Bovine Tuberculosis

Bovine tuberculosis (bTB) is a chronic, contagious, and zoonotic infectious disease of cattle caused primarily by Mycobacterium bovis, a member of the Mycobacterium tuberculosis complex. The disease is characterized by the formation of granulomatous lesions (tubercles) in various organs, most commonly the lungs and associated lymph nodes, but also in the pleura, liver, spleen, intestines, udder, and central nervous system. In cattle, the respiratory tract is the primary portal of entry, and the disease typically follows a slow, progressive course, often remaining subclinical for months to years. The economic impact of bTB is substantial due to reduced productivity, carcass condemnation, trade restrictions, and mandatory test-and-slaughter programs in many countries. In the context of bovine medicine, bTB is a notifiable disease with significant public health implications, as M. bovis can infect humans through consumption of unpasteurized milk or close contact with infected animals. The disease is classified under bovine respiratory and cardiovascular diseases because the respiratory system is the primary site of infection and the cardiovascular system may be affected secondarily through granulomatous lesions in the mediastinal lymph nodes or pericardium.

The primary causative agent of bovine tuberculosis is Mycobacterium bovis, an acid-fast, aerobic, slow-growing, Gram-positive-like bacterium belonging to the Mycobacterium tuberculosis complex. Other members of the complex, such as Mycobacterium tuberculosis and Mycobacterium caprae, can occasionally cause disease in cattle, but M. bovis is the most common and epidemiologically significant. The bacterium has a thick, lipid-rich cell wall containing mycolic acids, which contributes to its acid-fast staining property, resistance to environmental degradation, and ability to survive within macrophages. Virulence factors include the ESAT-6 and CFP-10 proteins, which are secreted by the ESX-1 secretion system and play a critical role in the lysis of macrophages and the inhibition of phagolysosome fusion, allowing intracellular survival. The bacterium can persist in the environment for several months, especially in cool, moist, and dark conditions, but is susceptible to direct sunlight, heat, and common disinfectants. Transmission occurs primarily via aerosol inhalation of infected respiratory droplets from coughing or sneezing cattle, but can also occur through ingestion of contaminated milk (especially in calves), or through skin wounds. The infectious dose is relatively low, and the bacterium has a long generation time (approximately 15-20 hours), contributing to the chronic nature of the disease.

Bovine tuberculosis has a worldwide distribution, with varying prevalence depending on the presence of wildlife reservoirs, control programs, and cattle management systems. In many developed countries, rigorous test-and-slaughter programs have reduced or eliminated the disease, but it remains endemic in parts of Africa, Asia, and Latin America. In the United Kingdom and New Zealand, wildlife reservoirs such as badgers and possums contribute to persistent infection in cattle herds. The disease affects cattle of all ages, but clinical disease is more common in adult cattle, particularly those over 2 years of age, due to the slow progression of lesions. Breed susceptibility varies, with dairy breeds (e.g., Holstein-Friesian) often showing higher prevalence due to intensive management and higher stocking densities, whereas beef breeds on pasture may have lower transmission rates. The disease is more prevalent in intensive dairy systems where cattle are housed indoors, facilitating aerosol transmission, and in herds with high turnover or commingling of animals from different sources. Morbidity within an infected herd can be high, with up to 50% of animals showing positive tuberculin skin tests, but clinical mortality is low unless the disease is advanced. Economic losses arise from reduced milk production (estimated at 5-10% in affected cows), weight loss, infertility, carcass condemnation, and the cost of testing and culling. The zoonotic risk is a major public health concern, particularly in regions where pasteurization is not universally practiced.

The pathophysiology of bovine tuberculosis begins with inhalation of M. bovis bacilli into the alveoli, where they are phagocytosed by alveolar macrophages. The bacterium resists intracellular killing by inhibiting phagolysosome fusion and neutralizing reactive oxygen species, allowing it to replicate within the macrophage. Infected macrophages migrate to regional lymph nodes (e.g., bronchial and mediastinal lymph nodes), where the bacterium disseminates via the lymphatic system. The host immune response, primarily cell-mediated immunity, leads to the formation of granulomas (tubercles) composed of a central caseous necrosis surrounded by epithelioid macrophages, multinucleated giant cells (Langhans cells), lymphocytes, and a fibrous capsule. In cattle, the lesions are typically well-encapsulated and may calcify over time. The primary complex (Ghon complex) consists of the lung lesion and the associated lymph node. As the disease progresses, lesions may enlarge, cavitate, and erode into bronchi, leading to shedding of bacilli in respiratory secretions. Hematogenous spread can occur, resulting in miliary tuberculosis affecting multiple organs, including the liver, spleen, kidneys, and udder. In the udder, granulomatous mastitis can develop, leading to shedding of M. bovis in milk, which is a major zoonotic risk. The cardiovascular system may be affected by compression of the heart or great vessels by enlarged mediastinal lymph nodes, or by granulomatous pericarditis, which can impair cardiac function. The chronic inflammatory response leads to cachexia, weight loss, and reduced productivity.

Intrinsic risk factors for bovine tuberculosis include age (adult cattle are more susceptible to clinical disease), genetic susceptibility (some breeds, such as Holstein, may have higher susceptibility), and immunosuppression due to concurrent diseases, stress, or poor nutrition. Extrinsic factors include high stocking density and poor ventilation in housed cattle, which increase the concentration of infectious aerosols. Overcrowding, inadequate biosecurity, and the introduction of untested cattle into a herd are major management-related risk factors. Wildlife reservoirs, such as badgers, possums, and wild boar, can serve as sources of infection for cattle on pasture. Poor hygiene in milking parlors and feeding unpasteurized milk to calves can facilitate transmission via the oral route. Lack of routine tuberculin testing and failure to quarantine new animals contribute to the spread of the disease. In addition, environmental factors such as cool, damp climates and soil conditions that favor bacterial survival may increase the risk of transmission.

Clinical signs of bovine tuberculosis are often insidious and may not appear until the disease is advanced. The most common signs are chronic cough, progressive weight loss, lethargy, and decreased milk production. As the disease progresses, respiratory signs may include dyspnea, tachypnea, and abnormal lung sounds on auscultation (e.g., crackles, wheezes, or reduced lung sounds over consolidated areas). Enlarged lymph nodes, particularly the retropharyngeal, bronchial, and mediastinal lymph nodes, may be palpable in some cases. If the udder is affected, there may be a chronic, non-responsive mastitis with a hard, nodular udder and no systemic signs. In advanced cases, animals may develop fever, anorexia, and emaciation. If the disease spreads to the central nervous system, neurological signs such as ataxia, circling, or seizures may occur. In some cases, the disease is detected only at slaughter through meat inspection, as many infected cattle appear clinically normal. The tuberculin skin test is the primary diagnostic tool for detecting subclinical infection.

Differential diagnoses for bovine tuberculosis include other chronic respiratory diseases such as bovine respiratory disease complex (BRDC) caused by viral and bacterial pathogens (e.g., Mannheimia haemolytica, Pasteurella multocida, Histophilus somni, Mycoplasma bovis), which typically present with acute onset of fever, nasal discharge, and pneumonia. Chronic suppurative pneumonia due to Trueperella pyogenes or other bacteria may also cause weight loss and cough. Parasitic conditions such as lungworm (Dictyocaulus viviparus) can cause coughing and respiratory distress, but are usually associated with pasture exposure and respond to anthelmintic treatment. Other granulomatous diseases, such as actinobacillosis (wooden tongue) or actinomycosis (lumpy jaw), can cause local lesions but are not systemic. Neoplasia, such as pulmonary adenocarcinoma, can cause chronic weight loss and respiratory signs. Additionally, other mycobacterial infections, such as Mycobacterium avium subspecies paratuberculosis (Johne's disease), cause chronic diarrhea and weight loss but primarily affect the intestine. The tuberculin skin test and interferon-gamma assay are crucial for differentiating bTB from these conditions.

The diagnostic algorithm for bovine tuberculosis begins with a thorough herd history and physical examination, focusing on chronic cough, weight loss, and reduced milk production. The primary screening test is the single intradermal tuberculin test (SITT) or the comparative cervical tuberculin test (CCT), which involves injecting purified protein derivative (PPD) from M. bovis and M. avium intradermally and measuring the skin thickness increase after 72 hours. A positive reaction to M. bovis PPD indicates exposure. Confirmatory tests include the interferon-gamma (IFN-γ) release assay, which measures cell-mediated immune response in whole blood, and the enzyme-linked immunosorbent assay (ELISA) for antibody detection. At necropsy, gross lesions and histopathology with acid-fast staining are definitive. Polymerase chain reaction (PCR) and culture of M. bovis from tissues or secretions are used for confirmation and strain typing. In live animals, thoracic radiography or ultrasonography may reveal pulmonary lesions, but these are not routinely used in field diagnosis. The algorithm should follow a stepwise approach: initial screening with tuberculin test, confirmation with IFN-γ or PCR, and finally, slaughter surveillance.

Laboratory findings in bovine tuberculosis are not specific but may include mild anemia, leukocytosis, and hyperglobulinemia. The tuberculin skin test is the primary laboratory-based diagnostic test, with a positive reaction defined as an increase in skin thickness of ≥4 mm for the SITT or a difference of ≥4 mm between bovine and avian PPD reactions for the CCT. The IFN-γ assay measures the release of IFN-γ from sensitized lymphocytes after stimulation with M. bovis antigens; a positive result is based on optical density cutoffs. ELISA can detect antibodies, but sensitivity is lower in early infection. PCR on tissue samples or respiratory secretions can detect M. bovis DNA with high sensitivity and specificity. Culture of M. bovis on selective media (e.g., Stonebrink or Middlebrook 7H11) is the gold standard but takes 4-8 weeks due to slow growth. Hematological and biochemical parameters are generally not diagnostic but may reflect chronic inflammation.

Imaging modalities are not commonly used in the diagnosis of bovine tuberculosis in live cattle, but thoracic radiography can reveal pulmonary nodules, consolidation, or cavitation in advanced cases. Ultrasonography of the thorax may show pleural effusion, lung consolidation, or enlarged mediastinal lymph nodes. In cases of suspected udder involvement, ultrasonography of the udder may reveal abscesses or granulomas. At necropsy, gross examination of the lungs and lymph nodes is the primary method, with lesions appearing as yellowish-white, caseous, calcified nodules. Computed tomography (CT) or magnetic resonance imaging (MRI) are rarely used in cattle due to cost and practicality.

Cytological examination of fine-needle aspirates from enlarged lymph nodes or udder lesions may reveal epithelioid macrophages, multinucleated giant cells, and caseous necrosis. Acid-fast staining (Ziehl-Neelsen) can demonstrate acid-fast bacilli. Histopathology of granulomatous lesions shows a characteristic structure: central caseous necrosis, surrounded by epithelioid macrophages, Langhans giant cells, lymphocytes, and a fibrous capsule. Calcification is common in cattle. The presence of acid-fast bacilli in tissue sections confirms the diagnosis. In the udder, granulomatous mastitis with caseous necrosis and acid-fast bacilli is pathognomonic. At necropsy, the distribution of lesions (e.g., lung and associated lymph nodes) is typical of the primary complex.

Treatment of bovine tuberculosis is not recommended in most countries due to the zoonotic risk and the chronic nature of the disease. The standard approach is test-and-slaughter, where positive animals are culled to eradicate the disease from the herd. In some rare cases, treatment with antimycobacterial drugs (e.g., isoniazid, rifampicin, ethambutol) has been attempted, but it is expensive, prolonged (6-12 months), and not effective in eliminating the infection, and it is prohibited in many jurisdictions. Therefore, treatment is not a viable option in practice. The focus is on prevention and control through biosecurity, testing, and culling.

The prognosis for an individual animal with bovine tuberculosis is poor, as the disease is progressive and fatal if left untreated. However, the prognosis for the herd depends on the effectiveness of the control program. With prompt identification and removal of infected animals, the herd can be cleared of the disease. The economic prognosis is guarded, as the disease can cause significant losses due to culling, trade restrictions, and reduced productivity. In countries with eradication programs, the long-term prognosis is favorable if biosecurity is maintained.

Follow-up for bovine tuberculosis involves regular herd testing with the tuberculin skin test or IFN-γ assay at intervals determined by the regulatory authority (e.g., every 60 days until the herd is declared free, then annually). All in-contact animals should be tested, and any positive reactors should be removed. Biosecurity measures, such as quarantine of new animals, testing before introduction, and preventing contact with wildlife, should be implemented. In endemic areas, vaccination with BCG (Bacille Calmette-Guérin) may be considered, but it can interfere with diagnostic tests. Herd health records should be maintained, and necropsy of all culled animals should be performed to monitor for lesions.

Clinical pearls: 1) Bovine tuberculosis often presents as a chronic cough and weight loss in adult cattle; consider it in the differential diagnosis of any chronic respiratory disease. 2) The tuberculin skin test is the cornerstone of diagnosis; ensure proper technique and interpretation. 3) The IFN-γ assay is useful for confirmatory testing and can be used in parallel with the skin test. 4) Always consider the zoonotic risk; wear protective equipment when handling suspect animals or tissues. 5) In herds with a history of bTB, implement strict biosecurity to prevent reintroduction. Pitfalls: 1) Do not rely solely on clinical signs, as many infected cattle are asymptomatic. 2) Avoid treating bTB with antibiotics, as it is ineffective and illegal in many regions. 3) Do not ignore the possibility of wildlife reservoirs; control measures should include wildlife management. 4) Ensure that all positive reactors are promptly removed to prevent further transmission. 5) Be aware of the potential for false-positive tuberculin reactions due to environmental mycobacteria; use the comparative test to differentiate.

There are no approved drug protocols for the treatment of bovine tuberculosis in cattle. The use of antimycobacterial drugs is prohibited in food animals in most countries due to the risk of drug residues and the potential for development of drug resistance. Therefore, no dosages, routes, or withdrawal times are provided. The only recommended control measure is the test-and-slaughter policy.

Evidence-based literature on bovine tuberculosis emphasizes the importance of test-and-slaughter programs for eradication. Studies have shown that the tuberculin skin test has a sensitivity of approximately 80-90% and specificity of >99% when used correctly. The IFN-γ assay has comparable sensitivity but may have lower specificity. Research on wildlife reservoirs, such as badgers in the UK and possums in New Zealand, has demonstrated that controlling wildlife is essential for successful eradication. Vaccination with BCG has shown variable efficacy in cattle, but it is not widely used due to interference with diagnostic tests. Meta-analyses of control programs indicate that a combination of regular testing, culling of reactors, and biosecurity measures is the most effective strategy. The economic impact of bTB has been quantified in several studies, with losses estimated at millions of dollars annually in affected countries. Expert consensus from the World Organisation for Animal Health (WOAH) and national veterinary associations supports the test-and-slaughter approach as the gold standard for control.