Bovine tuberculosis (bTB) is a chronic bacterial infection caused by Mycobacterium bovis, a close relative of the bacterium responsible for human tuberculosis. It primarily affects cattle but can spread to humans and a wide range of wildlife species. The disease remains a major concern for agriculture, food safety, and public health worldwide.
The Bacterium Behind the Disease
Mycobacterium bovis belongs to the same family of bacteria that causes tuberculosis in humans. The two species are so closely related that some M. bovis strains share biochemical and physical properties with M. tuberculosis, which can complicate laboratory identification. Despite these similarities, M. bovis has one clinically important difference: it is naturally resistant to pyrazinamide, one of the four standard drugs used to treat human TB. This built-in resistance means that human infections caused by M. bovis require a modified treatment approach, and misidentifying the species can lead to ineffective therapy.
How It Spreads Among Cattle
Cattle transmit M. bovis to each other primarily through three routes: respiratory droplets, aerosols, and the fecal-oral pathway. When an infected animal coughs or exhales, bacteria-laden droplets can be inhaled directly by nearby cattle or settle onto pasture, water troughs, and feed. Other animals then pick up the bacteria by grazing or drinking from contaminated sources. Urine and feces also carry the organism into the environment, where it can persist for weeks to months depending on temperature and sunlight exposure.
Close confinement accelerates spread. Barns, milking parlors, and shared feeding areas concentrate animals in ways that make respiratory transmission especially efficient. Herds that graze on open pasture still face risk, but the probability of transmission drops when animals have more space between them.
Wildlife Reservoirs
One of the most frustrating aspects of bovine TB control is that wild animals can harbor the bacterium and reintroduce it to livestock. The specific reservoir species varies by region: badgers in Great Britain, white-tailed deer and elk in North America, feral pigs across parts of Europe, brushtail possums in New Zealand, and Cape buffalo in South Africa. These wildlife populations sustain the infection independently of cattle, making eradication from livestock herds far more difficult. Even after a farmer eliminates the disease from a herd, contact with infected wildlife can restart the cycle.
Signs of Infection in Cattle
Most infected cattle show no visible symptoms, which is one reason the disease spreads so effectively. The incubation period ranges from as short as two months to several years. When signs do appear, they vary depending on where in the body the infection has taken hold. Chronic cough, weight loss, weakness, and a general decline in condition are common late-stage features.
Because bTB is always progressive once established, it produces a slow-building toxemia that gradually weakens the animal. Without intervention, the disease is ultimately fatal. Internal examination of infected cattle typically reveals granulomas, which are small, walled-off clusters of immune cells and bacteria, most often found in the lungs and lymph nodes. These lesions are sometimes discovered only at slaughter, in animals that appeared healthy on the farm.
How Cattle Are Tested
The standard diagnostic tool is the tuberculin skin test. A veterinarian measures the thickness of the skin at a specific site, injects a small amount of purified protein from the bacterium, then returns 72 hours later to check for swelling. A significant increase in skin thickness indicates the animal’s immune system has previously encountered M. bovis, suggesting infection. This is called a delayed hypersensitivity reaction.
Because cattle can also react to proteins from other, harmless mycobacteria found in soil and the environment, a comparative version of the test uses both bovine and avian tuberculin side by side. Comparing the two reactions helps distinguish a true bTB infection from a false alarm. No single test catches every infected animal, though. The sensitivity is below 100%, which means eradicating the disease from a herd typically requires multiple rounds of testing over months or years.
Transmission to Humans
M. bovis is a zoonotic pathogen, meaning it can jump from animals to people. The primary route of human infection is consuming unpasteurized dairy products, particularly raw milk and soft cheeses like queso fresco. In regions where people live in close quarters with cattle, direct aerosol transmission is also a risk, similar to how standard TB spreads person to person.
The burden falls heaviest on communities in low- and middle-income countries where cattle populations are growing, pasteurization infrastructure is limited, and people have frequent, close physical contact with livestock. Poverty, inadequate disease surveillance, and cultural preferences for raw dairy products all amplify the risk. A global meta-analysis found that roughly 1.4% of all human tuberculosis cases caused by the TB family of bacteria were identified as M. bovis when modern genetic testing methods were used. Older, less precise identification techniques produced higher estimates, around 12%, but these likely overcount due to misidentification.
Why Pasteurization Matters
Standard milk pasteurization was originally designed specifically to kill M. bovis. Heating milk to 72°C (about 162°F) for 15 seconds, known as high-temperature, short-time pasteurization, reliably destroys the bacterium. In countries with universal pasteurization, foodborne transmission of bovine TB to humans has dropped to near zero. The risk persists almost entirely through raw milk and artisanal dairy products that bypass this step.
Challenges in Treating Human Infections
When a person does contract M. bovis, treatment is complicated by the bacterium’s natural resistance to pyrazinamide. Standard human TB treatment relies on a four-drug regimen, and losing one of those drugs from the start means doctors must adjust the protocol and often extend the treatment timeline. Making matters worse, laboratory screening historically used pyrazinamide resistance as a shortcut to flag M. bovis infections, but this approach only catches about 82% of cases. Genetic testing that definitively distinguishes M. bovis from M. tuberculosis is more reliable but not always available in the settings where zoonotic TB is most common.
Economic Impact on Farmers
A positive bTB test triggers a cascade of financial consequences for livestock operations. Infected animals must be slaughtered, the rest of the herd is quarantined and cannot be sold through normal market channels, and the farm loses its disease-free certification. Research on cattle farms in southwestern Spain illustrates the costs concretely. Each slaughtered test-positive animal resulted in a net financial loss of about €270 after government compensation. But the real damage came from downstream effects: on a modest 25-head farm with three positive animals, the annual drop in sales revenue reached nearly €3,000, ballooning to over €7,400 across the two and a half years it takes to raise a replacement heifer to her first calving.
Replacement costs, lost sales, and movement restrictions compound quickly. Farms under quarantine can only sell animals to uncertified feedlots at reduced prices, and government compensation programs are designed to offset only the value of slaughtered animals, not the broader profit losses. For small and mid-sized operations, a single outbreak can threaten the viability of the entire business.
Control and Eradication Efforts
Most countries with established dairy and beef industries run national bTB control programs built around routine herd testing, slaughter of positive animals, movement restrictions, and post-mortem inspection at abattoirs. These programs have successfully eliminated or dramatically reduced the disease in much of North America, Western Europe, and Australasia. England and Ireland remain notable exceptions, where persistent wildlife reservoirs in badger populations have made eradication elusive despite decades of effort and billions in spending.
In sub-Saharan Africa, South Asia, and parts of Latin America, control is far less advanced. Limited veterinary infrastructure, porous borders for livestock trade, and the sheer scale of informal dairy production make systematic testing and slaughter programs difficult to implement. In these regions, protecting human health depends more heavily on expanding access to pasteurized milk than on eliminating the disease from cattle.