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    Paratuberculosis (Johne’s disease)

    Guernsey cow with clinical signs of Johne’s disease




    Standard culture method
    The standard method for diagnosis of Johne’s disease is isolation of M. paratuberculosis on standard bacteriological media. Shown here is one type of such media, Herrold’s egg yolk agar. Growth of slow-growing, small, white-yellow colonies only on media containing mycobactin (two left tubes) is indicative of M. paratuberculosis. This test is usually done on fecal samples and typically takes 12 to 16 weeks.


    Giant cell
    Giant cells result from fusion of several macrophages. Such cells are a hallmark of granulomatous inflammation and typical of the host response to M. paratuberculosis infection. The specific type giant cells most often seen in Johne’s disease are called Langhan’s giant cells.


    Scanning electron micrograph of M. paratuberculosis
    Scanning electron microscopes allow us to see the surface of extraordinarily small things like M. paratuberculosis bacteria. Notice how this bacterium grows in clumps and has a rough, waxy cell wall. The cell wall is responsible for the bacterium’s resistance to antibiotics, chemical disinfectants, and environmental factors like heat and cold.

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    AGID blood test for Johne’s disease

    A simple two-day test to confirm a diagnosis of Johne’s disease in cattle is the AGID (agar-gel immunodiffusion) test. Serum from the animal is place in one well, a positive control is place in another, and soluble antigen is place in the third (top well). Formation of a white line of precipitation between the sample well and the antigen well (arrow) is considered a positive test. This test is commercially available (Rapid Johne’s Test, ImmuCell Corp.) and can be done without need of special equipment. It also works well in small ruminants, however, the exact accuracy of the test has not been reported.


    Acid-fast stained tissue

    This is a high power magnification photomicrograph of a Ziehl-Neelsen acid-fast stained histopathology section of the bovine ileum. Use of special stains allows the mycobacteria (red rod-shaped bacteria) to be seen. M. paratuberculosis grows as clumps inside macrophages. Although macrophages are specialized cells that normally kill bacteria, M. paratuberculosis has evolved to not only survive, but actually thrive inside macrophages. Seeing acid-fast bacteria together with the typical host cell reaction, diffuse granulomatous inflammation, is diagnostic for paratuberculosis. However, isolation of M. paratuberculosis from such tissues in culture is important for a definitive diagnosis. In many cases of Johne’s disease, these bacteria may not be seen. Examples include the “paucibacillary” form of the disease in sheep or early stages of the infection in any species.


    Enlarged mesenteric lymph nodes:

    This photo shows the serosal (outside) surface of the ileum and the adjacent mesenteric lymph nodes (arrow). Lymph nodes function to trap bacteria that may penetrate the intestinal wall. In response to M. paratuberculosis infection in cattle mesenteric lymph nodes often become enlarged. The tissues in this picture came from an experimentally M. paratuberculosis-infected heifer (#13). The pattern of diagnostic tests results for this animal are shown in another photograph. Moderate enlargement, without caseous necrosis, of the lymph nodes is evident. From the serosal surface, the ileum appears normal. The mucosal (internal) surface is shown in another photograph.


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