The term Johne's disease immediately brings to mind an image of an emaciated, debilitated ruminant with bottlejaw and fluid, pipestream diarrhea. The clinical case, however, is merely the tip of the iceberg in terms of the total number of infected animals on the farm. If the clinically affected animal was born on the farm, a minimum of 25 other animals are probably infected and fewer than 30% of those will be detectable by currently available tests. Infected animals in the early stages of the disease progress slowly over a period of months or several years to stages where they may be diagnosed by antigen detection tests and antibody-based diagnostic tests. The duration of each stage of infection seems dependent upon age at the time of exposure and the dose of organisms ingested.
