Bovine tuberculosis

What it is

Bovine tuberculosis is a chronic disease caused by Mycobacterium bovis, a member of the Mycobacterium tuberculosis complex; M. caprae causes a similar disease. Cattle are infected mainly by inhalation of aerosols from infected animals, in close contact and in enclosed housing; calves can also be infected orally, through contaminated milk. The bacterium persists inside macrophages and triggers a cell-mediated immune response that forms the tubercle: a granuloma with a centre of caseous necrosis, which over time mineralises and becomes surrounded by a fibrous capsule. From the initial focus and its regional lymph node the infection can spread to other organs (generalised or miliary tuberculosis).

In some regions wildlife reservoirs hinder eradication, such as the badger in the United Kingdom and Ireland, wild boar and deer in the Iberian Peninsula or the white-tailed deer in North America. It is a zoonosis: people are infected mainly by consuming raw milk or unpasteurised dairy products and, to a lesser extent, by aerosols on farms and in abattoirs.

Who it affects

Cattle of any age and type, more often adults, especially in large, housed dairy herds or herds that buy animals frequently. Prevalence varies widely between countries: several countries are officially free, while in others it is endemic. Other domestic and wild ruminants, pigs and people can also be infected.

Anatomy involved

• Lungs: the most common primary focus is a granuloma in the lung parenchyma together with the lesion in its regional lymph node (primary complex). Tubercles are firm, yellowish-white nodules with caseous and sometimes calcified contents; in advanced disease they can coalesce and form cavities that open into the bronchi.
• Trachea and bronchi: when lesions communicate with the airways, the animal sheds bacilli in respiratory secretions and by coughing ("open" tuberculosis), the main route of spread between cattle. The tracheobronchial and mediastinal lymph nodes, located next to the tracheal bifurcation and in the mediastinum, are very often affected; when greatly enlarged they can compress the trachea or oesophagus and cause dyspnoea or bloat.

Clinical signs

The vast majority of infected animals show no signs and are only detected by official testing or at slaughter. In advanced disease there is progressive weight loss, fluctuating fever, intermittent moist chronic cough, dyspnoea, enlargement of superficial lymph nodes and recurrent ruminal bloat from compression of the oesophagus. Udder involvement causes mastitis with induration and shedding of bacilli in milk.

Diagnosis

• Intradermal tuberculin test: the basis of official programmes. Tuberculin is injected into the skin of the neck (or the caudal fold in some countries) and the increase in skin thickness is measured at 72 hours. The comparative test, with bovine and avian tuberculin, helps distinguish sensitisation by environmental mycobacteria or by M. avium subsp. paratuberculosis.
• Interferon-gamma blood test: an ancillary test that, used together with the tuberculin test, increases detection of infected animals.
• Meat inspection, histopathology and culture: suspicious lesions are confirmed by histopathology (granulomas with acid-fast bacilli on Ziehl-Neelsen staining), mycobacterial culture and PCR.
• Tests can fail early in infection and in very advanced animals that no longer react to tuberculin (anergy).

Treatment and control

It is not treated: in most countries treatment of cattle is prohibited and the disease is notifiable. Control relies on official test-and-slaughter programmes for reactor animals, with movement restrictions on affected herds, pre-movement testing, epidemiological investigation of outbreaks, abattoir surveillance and, where they exist, measures against wildlife reservoirs. Pasteurisation of milk protects people. BCG vaccine partially protects cattle but interferes with the tuberculin test, so in many countries it is not permitted.

Key studies

• Nuñez-Garcia et al. (2018), Bayesian meta-analysis of the sensitivity and specificity of 14 diagnostic tests, with data published between 1934 and 2009 in the United Kingdom and Ireland. The comparative intradermal test with standard interpretation had a median sensitivity of only 0.50 (95% credible interval 0.26–0.78) and a specificity of 1.00. Interferon-gamma had a sensitivity of 0.67 with tuberculins and 0.78 with the ESAT-6/CFP-10 antigens, with specificities of 0.98 and 0.99. The test is very reliable when positive but misses many infected animals.
• Willgert et al. (2025), systematic review of 91 studies on the impact of tuberculosis on health, welfare and production. 87% of positive animals had no clinical signs, which allows silent spread without routine surveillance, and 46% had visible lesions on post-mortem inspection. The data were sparse and sometimes contradictory.
• Collins et al. (2022), systematic review and meta-analysis of 83 studies on M. bovis in milk. Prevalence was 5% in milk from individual cows, 8% in tuberculin-positive cows and 5% in bulk-tank milk, with large heterogeneity between studies. It supports the zoonotic risk of raw milk.
• Müller et al. (2013), systematic review of zoonotic tuberculosis in humans in 61 countries. Outside Africa, the median proportion of human tuberculosis cases due to M. bovis or M. caprae was up to 1.4%; in the African countries studied it was 2.8%, with a crude estimate of 7 cases per 100,000 population per year. Data were missing for many regions.
• Fromsa et al. (2024), natural transmission experiment in Ethiopia with vaccinated and unvaccinated calves exposed for one year. BCG reduced infection of vaccinated calves by 58%, transmission from vaccinated animals that became infected by 74% and overall by 89%. It is a single experiment, carried out in one country.