First described more than 90 years ago in Germany, Johne's disease is of particular concern to farmers breeding and exporting pedigree cattle. Countries free of Johne's disease almost invariably require cattle to come from Johne's-free herds and to have all cattle pass one or more blood tests. Currently, few U.S. herds have the favored Johne's-free status. It is economically important because some animals may be so emaciated that they are condemned at slaughter. Origin of the infection The organism lives in the intestinal cells and lymph nodes, where it causes progressive thickening of the bowel wall of the lower small intestine and upper large intestine.
Infection sometimes extends to the lower large bowel and rectum.
The organism is passed out in feces and easily survives in manure and stagnant pond water for a year or more. New infections usually occur when young calves are exposed to the contaminated environment and ingest the organism by licking or sucking the dam's udder or vulval area. Infections also occur when young calves come in contact with dirty infected bedding in calving areas. Venereal transmission by artificial insemination or natural service has never been demonstrated. Although the organism has been cultured from the milk of clinical cases, it is not yet known whether such milk can act as a source of Johne's infection for calves.