William B. Thomas, James M. Fingeroth, and Ragnvi Hagman
Epidemiology is concerned with the frequency of disease, the course of disease, and the predilection of disease in relation to factors such as species, breed, and sex [1]. Epidemiologic data are useful for clinicians because awareness of disease frequency among different patient characteristics aids diagnosis. Knowledge of the course and severity of disease helps the veterinarian recommend treatment. Finally, identifying risk factors for a disease provides important clues to the underlying cause. This chapter reviews basic epidemiology and summarizes the current data regarding disc disease in chondrodystrophoid breed dogs.
Definitions
In epidemiology, the unit of study is a patient affected with a disease. Hence, the diagnosis of the particular disease is fundamental to all epidemiologic studies. Ideally, the criteria for diagnosis should be specified because this will affect the estimation for how common the disease is. For example, if the diagnosis of disc disease is based on clinical features of back pain, the estimate of disease frequency will be higher than if the diagnosis is based on imaging to confirm the diagnosis since not all dogs with mild clinical signs undergo further diagnostic testing.
The most basic question is, how common is the disease? The answer involves a count of the cases of the disease within a given place and time. It is common to relate the cases to the rest of the population, so that the number of cases of the disease is the numerator and all animals in the population is the denominator. If all animals in the population were examined, all the patients with the disease and without the disease could be discovered. But such a population survey is usually impractical. Instead, most veterinary studies involve patients who have come to medical attention. Much of the published information regarding disc disease arises from case series that involve cases collected from individual hospitals or databases that collect information from participating hospitals. One problem with such data is that there is little assurance that the cases or all patients in the hospital or database are representative of all animals. For example, not every animal in the population is presented for veterinary care. And the patients seen at a referral practice are not necessarily a reflection of patients seen at general practices.
Prevalence
The two basic measures of disease frequency are incidence and prevalence. Prevalence is the proportion of animals that have the disease during a specified period of time. It measures the frequency of existing disease. Prevalence is affected by several factors. If the disease is severe and many animals die early in the course of disease, the prevalence is depressed. If the duration of illness is long, the prevalence increases. Since prevalence rates are influenced by so many factors unrelated to disease causation, they do not provide strong evidence of causality [2].
Incidence
Incidence is the occurrence of new cases of disease that develop in the population over a specified time period. It measures the frequency of new diseases.
