B105 - TUBERCULOSIS (BOVINE)

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B105 - TUBERCULOSIS (BOVINE) 

Nature of the disease
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.
Classification
OIE List B disease
Susceptible species
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. In New Zealand, the introduced Australian possum is an important reservoir of infection.
Distribution
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.
In the Pacific, a number of countries are free of bovine tuberculosis (e.g. Cook Islands, New Caledonia, French Polynesia). 
It occurs in Australia, Fiji, New Zealand, Tonga and Western Samoa. 
In these countries it is subject to various levels of control/eradication programs. The disease has not been reported from Vanuatu since 1993, it is only present in some part of Australia.
Clinical signs 
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
Post-mortem findings 
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.
Differential diagnosis 
  • Contagious bovine pleuropneumonia
  • Bacterial pneumonia caused by pasteurellae or Corynebacterium pyogenes 
  • Inhalation pneumonia
  • Traumatic pericarditis
  • Chronic aberrant liver fluke infestation
Specimens required for diagnosis 
Diagnosis in the live animal is based on skin testing — intradermal tuberculin testing — and clinical examination. The latter is of value in advanced cases, which often do not react to the tuberculin test. In area where Mycobacterium avium is frequent, comparative tuberculin testing can be used to differentiate false positive. 

Serological methods like the gamma interferon test or ELISA are also available.

Diagnosis at slaughter is based on identifying typical lesions. It may not always be possible to distinguish tuberculosis lesions from other granulomas and neoplasms grossly. Histological examination in the laboratory, special staining of smears, and culturing of the organism can be undertaken.

Transmission   
Tuberculosis is primarily spread between herds by the movement of infected animals. But as Mycobacterium spp. are very resistant in the environment (surviving more than 2 months in pasture, and several months in sheds, resurgence from has to be considered. The role of wildlife reservoir has been stressed in some counties (possum in New Zealand, badgers  in United Kingdoms, deer in United States of America,...)

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. Less commonly are infections in utero, via the genital tract, or through skin abrasions. In Australia, under range conditions infection in cattle nearly always occurred by the respiratory route, while under intensive stocking infection by ingestion was just as common as by inhalation.

In respiratory cases bacilli are shed into the air by coughing. They are also shed in the faeces in advanced respiratory cases, in generalised cases and in alimentary cases. When the kidneys or udder are involved, bacilli may also be shed in the urine and milk. Ingestion of infected milk can be an important source of infection for calves.

Humans usually become infected by ingestion of infected milk, handling or consumption of meat from tuberculous animals, or inhalation where infection levels are high in cattle.

Risk of introduction   
Bovine tuberculosis is most likely to be introduced with imports of live cattle, embryo and semen. Such imports should come from free countries or certified tuberculosis-free herds, supplemented by tuberculin testing during quarantine.

The disease could also be introduced through the importation of contaminated meat and dairy products.

Control / vaccines  
The great majority of infections in cattle are progressive. 

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. Meat inspection to remove tuberculous animals from the food chain is also required. Infected carcases detected at abattoirs should be traced back to identify infected herds.

References
  • Office International des Epizooties, 2002
  • Tuberculosis in Cattle, In Merck Veterinary Manual, National Publishing Inc. Eight ed, 1998, Philadelphia, p 489-493
  • Tuberculosis, In Veterinary Medicine, Saunders, Eight ed, 1997, London p. 830-838