blood, mucus, or epithelial debris, and may be intermittent. Several weeks after the onset of diarrhea, a soft swelling may occur under the jaw. Known as "bottle jaw" or intermandibular edema, this symptom is due to protein loss from the bloodstream into the digestive tract. Paratuberculosis is progressive; affected animals become increasingly emaciated and usually die as the result of dehydration and severe cachexia. Signs are rarely evident until two or more years after the initial infection, which usually occurs shortly after birth. Animals are most susceptible to the infection in the first year of life. Newborns most often become infected by swallowing small amounts of infected manure from the birthing environment or udder of the mother. In addition, newborns may become infected while in the uterus or by swallowing bacteria passed in milk and colostrum. Animals exposed at an older age, or exposed to a very small dose of bacteria at a young age, are not likely to develop clinical disease until they are much older than two years. The clinical signs are similar in other ruminants. In sheep and goats, the wool or hair is often damaged and easily shed, and diarrhea is uncommon. In deer, paratuberculosis can progress rapidly. Intestinal disease has also been reported in rabbits and nonhuman primates. Unlike cattle and sheep, infections in deer often present with clinical illness in animals under one year of age. Pathophysiology The primary site targeted by Johne's disease is the lower part of the intestine known as 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. Peyer's patches are clusters of macrophages and lymphocytes organized much like lymph nodes. Covering Peyer's patches are a layer of cells called M cells. These cells function to sample the content of the lumen of the intestines and pass antigens (bacteria) through to the underlying cells of the Peyer's patch to "show" these antigens to the macrophages and lymphocytes. This is a means of "educating" the cells in a young animal about its environment, and is a protective mechanism designed to help the animal become immune to pathogens in its environment. Unfortunately, when M cells bring M. paratuberculosis to the Peyer's patch, the bacteria find an ideal place for growth. Macrophages in Peyer's patches engulf M. paratuberculosis for the purpose of