Communicable Diseases Module: 1. Basic Concepts in the Transmission of Communicable Diseases

Study Session 1  Basic Concepts in the Transmission of Communicable Diseases

Introduction

As you will recall from the Module on Health Education, Advocacy and Community Mobilisation, health is defined as a complete state of physical, mental and social well-being and not the mere absence of disease. The term disease refers to a disturbance in the normal functioning of the body and is used interchangeably with ‘illness’. Diseases may be classified as communicable or non-communicable. Communicable diseases are caused by infectious agents that can be transmitted to other people from an infected person, animal or a source in the environment. Communicable diseases constitute the leading cause of health problems in Ethiopia.

Before we describe each communicable disease relevant to Ethiopia in detail in later study sessions, it is important that you first learn about the basic concepts underlying communicable diseases. Understanding these basic concepts will help you a lot, as they form the basis for this Module.

In this first study session, we introduce you to definitions of important terms used in communicable diseases, the types of infectious agents that cause these diseases, the main factors involved in their transmission, and the stages in their natural development. This will help you to understand how measures for the prevention and control of communicable diseases are put into place at several levels of the health system, including in homes and at your Health Post – which is the focus of Study Session 2.

Learning Outcomes for Study Session 1

When you have studied this session, you should be able to:

1.1  Define and use correctly all of the key terms printed in bold. (SAQs 1.1 and 1.5)

1.2  Identify the main types of infectious agents. (SAQs 1.2 and 1.3)

1.3  Describe the main reservoirs of infectious agents. (SAQ 1.3)

1.4  Describe the chain of transmission of communicable diseases and explain how infectious agents are transmitted by direct and indirect modes. (SAQs 1.3 and 1.5)

1.5  Describe the characteristics of susceptible hosts and the main risk factors for development of communicable diseases. (SAQ 1.4)

1.6  Describe the stages in the natural history of communicable diseases. (SAQ 1.5)

1.1  What are communicable diseases?

As described in the introduction, the organisms that cause communicable diseases are called infectious agents, and their transmission to new uninfected people is what causes communicable diseases; (note that infectious diseases is an interchangeable term). Familiar examples of communicable diseases are malaria and tuberculosis. Diseases such as heart disease, cancer and diabetes mellitus, which are not caused by infectious agents and are not transmitted between people, are called non-communicable diseases.

1.1.1  The burden of communicable diseases in Ethiopia

Communicable diseases are the main cause of health problems in Ethiopia. According to the Ethiopian Federal Ministry of Health, communicable diseases accounted for most of the top ten causes of illness and death in 2008/09. As you can see in Table 1.1, most causes of outpatient visits are due to communicable diseases.

Table 1.2 shows that most causes of inpatient deaths are due to communicable diseases, including pneumonia, tuberculosis, HIV/AIDS and malaria. These and other communicable diseases will be discussed in detail in later study sessions of this module.

1.1.2  Endemic and epidemic diseases

Not all communicable diseases affect a particular group of people, such as a local community, a region, a country or indeed the whole world, in the same way over a period of time. Some communicable diseases persist in a community at a relatively constant level for a very long time and the number of individuals affected remains approximately the same. These communicable diseases are known as endemic to that particular group of people; for example, tuberculosis is endemic in the population of Ethiopia and many other African countries.

By contrast, the numbers affected by some communicable diseases can undergo a sudden increase over a few days or weeks, or the rise may continue for months or years. When a communicable disease affects a community in this way, it is referred to as an epidemic. Malaria is endemic in some areas of Ethiopia, and it also occurs as epidemics due to an increase in the number of cases suddenly at the beginning or end of the wet season.

1.1.3  Prevention and control measures

The health problems due to communicable diseases can be tackled by the application of relatively easy measures at different levels of the health system. Here, we will use some examples at the individual and community levels, which are relevant to your work as a Health Extension Practitioner.

Some measures can be applied before the occurrence of a communicable disease to protect a community from getting it, and to reduce the number of cases locally in the future. These are called prevention measures. For example, vaccination of children with the measles vaccine is a prevention measure, because the vaccine will protect children from getting measles. Vaccination refers to administration of vaccines to increase resistance of a person against infectious diseases.

Once a communicable disease occurs and is identified in an individual, measures can be applied to reduce the severity of the disease in that person, and to prevent transmission of the infectious agent to other members of the community. These are called control measures. For example, once a child becomes infected with measles, treatment helps reduce the severity of the disease, and possibly prevents the child’s death, but at the same time it decreases the risk of transmission to other children in the community. In this context, treatment of measles is considered a control measure.

Next we look at the main ways in which infectious agents are transmitted.

1.2  Factors involved in the transmission of communicable diseases

Transmission is a process in which several events happen one after the other in the form of a chain. Hence, this process is known as a chain of transmission (Figure 1.1). Six major factors can be identified: the infectious agent, the reservoir, the route of exit, the mode of transmission, the route of entry and the susceptible host. We will now consider each of these factors in turn.

Figure 1.1  Factors involved in the chain of communicable disease transmission.

1.2.1  Infectious agents

Sizes and types of infectious agents

Infectious agents can have varying sizes. Some, such as Plasmodium falciparum and all bacteria and viruses, are tiny and are called micro-organisms, because they can only be seen with the aid of microscopes. Others, such as the ascaris worm (Ascaris lumbricoides), can be easily seen with the naked eye. The different types of infectious agents are illustrated in Table 1.3 according to their size, starting with the largest and ending with the smallest, and are then discussed below.

Table 1.3  Different types of infectious agents: their number of cells, visibility and examples. (Adapted from The Open University, 2007, Water and Health in an Overcrowded World, Chapter 2)

Type of infectious agent	Number of cells	Visibility	Examples
Helminths	many	Visible with the naked eye	Ascaris worm causes ascariasis Its length reaches 15–30 cm
Protozoa	1	Visible with a standard microscope	Plasmodium falciparum causes malaria
Bacteria	1	Visible only with a special microscope; much smaller in size than protozoa	Vibrio cholerae causes cholera
Viruses	0	Visible only with a special microscope; much smaller in size than bacteria	HIV causes AIDS

Helminths are worms made up of many cells; for example, Ascaris lumbricoides.

Protozoa are micro-organisms made up of one cell; for example, Plasmodium falciparum.

Bacteria are also micro-organisms made up of one cell, but they are much smaller than protozoa and have a different structure; for example Vibrio cholerae, which causes cholera.

Viruses are infectious agents that do not have the structure of a cell. They are more like tiny boxes or particles and are much smaller than bacteria; for example, HIV (the Human Immunodeficiency Virus), which can lead to AIDS.

Though not as common as causes of communicable disease in humans, other types of infectious agents include fungi (e.g. ringworm is caused by a fungus infection), and mites (similar to insects), which cause scabies.

1.2.2  Reservoirs of infectious agents

Many infectious agents can survive in different organisms, or on non-living objects, or in the environment. Some can only persist and multiply inside human beings, whereas others can survive in other animals, or for example in soil or water. The place where the infectious agent is normally present before infecting a new human is called a reservoir. Without reservoirs, infectious agents could not survive and hence could not be transmitted to other people. Humans and animals which serve as reservoirs for infectious agents are known as infected hosts. Two examples are people infected with HIV and with the bacteria that cause tuberculosis; these infectious agents persist and multiply in the infected hosts and can be directly transmitted to new hosts.

Animals can also be reservoirs for the infectious agents of some communicable diseases. For example, dogs are a reservoir for the virus that causes rabies (Figure 1.3). Diseases such as rabies, where the infectious agents can be transmitted from animal hosts to susceptible humans, are called zoonoses (singular, zoonosis).

Figure 1.3  Rabies is a zoonosis, which can be transmitted from dogs to humans. (Photo: WHO at http://www.who.int/ rabies/ animal/ en/)

Non-living things like water, food and soil can also be reservoirs for infectious agents, but they are called vehicles (not infected hosts) because they are not alive. You will learn more about them later in this study session.

1.2.3  Route of exit

Before an infectious agent can be transmitted to other people, it must first get out of the infected host. The site on the infected host through which the infectious agent gets out is called the route of exit. Some common examples are described below.

The routes of exit from the respiratory tract are the nose and the mouth. Some infectious agents get out of the infected host in droplets expelled during coughing, sneezing, spitting or talking, and then get transmitted to others (Figure 1.4). For example, people with tuberculosis in their lungs usually have a persistent cough; Mycobacterium tuberculosis uses this as its route of exit.

Figure 1.4  Infectious agents in the respiratory tract can exit from infected hosts during coughing and be transmitted to others.

Gastrointestinal tract

Figure 1.5  Infectious agents can get out of the body with faeces and get transmitted to others.

The anus is the route of exit from the gastrointestinal tract (or gut). Some infectious agents leave the human body in the stool or faeces (Figure 1.5). For example, the infectious agents of shigellosis, a disease which can cause bloody diarrhoea, use this route of exit.

1.2.4  Modes of transmission

Once an infectious agent leaves a reservoir, it must get transmitted to a new host if it is to multiply and cause disease. The route by which an infectious agent is transmitted from a reservoir to another host is called the mode of transmission. It is important for you to identify different modes of transmission, because prevention and control measures differ depending on the type. Various direct and indirect modes of transmission are summarised in Table 1.3 and discussed below it.

1.2.5  Route of entry

Successful transmission of the infectious agent requires it to enter the host through a specific part of the body before it can cause disease. The site through which an infectious agent enters the host is called the route of entry.

Figure 1.6  Some infectious agents get into the body with contaminated food, water or on hands. (Photo: Basiro Davey)

1.2.6  Susceptible hosts and risk factors

After an infectious agent gets inside the body it has to multiply in order to cause the disease. In some hosts, infection leads to the disease developing, but in others it does not. Individuals who are likely to develop a communicable disease after exposure to the infectious agents are called susceptible hosts. Different individuals are not equally susceptible to infection, for a variety of reasons.

Factors that increase the susceptibility of a host to the development of a communicable disease are called risk factors. Some risk factors arise from outside the individual – for example, poor personal hygiene, or poor control of reservoirs of infection in the environment. Factors such as these increase the exposure of susceptible hosts to infectious agents, which makes the disease more likely to develop.

Additionally, some people in a community are more likely to develop the disease than others, even though they all have the same exposure to infectious agents. This is due to a low level of immunity within the more susceptible individuals. Immunity refers to the resistance of an individual to communicable diseases, because their white blood cells and antibodies (defensive proteins) are able to fight the infectious agents successfully. Low levels of immunity could be due to:

• diseases like HIV/AIDS which suppress immunity
• poorly developed or immature immunity, as in very young children
• not being vaccinated
• poor nutritional status (e.g. malnourished children)
• pregnancy.

Finally, look back at Figure 1.2. We can now summarise the chain of transmission as follows:

• the infectious agent gets out of the reservoir through a route of exit
• it gets transmitted to a susceptible host by a direct or indirect mode of transmission and it gets into the susceptible host through a route of entry
• if it multiplies sufficiently in the susceptible host it will cause a communicable disease.

1.3  Natural history of a communicable disease

The natural history of a communicable disease refers to the sequence of events that happen one after another, over a period of time, in a person who is not receiving treatment. Recognising these events helps you understand how particular interventions at different stages could prevent or control the disease. (You will learn about this in detail in Study Session 2.)

Events that occur in the natural history of a communicable disease are grouped into four stages: exposure, infection, infectious disease, and outcome (see Figure 1.6). We will briefly discuss each of them in turn.

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Figure 1.6  Stages in the natural history of communicable diseases.

1.3.1  Stage of exposure

In the stage of exposure, the susceptible host has come into close contact with the infectious agent, but it has not yet entered the host’s body cells. Examples of an exposed host include:

• a person who shakes hands with someone suffering from a common cold
• a child living in the same room as an adult with tuberculosis
• a person eating contaminated food or drinking contaminated water.

1.3.2  Stage of infection

At this stage the infectious agent has entered the host’s body and has begun multiplying. The entry and multiplication of an infectious agent inside the host is known as the stage of infection. For instance, a person who has eaten food contaminated with Salmonella typhii (the bacteria that cause typhoid fever) is said to be exposed; if the bacteria enter the cells lining the intestines and start multiplying, the person is said to be infected.

At this stage there are no clinical manifestations of the disease, a term referring to the typical symptoms and signs of that illness. Symptoms are the complaints the patient can tell you about (e.g. headache, vomiting, dizziness). Signs are the features that would only be detected by a trained health worker (e.g. high temperature, fast pulse rate, enlargement of organs in the abdomen).

1.3.3  Stage of infectious disease

At this stage the clinical manifestations of the disease are present in the infected host. For example, a person infected with Plasmodium falciparum, who has fever, vomiting and headache, is in the stage of infectious disease – in this case, malaria. The time interval between the onset (start) of infection and the first appearance of clinical manifestations of a disease is called the incubation period. For malaria caused by Plasmodium falciparum the incubation period ranges from 7 to 14 days.

Remember that not all infected hosts may develop the disease, and among those who do, the severity of the illness may differ, depending on the level of immunity of the host and the type of infectious agent. Infected hosts who have clinical manifestations of the disease are called active cases. Individuals who are infected, but who do not have clinical manifestations, are called carriers. Carriers and active cases can both transmit the infection to others.

Depending on the time course of a disease and how long the clinical manifestations persist, communicable diseases can be classified as acute or chronic. Acute diseases are characterised by rapid onset and short duration of illness. For instance, diarrhoea that starts suddenly and lasts less than 14 days is an acute diarrhoeal disease. Chronic diseases are characterised by prolonged duration of illness; for example, a chronic diarrhoeal disease lasts more than 14 days.

1.3.4  Stage of outcome

At this stage the disease may result in recovery, disability or death of the patient. For example, a child who fully recovers from a diarrhoeal disease, or is paralyzed from poliomyelitis, or dies from malaria, is in the stage of outcome.

In the next study session you will learn how communicable diseases are classified, and about the main types of prevention and control measures.

Summary of Study Session 1

In Study Session 1 you have learned that:

1. Communicable diseases are caused by infectious agents that can be transmitted to susceptible individuals from an infected person, or from other animals, objects or the environment.
2. Infectious agents include helminths, protozoa, bacteria, viruses and fungi.
3. Six factors are involved in the transmission of communicable diseases: the infectious agent, the reservoir, route of exit, mode of transmission, route of entry, and the susceptible host.
4. A reservoir is a human, another animal, or a non-living thing (such as soil), where the infectious agent normally lives.
5. Modes of transmission of an infectious agent can be directly through person-to-person contact, or across the placenta from mother to fetus. Indirect transmission can occur through air, vehicles such as water, food and contaminated objects, or via a vector such as a mosquito.
6. A susceptible host is a person or animal who can develop infection if exposed to the infectious agent. Susceptibility is increased if exposure is high, or the host’s immunity is low.
7. The natural history of an untreated communicable disease has four stages: stage of exposure, stage of infection, stage of infectious disease, and stage of outcome.

Self-Assessment Questions (SAQs) for Study Session 1

Now that you have completed this study session, you can assess how well you have achieved its Learning Outcomes by answering the questions below. Write your answers in your Study Diary and discuss them with your Tutor at the next Study Support Meeting. You can check your answers with the Notes on the Self-Assessment Questions at the end of this Module.

First read Abebe’s story and then answer the questions that follow it.