Infection with Mycobacterium tuberculosis complex

Infection is often subclinical for months or years, and infected animals may transmit without looking ill. When signs appear in cattle they develop slowly: progressive weight loss, weakness, poor appetite, low-grade fluctuating fever and enlarged lymph nodes that may rupture and drain. A moist, intermittent cough worse in cold weather or on exertion follows, with laboured breathing in advanced cases; diarrhoea or constipation may occur. Deer and camelids progress faster, and camelids and horses show more digestive signs. At necropsy there are caseous or calcified tubercles in lungs and bronchial, mediastinal and retropharyngeal lymph nodes, or pus-filled abscesses in deer.

Mammalian tuberculosis is a chronic granulomatous disease caused by acid-fast bacilli of the Mycobacterium tuberculosis complex, chiefly M. bovis, with M. caprae, M. tuberculosis, M. microti, M. orygis and M. pinnipedii also involved. Inhaled bacilli are taken up by alveolar macrophages; if not cleared they may lie dormant for years or multiply. Failed containment produces tubercles: cores of dead macrophages ringed by lymphoid cells and multinucleated giant cells, often caseous, encapsulated or calcified. After inhalation lesions sit in the lungs and cranio-thoracic lymph nodes; after ingestion, in abdominal organs and mesenteric nodes. Lymphatic or blood-borne spread gives miliary disease in lungs, liver, kidney, spleen, udder, gut and brain.

All mammals are considered susceptible; cattle are the main livestock host, and sheep, goats, pigs, horses, camelids, cats, dogs and wildlife are also infected. WOAH describes the disease as endemic and largely uncontrolled in Africa, Asia, Latin America and most Middle Eastern countries. Wildlife reservoirs maintain it in some regions, such as badgers in the UK and Ireland, possums in New Zealand, African buffalo in South Africa and wild boar in Iberia. Aerosol spread is the main route; young animals also acquire it through contaminated milk or colostrum, and exudates, faeces and semen can transmit it. Humans are infected through unpasteurised dairy products, raw meat or contact. It is WOAH-listed and notifiable.

Because signs are not distinctive, several conditions must be ruled out by sampling. Contagious bovine pleuropneumonia leaves lung sequestra; bovine farcy (Mycobacterium farcinogenes) adds skin and subcutaneous lesions and mastitis; old hydatid cysts can be calcified like tubercles. Actinobacillosis, actinomycosis (lumpy jaw), lung abscesses and bacterial or viral bronchopneumonia complete the list. Culture, PCR and tuberculin testing confirm or exclude tuberculosis.

Lungs, bronchial, mediastinal, retropharyngeal and mesenteric lymph nodes, liver, kidney, spleen, mammary gland, gastrointestinal tract, central nervous system

Tuberculin skin or gamma interferon testing with removal or segregation of reactors; biosecurity limiting wildlife contact; abattoir surveillance; milk pasteurisation. BCG vaccine is restricted in cattle because it interferes with tuberculin testing.

Chronic and often lifelong; animals may stay quiescent then reactivate when immunosuppressed. Advanced generalised (miliary) disease is invariably fatal untreated. Control relies on test-and-slaughter or segregation rather than treatment.

Sus scrofa