How to Investigate a Disease Outbreak

1. Introduction

While individual sick or injured horses can be challenging cases, problems affecting a large number of animals at the same time can be unsettling and even more difficult to solve. Typically, it is cases of respiratory disease, diarrhea, or abortion that make a practitioner wonder if they might be seeing the beginning of a disease outbreak. It is possible, however, to encounter cases of neurologic disease, sudden death, even colic or lameness that may be part of a larger problem affecting a number of animals. The basic premise of epidemiology, and specifically outbreak investigation, is that disease does not occur randomly. In general, a group of cases might be considered an outbreak whenever a larger proportion of the animals at risk are affected than one would normally expect. The number of affected animals required to be classified as an outbreak varies according to the characteristics of the disease and population affected. Also, there may be a pattern to the disease occurrence that is unusual and therefore warrants investigation. Disease patterns may be temporal, spatial, or related to certain characteristics of the animals involved. It is understanding these disease patterns and relating them to the patterns of potential risk factors that allows identification of measures to prevent new cases of disease and future outbreaks. Different combinations of infectious or toxic agents, individual and herd immunity, population age and movement, nutrition and environmental factors may contribute to an outbreak of disease. Sometimes it is possible to identify the cause or causes of an outbreak simply using keen observation and intuition gained through experience. Unfortunately, this approach can be biased easily by an individual’s interests, expertise, or past experiences, as well as the emotions of the practitioner or the client. This presentation outlines a systematic, unbiased approach to identify the contributing cause(s) or source of an outbreak with the ultimate goal of recommending control measures to stop the current outbreak and to prevent future outbreaks.