Preclinical and clinical manifestations of paratuberculosis (including pathology).

The term Johne's disease immediately brings to mind an image of an emaciated, debilitated ruminant with bottlejaw and fluid, pipestream diarrhea. However, the clinical case 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 less 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 that may be detected by antigen detection tests and antibody-based diagnostic tests. The duration of each stage of infection depends on age at the time of exposure and the dose of organisms ingested. The lack of tests sensitive enough to detect infected animals in the earliest stages of disease is a serious obstacle to eradication of Johne's disease. The animals with overt infection may be culled from the herd immediately on detection. However, the animals that are shedding organisms below the threshold of detectability and have not mounted an immune response remain behind to contaminate the environment and infect herdmates.