Communicable diseases are diseases that are the result of a causative organism spreading from one person to another or from animals to people.
They are among the major causes of illnesses inKenyaand the entireAfrica. These diseases affect people of all ages but more so children, due to their exposure to environmental conditions that support the spread. Communicable diseases are preventable if interventions are placed at the various levels of transmission of the disease.
In recent years our region also faces new and emerging diseases which are challenging public health as never before. Unfortunately, many of these diseases affect the poor and marginalized sections of society, and contribute not only to ill health and poverty at micro-level but also have serious socio-economic implications at the macro-level. Health workers have an important role to play in the control of these diseases by applying effective and efficient management, prevention and control measures.
As a health worker you need to be equipped with the capacity to target communicable diseases for their eradication.
I hope your answer included the following examples:
We shall now focus on the various classes of communicable diseases. There are various ways of classifying communicable diseases. The classification given below is considered to be the most appropriate for ease of understanding. The detailed description of each of the classes will be discussed in the respective units of this course.
Discuss with a colleague the classes of communicable diseases listed below. Explore any other method of communicable disease classifications that you might have read elsewhere.
The classes include:
You have learnt a lot so far.
Now try to define communicable disease.
List some of the characteristics of the communicable diseases.
Classify with clear example the various communicable diseases.
If you could answer these questions correctly, then you are doing well! Keep it up and proceed to the next section on epidemiology.
This section will focus on various aspects of epidemiology.
By the end of this section you should be able to answer the following questions:
How do we define epidemiology?
What is the scope of epidemiology?
What is disease natural history?
Which are the disease prevention levels?
What are the various epidemiologic methods?
How do we handle communicable disease outbreaks?
Epidemiology is the study of the distribution and determinants of disease among populations. This definition has however been broadened to include various illnesses and conditions that are not necessarily diseases but affect the health of populations such as chronic illnesses like diabetes, hypertension or ulcers, as well as such conditions as natural calamities, road traffic accidents, crimes and other social vices.
Epidemiology as a scientific discipline has developed over the years and has become a necessity in the field of health care for effective disease prevention and control. It is a common saying that “disease prevention is better (cheaper) than cure”. That saying aptly describes the role of epidemiology in our day to day life.
We have seen that epidemiology is the study of disease/conditions distribution and determinants. As a science, it utilises rational study methods in identifying the distribution patterns and determinants of diseases. From the studies, recommendations are generated on how to:
Figure 1 illustrates the scope of epidemiology is simple terms.
Figure 1: The scope of epidemiology
Let us now examine more carefully Figure 1. We find that the disease distribution factors are three: persons, time and place. In other words we look at who was affected, when and where.
Furthermore, with reference to the first factor, persons affected by the disease, we consider their age, sex, race, occupation etc. as well as any other common characteristics relating to those persons affected by the disease.
The second factor, time, relates to when the diseases is most likely to strike e.g. an epidemic, endemic, seasonal, cyclic, etc
Finally, the third factor, place, refers to the geographical distribution of a disease and the common characteristics that are favourable for that disease in the given locality. Some diseases are localized, regional, pandemic, etc.
Disease determinant factors include the agent, host and environment.
Agent refers to the disease causing organism characteristics such as habitation, breeding, migration, infectivity, climatic and environmental factors favouring its existence.
Host refers to the biological makeup of the individuals that makes them vulnerable to the specified illness such as physical condition, genetic make up, habits, etc.
Environment refers to the ecological conditions that favour the interaction of host and agent, for example swampy areas, bushes within house holds, sanitation etc.
I am sure you have come across the term Epidemiologic cycle, epidemiologic triangle, or disease causation triangle. All of these mean one and same thing. Read the content below to know more about it.
Here is a diagrammatic illustration of the interaction between disease determinant factors.
Figure 2: Disease causation cycle
The host, agent and environmental factors have to be conducive for the disease to occur. All communicable diseases require that the three factors are present for individuals to be affected.
Vector is the vehicle that some of the agents or disease causing organisms require so as to be moved from one point to the other. Some require it to complete their developmental cycle, e.g. a mosquito in the transmission of malaria. Not all communicable diseases require a vector for transmission.
Control and prevention measures can be implemented at the various levels of the chains shown in Figure 2. For example, to break the chain between host and environment for the transmission of malaria, you need to clear swampy areas, bushes and ensure proper disposal of waste. Alternatively, to break the chain between host and agent in the malaria disease one can take prophylactic anti-malaria drugs. To break the chain between the environment and the agent, one can spray the breeding sites for mosquitoes and to break the vector and host chain one can sleep under mosquito net.
a) Select any three communicable diseases and construct epidemiologic cycle for each.
b) Describe how the various chains for each of the diseases you have chosen in ‘(a) are broken for effective control and preventionmeasures
The disease transmission process has three components. These are: source, transmission route and susceptible host.
The source is the origin of the disease causing organism. This could be an infected person, animal, place or object.
The main transmission routes are;
A Susceptible host is an individual who has low resistance to a particular disease. This may be due to various factors such as:
Each and every communicable disease has its own unique source, route of transmission and susceptible hosts. Therefore, the principles of control and prevention should be geared towards:
For example, in the case of malaria, we can attack the source by clearing and spraying the mosquito breeding sites. Thus we interrupt the transmission route by spraying and killing the mosquitoes, hence the causative organism and lastly we can protect the susceptible host by giving prophylactic medications, and advising him or her to sleep under treated mosquito nets.
To understand the transmission process of communicable diseases it is paramount to understand the Natural history of a disease.
In order to understand the process of disease progression, we shall now go through the so called natural history of disease.
Natural history of disease refers to the process of disease/condition progression from the time it affects an individual to the time the individual recovers or dies, if appropriate measures are not instituted.
The process of disease progression has two distinct periods: pre-pathogenesis period and pathogenesis period.
The pre-pathogenesis period is the period before the disease infects an individual. The agent and the host are interacting in the environment. At this level, the host defense system is well capable of handling the agent (causative organisms) hence there is no disease.
The pathogenesis period is the period that starts when the body defense mechanism has been overcome by the agent (disease causing organisms), resulting to the host cells dying. This period has various stages.
Sub clinical horizon. At this stage the host cells have started dying but no major effects are felt yet and the host has no signs or symptoms of the disease. At this level only laboratory tests would reveal the extent of damage.
Clinical horizon. At this time the damage on the cells is so much that some of the host’s body functions are starting to fail. This manifests in signs and symptoms of a disease and the host is feeling uncomfortable or sick. If the host does not receive appropriate medical intervention then they will continue to the next stage.
Early disease stage. At this time the disease effects are real as a result of massive cell damages that are affecting tissues functions. The host needs appropriate intervention to correct the damage. If they fail to receive correct interventions then the disease gets in to the next level.
Advanced disease. At this level the damage to the host systems is massive and may be irreversible, leading to disability or permanent damage. The host may have one of three outcomes at this stage: recover, suffer permanent disability, convalescence or in worst cases death. Figure3 illustrates the whole process of disease causation and progression till death or the natural history of the disease or condition process.
Figure 3: the natural history of disease/condition.
Describe the natural history of a chronic disease such as diabetes.
Disease prevention refers to deliberate actions to halt or delay disease progression from one stage to the next. There are three levels of disease prevention: primary, secondary and tertiary.
For communicable disease this is the most common prevention method. Primary prevention targets the pre-pathogenesis period or the period before the disease infects the individual. The interventions include:
This mainly happens during the early stages of the disease process. The purpose is to prevent further damage to the host cells and tissues and thus avoid disease complications. Measures would include early diagnosis, screening and prompt treatment.
This takes place during the advanced stages of the disease progression to minimise the complications or reverse the effects. Main interventions are rehabilitative in nature and include physical therapy, occupational therapy, psychotherapy, corrective/rehabilitative surgery etc.
Epidemiologists use various methods and approaches in quantifying disease cases in a population. Some of these are: counts, ratio, proportion, percentage, and rates.
Count. This is the number of individuals with a specified quality in a defined population.
Ratio. This expresses the relationship between two numbers in the form of
E.g. The ratio of males to females is 1:3
Proportion. This is a specific type of ratio in which the numerator is included in the
denominator and the resultant value is expressed as a part of the whole.
X/X+Y or X per X+Y
E.g. proportion of males in the class is 4 per 10 students, or 40 per 100 students or 400 per 1000, etc.
Percentage. This is a proportion that is expressed per 100.
X per 100
E.g. 40 percent of the community members are males.
Percentage is a very common measure of disease in various epidemiological descriptive studies.
Rate. This is a special form of proportion that includes specification
It is the basic measure that most clearly expresses the probability or risk of disease in a defined population over a specific period of time. The rate is defined as follows;
Rate = no of events in a specified period x K
Population at risk of the specific event in a specified period
Where K is a constant that may be set (fixed) for certain rates or set by the investigator for convenience.
Here we talk about the incidence or prevalence of the disease/condition in the population.
The incidence of a disease or health condition refers to the number of persons in a population who develop the disease/condition during a specified period of time. The determination of incidence requires that a population is followed up over a period of time through a prospective study.
Incidence rate = Number of new cases in a population over a
specified duration of time in a given population x K
The population at risk of developing the disease or condition at the beginning of the duration of interest.
Kilanga village has a population of 1500 males and 2000 females. A prospective study is conducted over two years to establish the incidence of cancer of the prostate within the population during which 30 males developed the disease.
Calculate the incidence of the disease in the population
Incidence rate= 30 (individuals who developed the disease) = 0.02 cases
For ease of interpretation we multiply by K which we may take to be 100 for this case and it will be:
Incidence = 2 cases per hundred males in the population of Kilanga village within the two years of study.
The prevalence of a disease or condition refers to the total number of persons who have the disease/condition of interest at a particular time. Prevalence rates are calculated at a point in time (point prevalence) or over duration of time (period prevalence). The information required for point prevalence is obtained from cross sectional studies and in period prevalence is obtained from retrogressive studies.
Point prevalence = number of existing cases of
disease/condition at a specified point in time x K
total population at that point in time.
Data collected in March at Kilanga village revealed that there were 140 cases of malaria at the time of data collection. If kilanga village had a total population of 3500 persons, calculate the point prevalence of malaria at that tim.e
Point prevalence = 140 (cases ) = 0.04 cases
For ease of interpretation we multiply by K in this case we take it to be 100. Hence the report will read:
Point prevalence of malaria for Kilanga village in March was 4 cases per 100 people.
Period prevalence = number of existing cases of
disease/condition over a specified duration of time. x K
average population over the specified duration of time.
Records obtained from Kilanga village indicate that for the previous six months, inclusive of March, the number of malaria cases was 700. Calculate the period prevalence of the disease assuming that the population remained constant at 3500 over that duration.
Period prevalence = 700 (cases) = 0.2 cases
Hence for ease of interpretation we multiply by K which we may take to be 100.
The period prevalence will be 20 cases of malaria per 100 persons in Kilanga village over the last six months.
We need to know the incidence and prevalence rates because:
Epidemic. Tthis is the occurrence of disease cases at a frequency that is higher than the normal for the population in a given period of time.
Endemic. This is the occurrence of disease/condition in a given population at frequencies that are constant over duration of time. However, these frequencies are higher than would be expected for that population. Figure 4 illustrates this.
Figure 4: The epidemic and endemic patterns of communicable diseases
The most commonly used rates are the measures of natality, morbidity and mortality. We shall now see how to calculate these rates.
i. Crude birth rate = Number of live births during time interval x 1000
Estimated mid-interval population
ii. Fertility rate = Number of live births during time interval x 1000
Number of women aged 15 – 44 at mid-interval
i. Crude death rate = Number of deaths during time interval x 1000
Estimated mid-interval population
ii. Specific death rate = Number of deaths in sub group during time interval x 1000
Estimated mid-interval population of subgroup
iii. Cause specific death rate =
Number of deaths from specified cause during time interval x 1000
Estimated mid-interval population
iv. Infant mortality rate =
Number of deaths of infants age < 1year during time interval x 1000
Total live births during time interval
v. Neonatal mortality rate =
Number of deaths of infants age < 28days during time interval x 1000
Total live births during time interval
vi. Post neonatal mortality rate =
Number of deaths of infants age ≥ 28days but < 1year during time interval x 1000 Total live births during time interval
Get more data from your hospital records office and do calculations on percentages, rates and ratios to determine the trends of both mortalities and morbidities.
You have learnt a lot so far. Let us see how much of it you can remember!
What are the stages of the natural history of disease?
What is prevalence rate?
What is incidence rate?
State at least five specific rates that are commonly used?
If you had any problems when answering all these questions accurately, then read the specific section again. If your answers were correct and clear, then you are doing very well. Congratulations and move on to the next section.
Epidemiologic methods are the various approaches in which data is gathered, analyzed and reports written with regard to various diseases distribution and determinants. They enable us to plan and evaluate disease management in communities.
These methods are classified as:
These are basic studies that seek to describe the situation as it is. These methods utilize simple descriptive statistics such as percentages tables and graphs to describe the disease situation. These studies generate hypothesis.
Individual based studies. These studies reflect on single individuals as the unit of study. Here we have:
Population based studies. These include:
These studies are more advanced in methodology and analysis and their findings are more generalisable as compared to descriptive studies. While descriptive studies generate hypothesis, these ones test hypothesis.
Prospective studies (Cohort studies). These studies start with individuals with an exposure of interest (cohorts) and are followed up over time as they are compared with individuals who do not have the exposure of interest. Analysis is then done to find out if those who had exposure will develop the outcome of interest to a larger extent than those without exposure.
For example, people working on farms (exposed to zoonotic disease) are followed up over time to establish if they will manifest the zoonotic diseases. Simultaneously, a similar group of individuals who do not work on farms are followed up to establish if they will also develop the same zoonotic disease.
Retrospective (case control studies). These studies are easier to conduct, consume less time and are cheaper than the cohort studies. Two groups are selected from a population, of which one group with the disease of interest (case) and another without (control). Then they are asked about their past in order to establish if they had a common exposure that might have caused the disease. These studies are however prone to recall biases.
For example, in a community, people who have amoebiasis may be asked about their food (sources, preparation, storage and consumption) they have been consuming. Another group from the same community who do not suffer from amoebiasis is also asked the same questions then the data is analyzed.
Experimental studies. These are studies conducted in a controlled environment where the researcher has control over all variables. It is a laboratory set up kind of study. They are very useful in the isolation of the actual disease causative agent. They are very strict and also very expensive. They require highly specialized personnel and equipment.
Community trials. These are studies involving and entire community. For example, in one community the water may be fluoridate while in another it may not. The two communities are then followed up and with time if those taking water that is not fluoridated start complaining of teeth problems, then the effects of fluoride are confirmed.
Epidemiological data is important in the control of communicable diseases because it:
Health Surveillance may be defined as the tracking and forecasting of any health event or health determinant through the:
Surveillance products are produced for a specific public health purpose or policy objective. Surveillance should be purposeful, economical, and action oriented. It should not only detect emerging health risks, but should also include systems that allow public health officials to monitor and evaluate progress in health protection and disease prevention.
Surveillance uses whatever data sources will provide the necessary information. Surveillance systems may share data with personal health services information systems, but the end-products are different.
Most of the data currently available from health facilities are originally generated for administrative purposes. They can serve as raw material for health services management and research, as well as for disease and health surveillance if procedures for capturing and handling administrative data are appropriately adapted.
In general, surveillance data can originate from any of four classes of source:
Effective communicable diseases control relies on an effective surveillance and response system that promotes better coordination and integration of surveillance function. Recognizing this, the Initiative to Strengthen the Disease Surveillance system that promotes the integration of surveillance activities inEthiopiawas started in 1996.
In 1998 WHO/AFRO, following the resolution of the 48th assembly, started promoting the Integrated Disease Surveillance and Response (IDSR) for all member states to adopt as the main strategy to strengthen their National Disease Surveillance System.
The specific goals of IDSR are to:
The basic ingredients of IDSR include:
It is important that surveillance and laboratory data collected should be translated into specific and timely public health actions.
Integrated disease surveillance envisages all surveillance activities in a country as a common public service that carry out many functions using similar structures, processes and personnel. The surveillance activities that are well developed in one area may act as driving forces for strengthening other surveillance activities, offering possible synergies and common resources.
Surveillance is based on collecting only the information that is required to achieve objectives for disease control. Data requested may differ from disease to disease and some diseases may have specific information needs, requiring specialized systems.
With the global momentum to scale up response to communicable diseases, public health practitioners need to review constantly their performance in detecting and responding to communicable diseases. At the same time, they should remain accountable for their activities and policies to a variety of stakeholders.
People at different levels of surveillance need to report accurate, timely and reliable data to national authorities, so as to ensure timely and effective responses that will contain communicable disease outbreaks. Such data should also be availed to donors, so as to secure funding to strengthen surveillance and response activities in relation to communicable diseases. Most importantly, all surveillance levels in the member countries, should be able to utilize the surveillance information locally to address and resolve problems related to the control of communicable diseases.
The International Health Regulations emphasize the commitment of Member States to the goal of global health security. This will require all Member States to maintain a functional and effective surveillance and response system that is able to detect, investigate and respond to public health emergencies of national and international concern. Against this background and in response to requests from Member States, the World Health Organization (WHO), in consultation with technical partners and Member States, has developed a framework for monitoring and evaluating communicable disease surveillance and response systems. This framework aims to strengthen surveillance and response activities and builds on the experiences gained by Member States in monitoring and evaluating their surveillance and response systems. The framework contains a list of indicators that are proposed for purposes of monitoring and evaluating surveillance and response systems to communicable diseases. (See figure 5).
The following definitions have been adopted for purposes of monitoring and evaluating communicable disease surveillance and response systems.
Monitoring is the routine (continuous) tracking of the performance of the surveillance and response systems.
Evaluating is the periodic assessment of changes in targeted results (objectives) that can be attributed to the surveillance and response system. Evaluation attempts to attribute changes in outputs, outcomes and impacts (negative or positive, targeted or non-targeted) of the surveillance and response system.
Figure 5: The conceptual frame work of surveillance and response for communicable diseases
You have now completed the content on disease surveillance. Answer the following questions to see if you really have understood the content.
I am sure you have NOT forgotten what a disease epidemic is. Outbreaks or epidemics are the occurrence of a disease in excess of its expected frequency. Epidemics of infectious disease, such as cholera, meningococcal meningitis, typhoid, typhus and plague are an important threat in many countries. Outbreaks of such diseases affect many people and are caused by infections, dietary deficiencies or poisoning. Modern epidemiology rose out of studies of such diseases.
Epidemics may be categorised into two,that is, point-source epidemic and contagious disease epidemics.
In this type of epidemics many susceptible individuals are exposed, more or less simultaneously, to a source of pathogenic organisms or toxins, for example, when guests at a feast eat meat contaminated with staphylococci. This results in an explosive increase in the number of cases of disease over a short period.
This type of an epidemic results when organisms are propagated in the community by passage from person to person, so that the initial rise in the number of cases is less abrupt than in point-source epidemic.
Outbreak investigations are a type of fast-paced epidemiologic research, undertaken to determine the cause of the outbreak and what remedial actions are required. These investigations are typically retrospective, occur in real time often under intense public and political pressure, begin without hypotheses, are iterative, and are closely tied to the implementation of public health measures to contain the outbreak. Outbreak investigations also involve considerable challenges in communication, including essential risk communication to the public.
The investigation and management of infectious disease outbreaks is typically local and provincial, at least in the first instance. Other levels of government may assist, and the epidemic may even be managed by national or international bodies, but as a general rule, the first line of defense is local.
Recognition of a new threat has different permutations and challenges, depending upon whether the agent is known or unknown, whether the known agent is a notifiable or non-notifiable disease, and the extent of knowledge about how to contain the agent most effectively and efficiently.
Detection demands the timely upward reporting of data through the public health hierarchy - local, regional, provincial, national, global - and the collation and analysis of case data at the lowest level where a cluster of cases can be recognized.
On occasion, cases may be scattered so widely that an outbreak is not detectable at the local or even provincial level
We are now going to look at what epidemics investigation involves.
a. Rapid epidemiologic assessment is essential at the beginning of an outbreak or epidemic so as to define the scope of the problem and start mobilization of containment strategies.
The next step is epidemic investigation and it aims at identifying the aetiology and the modes of transmission of an infectious agent, thereby guiding appropriate measures to prevent further transmission. An ongoing outbreak is generally a health emergency. Approaches to its investigation require different modes of operation, different command-and-control structures and unified leadership. Investigators should be insulated from the constant demands of data flow.
b. Establishing a case definition is central to disease surveillance and outbreak containment.
c. Establishing aetiology is usually straightforward for known agents, provided the requisite logistical arrangements and laboratory capacity are in place. The ability to monitor data at the national and even provincial level may be undercut because of poor information systems and the lack of data sharing protocols. If epidemiologic and laboratory data are disintegrated, this will compromise the epidemic investigation efforts.
d. Confirmation of cases presupposes the existence of a definitive test to ascertain true cases. When the agent is unknown, this takes some time.
e. Cases finding and defining the scope of the problem by the outbreak management and investigation team is paramount.
f. Generating descriptive epidemiologic information should be done immediately by the investigative team as data accumulate during the outbreak.
g. Generating and testing hypotheses should follow so as to have a working proposition of what may be going on.
h. Reporting of findings of epidemiologic investigations to national and international bodies is a critical part of an outbreak investigation for several reasons. Understanding a disease allows other jurisdictions to put in place appropriate measures for its control and to learn from the experience of others.
Outbreaks are often highly visible and are conducted under intense public, political and media scrutiny. Communications with the media, clinical personnel, governments, and the public are all extremely important.
i. The control of an epidemic through public health measures is the immediate purpose for epidemic investigations. With disease spreading, decisions on public health interventions need to be taken quickly even though often with incomplete information. The actions that are taken in controlling any epidemic have very significant costs and may be controversial or highly unpopular.
The measures would include, case detection, isolation of cases, follow up and quarantine of contacts, strict infection control measures in hospitals, closure of hospitals, airline passenger screening and travel advisories.
What is epidemic disease response?
Outline the steps involved in an epidemic disease response.
InKenyaaccording to the Public Health Act, Cap 254 : 17(1) there are diseases that should be notified to the higher authorities above your office as a health worker once you come across them.
You have now come to the end of this section. I hope you enjoyed it well enough to learn all that it contains. There are a few questions that will enable you to know if you have comprehended the content of Section 2.