Mycobacterium bovis
Mycobacterium bovis | |
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Attenuated strain of M. bovis used in the Bacillus Calmette-Guérin vaccine
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Scientific classification | |
Domain: | Bacteria |
Phylum: | Actinomycetota |
Class: | Actinomycetia |
Order: | Mycobacteriales |
Family: | Mycobacteriaceae |
Genus: | Mycobacterium |
Species: | M. bovis
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Binomial name | |
Mycobacterium bovis Karlson & Lessel 1970,
ATCC 19210 |
[2]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.[3]
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) in tissue smears, large slender beaded rods in culture. They have no
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.[citation needed]
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.[citation needed]
They are nonspore-forming.
Culture and biochemical features
Growth requirements
M. bovis is a
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.[citation needed]
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.[4] Pyruvate kinase catalyses the final step of glycolysis, the dephosphorylation of phosphorenolpyruvate to pyruvate. Therefore, in M. bovis, glycolytic intermediates are unable to enter into 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.[citation needed]
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.[5]
M. bovis is usually transmitted to humans by consuming
Bovine tuberculosis is a chronic infectious disease which affects a broad range of mammalian hosts, including humans,
Application to biotechnology
M. bovis is the ancestor of the most widely used vaccine against tuberculosis, M. bovis bacillus Calmette-Guérin (BCG) which was isolated after subculturing on glycerine potato medium 239 times during 13 years starting from an initial virulent strain .[citation needed]
Epidemiology and control
Testing
Skin testing is possible in cattle. Casal et al. 2012 tried both
New Zealand
In
The TB-free New Zealand programme is regarded as "world-leading".[11] 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%).[12]
Possums are controlled through a combination of trapping, ground-baiting, and where other methods are impractical, aerial treatment with 1080 poison.[13]
From 1979 to 1984, possum control was stopped due to lack of funding. From that point until 1994, TB rates in herds steadily increased.[14] The area of New Zealand harbouring TB-infected wild animals expanded from about 10% of the country to 40%.[citation needed]
The fact that possums are such effective transmitters of TB appears to be facilitated by their behaviour once they get the disease.[15]
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.[16] According to DEFRA and the Health Protection Agency, the risk to people contracting TB from cattle in Great Britain would be low.[citation needed]
In the UK, many other mammals have been found to be infected with M. bovis, although the frequency of isolation is generally much less than cattle and badgers. In some areas of south-west England,
In some localised areas, the risk of transmission to cattle from fallow deer has been argued to be greater than it is from badgers.[22][23]
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.[24][25]
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 during 2013. These are the first documented cases of cat-to-human transmission.[26]
In a 2010 opinion piece in
In July 2010, the second issue of the discussion document Bovine TB, Time for a Rethink [28] was published by Rethink Bovine TB, an independent research group. The paper considers current policy in England and Wales. It proposes an alternative solution that is both practical and cost effective. In the paper, evidence is drawn from DEFRA and the work by Professors Paul and David Torgerson.[27]
In March 2012, think tank the Bow Group published a target paper urging the government to reconsider its plans to cull thousands of badgers to control bovine TB, stating that the findings of Labour's major badger-culling trials several years prior were that culling does not work. The paper was authored by Graham Godwin-Pearson with a foreword by singer Brian May and contributions by leading tuberculosis scientists, including Lord Krebs.[29][30][31]
In 2017, Rachel Tanner and Helen McShane, of the Jenner Institute, Oxford, published research on replacing, reducing, and refining the use of animals in tuberculosis vaccine research.[32]
United States
According to Barbara Gutmann Rosenkrantz, the late 19th-century discovery of the relationship between bovine and human tuberculosis led to state and federal attempts to stamp out bovine tuberculosis. The campaigns for clean milk and meat frightened city people into supporting controls, although at the time, little evidence showed that tuberculosis was spread to humans through infected meat or milk. The campaigns against impure meat and milk led to tension between the developing veterinarian profession and the medical profession, each claiming that area as part of their own expertise.[34]
By 1917, 5% of American cattle were infected with M. bovis (bovine tuberculosis or bTB), including 10% dairy animals and 1–2% of beef cattle. The rates were going up. Around 1900, 15,000 Americans, mostly children, died each year from bTB, and many more suffered pain and disfigurement.[35] [36]
Threatened by a sales cutoff ordered by urban public health officials, Vermont state government officials launched an innovative eradication campaign against bTB on farms, 1877 to 1936. They made use of the latest German research, and thereby kept the New York City and Boston markets.[37] Vermont was exceptional, for across the country many farmers strenuously resisted bovine tuberculosis eradication as an expensive violation of their libertarian right to farm.[38]
In recent decades, M. bovis infections in cattle herds in the United States are not common. M. bovis is endemic in white-tailed deer (Odocoileus virginianus) in the northeastern portion of Michigan and northern Minnesota, and sporadically imported from Mexico. Only the white-tailed deer has been confirmed as a maintenance host in the Michigan outbreak of bTB, although other mammals such as raccoons (Procyon lotor), opossums (Didelphis virginiana), and coyotes (Canis latrans) can serve as spill-over and dead-end hosts.[39] The fact that white-tailed deer are a maintenance host for M. bovis remains a significant barrier to the US nationwide eradication of the disease in livestock. In 2008, 733,998 licensed deer hunters harvested around 489,922 white-tailed deer in attempts to control the disease spread. These hunters purchased more than 1.5 million deer-harvest tags. The economic value of deer hunting to Michigan's economy in the drive to eradicate TB is substantial. For example, in 2006, hunters spent US$507 million hunting white-tailed deer in Michigan.[40]
Global
The disease is found in cattle throughout the globe, but some countries have been able to reduce or limit the incidence of the disease through a process of "test and cull" of the cattle stock. Most of Europe and several Caribbean countries (including Cuba) are virtually free of M. bovis. Australia is officially free of the disease since the successful BTEC program, but residual infections might exist in feral
M. bovis can be transmitted from human to human; an outbreak occurred in Birmingham, England, in 2004,[42] and from human to cattle,[43][44] but such occurrences are rare.
In Mexico, the disease is prevalent and rising among humans.[45]
Zoonotic tuberculosis
The infection of humans with M. bovis is referred to as zoonotic tuberculosis.[46] In 2017, the World Health Organization (WHO), World Organization for Animal Health (OIE), Food and Agriculture Organization (FAO), and The International Union Against Tuberculosis and Lung Disease (The Union), published the first Roadmap for Zoonotic Tuberculosis, recognizing zoonotic tuberculosis as a prominent global health problem.[47] The main route of transmission is through the consumption of unpasteurized milk or other dairy products, although transmission via inhalation and via consumption of poorly cooked meat has also been reported.[47] In 2018, based on the most recent Global Tuberculosis Report, an estimated 142,000 new cases of zoonotic tuberculosis, and 12,500 deaths due to the disease occurred.[48] Cases of zoonotic tuberculosis have been reported in Africa, the Americas, Europe, the Eastern Mediterranean, and the Western Pacific.[49] Human zoonotic tuberculosis cases are linked to the presence of bovine tuberculosis in cattle, and regions without adequate disease control measures and/or disease surveillance are at higher risk.[49] It is difficult to clinically distinguish zoonotic tuberculosis from tuberculosis caused by Mycobacterium tuberculosis in people, and the current most commonly used diagnostics cannot effectively distinguish between M. bovis and M. tuberculosis, which contributes to an underestimation of total cases worldwide.[50] Controlling this disease requires animal health, food safety, and human health sectors to work together under a One Health approach (multi-disciplinary collaborations to improve the health of animals, people, and the environment).[51]
The 2017 Roadmap identified ten priority areas for addressing zoonotic tuberculosis, which include collecting more accurate data, improving diagnostics, closing research gaps, improving food safety, reducing M. bovis in animal populations, identifying risk factors for transmission, increasing awareness, developing policies, implementing interventions, and increasing investments.[47] To align with goals outlined in the Stop TB Partnership Global Plan to End TB 2016-2020,[52] The Roadmap outlines specific milestones and goals to be met within this time frame.[47]
Treatment
M. bovis is innately resistant to pyrazinamide, so the standard human treatment is isoniazid and rifampicin for 9 months.[53] Most cattle that test positive are killed.[54]
See also
- Christopher Morcom
- Badger culling in the United Kingdom
- Veterinary medicine
- Paratuberculosis
- Mycobacterium avium complex
References
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External links
- TB free New Zealand Archived 2011-08-30 at the Wayback Machine - TB control programme in New Zealand
- Bovine TB information on Department of Conservation website - The use of 1080 for pest control in New Zealand - Possums as reservoirs of bovine tuberculosis
- Information about bovine TB on 1080: The Facts website - Facts about how 1080 poison is used to control bovine TB in New Zealand
- Background on immunology and testing for Bovine TB - The background on immunology and testing for Bovine Tuberculosis.
- Mycobacterium bovis in African wildlife Mycobacterium bovis in African wildlife
- Tuberculosis - Mycobacterium bovis - Health Protection Agency