Disease and Epizootiology — Basic Principles

Abstract

The concept of disease is surprisingly difficult to define in terms that are sufficiently broad for application to the wide range of conditions that occur in free-ranging wild animals, and that are still sufficiently narrow to separate disease from other factors, such as predation, that effect wildlife negatively. Disease might be defined as any departure from health, but this leads to a circular discussion of the meaning of health and normality. Disease in wild animals is often considered only in terms of death or obvious physical disability, probably because these are readily identified parameters. However, the effect of disease on wild populations may be much greater than is evident by simply counting the dead or maimed, even if it were possible to do so accurately. The impact of DDT and certain other chlorinated hydrocarbon insecticides on some raptorial and piscivorous birds provides an excellent illustration of this fact. These compounds have low direct toxicity and rarely resulted in the death of birds or in obvious clinical signs of intoxication, yet they had profound population effects through decreased recruitment as a result of increased egg breakage.

“Up to the present time it has been customary to believe that wild animals possess a high standard of health, which is rigidly maintained by the action of natural selection, and which serves as the general, though unattainable, ideal of bodily health for a highly diseased human civilization. This belief is partly true and partly false”. (Elton, 1931)