COMMUNICABLE DISEASE

TUGUME DANIEL
BNS, MNS
 Introduction
Diseases transmitted from one person to another or from a reservoir to a susceptible host.
Diseases caused by germs or pathogens e.g. viruses, bacteria, parasites, worms etc.

If diseases are communicable, then they present in an epidemic or endemic form, while if non-communicable
as acute or chronic, as follows:
Communicable:
 epidemic (e.g. measles);
 endemic (e.g. malaria).
Non-communicable:
 acute (e.g. accidents);
 chronic (e.g. coronary heart disease).
Communicable diseases are common in developing countries and non-communicable in the
developed world.
Elements of communicable diseases
The key to any communicable disease is to think of it in terms of agent, transmission, host and
environment.
There needs to be a causative agent, which requires a means of transmission from one host to
another, but the outcome of infection will be influenced by the environment in which the disease
is transmitted.
The Agent
The agent can be an organism (virus, bacteria, rickettsia, protozoan, helminth, fungus or
arthropod), a physical or a chemical agent (toxin or poison).
If the agent is an organism, it needs to multiply and find a means of transmission and survival.
Transmission
Communicable diseases fall into a number of transmission patterns which include:
Direct transmission includes person to- person contact as from dirty fingers or via food and
water in the diarrhoeal diseases.
Direct transmission also occurs through droplet infection in the respiratory diseases.
Autoinfection can occur where humans contaminate themselves directly from their external
orifices.
Examples are transmission of Enterobius from anal scratching or infection of skin abrasions
with bacteria from nose-picking.
Human reservoir with intermediate host
The adults of schistosomiasis live in humans, but for transmission to another human, the parasite
must undergo developmental stages in a snail as intermediate host.
Vector
A vector carries the infection from one host to another either as part of the transmission process,
such as a mosquito, or it can be mechanical, for example, through the housefly, which
inadvertently transmits organisms to the host on its feet and mouth parts.
All vectors of importance are either insects, mosquitoes, flies, fleas, lice, etc. or, ticks and mites.
Zoonosis
From animals to man
Plants
Vegetable material that is eaten by the host can serve as a method of transmission
Host Factors
If the agent is transmitted to a new host, its successful invasion and persistence will depend upon a
number of host factors.
Susceptibility: age, sex, pregnancy etc
Inherent defence mechanisms
Any infecting organism must be able to overcome the body’s inherent defence mechanisms.
Resistance
The person’s susceptibility and defence mechanisms may be altered by the resistance of the
individual.
This may be lowered by the following: nutrition, trauma and debilitating conditions, multiple
infections etc
Immunity: acquired or passive
The Environment
The transmission cycle used by the agent to reach the host takes place within an environment
that determines the success and severity of the infection.
Communicable Disease Theory
Force of Infection
In a communicable disease, the number of new cases occurring in a period of time is dependent
on the number of infectious persons within a susceptible population and the degree of contact
between them.
Rate at which susceptible individuals in a population acquire an infectious disease in that
population, per unit time.
It is also known as the incidence rate or hazard rate.
An incidence rate describes how quickly disease occurs in a population.
Incidence = (New Cases) / (Population x Timeframe)
An example will help demonstrate this equation and is provided below.
You watch a group of the 5,000 people in your town.
During a five-year period, 25 individuals are newly diagnosed with diabetes mellitus.
What is the annual incidence of diabetes mellitus for your town?
(25 new cases diabetes mellitus)/(5,000 people x 5 years) =
(25 new cases) / (25,000 people-year) =
0.001 cases/people-year =
1 case / 1000 people-year
The above can be interpreted as "If we watch 1,000 people in the town for one
year we would expect one person, on average, to be newly diagnosed with
diabetes mellitus during the year of observation.“
If the susceptible population is sufficiently large to maintain a permanent pool of
susceptibles (as would happen in a disease where there is little or no immunity)
and the force of infection is constant, then newly infected individuals will
continue to be produced, while infectious individuals remain in the population.
One healthy carrier might continue to infect a large number of individuals over a
long period of time, or a brief devastating epidemic, with a short period of
infectiousness, may infect a large number of people over a short period of time.
The proportion of susceptible individuals can be reduced by mortality, immunity
or emigration, or increased by birth or immigration.
Epidemic Theory
Epidemic means an excess of cases in the community from that normally expected, or the
appearance of a new infection.
The point at which an endemic disease becomes epidemic depends on the usual presence of the
disease and its rate.
With an unusual disease, a few cases could be an epidemic, whereas with a common disease
(e.g. gastroenteritis), an epidemic occurs when the usual rate of the disease is substantially
exceeded
Epidemic contrasts with endemic, which means the continuous presence of an infection in the
community and is described by incidence and prevalence measurements.
Once an individual has experienced an episode of the disease (whether manifest or not), he or she
may develop immunity (either temporary or permanent) or die.
When a certain number of individuals have developed immunity then there are insufficient
susceptibles and the infection dies out.
This collective permanent immunity (as occurring in viral infections) is called the herd
immunity.
After a period of 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 (the theoretical minimum host population size required to
maintain an infecting agent)
Endemicity
An endemic disease implies that there is a constant rate of infection occurring in the community.
As new individuals are born, they become infected, are cured (including self-cure), retain the
infection for life or become immune.
Control Principles and Methods
Control can be directed either at the agent, the route of transmission, the host or the environment.
Sometimes it is necessary to use several control strategies.
The Agent: Destruction of the agent can be by specific treatment, using drugs that kill the agent
in vivo, or if it is outside the body, by the use of antiseptics, sterilization, incineration or
radiation.
Transmission:
Quarantine or isolation: Keeping the agent at a sufficient distance and for a sufficient length of
time away from the host until it dies or becomes inactive can be effective in preventing
transmission.
Contacts: People who might have become infected because of their close proximity to a case are
called contacts. They can be isolated, given prophylactic treatment or kept under surveillance.
Environmental health Methods of personal hygiene, water supplies and sanitation are particularly
effective against all agents transmitted by the faecal–oral route whether by direct transmission or
complex parasitic cycles involving intermediate hosts.
Animals Whether they act as reservoirs or intermediate host animals can be controlled by
destruction or vaccination (e.g. against rabies). If animals are to be eaten, their carcasses can be
inspected to make sure that they are free of parasitic stages.
The excretions or tissues of an animal can be infectious; so protective clothing and gloves should
be worn when handling animals.
Cooking: Proper cooking renders plant and animal produce safe for consumption, although some
toxins are heat-resistant. Food should be prepared hygienically before cooking and stored
properly afterwards.
Vector control is one of the most highly developed methods of interrupting transmission
because the parasite utilizes a vulnerable stage for development and transport.
Attack on vectors can either be on their larval stage by using larvicides and methods of biological
control, or while they are adults with adulticides.
Host: The host can be protected by physical methods (mosquito nets, clothing, housing, etc.), by
vaccination against specific diseases or by taking regular prophylaxis.
Environment: The environment of the host can be improved by education, assistance
(agricultural advice, house building, subsidies, loans, etc.), and improvement of communications
(to market his produce, reach health facilities, attend school, etc.).
In the course of time, these will be the most effective methods of preventing continuation of the
transmission cycle.