
do people want and what are their needs?)?
5. What needs to be done to reach the goals? With what (sub)populations shall work be done,
and in what sequence, to solve the problem?

How can factors be changed?
Basic factors to intervene:
1. Change/kill agent
2. Raise host resistance (e.g. vaccines)
3. Modify environment (e.g. remove mosquitos breeding places)
4. Separate agent from host/ interrupt transmission (e.g. isolation)

An ideal intervention target has three conditions: it is a costly problem, it had moderate to high
prevalence in a defined subpopulation, and effective interventions to address it are not unduly
expensive.
The prevention paradox: preventive interventions often must involve many, many people in order to
help just a few (e.g. thousands of people are wearing their automobile seatbelts for thousands of
days each, for just one life to be saved).

1

, Communicable diseases: chapter 1 & 2
: (EIDM)‘linking the problem to the solution’
COMMUNICABLE DISEASES
A communicable disease is an illness that is transmitted from a person, animal, or inanimate source
to another person either directly, with the assistance of an intermediate host or by a vector.
Communicable diseases present in an epidemic or endemic form. An epidemic is the introduction of
a new infection or the presence of an illness in excess of normal expectancy. An endemic disease is
constantly present in a geographical area or population group, e.g. malaria.

The key to any communicable disease is to think of it in terms of agent, transmission, host and
environment. A vector carries the infection from one host to another either as part of the
transmission process, as in the mosquito, or mechanically, as in the house fly, which inadvertently
transmits organisms to the host on its feet and mouthparts.

Whether a host gets infected by a new agent, can depend on numerous factors, e.g. age, sex, genes,
pregnancy. An infecting organism must overcome the body’s inherent defence mechanisms, both
physical, such as the skin, mucous-secreting membranes or acidity of the stomach; and inflammatory
mechanisms.

Experience of previous infection by a host can lead to the development of immunity. This consists of
two parts: a cellular and a humoral response.
• In the cellular response, T-lymphocytes destroy host cells exhibiting this foreign material.
• In the humoral response, B-lymphocytes are stimulated to synthesize specific antibodies,
tailor-made to the antigen. B-cell activation will stimulate memory cells which reside in the
bone marrow to produce long-term protection.

Immunity can be either active or passive. Active immunity follows an infection (with or without
symptoms) or vaccination with attenuated (live or dead) organisms. Passive immunity is the transfer
of antibodies from a mother to her child via the placenta. Passive immunity is short lived.

SUMMARY
• A communicable disease requires an agent and a means of transmitting the infection to a
susceptible host within an amenable environment.
• The agent needs to be able to multiply and survive if it is to have an effect on the host.2
Transmission can be direct, via an intermediate host or by a vector.

2

, • The host’s susceptibility is influenced by age, sex, defence mechanisms and immunity.
• The social environment is modified by education and resources and altered by movements of
communities or individuals.
• The physical environment is affected by the nature of the surroundings, seasonality and
climate change.

The force of infection = Number of infectious individuals × Transmission rate

EPIDEMICS
Epidemics can occur unexpectedly, as when a new disease enters a community, or can occur
regularly at certain times of the year, as in epidemics of measles. Epidemic contrasts with endemic,
which means the continuous presence of an infection in the community and is described by incidence
and prevalence measurements.

Characteristics of an epidemic are as follows:
• Latent period, the time interval from initial infection until the start of infectiousness.
• Incubation period, the time interval from initial infection until the onset of clinical disease.
The incubation period varies from disease to disease and for a particular disease has a range.
This range extends from a minimum incubation period to a maximum incubation period
• Period of communicability, the period during which an individual is infectious. The infectious
period can start before the disease process commences (e.g. hepatitis) or after (e.g. sleeping
sickness). In some diseases, such as diphtheria and streptococcal infections, infectiousness
starts from the date of first exposure.

Potential exam question: what source of infection could the figures represent?

Common-source epidemics can
further be divided into a point-source
epidemic resulting from a single
exposure, such as a food poisoning
episode, or an extended epidemic
resulting from repeated multiple
exposures over a length of time (e.g. a
contaminated well).

In a propagated-source epidemic, the agent is spread through serial transfer from host to host. With
a disease having a reasonably long incubation period, the initial peaks will be separated by the
median incubation periods. Chickenpox (varicella) can start as an epidemic in one school; then
mingling children will lead to transfer to another school, leading to a series of propagated epidemics.

3

, In the investigation of any outbreak of a disease, the basic approach is to gather information on:
1. Persons: age, sex, occupation, ethnic group, etc.
2. Place: country, district, town, village, household, geographical features
3. Time: annual, monthly (seasonal), day and hour (nocturnal/diurnal)

Diseases with a high secondary attack rate are very contagious.

Basic reproductive rate: the extent to which infected people increase. For example, if one case gave
rise to two and these two to four, etc., the basic reproductive rate would be 2. This is the most
extreme situation. If the basic reproductive rate is less than 1, the infection dies out.
When a certain number of individuals have developed immunity, then there are insufficient
susceptibles and the infection dies out. This collective permanent immunity (as occurs in viral
infections) is called herd immunity. After some time, depending on the size of the population, this
herd immunity becomes diluted by new individuals born (or by immigration), and a new epidemic
can take place. This is called the critical population.

An extension of the concept of herd immunity shows that not everyone in a population needs to be
vaccinated to prevent an epidemic. On the same principle as calculating the critical population, the
critical rate of vaccination coverage can also be worked out – in other words, the population that
will need to be successfully vaccinated to reduce the population at risk below the epidemic
threshold.

Within a community, prevalence rates can vary. These areas of increased prevalence within a
community are called foci. Two types of foci occur:
• Host focality, where some individuals have more severe infection than others, e.g. worm load
in schistosomiasis.
• Geographical focality, where certain localities have a higher prevalence rate than others.
Malaria exhibits geographical focality.

SUMMARY
• A disease can either be epidemic or endemic.
• Epidemic diseases can be common source or propagated, but by measuring the minimum
and maximum incubation period from the first case, the time of infection can be determined.
• The size of the population will determine the frequency of epidemics and the number that
need to be vaccinated to produce herd immunity.
• An endemic disease is described by its incidence and prevalence, but foci of infection can
also occur.
• Mathematical models can look at different parameters of a disease and determine the best
strategy for control.

4

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