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==Description==
==Description==
Inhalation of ruminal gases is the most common route of entry for the mycobacterium organism, and spread of the disease is usually via cow-to-cow contact. Cattle can also become infected by ingestion of the causative agent; this is the usual route of entry when the badger is involved, by infecting grazing land or water troughs. Calves with infected dams can become affected via the milk, and intrauterine infection at coitus has been reported.
Inhalation of ruminal gases is the most common route of entry for the mycobacterium organism, and spread of the disease is usually via cow-to-cow contact. Cattle can also become infected by ingestion of the causative agent; this is the usual route of entry when the badger is involved, by infecting grazing land or water troughs. Calves with infected dams can become affected via the milk, and intrauterine infection at coitus has been reported.
A higher level of infection is required to establish the alimentary form of the disease and this is reflected by its lower incidence in comparison to the respiratory form. In the respiratory form the mycobacteria are inhaled, gaining entry to the respiratory system. The organisms are phagocytosed by alveolar [[Macrophages|macrophages]] where they multiple and result in a characteristic [[Lungs Inflammatory - Pathology#Granulomatous pneumonia|granulomatous inflammation]] and tubercle formation. Ultimately, caseous necrosis develops and the nodule ruptures, releasing the organism and infection spreads. The cell-mediated immune system is activated and produces cytotoxic T-lymphocytes, which attack and destroy infected cells, leading to a type IV hypersensitivity reaction.
A higher level of infection is required to establish the alimentary form of the disease and this is reflected by its lower incidence in comparison to the respiratory form. In the respiratory form the mycobacteria are inhaled, gaining entry to the respiratory system. The organisms are phagocytosed by alveolar [[Macrophages|macrophages]] where they multiple and result in a characteristic [[Lungs Inflammatory - Pathology#Granulomatous pneumonia|granulomatous inflammation]] and tubercle formation. Ultimately, caseous necrosis develops and the nodule ruptures, releasing the organism and infection spreads. The cell-mediated immune system is activated and produces cytotoxic T-lymphocytes, which attack and destroy infected cells, leading to a [[Type IV Hypersensitivity - WikiBlood|type IV hypersensitivity reaction]].
Historically bovine TB was a major cause of human TB, but the introduction of tuberculin testing and slaughter, meat inspection at abattoirs, the pasteurisation of milk and the BCG vaccination has dramatically reduced transmission to humans and bovine TB as a cause of human disease is now very low indeed.
Historically bovine TB was a major cause of human TB, but the introduction of tuberculin testing and slaughter, meat inspection at abattoirs, the pasteurisation of milk and the BCG vaccination has dramatically reduced transmission to humans and bovine TB as a cause of human disease is now very low indeed.