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Cattle can become infected with TB by several routes, but the respiratory and alimentary routes are most common. Based on the distribution of lesions found in TB reactor cattle, infection of cattle by the bovine TB bacterium (Mycobacterium bovis – M. bovis) is thought to occur mainly by the respiratory route. This is supported by experimental evidence when it was found that the minimum infectious dose to establish infection by the oral route was two or three orders of magnitude greater than that of the respiratory route. It follows that respiratory excretion and inhalation of M. bovis is considered to be the main route for cattle to cattle transmission, followed by the digestive (oral) route (for instance by ingestion of heavily contaminated forage/grazing, or when young calves are fed discarded/surplus raw milk from dairy cows with TB of the udder).
Other routes of infection e.g. by spread across the placenta to the foetus, or through infection of the genitals, are now very infrequent in the UK as continuous testing and culling of TB positive (reactor) cattle ensures that most infected animals are detected in the initial stages of infection.
Indirect transmission via the respiratory route could potentially occur from inhalation of contaminated aerosols generated during spreading of infective material e.g. slurry. Transmission by inhalation of contaminated dust particles, droplets of contaminated water or aerosols generated during eructation when feeding on contaminated pastures has also been suggested but supporting data is sparse.
Lesions seen in a typical bovine TB reactor in the UK usually involve the lymph nodes behind the pharynx (throat), the lungs and associated lymph nodes in the chest cavity, but other organs may be involved more rarely depending on the route of infection. If infection is via ingestion of contaminated feedstuffs, lesions are usually found in the lymph nodes that drain the throat (retropharyngeal), gut (mesenteric) and occasionally in the liver.
Lesions may remain localised at the portal of entry of the bacterium and the corresponding local lymph node(s), or may spread through the body to other organs, e.g. spleen, kidneys and udder. Clinical signs may not be seen for months or years post infection until there is enough tissue damage to cause organ dysfunction. Even so, any signs tend to be unspecific. Clinical signs are rarely seen nowadays in live cattle in the UK, as TB control programmes using immunological tests based on tuberculin detect infected animals at an early stage.