The skin test frequently becomes negative as immune reactivity declines in more advanced stages. Antibody production does not occur until the intermediate stages of the disease and often disappears in later stages. By the time the quick serologic tests become positive, animals are usually shedding enough organisms to be detected by fecal culture. Both tests may be useful in supplementing information from other diagnostic tests. The IV Johnin test and staining of fecal smears or rectal scrapings for clumps of intracellular acid-fast bacteria can be helpful in the diagnosis of clinical cases.
Smears require careful and thorough examination, and interpretation can be difficult.
In advanced clinical cases, rectal biopsies may be diagnostic. Control Control and eradication of Johne's disease depends upon reducing contamination of the environment and eliminating exposure to the organism, especially for young calves. This requires identification and culling of infected shedders and necessary changes in management practices for youngstock. Fecal culture at 6-month intervals is used to identify infected shedders so that they can be removed as soon as possible. Any animals showing diarrhea and weight loss should be culled immediately or tested and isolated pending test results.