Correspondence

Abstract

An unfortunate error occurred during the editorial process for the paper by R.N. Chesterton et al. entitled Environmental and behavioural factors affecting the prevalence of foot lameness in New Zealand dairy herds — a case-control study ⁽¹⁾ in the last issue of the New Zealand Veterinary Journal. Throughout the paper the term “incidence” was used in our manuscript, but this was changed to “prevalence” in the published text. As an epidemiologist, I take particular care with the use of these terms, which have precise epidemiological meanings. Most veterinarians who graduated before epidemiology became an important discipline in veterinary schools make the error of using “incidence” in all cases where they wish to comment on the occurrence of disease. In order to accurately describe disease patterns in epidemiological terms, it is necessary to distinguish carefully between new cases of disease and the total number of animals affected by disease at a particular time or over a period of time. In acute diseases of short duration the difference becomes minor, but it is of great importance for diseases which may last some time, since the prevalence will then be much higher than the incidence. Incidence is the number of new cases of disease per unit time, whereas prevalence is the number of cases in total — both new and chronic cases. We distinguish point prevalence which is the number of affected animals at a single point in time (determined by a survey or similar method) from period prevalence, which is a count of the total number of animals affected over an extended period of time (measured in the course of routine testing in a disease control programme, for example).