Mycobacterium bovis

Mycobacterium bovis
Attenuated strain of M. bovis used in the Bacillus Calmette-Guérin vaccine
Scientific classification
Kingdom:	Bacteria
Phylum:	Actinobacteria
Order:	Actinomycetales
Suborder:	Corynebacterineae
Family:	Mycobacteriaceae
Genus:	Mycobacterium
Species:	M. bovis
Binomial name
Mycobacterium bovis Karlson & Lessel 1970,[1] ATCC 19210

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). Related to M. tuberculosis—the bacterium which causes tuberculosis in humans—M. bovis can also jump the species barrier and cause tuberculosis in humans.^[2]

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.^{[citation needed]}

M. bovis is usually transmitted to humans via infected milk, although it can also spread via aerosol droplets. Actual infections in humans are rare, mostly due to pasteurisation killing any bacteria in infected milk; as well, cattle are randomly tested for the disease and immediately culled if infected, but can still be used 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.^[3]

Bovine TB is a chronic infectious disease which affects a broad range of mammalian hosts including humans, cattle, deer, llamas, pigs, domestic cats, wild carnivores (fox, coyotes, badgers), possums, mustelids and rodents; it rarely affects equids or sheep.^[4][5] The disease can be transmitted in several ways, for example, badgers excrete M. bovis 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.

Epidemiology and control

New Zealand

See also: Animal Health Board (New Zealand)

In New Zealand, the common brushtail possum is the main vector for the spread of M. bovis. The disease now endemic in possums across approximately 38% of New Zealand (known as ‘vector risk areas’). In these areas, nearly 70% of new herd infections can be traced back to possums or ferrets. The Biosecurity Act 1993, which established a National Pest Management Strategy, is the legislation behind control of the disease in New Zealand. The Animal Health Board (AHB) 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.^[6]

The TBfree New Zealand programme is regarded as "world-leading".^[7] 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 possum control 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%).^[8]

Possums are controlled through a combination of trapping, ground-baiting and, where other methods are impractical, aerial treament with 1080 poison.^[9]

From 1979 - 1984, possum control was stopped due to lack of funding. In spite of regular and frequent TB testing of cattle herds, the number of infected herds snowballed and continued to increase until 1994.^[10] The area of New Zealand where there were TB wild animals expanded from about 10 to 40%.

That possums are such effective transmitters of TB appears to be facilitated by their behaviour once they succumb to the disease. Terminally ill TB possums will show increasingly erratic behaviour, such as venturing out during the daytime to get enough food to eat, and seeking out buildings in which to keep warm. As a consequence, they may wander on to paddocks, where they naturally attract the attention of inquisitive cattle and deer. This behaviour has been captured on video.^[11]

United Kingdom

In the 1930s, 40% of cows in the UK were infected with M. bovis and there were 50,000 new cases of human M. bovis infection every year.^[12] According to DEFRA and the Health Protection Agency, the risk to people contracting TB from cattle in Great Britain today is very low. The HPA has said that three quarters of the 440 human cases reported to the HPA between 1994 and 2006 were aged 50 years and above and only 44 cases (16%) were known to be non-UK born.

Badgers (Meles meles) were first identified as carriers of M. bovis 30 years ago, but the report of an independent review committee in 1997 concluded badgers made an important contribution to the spread of M. bovis between herds of cattle.^[13] This was the major cause of the current battle between animal conservationists (keen to save the badger) and farmers (keen to cull badgers, to reduce livestock losses). The Randomised Badger Culling Trial ^[14] (designed, overseen and analysed by the Independent Scientific Group on Cattle TB, or ISG ^[15]) was 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,^[16] 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 regimes 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."^[17]

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, deer, especially fallow deer due to their gregarious behaviour, have been implicated as a possible maintenance host for transmission of bovine TB,^[18][19] a disease which in the UK in 2005 cost £90 million in attempts to eradicate.^[20] It has been argued that in some localised areas, the risk of transmission to cattle from fallow deer is greater than it is from badgers.^[18][19]

In a 2010 opinion piece in Trends in Microbiology, Paul and David Torgerson argued that bovine tuberculosis is a negligible public health problem in the UK, providing milk is pasteurized. Bovine TB is very rarely spread by aerosol from cattle to humans. Therefore, the bovine tuberculosis control programme in the UK in its present form is a misallocation of resources and provides no benefit to society. Indeed, there is even very little evidence of a positive cost benefit to the livestock industry, as few studies have been undertaken on the direct costs of bovine TB to animal production. Milk pasteurisation was the single public health intervention that prevented the transmission of bovine TB to humans, and there is no justification for the present test and cull policy in the UK.^[21]

United States of America

In the United States, there is endemic M. bovis in white-tailed deer (Odocoileus virginianus) in the northeastern portion of the state of Michigan and in northern Minnesota, and sporadic import of the disease from Mexico. The white-tailed deer has been confirmed as the sole maintenance host in the Michigan outbreak of bovine tuberculosis which remains a significant barrier to the US nationwide eradication of the disease in livestock. In 2008, 733,998 licensed deer hunters harvested approximately 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.^[22]

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 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 water buffalo in isolated parts of the Northern Territory. In Canada, there are affected wild elk and white-tailed deer in and around Riding Mountain National Park in Manitoba. To improve control and eliminate bovine TB, the Canadian Food Inspection Agency (CFIA) has split Manitoba into two management areas: The Riding Mountain TB eradication area (RMEA), the area where the disease has been found and the Manitoba TB Eradication Area (MTEA), the rest of the province outside RMEA where the disease has not been found.^[23] The disease has also been found in wild buffalo in South Africa.

Mycobacterium bovis can be transmitted from human to human; there was an outbreak in Birmingham, England in 2004,^[24] and from human to cattle,^[25][26] but such occurrences are rare.

Treatment

See: Tuberculosis treatment

M. bovis is innately resistant to pyrazinamide: therefore the standard treatment is isoniazid and rifampicin for 9 months. However, most cattle with TB will be culled.

See also

• Agriculture
• Veterinary medicine

References

1. ^ Karlson, A. G.; Lessel, E. F. (1970). "Mycobacterium bovis nom. nov.". International Journal of Systematic Bacteriology 20 (3): 273–282. doi:10.1099/00207713-20-3-273.  edit
2. ^ Grange, John M.; Malcolm D. Yates and Isabel N. de Kantor. (1996). "Guidelines for speciation within the Mycobacterium tuberculosis complex. Second edition" (PDF). World Health Organization.. http://whqlibdoc.who.int/hq/1996/WHO_EMC_ZOO_96.4.pdf. Retrieved 2007-08-02. 
3. ^ O'Reilly LM, Daborn CJ. (August 1995). "The epidemiology of Mycobacterium bovis infections in animals and man: a review". Tuber Lung Dis. 76 (Suppl 1): 1–46. doi:10.1016/0962-8479(95)90591-X. PMID 7579326. 
4. ^ Delahay, R.J., De Leeuw, A.N.S., Barlow, A.M., Clifton-Hadley, R.S. and Cheeseman, C.L. (2002). The status of Mycobacterium bovis infection in UK wild mammals: A review. The Veterinary Journal, 164: 90-105
5. ^ Phillips, C.J.C., Foster, C.R.W., Morris, P.A. and Teverson R. (2001). The transmission of Mycobacterium bovis infection to cattle. Research in Veterinary Science, 74: 1-15
6. ^ "TBfree New Zealand programme". http://www.ahb.org.nz/Default.aspx?tabid=116. 
7. ^ "Bovine TB control: What are other countries doing?". 21 July 2011. http://www.fwi.co.uk/Articles/2011/07/19/127893/Bovine-TB-control-what-are-other-countries-doing.htm. 
8. ^ Kean, J.M.; Barlow, N. D. and Hickling, G.J. (1999). "Evaluating potential sources of bovine tuberculosis infection in a New Zealand cattle herd". New Zealand Journal of Agricultural Research (New Zealand Journal of Agricultural Research.) 42: 101. doi:10.1080/00288233.1999.9513358. 
9. ^ "The use of 1080 for pest control - 3.1 Possums as reservoirs of bovine tuberculosis". 2011. http://www.doc.govt.nz/publications/conservation/threats-and-impacts/animal-pests/the-use-of-1080-for-pest-control/3-why-we-use-1080-for-pest-control/3_1-possums-as-reservoirs-of-bovine-tuberculosis/. 
10. ^ "Future freedom from bovine TB, Graham Nugent (Landcare Research)". 2011. http://www.ourfuture.net.nz/Stories/180. 
11. ^ "Dr Paul Livingstone letter to the editor". Gisborne Herald, 26 May 2011. http://www.gisborneherald.co.nz/article/?id=22957. 
12. ^ Reynolds D (2006). "A review of tuberculosis science and policy in Great Britain". Vet Microbiol 112 (2–4): 119–126. doi:10.1016/j.vetmic.2005.11.042. PMID 16343818. 
13. ^ Krebs JR, Anderson T, Clutton-Brock WT (1997). Bovine tuberculosis in cattle and badgers: an independent scientific review. London: Ministry of Agriculture, Fisheries and Food. 
14. ^ defra. "Bovine TB: Randomised Badger Culling Trial (RBCT)". http://www.defra.gov.uk/animalh/tb/culling/index.htm. Retrieved 2007-07-30. 
15. ^ defra. "Bovine TB: The Independent Scientific Group on Cattle TB". http://www.defra.gov.uk/animalh/tb/isg/index.htm. Retrieved 2007-07-30. 
16. ^ Independent Scientific Group on Cattle TB. "Bovine TB: The Scientific Evidence; Final Report of the Independent Scientific Group on Cattle TB Presented to the Secretary of State for Environment, Food and Rural Affairs The Rt Hon David Miliband MP, June 2007" (PDF). http://www.defra.gov.uk/animalh/tb/isg/pdf/final_report.pdf. Retrieved 2007-07-30. 
17. ^ Daily Hansard. "Daily Hansard, House of Lords; Thursday, 26 July 2007.". http://www.publications.parliament.uk/pa/ld200607/ldhansrd/text/70726-0001.htm#st_11. Retrieved 2007-07-30. 
18. ^ ^{a b} Delahay, R.J., Smith, G.C., Barlow, A.M., Walker, N., Harris, A., Clifton-Hadley, R.S. and Cheeseman, C.L. (2007). Bovine tuberculosis infection in wild mammals in the South-West region of England: A survey of prevalence and a semi-quantitative assessment of the relative risks to cattle. The Veterinary Journal, 173: 287-301
19. ^ ^{a b} Ward, A.I., Smith, G.C., Etherington, T.R. and Delahay, R.J. (2009). Estimating the risk of cattle exposure to tuberculosis posed by wild deer relative to badgers in England and Wales. Journal of Wildlife Diseases, 45: 1104–1120
20. ^ The Veterinary Record, (2008). Bovine TB: EFRACom calls for a multifaceted approach using all available methods. The Veterinary Record, 162: 258-259
21. ^ Torgerson, PR; Torgerson, DJ (2010). "Public health and bovine tuberculosis: what's all the fuss about?". Trends in Microbiology 18 (2): 67–72. doi:10.1016/j.tim.2009.11.002. PMID 19944609. 
22. ^ O’Brien, D.J., Schmitt, S.M., Fitzgerald, S.D. and Berry, D.E. (2011). Management of bovine tuberculosis in Michigan wildlife: Current status and near term prospects. Veterinary Microbiology, 151: 179-187
23. ^ " Bovine Tuberculosis (TB) Management - In the Riding Mountain Area, Manitoba Conservation
24. ^ Debora MacKenzie. "TB travelled from a farm to a night club". Archived from the original on 2007-09-28. http://web.archive.org/web/20070928023424/http://www.promedmail.org/pls/promed/f?p=2400:1001:::NO::F2400_P1001_BACK_PAGE,F2400_P1001_PUB_MAIL_ID:1000,34753. Retrieved 2007-03-21. 
25. ^ Griffith AS and Munro WT (1944). "Human pulmonary tuberculosis of bovine origin in Great Britain". J Hyg 43 (4): 229–40. doi:10.1017/S0022172400012894. 
26. ^ Tice FJ (1944). "Man, a source of bovine tuberculosis in cattle". Cornell Vet 34: 363–5. 

External links

• TB free New Zealand - 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