Mycobacterium bovis is a slow-growing (16- to 20-hour generation time) aerobic bacterium and the causative agent of tuberculosis in cattle (known as bovine TB). It is related to Mycobacterium tuberculosis, the bacterium which causes tuberculosis in humans. M. bovis can jump the species barrier and cause tuberculosis-like infection in humans and other mammals.
Bacterium morphology and staining
The bacteria are curved or straight rods. They sometimes form filaments, which fragment into bacilli or cocci once disturbed. In tissues, they form slender rods, straight or curved, or club-shaped. Short, relatively plump bacilli (rods) are seen in tissue smears, and large slender beaded rods in culture. They have no flagella or fimbria, and no capsule.
Mycobacterium tuberculosis group bacteria are 1.0–4.0 μm long by 0.2–0.3 μm wide in tissues. In culture, they may appear as cocci, or as bacilli up to 6–8 μm long.
The bacteria stain Gram-positive, acid-fast. The cell wall contains as high as 60% lipid, giving the mycobacteria their hydrophobic characteristics, slow growth, and resistance to desiccation, disinfectants, acids, and antibodies. (Mycobacterium family). They are not easy to stain with aniline dyes; although they are Gram-positive, confirming this may be difficult. Ziehl–Neelsen staining results in stain pink with hot carbol fuscin and then resist decolourisation with 3% hydrochloric acid in 95% alcohol (i.e. they are acid-alcohol fast); following washing, the slide is counterstained with e.g. methylene blue.
They are nonspore-forming.
Culture and biochemical features Growth requirements M. bovis is a facultative intracellular parasite. For in vitro growth, special culture media are required; for example, Dorset's egg medium incorporates egg yolk, phosphate buffer, magnesium salts, and sodium pyruvate; amino acids may be added, but glycerol is not included, as it is inhibitory. Culture generally requires several weeks at 37 °C to reach colonies visible to the unaided human eye. It is strictly aerobic and grows at 37, but not at 25 °C. Optimal growth occurs at 37-38 °C. The species does not reduce nitrate or niacin, and is resistant to pyrazinamide. It is sensitive to thiophene-2-carboxylic acid hydrazide.
Appearance of colonies Initially (after 3–4 weeks), its minute, dull flakes, begin to thicken to form dry, irregular masses standing high above the culture medium surface. Eventually, confluent growth is seen over the whole culture surface, forming a rough, waxy blanket, becoming thick and wrinkled and reaching up the sides of the container. Colonies are yellow when first visible, darkening to deep yellow and eventually brick red, if exposed to light. In fluid media, growth is on the surface only, unless a wetting agent (e.g. Tween 80) is added to the medium.
Cell structure and metabolism M. bovis is similar in structure and metabolism to M. tuberculosis. M. bovis is a Gram-positive, acid-fast, rod-shaped, aerobic bacterium. Unlike M. tuberculosis, M. bovis lacks pyruvate kinase activity, due to pykA containing a point mutation that affects binding of Mg2+ cofactor. Pyruvate kinase catalyses the final step of glycolysis, the dephosphorylation of phosphorenolpyruvate to pyruvate. Therefore, in M. bovis, glycolytic intermediates cannot enter oxidative metabolism. Although no specific studies have been performed, M. bovis seemingly must rely on amino acids or fatty acids as an alternative carbon source for energy metabolism.
Pathogenesis
During the first half of the 20th century, M. bovis is estimated to have been responsible for more losses among farm animals than all other infectious diseases combined. Infection occurs if the bacterium is ingested or inhaled.
M. bovis is usually transmitted to humans by consuming raw milk from infected cows, although it can also spread via aerosol droplets. Actual infections in humans are nowadays rare in developed countries, mainly because pasteurisation kills M. bovis bacteria in infected milk. In the UK, cattle are tested for the disease as part of an eradication program and culled if they test positive. Such cattle can still enter the human food chain, but only after a meat inspector or a government veterinary surgeon has inspected the carcass and certified that it is fit for human consumption. However, in areas of the developing world where pasteurisation is not routine, M. bovis is a relatively common cause of human tuberculosis.
Bovine tuberculosis is a chronic infectious disease that affects a broad range of mammalian hosts, including humans, cattle, deer, llamas, pigs, domestic cats, wild carnivores (foxes, coyotes) and omnivores (common brushtail possum, mustelids and rodents); it rarely affects equids or sheep. The disease can be transmitted in several ways; for example, it can be spread in exhaled air, sputum, urine, faeces, and pus, so the disease can be transmitted by direct contact, contact with the excreta of an infected animal, or inhalation of aerosols, depending on the species involved.
Application to biotechnology M. bovis is the ancestor of the most widely used vaccine against tuberculosis, M. bovis bacillus Calmette-Guérin (BCG). The attenuated BCG strain was produced by serial passage starting from an initial virulent strain, subculturing on glycerine potato medium 230 times over a span of 13 years.
In non-human animals Control Testing Tuberculin skin testing is possible in cattle. Casal et al. 2012 tried both recombinant protein and overlapping peptide provocation, finding the peptide test to be less sensitive.
The tuberculin test causes false positives in BCG-vaccinated cattle. The Detect Infected among Vaccinated Animals (DIVA) skin test is designed to detect only infected animals among vaccinated ones. Phase 1 trials were completed in the UK in 2022, and phase 2 by 2025. (Technically speaking, DIVA is a descriptive term for all tests able to perform this kind of differentiation. The specific form of test being tested in the UK is DST-F, a fusion protein of three selected antigens not found in BCG. In this context, it makes BCG a marker vaccine.)
Vaccination The BCG vaccine can be used in cattle. The CattleBCG vaccine, based on the BCG Danish strain, is the leading candidate in the UK. Its future implementation depends on the success of the DST-F trials.
Epidemiology and responses
China M. bovis is prevalent among the Chinese deer population at 16.1%. It even more common among yaks with a seroprevalence of 48.7%.
New Zealand In New Zealand, the introduced common brushtail possum is a vector for the spread of M. bovis. The Biosecurity Act 1993, which established a national pest-management strategy, is the legislation behind the control of the disease in New Zealand. The Animal Health Board operates a nationwide programme of cattle testing and possum control, with the goal of eradicating M. bovis from wild vector species across 2.5 million hectares – or one-quarter – of New Zealand's at-risk areas, by 2026, and eventually eradicating the disease entirely.
The TB-free New Zealand programme is regarded as "world-leading". It has successfully reduced cattle- and deer-herd infection rates from more than 1700 in 1994 to fewer than 100 herds in July 2011. Much of this success can be attributed to sustained cattle controls reducing cross-infection and breaking the disease cycle. For example, at Hohotaka, in New Zealand's central North Island, control work from 1988 to 1994 achieved a sustained mean reduction of 87.5% in the density of TB‐infected possums. As expected, annual TB incidence in local cattle herds consequently declined by a similar amount (83.4%).
Possums are controlled through a combination of trapping, ground-baiting, and where other methods are impractical, aerial treatment with 1080 poison.
From 1979 to 1984, possum control was stopped due to lack of funding. From that point until 1994, TB rates in herds steadily increased. The area of New Zealand harbouring TB-infected wild animals expanded from about 10% of the country to 40%.
The fact that possums are such effective transmitters of TB appears to be facilitated by their behaviour once they get the disease.
United Kingdom
In the 1930s, 40% of cattle in the UK were infected with M. bovis, and 50,000 new cases of human M. bovis infection were reported every year. According to DEFRA and the Health Protection Agency, the risk to people contracting TB from cattle in Great Britain would be low.
Badgers (Meles meles) were first identified as carriers of M. bovis in 1971, but the report of an independent review committee in 1997 (the Krebs Report) concluded: "strong circumstantial evidence [exists] to suggest that badgers represent a significant source of M. bovis infection in cattle... [h]owever, the causal link... has not been proven". In essence, the contribution of badgers 'to the TB problem in British cattle' was at this point a hypothesis that needed to be tested, according to the report. The subsequent Randomised Badger Culling Trial (designed, overseen and analysed by the Independent Scientific Group on Cattle TB, or ISG) examined this hypothesis by conducting a large field trial of widescale (proactive) culling and localised reactive culling (in comparison with areas which received no badger culling). In their final report, the ISG concluded: "First, while badgers are clearly a source of cattle TB, careful evaluation of our own and others' data indicates that badger culling can make no meaningful contribution to cattle TB control in Britain. Indeed, some policies under consideration are likely to make matters worse rather than better. Second, weaknesses in cattle-testing regimens mean that cattle themselves contribute significantly to the persistence and spread of disease in all areas where TB occurs, and in some parts of Britain are likely to be the main source of infection. Scientific findings indicate that the rising incidence of disease can be reversed, and geographical spread contained, by the rigid application of cattle-based control measures alone." On 26 July 2007, the Minister of State, Department for Environment, Food and Rural Affairs (Lord Rooker) said, "My Lords, we welcome the Independent Scientific Group's final report, which further improves the evidence base. We are carefully considering the issues that the report raises, and will continue to work with industry, government advisers, and scientific experts in reaching policy decisions on these issues."
In the UK, many other mammals are infected with M. bovis, although the frequency of isolation is generally much less than cattle and badgers. In some areas of south-west England, deer, especially fallow deer due to their gregarious behaviour, have been implicated as possible maintenance hosts for transmission of bovine TB.
In some localised areas, the risk of transmission to cattle from fallow deer has been argued to be greater than it is from badgers.
One of the reasons that the Department for Environment, Food, and Rural Affairs requires infected or suspected cattle to be culled is to meet EU regulations for the export of meat and dairy products to other member states. Meat and dairy products can still be sold in the UK into the human food chain, providing the relevant carcass inspections and milk pasteurisation have been applied.
The Spread of the disease to humans by domestic pets became evident in March 2014 when Public Health England announced two people in England developed bTB infections after contact with a domestic cat. The two human cases were linked to 9 cases of bTB infection in cats in Berkshire and Hampshire in 2013. These are the first documented cases of cat-to-human transmission.