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2. Epidemiology: some basic concepts and definitions

2.1 Introduction
2.2 Intrinsic determinants of disease
2.3 Extrinsic determinants of disease
2.4 Describing disease events in populations

2.1 Introduction

A question frequently asked is, "What is epidemiology"? There are many different definitions of the term. In the main, people attempting to define epidemiology have normally done so in the context of their own particular interests or needs. A useful general definition is that given by Schwabe et al (1977), which defines epidemiology as the study of disease in populations. It thus differs from the more conventional medical approaches to the study of disease that are normally concerned with the study of disease processes in affected individuals. While the objective of the latter is to find cures for diseases in individuals already affected, epidemiology is basically concerned with the reasons why those individuals became diseased in the first place.

Inherent in the epidemiological approach is the belief that the frequency of occurrence of a disease in a population is governed by the interaction of a large number of different factors or determinants. The epidemiologist believes that by studying these interactions it may become possible to manipulate some of the determinants involved, and so reduce the frequency with which the disease in question occurs m a population.

At this stage it is necessary to ascertain what is meant by the terms population and determinant.

A population can be defined as the complete collection of individuals that have some particular characteristic(s) in common. Depending on the characteristic(s) being considered, a population can be very large or very small. For example, one may wish to study a particular disease in a particular cattle population in a particular country. That cattle population could consist of:

All the cattle in the country
All the dairy cattle in the country
All the dairy cattle of a certain breed in the country etc.

Another term often used in epidemiological studies is population at risk. This is usually a subset of the original, defined population and comprises the total number of individuals in that original population that are considered capable of acquiring the particular disease or disease characteristic being studied.

For instance, we might be interested in studying the frequency with which abortion occurs in a population of dairy cattle of a certain breed in a certain country. The population at risk would not be all the individual animals of that particular dairy breed in that country, since this would include males, steers and immature females, all of which would not or could not be pregnant and therefore could not abort! It would consist of female cattle of that breed which were of breeding age. However, if the characteristic being studied was infection by one of the infectious agents that can cause abortion, such as Brucella abortus, the population at risk would have to include all calves, adult males, steers and immature females of the particular breed in question, since all these individuals could potentially become infected with this organism.

A determinant is any factor or variable that can affect the frequency with which a disease occurs in a population. Determinants can be broadly classified as being either intrinsic or extrinsic in nature. Intrinsic determinants are physical or physiological characteristics of the host or disease agent (or intermediate host or vector, if present) which are generally determined genetically. Extrinsic determinants are normally associated with some form of environmental influence on the host or disease agent (or intermediate host or vector, if present). They may also include interventions made by man into the disease process by the use of drugs, vaccines, dips, movement controls and quarantines. The role of determinants in the disease process is discussed in more detail later on in this chapter.

Since the determinants of disease are often varied, the epidemiologist may have to draw on a number of different scientific disciplines and techniques if he is to study them. The epidemiological approach is, therefore, a holistic one and the "art" of epidemiology lies in the ability of the epidemiologist to coordinate the use of such disciplines and techniques in a disease investigation, and to produce from the results generated a composite and comprehensive picture of how a particular disease maintains itself in nature.

If we accept the premise that the frequency with which a disease occurs in a population is governed by a large number of determinants, it would be expected that some of these, particularly the extrinsic ones, would vary in space and time. It follows, therefore, that disease is a dynamic process. The type and pattern of diseases in livestock differ from country to country, area to area, species to species and production system to production system. Furthermore, the range and importance of the disease problems encountered may change dramatically over time within the criteria mentioned. The effective control of disease depends as much on a thorough understanding of the many complex factors that govern the changes taking place in a disease process as it does on the provision of veterinary inputs such as drugs, vaccines and dips.

2.2 Intrinsic determinants of disease

2.2.1 Disease agents as determinants of disease
2.2.2 Host determinant

2.2.1 Disease agents as determinants of disease

Agents associated with disease can be categorized into two broad groups:

· "Living" agents, such as viruses, bacteria, rickettsia, protozoa, helminths, arthropods etc.

· "Non-living" agents, such as heat and cold, water, nutrients, toxic substances etc.

Since infectious diseases of livestock are generally regarded as being of prime importance in Africa, the following discussion is concerned principally with the determinants associated with the so-called living disease agents.

In instances of infectious disease, the presence or absence of the aetiological agent is the main determining factor in the epidemiology of the disease. Obviously, disease cannot occur in the absence of the agent, but, conversely, disease need not always result from the presence of the agent. This leads us to the important epidemiological distinction between infection and disease.

· Infection can be defined as the invasion of a living organism, the host, by another living organism, the agent.

· Disease can be defined as a derangement in the function of the whole body of the host or any of its parts.

Infectivity, virulence and pathogenicity

Whether infection takes place or not may depend on a whole range of determinants, both intrinsic and extrinsic, which affect the host and the agent (and the intermediate host or the vector, if present).

Infectivity is a measure of the ability of a disease agent to establish itself in the host. This term can be used qualitatively, when an agent is referred to as being of low, medium or high infectivity, or quantitatively. Attempts to quantify infectivity normally involve the use of a statistic known as ID50. This refers to the individual dose or numbers of the agent required to infect 50% of a specified population of susceptible animals under controlled environmental condition.

Having become infected, the host may or may not become diseased, and this is again determined by a range of intrinsic and extrinsic determinants affecting the agent and the host. Two terms - virulence and pathogenicity - are often used to describe the ability of the agent to cause disease.

Virulence can be defined as a measure of the severity of a disease caused by a specified agent. In its strict sense, virulence is a laboratory term and is used to measure the varying ability of disease agents to produce disease under controlled conditions. It is often quantified by a statistic known as LD50 which refers to the individual dose or numbers of the agent which will kill 50% of a specified population of susceptible animals under controlled environmental conditions.

Pathogenicity is an epidemiological term used to describe the ability of a particular disease agent of known virulence to produce disease in a range of hosts under a range of environmental conditions.

Host/agent relationships

The relationships between infection and disease are frequently dynamic in nature. They centre on the "balance" that can be achieved between the resistance mechanism of the host and the infectivity and virulence of the agent. Disease outbreaks caused by the introduction of an agent into a susceptible host population which has not been previously exposed to that agent normally result in a disease of high pathogenicity with commensurate severe losses in the host population. Such a process is actually detrimental to the agent's survival, since by killing off the host population it adversely affects both its ability to reproduce and its chances of gaining access to new susceptible hosts. An agent can therefore improve its chances of survival by increasing its infectivity and decreasing its pathogenicity, and some agents have a natural tendency to do this under certain circumstances.

Since a commensal or parasitic relationship confers no benefits to the hosts, they tend to develop means of resisting infection by disease agents. While the agents, in order to survive, develop methods of circumventing the hosts' defences. Disease agents normally have much shorter generation intervals and can multiply much more rapidly than their hosts, and therefore tend to evolve much quicker. This rapid evolution usually enables the agents to keep comfortably ahead of the hosts' defence mechanisms. There are many mechanisms by which infectious agents can avoid or overcome the defences of the host. The two mechanisms whose consequences are of particular importance in the field of livestock disease control are the carrier state and antigenic variation.

Creation of the carrier state. The term "carrier" is used to describe an individual that is infected by a disease agent and is capable of disseminating that disease agent but shows no sign of clinical disease. Three types of carrier state are recognised:

· The true carrier, which is an infected individual capable of disseminating the infectious agent but which never exhibits clinical signs of disease. True carriers occur in various diseases, including salmonellosis.

· The incubatory carrier, which is an infected individual capable of disseminating the infectious agent while the disease is still in the incubatory stage. In foot-and-mouth disease, for instance, infected animals are most infectious 12 to 24 hours before the clinical signs of the disease appear.

· The convalescent carrier, which is an individual that continues to disseminate the infectious agent after the clinical signs of the disease have disappeared. Convalescent carriers occur in such diseases as contagious bovine pleuropneumonia.

Antigenic variation. Some species of disease agent seek to evade the hosts' defence mechanisms by altering their antigenic characteristics. The most extreme case of antigenic variation occurs in trypanosomiasis, where infection in the host usually takes the form of a series of parasitaemias each one of which involves a form of trypanosome antigenically different from the preceding one. This type of antigenic variation occurs during the course of a single infection.

Another type of antigenic variation occurs in certain agents, such as the foot-and-mouth disease virus, that are highly infectious in nature and that depend for their survival on a continuous cycling through host populations of relatively long-lived animals. The ability to reinfect the same host at a later date is obviously desirable for the agent's survival, and this is dependent on the generation of a relatively short-lived immunity combined with the ability of the agent to undergo antigenic variation during its passage through the host population. In such circumstances there is a strong selection pressure for antigenic variants. The two main types of variation are:

· Antigenic drift, which involves only minor changes in antigenicity, so that hosts previously infected with the agent retain a certain degree of immunity to the drifted strain.

· Antigenic shift, which involves a major change in antigenicity, so that previously infected individuals possess little or no immunity to the shifted agent.

Antigenic shifts are of particular significance when the control of a disease is being attempted by vaccination, since in effect they represent the introduction of a new agent against which the existing vaccine is likely to confer little or no immunity.

The capacity of parasites to evolve rapidly has important implications in other areas of disease control. The very act of introducing a control measure or disease treatment may, in itself, create conditions whereby a strong pressure is exerted on the agent population to select strains which are resistant to the measures or treatments imposed. The evolution of such resistant strains will, in turn, jeopardise the effectiveness of the control measure or treatment. Resistant strains of agents are most likely to develop when the measures or treatments are carried out on a wide scale but improperly - as, for example, in the case of antibiotic resistance arising through the widespread, unsupervised use of antibiotics by livestock producers.

Other terms used to further define host/agent relationships include:

· Incubation period, which is the period of time that elapses from the infection of the host by the agent to the appearance of clinical symptoms.

· Prepatent period, which is the period between the infection of the host by the agent and the detection of the agent in the tissues or secretions of the host.

· Period of communicability, which is the period of time during which an infected host remains capable of transmitting the infective agent.

Methods of transmitting infectious agents

Ascertaining the means by which disease agents are transmitted is a major objective in epidemiological studies, since once the mechanisms by which a particular disease is transmitted are understood, it may become possible to introduce measures to prevent transmission from taking place.

There are three main ways by which disease agents are transmitted from infected to susceptible hosts. An agent may be transmitted through contact between infected and susceptible individuals, or it may be conveyed between these individuals by means of an inanimate object or via another animal serving as a vector or intermediate host. These methods of transmission are not mutually exclusive; the same disease agent may be transmitted by more than one of the following ways.

Contact transmission. In contact transmissions the agent is conveyed between hosts through direct physical contact, as in the case of venereally transmitted diseases such as vibriosis or trichomoniasis, or through indirect contact.

In cases of indirect contact the agent is normally contained in the excretions, secretions or exhalations of the infected host i.e. in the faeces, urine, milk, saliva, placenta and placental fluids, or as aerosols or droplets in the breath. Susceptible hosts contract the infection either by direct exposure to these or through exposure to substances contaminated by them. Diseases spread in this fashion include rinderpest, foot-and-mouth disease, Newcastle disease, and contagious bovine pleuropneumonia.

Contact transmissions can be further distinguished according to whether they occur horizontally between individuals of the same generation or vertically between individuals of different generations. In vertical transmissions the infectious agent is usually passed from dam to offspring either in the uterus or through the colostrum.

The main factors determining whether or not transmission takes place in contact-transmitted diseases are:

- The ability of the agent to survive in the environment. Rinderpest virus, for example, is easily destroyed in the environment, so contact between infected and susceptible individuals must be close and immediate for transmission to take place, whereas, under certain circumstances, foot-and-mouth disease can spread between widely separated stock.

- The extent of the contact that occurs between infected and susceptible individuals of the host populations and their mobility within these populations. The control of livestock movements is, therefore, a vital factor in the control of contact-transmitted diseases which, in Africa, normally occur more frequently during the dry season when livestock movements are at their highest.

Vehicular transmission. In vehicular transmission the agent is transferred between infected and susceptible hosts by means of an inanimate substance or object (sometimes called fomite), such as water, foodstuffs, bedding materials, veterinary equipment and pharmaceuticals, or on the skin, hair or mouthparts of animals. In contrast to indirect transmission, the survival time of the agent in or on the vehicle is usually prolonged. This means, in effect, that vehicular transmission can take place over greater distances and over longer time periods. Hygiene, disinfection and control over the distribution of likely vehicles of transmission are important factors in the control of vehically transmitted diseases.

Certain agents may take the opportunity to reproduce themselves during vehicular transmission. This occurs in the transmission of food-borne bacteria, such as salmonella and coliforms, and underlines the importance of strict hygiene in the handling of foodstuffs and livestock feeds, since a small initial contamination may eventually result in the gross contamination of a whole batch of food or feed.

Vectors and intermediate hosts. Confusion frequently arises between the terms "vector", "intermediate host" and "definitive host". The latter two terms are essentially parasitological terms and describe the different types of hosts that are biologically necessary in the lives of agents with relatively complicated life cycles.

· A definitive host is a host in which the agent undergoes a sexual phase of its development.

· An intermediate host is a host in which the agent undergoes an asexual phase of its development.

The definitive host is usually a vertebrate, while intermediate hosts can be either vertebrates or invertebrates.

· A vector is an invertebrate animal that actively transmits an infectious agent between infected and susceptible vertebrates.

Essentially, vectors can transmit infectious agents in two ways. They can serve as a vehicle whereby the infectious agent is conveyed from one host to another without undergoing a stage of development or multiplication. This is known as mechanical transmission. Alternatively, the infectious agent can undergo some stage of development or multiplication in the vector - this is known as biological transmission - and in this case the vector is serving either as an intermediate or definitive host, depending on which stage of the development cycle of the agent takes place within it. Vertebrate intermediate hosts play the same role in the transmission of their disease agents as biological vectors.

In mechanical transmission the agent is carried on the skin or mouthparts of the vector from an infected to a susceptible host. The survival time of the agent in or on the vector is usually short, and as a result the transmission of the agent has to be accomplished rapidly. The carriers are normally winged haematophagous insects, and transmission usually takes place when susceptible and infected hosts are in close proximity and when large numbers of vectors are present.

In biological transmission, since the agent develops in the vector, a period of time elapses between the acquisition of the infectious agent by the vector and its becoming infective. Once it has become infective, the vector may remain so, normally for a considerable period if not the rest of its life. This provides more than a single opportunity for disease transmission.

In addition, vectors may be able to pass the agent on to their own offspring transovarially. Transovarial transmission enables an infectious agent to be maintained in a vector population through many generations without that population having to be reinfected, and, as such, the vector population remains a continuous source of risk. If transovarial transmission does not occur, at least one stage in each generation of the vector must become infected before transmission of the agent can take place.

Arthropod vectors that undergo metamorphosis have the capacity to pass an agent from one developmental stage to the next. This is known as transtadial transmission. Usually in transtadial transmission, one developmental stage becomes infected with the disease agent and the following stage transmits it. If different developmental stages feed on different host species, transtadial transmission can provide a mechanism for an inter-species transmission of disease agents.

2.2.2 Host determinants

The main intrinsic determinants in the host which can influence the frequency of occurrence of infection and disease are species, breed, age and sex.

Species susceptibilities and natural reservoirs

Most disease agents are capable of infecting a range of animal species, both vertebrate and invertebrate. The severity of the disease resulting from such infections may, however, vary between the species concerned. While certain host species may be refractory to infection with certain disease agents, e.g. equines to the foot-and-mouth disease virus, very few disease agents are in fact restricted to one host species.

The multi-species susceptibility to disease agents is particularly important if the species concerned are able to maintain the disease agent within their populations i.e. to function as natural reservoirs of infection. The failure of programmes aimed at controlling a certain disease in one species has often been blamed on the presence of natural reservoir species, because they can reintroduce the infectious agent.

When investigating the potential of a certain species to act as a natural reservoir of a particular disease agent, and the implications this would have on disease control policy, the following considerations need to be borne in mind:

Infection with the disease agent. Although it may be possible to infect a certain host species with a disease agent under laboratory conditions, this may only be achievable by using a method of transmission that does not occur naturally (e.g. intracerebral inoculation). If this is the case, that particular host species is unlikely to play a significant role in the epidemiology of the disease.

Ability of a host species to maintain a disease agent. It may prove possible to demonstrate that a particular host species can be infected by a certain disease agent and that that infection can be accomplished by a natural means of transmission. A further question then needs to be asked, namely, is that species capable of maintaining the agent within its populations for significant periods of time? If this is not the case, then although that particular species may be involved in the localised spread of the disease agent during an outbreak, it will not serve as a continuous source of infection. As such, the importance of that species in the overall epidemiology of the disease may be reduced, and it may become possible to contemplate a disease control programme in which control measures do not have to be applied to that particular host species. In rinderpest control, for example, it has proved possible to control and perhaps even eradicate the disease by concentrating control measures solely on cattle populations, in spite of the presence of species of wild game which are also susceptible to the disease.

Transmission from the natural reservoir. Even if a species can function as a natural reservoir for a particular disease agent, transmission from that reservoir to domestic livestock may only occur rarely and in certain, clearly defined circumstances. If this is the case, the reservoir species is unlikely to cause a major problem in the initial control of the disease in question. However, when the frequency of occurrence of the disease has been reduced to a low level, and eradication of the disease becomes a possibility, the implications of the presence of reservoir host species for the success of the proposed eradication programme may have to be re-assessed.

Breed susceptibilities

Within a host species, wide ranges of susceptibility to a particular disease are often observed between different breeds. In Africa, for example, certain breeds of cattle, horses, sheep and goats are more tolerant of trypanosomiasis than others. Bos taurus breeds of cattle are generally more susceptible to ticks and tick-borne diseases than Bos indicus. It is important, however, to distinguish between the differences in susceptibility that are genuinely related to breed or species and the differences that may arise as a result of previous exposure to infection.

Within breeds too, differences in susceptibility to the same disease agent have been noted between strains or families. This has led, in recent years, to the development of breeding programmes designed to select for disease resistance. Selective breeding has been pioneered in the poultry industry where a large number of different "lines" of poultry have been developed that are resistant to such diseases as Marek's disease, salmonellosis, and even vitamin D and manganese deficiencies. Pigs, too, can be selected for their resistance to atrophic rhinitis and some forms of colibacillosis. There are breeding programmes in Australia selecting for tick resistance in cattle, and in Great Britain there is increasing evidence that a similar approach could be adopted for the control of certain forms of mastitis and metabolic disorders in high-yielding dairy cattle. In Africa, trypanotolerant breeds of livestock are receiving increasing attention as a possible solution to the trypanosomiasis problem m certain areas.

Breeding for disease resistance is probably most applicable as a disease control option in instances where particular disease agents are ubiquitous in the environment, or of non-infectious diseases caused by multi-causal determinants, or where other methods of control have proved unsatisfactory.

Differences in species or breed susceptibility to disease must be taken into account when introducing new breeds or species into new environments. The new breed or species may be exposed to disease agents to which the local breeds or species are resistant but to which the new breed or species is highly susceptible. Conversely, the imported breed or species may itself introduce a new disease agent to which it is resistant but to which local breeds or species are susceptible. This factor has become the cause for much concern in recent years given the rapid development of international transport facilities whereby livestock and their products can easily be conveyed from one part of the world to another. Furthermore, because of improvements in the disease investigation and diagnostic facilities of many veterinary services, disease agents are being identified that cause little or no disease in indigenous livestock populations but which have the potential to cause a severe problem in the more susceptible livestock populations of other countries should these agents be imported. Bluetongue is an example of a disease which has attained prominence in this way.

Age susceptibilities

Differences in susceptibility to disease are often seen between different age groups. For example, young animals are generally less susceptible to tick-borne diseases than older animals. There is, however, often a problem in distinguishing between true age resistance in young animals and passive resistance occasioned by the transfer of maternal antibodies via the placenta or in the colostrum. A false impression of age susceptibility may also be created when a highly infectious disease occurs frequently in a population. It may, for instance, appear that only young individuals are affected by the disease in question. This may not be due to a difference in age susceptibility but simply because the older individuals, who had been infected previously, represent a surviving and immune population.

Sex associations in disease

In these associations the clinical signs of disease are associated with sexual attributes, as in the case of diseases of the reproductive tract, rather than with the fact that males may be more susceptible than females or vice versa. Sometimes, too, one particular sex may be regarded by farmers as being of greater value than the other and will therefore receive a correspondingly greater amount of care and attention when sick.

2.3 Extrinsic determinants of disease

2.3.1 Climate
2.3.2 Soils
2.3.3 Man

Extrinsic determinants of disease are important in epidemiology in that they can have effects on the host, on the agent, and on the interactions between the host and the agent. They can also affect any intermediate hosts or vectors involved in the transmission of a disease, and thus determine the type and extent of the disease transmission taking place.

There are three major extrinsic determinants. The first two are climate and soils, which, by interacting in a variety of ways, affect the environment of the host, the agent, and the intermediate host or vector, if they are present. The third major factor is man, who, uniquely among animals, has the ability to modify both the environment in which he lives and the environment in which he keeps his livestock.

2.3.1 Climate

When considering climate as a determinant of disease, a distinction is normally made between the macroclimate or weather, and the microclimate. The term microclimate refers to the actual climatic conditions prevailing in the specific, restricted environment where the host, agent, vector or intermediate host actually live. While man is as yet largely incapable of deliberately manipulating macroclimates, he can control and manipulate microclimates to some extent.

Macroclimates. A large number of different factors combine to make up the microclimate. Some of these factors (heat, cold, rainfall, wind, humidity etc) can act as disease agents in their own right, either individually or in combinations. As such they can cause disease in young and newborn animals which are particularly sensitive to heat, cold and dehydration. In older animals they tend to act more as indirect determinants of disease in that they can produce either alone or in combinations with other managemental and nutritional determinants - "stress" conditions in the host, which may lower its resistance both to infection and, if infection takes place, to disease.

Macroclimates can also affect the ability of a disease agent, or its intermediate host or vector, to survive in the environment. If the effects of weather on disease agents and their intermediate hosts or vectors are known, it may be possible to predict when host populations are at a particular risk of contracting disease and thereby to implement appropriate control measures at strategic times. This approach has been used with success in the control of such diseases as helminthiasis, ticks and tick-borne diseases, trypanosomiasis, foot-and-mouth disease, and in mineral and other nutritional deficiencies.

Microclimates. While macroclimates can have a direct effect on microclimates, the study of macroclimates alone can frequently be misleading in achieving an understanding of the epidemiology of a disease. Regions where existing macroclimatic conditions might be thought unsuitable for the transmission of a disease may, in fact, contain limited areas where the microclimatic conditions are suitable for the survival of the disease agent and its vector or intermediate host. (An example may be a water hole or an irrigated pasture in an arid environment). Such areas often provide enhanced conditions for disease transmission, since they may prove attractive to livestock, particularly at those times of the year when the macroclimate is at its most severe. If the host and the agent (and the vector or intermediate host, if they exist) are in close contact, the transmission of disease can be effected rapidly and easily. Thus, in arid areas, the transmission of such diseases as helminthiasis and trypanosomiasis may in fact take place during the dry season when the hosts, the agent and the vector are all concentrated around permanent water sources. High contact rates in these areas also favour the introduction and transmission of rinderpest, foot-and-mouth disease and contagious bovine pleuropneumonia.

2.3.2 Soils

By interacting with climate, soils determine vegetation and the environment in which the livestock are kept. The main effect of vegetation is on nutrition. Soils therefore act indirectly as determinants of disease by causing starvation, if there is little or no vegetation, or nutritiorial imbalances such as protein, energy, vitamin or mineral deficiencies. Malnutrition can be the direct cause of disease, or it can stress the host and thus increase its susceptibility to infection and disease from other sources. Soils can also have an effect on the ability of the agent to survive in the environment, through such factors as waterlogging, pH etc.

2.3.3 Man

Man is often able to create favourable, artificial microclimates for livestock rearing by providing such inputs as housing, water supplies, irrigation etc. Unfortunately, this often results in the creation of conditions favourable for the survival of disease agents and their intermediate hosts or vectors. This means that, by altering the environment, man can alter the determinants of the diseases present in that environment. The changes in determinants will favour some diseases and be detrimental to others. Thus changes in systems and methods of production will result in changes in the relative importance of the diseases present, with perhaps some new diseases being introduced and others disappearing. The epidemiologist should be alert to such changes and should attempt to predict the likely effect that these will have on the overall disease picture, so that potentially dangerous situations can be averted or controlled.

Man is also able to interfere directly in the disease process through the use of drugs, vaccines, movement controls, quarantines etc. Among the main tasks of the epidemiologist is the investigation of the efficacy such measures, as well as to design ways in which they can be used most efficiently and to monitor the effects of their introduction on disease incidence.

2.4 Describing disease events in populations

The first priority in investigating the epidemiology of a disease is to describe accurately the nature of the problem being investigated. Comprehensive and accurate description of disease problems often provides valuable insights into the epidemiology of the disease being investigated and allows hypotheses about likely determinants to be formulated.

A description of a disease problem should specify the disease and the population at risk, give information on the distribution of events in time and space, and include an attempt to quantify disease events.

Disease diagnosis. If the disease is infectious in nature, the disease agent involved should also be identified. For the disease agent to be infectious it must fulfil Koch's postulates that:

- The agent should be present in all cases of the disease;
- It can be isolated and grown in pure culture; and
- It should be capable of producing the disease when innoculated into healthy animals.

One of the problems associated with these postulates is that they do not take into account the differences between different strains of agents, particularly in their virulence, pathogenicity, and infectivity, which may be important in the epidemiology of the disease. We shall have more to say on the problems of disease diagnosis in Chapter 4.

Populations at risk. These can be identified by studying the distribution of the disease within host populations by species, breed, age and sex. Descriptions of population densities and movements are also of great value, particularly when the disease is transmitted by contact.

Distribution of disease events in time and space. This generally involves looking for the "clustering" of disease events in time, space or both.

The clustering of disease events in space can often be demonstrated by the use of conventional mapping techniques. This type of clustering may indicate the presence of a particular determinant or determinants (e.g. a vector, a mineral deficiency etc) in an area. It should be remembered, however, that clustering in space occurs naturally in the case of contact - transmitted diseases, and that it may also be a function of host-population density.

The clustering of disease events in time may indicate that the host population was exposed to a common source of the disease or its determinant. Outbreaks of diseases transmitted by such vehicles as water or foodstuffs frequently exhibit clustering in time, as in the case of food poisonings. Seasonal clustering of disease events often indicates the influence of climatic determinants in some form or other.

The distribution of disease events in populations in time and space can be described by three basic descriptive terms. These are: endemic, epidemic and sporadic.

An endemic disease is a disease that occurs in a population with predictable regularity and with only minor deviations from its expected frequency of occurrence. In endemic diseases, disease events are clustered in space but not in time. Note that a disease may be endemic in a population at any frequency level, provided that it occurs with predictable regularity. Additional terms can be used to describe endemic diseases according to their frequency of occurrence. Thus:

· Hyperendemic is an endemic disease that affects a high proportion of the population at risk.

· Mesoendemic is an endemic disease that affects a moderate proportion of the population at risk.

· Hypoendemic is an endemic disease that affects a small proportion of the population at risk.

An epidemic disease is a disease that occurs in a population in excess of its normally expected frequency of occurrence. In an epidemic disease, disease events are clustered in time and space. Note that a disease may be epidemic even at a low frequency of occurrence, provided that it occurs in excess of its expected frequency.

A pandemic is a large epidemic affecting several countries or even one or more continents.

A sporadic disease is a disease that is normally absent from a population but which can occur in that population, although rarely and without predictable regularity.

Many epidemics of infectious disease occur in a regular, cyclical fashion over a prolonged period of time. This is because with an increasing frequency of occurrence of the disease in a host population, the number of susceptible hosts decreases as individuals within that population become infected, and then either die or recover and become immune to reinfection. As the number of susceptible hosts decreases, so does the opportunity for disease transmission. This, in turn, means that the frequency of occurrence of new cases of the disease declines. A period of time then elapses during which new susceptible individuals are born into the host population. The number of susceptible hosts in the population thus increases, and the opportunities for the disease agent to find a susceptible host are enhanced. As a result the frequency of occurrence of the disease may increase and a new epidemic may take place.

When assessing the efficacy of measures introduced to control epidemics, an attempt should be made to distinguish between a decline in the frequency of occurrence of the disease due to a control measure, and a natural decline in the epidemic cycle. Epidemics can be prevented if the level of immunity in the host population can be sustained. It is important, therefore, in instances where the control of an infectious disease is being attempted by vaccination, that coverage be maintained in the. host population even when the disease is occurring rarely.

Quantification of disease events. Any description of a disease problem should include an attempt at quantification. The methods by which disease events in populations are quantified are described in Chapter 3.

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