Bovine tuberculosis (bovine TB) is a contagious chronic disease of cattle caused by Mycobacterium bovis and associated with progressive emaciation and tubercle (granuloma) formation involving most usually the respiratory system but also other organs. As well as being of great economic importance to the livestock industry, because humans can be infected, it is also an important public health issue.
|Cattle and buffaloes are the principal hosts for Mycobacterium bovis and are responsible for maintaining the disease. M. bovis can also cause disease in deers, pigs and humans and, occasionally, in horses, dogs, cats and sheep.|
|Bovine TB is widespread throughout the world. It is subject to control programs in a number of countries. In the absence of control measures it tends to be more common in colder climates, because housing of animals favours spread.|
Infection most commonly occurs via the respiratory tract and the alimentary tract with cattle considered to be much more susceptible to infection by inhalation. Pigs on the other hand are more likely to be infected by ingestion of contaminated foodstuffs.
In the early stages, there are no clinical signs. In advanced stages cattle have:
- Fluctuating temperature
- Anorexia and loss of body condition
- Enlarged lymph nodes
- Persistent cough progressing to dyspnoea and increases respiratory rate if pulmonary tuberculosis
- Induration of the udder
|M. bovis mainly enters the body via the respiratory tract or the alimentary tract, with the former being the most common. In the lungs localised bronchiolitis is followed by ‘tubercle’, formation — an abscess with necrotic focus and caseation and sometimes calcification surrounded by a fibrous capsule. Tubercles have a yellowish appearance, and a caseous, caseo-calcareous or calcified consistency.Findings at post-mortem vary from single small focus usually in the lung to numerous, sometimes confluent lesions in several organs. Tubercles may be found in bronchial, mediastinal, retropharyngeal and portal lymph nodes. Lesions in the lungs, liver, spleen, body cavities and female genitalia can be found in advanced cases.|
|Contagious bovine pleuropneumonia Bacterial pneumonia caused by Pasteurella or Corynebacterium pyogenes Inhalation pneumonia Traumatic pericarditis Chronic aberrant liver fluke infestation|
|Control / vaccines|
|Vaccination with BCG confers poor protection in animal and interferes with tuberculin test and therefore is not practiced. Some treatments (e.g. isoniazid, streptomycin) have been shown to have some efficacy, however it is limited and include risks of zoonotic transmission (by non-removal of infective animals) and drug resistance. Consequently, treatment of infected animals is not recommended and is illegal in a number of countries. The preferred option is to eradicate the disease by test and slaughter. This involves, on a herd basis, repeat tuberculin testing and removal of reactors until the whole herd has passed ‘clean’ at two successive tests. Tests in infected herds should be conducted every 3 months. Herds should be considered free of TB after two negative tests distant from 6 months. Hygienic measures on the farm (cleaning and disinfecting) and control of movement of infected cattle are very important. From the public health perspective, pasteurisation of milk is essential to inactivate tubercle bacilli.|