The effect of M. paratuberculosis on the intestinal tract is the same whether this bacterium infects dairy or beef cattle. INFECTION | MULTIPLICATION EARLY | MID | TERMINAL STAGES Infected cows are the primary source of infection for calves. The vast majority of infections in young animals are acquired by ingestion of M. paratuberculosis. In most cases, this occurs through the calves' drinking contaminated milk. The bacteria pass down the intestinal tract until reaching the lower part of the small intestine (the ileum). The wall of the ileum contains a large number of pockets of lymphoid tissue known as Peyer's patches that lie just beneath the interior surface of the intestine. The highest concentration of Peyer's patches are in the terminal ileum and it is for this anatomical reason that the infection begins here. The adjacent photo shows the anatomy of the bovine gastrointestinal tract. The arrow points to the part of the intestine, the ileum, where M. paratuberculosis infections generally begin and pathology results.		When calves ingest M. paratuberculosis, it invades the last part of the small intestine called the ileum. There it replicates, causing inflammation that impairs intestinal function and causes diarrhea. Tissue changes caused by this infection can be seen with the unaided eye in some cases as well as with a microscope. This section shows the sequence of pathologic changes that occur with Johne's disease in beef cattle.

Peyer's patches are clusters of cells called macrophages and lymphocytes that are part of the calve's immune system and are organized similarly to the cells in lymph nodes. Covering the Peyer's patches are a layer of cells called M cells. These cells circulate into the lumen of the intestine where they ingest bacteria, viruses and other antigens before returning to the Peyer's patch to hand over these antigens and organisms to the macrophages. Macrophages are designed to kill invading microbes and "show" them to lymphocytes, the primarily cell of the immune system. This is a protective mechanism designed to "educate" the young calf's immune system and help it cope with microbial pathogens in its environment. This photo is a scanning electron micrograph of the surface of a Peyer's patch in the intestinal wall of a calf.

Unfortunately for calves, after the M cells bring M. paratuberculosis to the Peyer's patch and they are engulfed by macrophages, the bacterium finds itself in an ideal location. Macrophages are efficient microbial killers in most cases. For reasons that are only partially understood, macrophages fail to kill this kind of bacteria. Bacteria in the genus Mycobacterium have evolved over thousands of years to resist killing by animal macrophages and have actually co-opted this cell for its home. (For people interested in the latest information about the relationship between mycobacteria and host cells, read an excellent article: "How to establish a lasting relationship with your host: Lessons learned from Mycobacterium spp." in the journal Immunology and Cell Biology volume 78, pages 301-310, 2000.)

The calf's immune system reacts to the M. paratuberculosis invasion by recruiting more macrophages and lymphocytes to the site of infection. The lymphocytes release a variety of chemical signals, called cytokines, in an attempt to increase the bacterial killing power of the macrophages and recruit more cells to fight off the infection. The cytokine gamma interferon, is most likely to be detected at this time. Macrophages fuse together to form large multi-nucleated cells, called giant cells, in an apparent attempt to kill the mycobacteria.

Infiltration of infected tissues with millions of lymphocytes and macrophages progresses over years leading to visible thickening of the intestine. The mucosal surface becomes corrugated and granular in appearance. The ileum is no longer thin and pliable, and can not be easily stretched. At necropsy, the lower duodenum, a portion of the gastrointestinal tract that is usually not affected, can be used as an example of what the ileum should look and feel like. The photo shown (generously provided by the late Dr. B.J. Jørgensen of the Veterinary Serum Laboratory in Copenhagen, Denmark) shows an ileum thickened due to Johne's disease (top half of photo) and a normal ileum (bottom half of photo).

The serosal (outer) surface of the ileum may be tracked with thickened and dilated lymphatics. These clearly visible tube-like vessels have thickened walls and are congested with lymphocytes, a phenomenon called cording. The photo shown here was taken by veterinary pathologist Dr. A.J. Cooley. It shows both the corrugated mucosal (inside) gut surface as well as the serosal (outside) surface. The arrow points to a dilated lymph vessel.

Lymphatic vessels carry lymph, a fluid component of the immune system, to the local lymph nodes. In infected cattle M. paratuberculosis bacteria are also carried along inside the macrophages transported by the lymph. When they reach the lymph nodes they are trapped there. Lymph nodes filter out and kill harmful bacteria and serve as the second line of defense against infection after the intestinal wall. Just as in Peyer's patches, M. paratuberculosis is undisturbed by the abundant macrophages in the lymph nodes and continues to multiply. As immune cells are recruited to fight the infection the lymph nodes become enlarged and pale in color. This photo, taken from calves experimentally challenged with M. paratuberculosis, shows enlarged, pale mesenteric lymph nodes (arrow).

Most of the characteristic pathology of Johne's disease is limited to the intestinal tract and local lymph nodes. A particular finding in some cattle with Johne's disease is calcified plaques in the wall of the aorta, the largest blood vessel leading from the heart. The adjacent photo shows this classic, or pathognomonic, finding in Johne's disease (arrow at bovine aorta plaque). This exceptional photo was donated by veterinary pathologist and outstanding photographer Dr. A.J. Cooley.

In the last phases of this infection, the animal begins to produce antibody, an immune protein that circulates in the blood. While protective for other diseases, antibody provides no protection against M. paratuberculosis multiplication. In fact, detection of antibody to M. paratuberculosis is an indicator that clinical signs of disease and death from the infection will soon follow. It is not known how often spontaneous recovery from infection occurs, if it occurs at all. It appears that most infections slowly and inevitably progress until the animal dies from the pathology induced in the intestine. The course of the infection may take so long that some animals may die from other causes prior to succumbing to Johne's disease.

When the ileum becomes thickened by granulomatous inflammation, diarrhea and a condition known as a protein-losing enteropathy occurs (i.e. protein absorption is impaired and excess protein is lost in the feces). The result can be measured as subnormal levels of total serum proteins. One serum protein in particular, serum albumin, may be extremely low in advanced stages of Johne's disease. This hypoalbuminemia impairs the ability of the animal to retain fluids within the vasculature (blood vessels) leading to its leaking out into the tissue (edema). In cattle this is most noticeable in the submandibular region: a condition referred to as "bottle jaw" (sorry beef folks, this dairy cow is my only bottle jaw photo.)

The majority of organs besides the gastrointestinal tract generally appear normal. In advanced Johne's disease, when animals have lost a marked amount of weight, the usual fat layers surrounding the kidney, heart and even in the bone marrow may be missing completely.

During the final stage of this disease, M. paratuberculosis may disseminate beyond the ileum and lymph nodes. For instance, the organism can be isolated by culture from tissue samples taken from internal organs such as liver, spleen, mammary gland and uterus of cattle with late stage Johne's disease. During this bacteremic stage, minimal tissue response to these bacteria is seen and it is thought that the animal's immune system has essentially shut down and is no longer able to respond to the infection. Diagnostic tests usually detect high antibody levels and little to none of the cytokine gamma interferon response at this stage of infection.