Clinical signs of paratuberculosis are a slowly progressive wasting and diarrhoea, which is intermittent at first,
becoming progressively more severe until it is constantly present in bovines (13). Diarrhoea is less common in
small ruminants.
Early lesions occur in the walls of the small intestine and the draining mesenteric lymph nodes, and infection is
confined to these sites at this stage. As the disease progresses, gross lesions occur in the ileum, jejunum,
terminal small intestine, caecum and colon, and in the mesenteric lymph nodes. Mycobacterium avium subsp.
paratuberculosis is present in the lesions and, terminally, throughout the body. The intestinal lesions are
responsible for a protein leak and a protein malabsorption syndrome, which lead to muscular wasting. Clinical
signs usually first appear in young adulthood, but the disease can occur in animals at any age over 1–2 years.
Within a few weeks of infection, a phase of multiplication of M. paratuberculosis begins in the walls of the small
intestine. Depending on the resistance of the individual, this infection is eliminated or the animal remains infected
as a healthy carrier. The proportion of animals in these categories is unknown. A later phase of multiplication of
the organisms in a proportion of carriers leads to the extension of lesions, interference with gut metabolism and
clinical signs of disease. Subclinical carriers excrete variable numbers of M. paratuberculosis in the faeces. In
most cases larger numbers of organisms are excreted as clinical disease develops.