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2.3 Prevalence and Incidence

Samuel Stewart, DVM, DACVECC and Chand Khanna, DVM, PhD, DACVIM (Onc), DACVP (Hon)

Ethos Veterinary Health, Woburn, MA, USA

BASICS

2.3.1 Summary

Prevalence and incidence are both terms used to describe the occurrence of a disease over a period of time. Prevalence is defined by the number of individuals alive with a disease, while incidence is defined by the number of newly diagnosed cases of a disease divided by the population at risk of developing that disease. Incidence is often not able to be described in veterinary medicine as the population at risk of developing a given disease process is generally not known and would require a pet census. Additionally, it assumes that there is a steady rate of disease occurrence over a specified period of time, which is not always true of disease states. In veterinary studies prevalence is more commonly used as it does not require definition of the at‐risk patient population.

2.3.2 Terms Defined

Epidemiology: The study of how diseases occur in given patient populations and why.

Incidence: The rate of newly diagnosed cases of a given disease process in a specified period of time.

Prevalence: The number of cases of a disease process that occur in a specified period of time.

MAIN CONCEPTS

2.3.3 Introduction

Epidemiology is the study and analysis of diseases in a given population of people or animals. It focuses on factors of disease, such as cause, risk factors, frequency, and distribution/pattern. The purpose for the study of these factors is the goal of achieving control of the disease process (i.e., means to reduce its occurrence) or to define the size of a market before developing and commercializing an innovation for disease.

Prevalence and incidence are terms commonly used when describing the study of epidemiology. Prevalence is the actual number of cases that are alive and affected by a specific disease in a given period of time. Incidence is the calculated risk of an individual acquiring a disease in a given period of time. Both variables can be defined as disease occurrence within a certain amount of time (rate) or within a specified population (proportion).

Prevalence and incidence are expressed as fractions, which allows for the estimation between the probability of disease and the various factors that can influence that probability. The numerator of the fraction represents the number of patients that have the disease (for prevalence) or the number of newly diagnosed cases of disease (for incidence). The denominator represents the size of the population from which the individuals were drawn (for prevalence) or the size of the at‐risk group of individuals (for incidence). The denominator can also be expressed in relation to time (e.g., cases of disease/population time).

2.3.4 Key Differences

Prevalence can be expressed over a period of time, referred to as period prevalence, and can also be expressed at a specific point in time, known as point prevalence. Point prevalence does not provide as good a measurement of disease as period prevalence as it is only accounting for cases alive with that disease at a given point in time and does not look at new cases and deaths over a defined time period.

Incidence refers to the number of newly diagnosed cases of a given disease. It is expressed as a fraction of the number of new cases of disease divided by the population at risk of developing the disease (e.g., 100 cases per 100 000 people = 0.001). In essence, incidence allows for determination of an individual's probability of being diagnosed with a disease during a given period of time.

There are two ways in which incidence can be expressed: incidence risk and incidence rate. Incidence risk (also known as cumulative incidence) refers to number of at‐risk individuals in a given population that become new cases of disease during the defined follow‐up period (e.g., 30 cases per 1000 individuals per three years). There are multiple ways in which the follow‐up period can be defined, such as a fixed duration (e.g., three years), the lifetime of the individual (e.g., lifetime incidence risk of kidney disease), or based on the duration of a given epidemic. Incidence rate (also known as incidence density) refers to the number of new cases of a disease per person‐time in the at‐risk population. This approach requires dividing the number of new cases by the number of years being evaluated. Using the example above, an incidence risk of 30 cases per 1000 individuals per three years would equal an incidence rate of 10 cases per 1000 individual years.

The natural progression of a disease greatly defines the relationship between incidence and prevalence. Diseases that have a high rate of spontaneous resolution will result in high incidence of disease but possibly low prevalence as the disease will be resolved in many of the patients depending on the period of time being evaluated. Likewise, in diseases that have low cure rates but have treatments that provide prolonged survival rates, the incidence will contribute to a continuous growth of the prevalence.

2.3.5 Why is Incidence So Much Harder to Determine?

Incidence is defined by the population at risk of developing a disease. In human medicine there is often stringent monitoring and reporting of disease, so the at‐risk population is able to be defined. Therefore, the value of incidence data is directly related to the accuracy of disease diagnosis and its subsequent reporting. While there are reporting requirements for certain disease processes in veterinary medicine that have a potentially high impact on public health, many of these cases can be misdiagnosed or fail to be reported. Furthermore, diseases outside the reportable conditions do not have any requirement for monitoring and the at‐risk patient population is even more difficult to define.

The incidence of a disease is a variable that is constantly in flux. It is affected by numerous factors, such as changes in the environment of the investigated population and improvements in preventive measures to prevent the occurrence of disease. Defining incidence as a rate, rather than pure incidence, allows for this context to be taken into consideration. This approach, however, also makes the assumption that disease occurrence is evenly distributed over that period of time, which is not necessarily an accurate reflection of the true disease occurrence.

It is also not always appropriate to report the incidence of untreated cases, as the actual determination of this rate is often difficult to achieve. This would require the differentiation of new and old cases of disease and whether an individual is from the at‐risk population. It is generally more appropriate to report the number of new cases that are treated as this is a known number.

2.3.6 When/Why Do We Really Need Incidence?

In general, when studying disease, it is ideal to investigate incidence rate as it is not affected by extraneous factors related to the disease, such as the type of care a patient receives or the disease's fatality rate. This removal of influence makes it easier to compare different patient populations and be able to define differences. Incidence is also more valuable when evaluating diseases that occur over short periods of time. Prevalence does not account for the number of survivors and deaths, and therefore can underestimate the severity of the issue.

2.3.7 Always Prevalence in Veterinary Medicine

Since the calculation of prevalence does not require determination of the at‐risk population, it is more commonly used in veterinary medicine. It does allow for more variability in outcomes due to extraneous factors, which can complicate the interpretation of the prevalence data collected. Prevalence, however, is more useful in determining the weight of chronic diseases, whereas incidence is solely focused on new cases of disease.

EXAMPLES

Hemangiosarcoma is an aggressive form of cancer that is most commonly encountered in German shepherd dogs, Labrador retrievers, and golden retrievers. There are an estimated 75 000 cases of hemangiosarcoma diagnosed in dogs in the US per year. There are around 78 million dogs in the US, with the breeds most commonly affected by hemangiosarcoma representing around 20% of these (15.6 million). The incidence rate would be defined as the number of new cases per year (75 000) divided by the at‐risk population per unit time (15.6 million/year), which would give an incidence rate of 0.005. Most dogs with hemangiosarcoma succumb to their disease within 6–12 months of being diagnosed, depending on the treatment that is pursued. Looking at a one‐year period, you can make the assumption that around one quarter of hemangiosarcoma cases from the previous year will still be alive (18 750). Add this to the number of new cases that will be diagnosed that year (75 000 + 18 750 = 93 750). If this was to be defined based on the total population, the prevalence would be 0.001.

TAKE‐AWAYS

 Epidemiology is the study of disease in a population for the purposes of disease control as well as to define the market for the development and commercialization of diagnostics and therapeutics for that disease process.

 Prevalence is defined by the number of individuals alive with a given disease, while incidence is the number of newly diagnosed cases of disease divided by the population at risk of developing that disease.

 Incidence is difficult to accurately determine as it overlooks the potential for unreported disease events and misdiagnosed disease, and assumes that new cases of disease occur at a steady rate over a given period of time.

 Prevalence is more commonly utilized in veterinary medicine as it does not require defining the population that is at risk of developing that disease.

 Prevalence is preferred for the evaluation of chronic disease conditions as it does not account for survivors and deaths, which are important for the evaluation of shorter term impacts of disease.

MISCELLANEOUS

Recommended Reading

1 Noah, D. L., & Ostrowski, S. R. Basic Principles of Epidemiology. www.merckvetmanual.com/public‐health/public‐health‐primer/basic‐principles‐of‐epidemiology

2 Perez, A.M. (2015). Past, present, and future of veterinary epidemiology and economics: one health, many challenges, no silver bullets. Frontiers in Veterinary Science 2: 60.

3 Stevenson, M. (2008). An Introduction to Veterinary Epidemiology. www.massey.ac.nz/massey/fms/Colleges/College%20of%20Sciences/Epicenter/docs/ASVCS/Stevenson_intro_epidemiology‐web_2008.pdf

4 Thrusfield, M. (2013). Veterinary Epidemiology. Weinheim, Germany: Wiley.