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Book summary Communicable Diseases
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Communicable diseases: a globalperspective
1.Elements of communicable diseases
Communicable disease: illness transferred from a person, animal or source to another person either
directly, with the assistance of a host, or by a vector. CD are dependent on the person being
susceptible to infection. Children or isolated communities are more liable to become infected since it
is their first encounter.
Conditions that encourage transmission:
- overcrowding
- poor hygiene
- tropical environment
and therefore associated with poverty
Epidemic: introduction of a new infection or the presence of an illness in excess of normal
expectancy
- seasonal (influenza)
- number of susceptible persons is sufficient (measles)
Any unknown infection will be epidemic when first introduced.
Endemic: disease that is constantly present in a geographic area or population group
- tropical countries (malaria)
Strict difference between CD en NCD is being less defined, since new suspect organisms are
discovered which have a communicable origin (HPV → cervix cancer^)
Cleanliness → we are less likely prevents mnor infections that stimulate the immune system (mostly
in developed countries).
Agent: organism (virus, bacteria, fungus) or physical/chemical (toxin, poison). If it is an organism, it
needs to multiply, find transmission means and survive.
Multiplication
- sexual: great scope of variety → natural selection acts on individuals → variations of
adaptility occur. Both male and female parasites must meet before reproduction.
- asexual: a succession of replicas → natural selection will act on batches or trains rather than
on individuals. One parasite requires it to be transmitted.
This is relevant in treatment and control since:
,… if a treatment is successful in destroying an asexual organism, then it will be succesfull against all
the other individuals of that strain or organism (unless there is a mutation with individual resistance)
…sexual reproduction produces individuals of different vigour → some individuals will succumd to
treatment, others not.
- Having two sexes can be a disadvantage since methods of control can be devised to attack
one of the sexes or reduce the chances of individuals meeting.
Survival of parasite agents methods:
- Reservoirs (agent lives in host)
- Parasite adaptability → reaction of host is minder → adaptability increases → parasite lives
in host and uses it as reservoir with few ill effects
Parasite adaptability might not be bad since it reduces the allergic reactions from the body
- Persistence
- Latency (the production by the organism of a developmental stage in the environment that is
not infective to a new host)
Illness by agents
‘ agents infect new host and produce illness, severity dependent on toxicity and virulence ‘
Virulence decreases while passing hosts (individuals) - this is a method used in developing vaccines
For an infectious agent, a minimum number of organisms is required to overcome the defenses of
the host and cause the disease. Once this number is passed, the severity will stay the same whether
there is a small or big dose (in most infections).
Sometimes the severity is dependent on the dose (food poisoning).
Infections with a low infective dose can spread by person-to-person contact; this means that the
provision of a safe water supply will have little effect.
Organisms for which a high infective dose is required to produce the disease → improving water
quality and the reduction of patterns in the sewage → beneficial to the Community in protecting
them from these infections
This will influence the interventions!
Transmission cycles
- Direct transmission; includes person to person contact (dirty fingers, food or water)
- Human reservoir with intermediate host; the adults live in humans but for transmission to
another human the parasite must undergo developmental stages
- Animals as intermediate host or reservoir; animals can be intermediate hosts where larval
tapeworm must develop in the animal muscle before they infect humans
- Vectors; caries the infection from one host to another
Direct insect to human as in malaria
Insect to animal with humans entering the cycle as an abnormal host
Insect to animal including humans from which it is transmitted to other humans by the same
or another insect vector
Zoonosis
Diseases in which there is an animal reservoir or intermediate host. Infections that are Naturally
transmitted between Animals and humans.
, - Domestic: those invited animals that live in close proximity to humans such as farm animals
and Pets
- Synanthropic: animals that live in close association with humans but are not invited such as
rats
- Exco Anthropic: animals that are not inclusive association with humans such as monkeys
This is important since it indicates the importance of the disease.
In zoonosis the animal is all important in control. Depending on the animal measures will be taken.
Diseases in which there is an animal reservoir intermediate host or vector are complex and difficult
to control.
Carrier: transmits the infective agent but does not manifest the disease.
- incubating carriers who are infectious but unaware that they are in the early stages of the
disease
- asymptomatic carriers who remain well throughout the infection and;
- who continue to be infectious after the clinical disease has passed
Identification of carriers is difficult, like in asymptomatic states. Because:
- routine culture technique will not detect it since the organism is in such reduced numbers
- clinically well people object to having investigations performed on them - coverage is
incomplete
Cyclical subclinical transmission: infection is transmitted within an immune family but asymptomatic.
When someone who is susceptible enters this cycle, a clinical outbreak occurs.
Host factors (let's say the agent is transmitted to a new host. It’s invasion and continuation will
depend on several factors)
- Susceptibility
Genetic- some will be highly susceptible and others might be quite resistant. The wider the
genetic separation, the greater the possibility of producing offspring resistant to some
infections
Age - different diseases affect particular age groups
Sex - producing many individuals of different vigour is beneficial (mammals evolving with two
sexes to evade parasites). The more genetically different → the greater the ability to combat
disease. There are some diseases more common in certain sexes.
Pregnancy - more susceptible to infections
- Inherent defence mechanisms
Physical - skin, membranes or acidity in the stomach.
Inflammatory - the reaction
- Immunity (experience of previous infection by a host → immunity)
Active - infection or vaccination
Passive - transfer of antibodies from mother to child (short-lived immunity) or antibodies in
immune serums
- Resistance
Nutrition
Trauma - such as intramuscular injection → paralytic disease
Multiple infections
, Immunodeficiency - interferes with the host immune respons
The environment
The transmission cycle used by the agents to reach the Host takes place within an environment that
determines the success and severity of the infection.
a) Social environment
- Education - knowledge for CD to be prevented. This might be a long and generative
process. Improvement in the level of education and understanding was probably the
most important reason that CD largely disappeared from the developed world.
Education → demand for improving living standards -> living standards rose →
decline in CD.
- Resources and economics - lack of resources → poverty → reduces the ability to
combat disease. A developing society → education, or the ability to perform,
becomes a resource. Diseases are best prevented by educating people to overcome
them.
Health economics ‘making the most use of resources and balancing what is needed
with what is available’. F.i. limiting unnecessary expenditure on treatment. Financial
restrictions will limit what can be supplied.
Cost-effectiveness: programmes yield the greatest health improvement for available
resources (vaccination programme)
Cost-benefit producing the greatest benefit per unit of cost (long-term planning)
Measuring the benefit of these interventions is difficult.
Development of methods listed above: marginal costs
Three different strategies of vaccination programme:
1) fixed units: reaching the greatest number of children for the leats cost. Hight
coverage → more costs/child
2) mobile clinics: reach the last remaining children
3) outreach programmes: for existing clinics.
Disease control - some are more expensive (longer-term, more permanent control)
Best economic strategy: the use of methods.
- Communities and movements: demographic changes → urban areas. Slums have
developed → diseases of poverty thrive + imbalance of sexes → STD’s. Nomad
people are hard to reach for Child health organisations and therefor children are not
vaccinated. Migration and refugee camps → hight incidence rates CD.
b) Physical environment
- Topography: the nature of the psycical surroundings can influence the diseases that
are found there. Human activity → change the landscape → making it more suitable
for CD transmission.
- Climate: listed below are the elements that have a marked influence on where
diseases are found and the ways in which they are to be controlled
Temperature - tropics are the main area for CD. Temperature also regulates the
amount of CD (more rapid development in the tropics).
Rainfall - most essential element in human livelihood. Irregular rainfall → failed
crops → malnutrition → reduction of resistance to infection. Rainfall also influences
breeding of diseases.
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