Other names
In the past, tuberculosis has been called consumption, because it seemed to consume people from within, with a bloody cough, fever, pallor, and long relentless wasting. Other names included phthisis (Greek for consumption) and phthisis pulmonalis; scrofula (in adults), affecting the lymphatic system and resulting in swollen neck glands; tabes mesenterica, TB of the abdomen and lupus vulgaris, TB of the skin; wasting disease; white plague, because sufferers appear markedly pale; king’s evil, because it was believed that a king’s touch would heal scrofula; and Pott’s disease, or gibbus of the spine and joints.[7][8] Miliary tuberculosis—now commonly known as disseminated TB—occurs when the infection invades the circulatory system resulting in lesions which have the appearance of millet seeds on X-ray.[7][9]
[edit] Symptoms
Further information: Tuberculosis classification
When the disease becomes active, 75% of the cases are pulmonary TB. Symptoms include chest pain, coughing up blood, and a productive, prolonged cough for more than three weeks. Systemic symptoms include fever, chills, night sweats, appetite loss, weight loss, pallor, and often a tendency to fatigue very easily.[2]
In the other 25% of active cases, the infection moves from the lungs, causing other kinds of TB more common in immunosuppressed persons and young children. Extrapulmonary infection sites include the pleura, the central nervous system in meningitis, the lymphatic system in scrofula of the neck, the genitourinary system in urogenital tuberculosis, and bones and joints in Pott’s disease of the spine. An especially serious form is disseminated TB, more commonly known as miliary tuberculosis. Although extrapulmonary TB is not contagious, it may co-exist with pulmonary TB, which is contagious.[10]
[edit] Bacterial species
Main article: Mycobacterium tuberculosis
Scanning electron micrograph of Mycobacterium tuberculosisThe primary cause of TB, Mycobacterium tuberculosis, is an aerobic bacterium that divides every 16 to 20 hours, an extremely slow rate compared with other bacteria, which usually divide in less than an hour.[11] (For example, one of the fastest-growing bacteria is a strain of E. coli that can divide roughly every 20 minutes.) Since MTB has a cell wall but lacks a phospholipid outer membrane, it is classified as a Gram-positive bacterium. However, if a Gram stain is performed, MTB either stains very weakly Gram-positive or does not retain dye due to the high lipid & mycolic acid content of its cell wall.[12] MTB is a small rod-like bacillus that can withstand weak disinfectants and survive in a dry state for weeks. In nature, the bacterium can grow only within the cells of a host organism, but M. tuberculosis can be cultured in vitro.[13]
Using histological stains on expectorate samples from phlegm (also called sputum), scientists can identify MTB under a regular microscope. Since MTB retains certain stains after being treated with acidic solution, it is classified as an acid-fast bacillus (AFB).[12] The most common staining technique, the Ziehl-Neelsen stain, dyes AFBs a bright red that stands out clearly against a blue background. Other ways to visualize AFBs include an auramine-rhodamine stain and fluorescent microscopy.
The M. tuberculosis complex includes three other TB-causing mycobacteria: M. bovis, M. africanum and M. microti. The first two only very rarely cause disease in immunocompetent people. On the other hand, although M. microti is not usually pathogenic, it is possible that the prevalence of M. microti infections has been underestimated.[14]
Other known pathogenic mycobacteria include Mycobacterium leprae, Mycobacterium avium and M. kansasii. The last two are part of the nontuberculous mycobacteria (NTM) group. Nontuberculous mycobacteria cause neither TB nor leprosy, but they do cause pulmonary diseases resembling TB.[15]
[edit] Evolution
During its evolution, M. tuberculosis has lost numerous coding and non-coding regions in its genome, losses that can be used to distinguish between strains of the bacteria. The implication is that M. tuberculosis strains differ geographically, so their genetic differences can be used to track the origins and movement of each strain.[16]
[edit] Transmission
Further information: Transmission (medicine)
When people suffering from active pulmonary TB cough, sneeze, speak, or spit, they expel infectious aerosol droplets 0.5 to 5 µm in diameter. A single sneeze, for instance, can release up to 40,000 droplets.[17] Each one of these droplets may transmit the disease, since the infectious dose of tuberculosis is very low and the inhalation of just a single bacterium can cause a new infection.[18]
Tuberculosis is spread by aerosols created by coughing or sneezing.People with prolonged, frequent, or intense contact are at particularly high risk of becoming infected, with an estimated 22% infection rate. A person with active but untreated tuberculosis can infect 10–15 other people per year.[2] Others at risk include people in areas where TB is common, people who inject drugs using unsanitary needles, residents and employees of high-risk congregate settings, medically under-served and low-income populations, high-risk racial or ethnic minority populations, children exposed to adults in high-risk categories, patients immunocompromised by conditions such as HIV/AIDS, people who take immunosuppressant drugs, and health care workers serving these high-risk clients.[19]
Transmission can only occur from people with active—not latent—TB. The probability of transmission from one person to another depends upon the number of infectious droplets expelled by a carrier, the effectiveness of ventilation, the duration of exposure, and the virulence of the M. tuberculosis strain.[10] The chain of transmission can therefore be broken by isolating patients with active disease and starting effective anti-tuberculous therapy. After two weeks of such treatment, people with non-resistant active TB generally cease to be contagious. If someone does become infectous, then it will take at least from 21 days, or 3–4 weeks, before the new infectous person can transmit the disease to others.[20] TB can also be transmitted by eating meat if the cattle is infected with TB. Mycobacterium bovis causes TB in cattle. Details below.
[edit] Pathogenesis
Mycobacterium tuberculosis (stained red) in sputumAbout 90% of those infected with Mycobacterium tuberculosis have asymptomatic, latent TB infection (sometimes called LTBI), with only a 10% lifetime chance that a latent infection will progress to TB disease. However, if untreated, the death rate for these active TB cases is more than 50%.[21]
TB infection begins when the mycobacteria reach the pulmonary alveoli, where they invade and replicate within alveolar macrophages.[22] The primary site of infection in the lungs is called the Ghon focus. Bacteria are picked up by dendritic cells, which do not allow replication, although these cells can transport the bacilli to local (mediastinal) lymph nodes. Further spread is through the bloodstream to the more distant tissues and organs where secondary TB lesions can develop in lung apices, peripheral lymph nodes, kidneys, brain, and bone.[23] All parts of the body can be affected by the disease, though it rarely affects the heart, skeletal muscles, pancreas and thyroid.[24]
Tuberculosis is classified as one of the granulomatous inflammatory conditions. Macrophages, T lymphocytes, B lymphocytes and fibroblasts are among the cells that aggregate to form a granuloma, with lymphocytes surrounding the infected macrophages. The granuloma functions not only to prevent dissemination of the mycobacteria, but also provides a local environment for communication of cells of the immune system. Within the granuloma, T lymphocytes (CD4+) secrete cytokines such as interferon gamma, which activates macrophages to destroy the bacteria with which they are infected.[25] T lymphocytes (CD8+) can also directly kill infected cells.[22]
Importantly, bacteria are not always eliminated within the granuloma, but can become dormant, resulting in a latent infection. Another feature of the granulomas of human tuberculosis is the development of cell death, also called necrosis, in the center of tubercles. To the naked eye this has the texture of soft white cheese and was termed caseous necrosis.[26]
If TB bacteria gain entry to the bloodstream from an area of damaged tissue they spread through the body and set up many foci of infection, all appearing as tiny white tubercles in the tissues. This severe form of TB disease is most common in infants and the elderly and is called miliary tuberculosis. Patients with this disseminated TB have a fatality rate of approximately 20%, even with intensive treatment.[27]
In many patients the infection waxes and wanes. Tissue destruction and necrosis are balanced by healing and fibrosis.[26] Affected tissue is replaced by scarring and cavities filled with cheese-like white necrotic material. During active disease, some of these cavities are joined to the air passages bronchi and this material can be coughed up. It contains living bacteria and can therefore pass on infection. Treatment with appropriate antibiotics kills bacteria and allows healing to take place. Upon cure, affected areas are eventually replaced by scar tissue.[26]
[edit] Diagnosis
For more details on this topic, see Tuberculosis diagnosis.
Mantoux tuberculin skin testTuberculosis can be a difficult disease to diagnose, mainly due to the difficulty in culturing this slow-growing organism in the laboratory (4–12 weeks for blood culture). A complete medical evaluation for TB must include a medical history, a chest X-ray, and a physical examination. Tuberculosis radiology is used in the diagnosis of TB. It may also include a tuberculin skin test, a serological test, microbiological smears and cultures. The interpretation of the tuberculin skin test depends upon the person’s risk factors for infection and progression to TB disease, such as exposure to other cases of TB or immunosuppression.[10]
Currently, latent infection is diagnosed in a non-immunized person by a tuberculin skin test, which yields a delayed hypersensitivity type response to an extract made from M. tuberculosis. Those immunized for TB or with past-cleared infection will respond with delayed hypersensitivity parallel to those currently in a state of infection, so the test must be used with caution, particularly with regard to persons from countries where TB immunization is common.[28] New TB tests are being developed that offer the hope of cheap, fast and more accurate TB testing. These use polymerase chain reaction detection of bacterial DNA and antibody assays to detect the release of interferon gamma in response to mycobacteria.[29] These tests are not affected by immunization, so generate fewer false positive results.[30] Rapid and inexpensive diagnosis will be particularly valuable in the developing world.
[edit] Progression
Progression from TB infection to TB disease occurs when the TB bacilli overcome the immune system defenses and begin to multiply. In primary TB disease—1–5% of cases—this occurs soon after infection. However, in the majority of cases, a latent infection occurs that has no obvious symptoms. These dormant bacilli can produce tuberculosis in 2–23% of these latent cases, often many years after infection.[31] The risk of reactivation increases with immunosuppression, such as that caused by infection with HIV. In patients co-infected with M. tuberculosis and HIV, the risk of reactivation increases to 10% per year.[21]
Other conditions that increase risk include drug injection, mainly due to the lifestyle of IV drug users; recent TB infection or a history of inadequately treated TB; chest X-ray suggestive of previous TB, showing fibrotic lesions and nodules; diabetes mellitus; silicosis; prolonged corticosteroid therapy and other immunosuppressive therapy; head and neck cancers; hematologic and reticuloendothelial diseases, such as leukemia and Hodgkin’s disease; end-stage kidney disease; intestinal bypass or gastrectomy; chronic malabsorption syndromes; or low body weight.[10]
Twin studies in the 1950s showed that the course of TB infection was highly dependent on genetics. At that time, it was rare that one identical twin would die and the other live.[32]
Some drugs, including rheumatoid arthritis drugs that work by blocking tumor necrosis factor-alpha (an inflammation-causing cytokine), raise the risk of activating a latent infection due to the importance of this cytokine in the immune defense against TB.[33]
[edit] Treatment
For more details on this topic, see Tuberculosis treatment.
Treatment for TB uses antibiotics to kill the bacteria. The two antibiotics most commonly used are rifampicin and isoniazid. However, instead of the short course of antibiotics typically used to cure other bacterial infections, TB requires much longer periods of treatment (around 6 to 12 months) to entirely eliminate mycobacteria from the body.[10] Latent TB treatment usually uses a single antibiotic, while active TB disease is best treated with combinations of several antibiotics, to reduce the risk of the bacteria developing antibiotic resistance.[34] People with latent infections are treated to prevent them from progressing to active TB disease later in life. However, treatment using Rifampin and Pyrazinamide is not risk-free. The Centers for Disease Control and Prevention (CDC) notified healthcare professionals of revised recommendations against the use of rifampin plus pyrazinamide for treatment of latent tuberculosis infection, due to high rates of hospitalization and death from liver injury associated with the combined use of these drugs.[35]
Drug resistant tuberculosis is transmitted in the same way as regular TB. Primary resistance occurs in persons who are infected with a resistant strain of TB. A patient with fully-susceptible TB develops secondary resistance (acquired resistance) during TB therapy because of inadequate treatment, not taking the prescribed regimen appropriately, or using low quality medication.[34] Drug-resistant TB is a public health issue in many developing countries, as treatment is longer and requires more expensive drugs. Multi-drug resistant TB (MDR-TB) is defined as resistance to the two most effective first line TB drugs: rifampicin and isoniazid. Extensively drug-resistant TB (XDR-TB) is also resistant to three or more of the six classes of second-line drugs.[4]
In ancient times, available treatments focused more on dietary parameters. Pliny the Elder described several methods in his Natural History: “wolf’s liver taken in thin wine, the lard of a sow that has been fed upon grass, or the flesh of a she-ass taken in broth”.[36] While these particular remedies haven’t been tested scientifically, it has been demonstrated that malnourished mice receiving a 2% protein diet suffer far higher mortality from tuberculosis than those receiving 20% protein receiving the same infectious challenge dose, and the progressively fatal course of the illness could be reversed by restoring the mice to the normal diet.[37] Moreover, statistics for immigrants in South London reveal an 8.5 fold increased risk of tuberculosis in (primarily Hindu Asian) lacto vegetarians, who frequently suffer protein malnutrition, compared to those of similar cultural backgrounds who ate meat and fish daily.[38]
[edit] Prevention
TB prevention and control takes two parallel approaches. In the first, people with TB and their contacts are identified and then treated. Identification of infections often involves testing high-risk groups for TB. In the second approach, children are vaccinated to protect them from TB. Unfortunately, no vaccine is available that provides reliable protection for adults. However, in tropical areas where the levels of other species of mycobacteria are high, exposure to nontuberculous mycobacteria gives some protection against TB.[39]
[edit] Vaccines
Many countries use Bacillus Calmette-Guérin (BCG) vaccine as part of their TB control programs, especially for infants. This was the first vaccine for TB and developed at the Pasteur Institute in France between 1905 and 1921.[40] However, mass vaccination with BCG did not start until after World War II.[41] The protective efficacy of BCG for preventing serious forms of TB (e.g. meningitis) in children is greater than 80%; its protective efficacy for preventing pulmonary TB in adolescents and adults is variable, ranging from 0 to 80%.[42]
In South Africa, the country with the highest prevalence of TB, BCG is given to all children under age three.[43] However, BCG is less effective in areas where mycobacteria are less prevalent; therefore BCG is not given to the entire population in these countries. In the USA, for example, BCG vaccine is not recommended except for people who meet specific criteria:[10]
Infants or children with negative skin test results who are continually exposed to untreated or ineffectively treated patients or will be continually exposed to multidrug-resistant TB.
Healthcare workers considered on an individual basis in settings in which a high percentage of MDR-TB patients has been found, transmission of MDR-TB is likely, and TB control precautions have been implemented and were not successful.
BCG provides some protection against severe forms of pediatric TB, but has been shown to be unreliable against adult pulmonary TB, which accounts for most of the disease burden worldwide. Currently, there are more cases of TB on the planet than at any other time in history and most agree there is an urgent need for a newer, more effective vaccine that would prevent all forms of TB—including drug resistant strains—in all age groups and among people with HIV.[44]
Several new vaccines to prevent TB infection are being developed. The first recombinant tuberculosis vaccine entered clinical trials in the United States in 2004, sponsored by the National Institute of Allergy and Infectious Diseases (NIAID).[45] A 2005 study showed that a DNA TB vaccine given with conventional chemotherapy can accelerate the disappearance of bacteria as well as protect against re-infection in mice; it may take four to five years to be available in humans.[46] A very promising TB vaccine, MVA85A, is currently in phase II trials in South Africa by a group led by Oxford University,[47] and is based on a genetically modified vaccinia virus. Many other strategies are also being used to develop novel vaccines. In order to encourage further discovery, researchers and policymakers are promoting new economic models of vaccine development including prizes, tax incentives and advance market commitments.[48][49]
The Bill and Melinda Gates Foundation has been a strong supporter of new TB vaccine development. Most recently, they announced a $200 million grant to the Aeras Global TB Vaccine Foundation for clinical trials on up to six different TB vaccine candidates currently in the pipeline.[50]
[edit] Epidemiology
Annual number of new reported TB cases. Data from WHO.[51]
World TB incidence. Cases per 100,000; Red = >300, orange = 200–300; yellow = 100–200; green 50–100; blue = <50 and grey = n/a. Data from WHO, 2006.[51]According to the World Health Organization (WHO), nearly 2 billion people—one third of the world's population—have been exposed to the tuberculosis pathogen.[52] Annually, 8 million people become ill with tuberculosis, and 2 million people die from the disease worldwide.[53] In 2004, around 14.6 million people had active TB disease with 9 million new cases. The annual incidence rate varies from 356 per 100,000 in Africa to 41 per 100,000 in the Americas.[2] Tuberculosis is the world's greatest infectious killer of women of reproductive age and the leading cause of death among people with HIV/AIDS.[54]
In 2005, the country with the highest estimated incidence of TB was Swaziland, with 1262 cases per 100,000 people. India has the largest number of infections, with over 1.8 million cases.[55] In developed countries, tuberculosis is less common and is mainly an urban disease. In the United Kingdom, TB incidences range from 40 per 100,000 in London to less than 5 per 100,000 in the rural South West of England;[56] the national average is 13 per 100,000. The highest rates in Western Europe are in Portugal (42 per 100,000) and Spain (20 per 100,000). These rates compare with 113 per 100,000 in China and 64 per 100,000 in Brazil. In the United States, the overall tuberculosis case rate was 4.9 per 100,000 persons in 2004.[53]
The incidence of TB varies with age. In Africa, TB primarily affects adolescents and young adults.[57] However, in countries where TB has gone from high to low incidence, such as the United States, TB is mainly a disease of older people.[58]
There are a number of known factors that make people more susceptible to TB infection: worldwide the most important of these is HIV. Co-infection with HIV is a particular problem in Sub-Saharan Africa, due to the high incidence of HIV in these countries.[51][59] Smoking more than 20 cigarettes a day also increases the risk of TB by two to four times.[60][61] Diabetes mellitus is also an important risk factor that is growing in importance in developing countries.[62]
[edit] History
Tubercular decay has been found in the spines of Egyptian mummies. Pictured: Egyptian mummy in the British MuseumTuberculosis has been present in humans since antiquity. The earliest unambiguous detection of Mycobacterium tuberculosis is in the remains of bison dated 18,000 years before the present.[63] However, whether tuberculosis originated in cattle and then transferred to humans, or diverged from a common ancestor, is currently unclear.[64] Skeletal remains show prehistoric humans (4000 BC) had TB, and tubercular decay has been found in the spines of mummies from 3000–2400 BC.[65] Phthisis is a Greek term for tuberculosis; around 460 BC, Hippocrates identified phthisis as the most widespread disease of the times involving coughing up blood and fever, which was almost always fatal.[66] Genetic studies suggest that TB was present in South America for about 2,000 years.[67] In South America, the earliest evidence of tuberculosis is associated with the Paracas-Caverna culture (circa 750 BC to circa 100 AD).[68]
[edit] Folklore
Before the Industrial Revolution, tuberculosis may sometimes have been regarded as vampirism. When one member of a family died from it, the other members that were infected would lose their health slowly. People believed that this was caused by the original victim draining the life from the other family members. Furthermore, people who had TB exhibited symptoms similar to what people considered to be vampire traits. People with TB often have symptoms such as red, swollen eyes (which also creates a sensitivity to bright light), pale skin and coughing blood, suggesting the idea that the only way for the afflicted to replenish this loss of blood was by sucking blood.[69] Another folk belief attributed it to being forced, nightly, to attend fairy revels, so that the victim wasted away owing to lack of rest; this belief was most common when a strong connection was seen between the fairies and the dead.[70] Similarly, but less commonly, it was attributed to the victims being "hagridden"—being transformed into horses by witches (hags) to travel to their nightly meetings, again resulting in a lack of rest.[70]
TB was romanticized in the nineteenth century. Many people believed TB produced feelings of euphoria referred to as "Spes phthisica" or "hope of the consumptive". It was believed that TB sufferers who were artists had bursts of creativity as the disease progressed. It was also believed that TB sufferers acquired a final burst of energy just before they died which made women more beautiful and men more creative.[71] In the early 20th century, some believed TB to be caused by masturbation.[72]
[edit] Study and treatment
The study of tuberculosis dates back to The Canon of Medicine written by Ibn Sina (Avicenna) in the 1020s. He was the first physician to identify pulmonary tuberculosis as a contagious disease, the first to recognise the association with diabetes, and the first to suggest that it could spread through contact with soil and water.[73][74] He developed the method of quarantine in order to limit the spread of tuberculosis.[75]
Although it was established that the pulmonary form was associated with 'tubercles' by Dr Richard Morton in 1689,[76][77] due to the variety of its symptoms, TB was not identified as a single disease until the 1820s and was not named 'tuberculosis' until 1839 by J. L. Schönlein.[78] During the years 1838–1845, Dr. John Croghan, the owner of Mammoth Cave, brought a number of tuberculosis sufferers into the cave in the hope of curing the disease with the constant temperature and purity of the cave air: they died within a year.[79] The first TB sanatorium opened in 1859 in Görbersdorf, Germany (today Soko?owsko, Poland) by Hermann Brehmer.[80]
In regard to this claim, The Times for January 15, 1859, page 5, column 5, carries an advertisement seeking funds for the Bournemouth Sanatorium for Consumption, referring to the balance sheet for the past year, and offering an annual report to prospective donors, implying that this sanatorium was in existence at least in 1858.
Dr. Robert Koch discovered the tuberculosis bacilli.The bacillus causing tuberculosis, Mycobacterium tuberculosis, was identified and described on March 24, 1882 by Robert Koch. He received the Nobel Prize in physiology or medicine in 1905 for this discovery.[81] Koch did not believe that bovine (cattle) and human tuberculosis were similar, which delayed the recognition of infected milk as a source of infection. Later, this source was eliminated by the pasteurization process. Koch announced a glycerine extract of the tubercle bacilli as a "remedy" for tuberculosis in 1890, calling it 'tuberculin'. It was not effective, but was later adapted as a test for pre-symptomatic tuberculosis.[82]
The first genuine success in immunizing against tuberculosis was developed from attenuated bovine-strain tuberculosis by Albert Calmette and Camille Guérin in 1906. It was called "BCG" (Bacillus of Calmette and Guérin). The BCG vaccine was first used on humans in 1921 in France,[40] but it was not until after World War II that BCG received widespread acceptance in the USA, Great Britain, and Germany.[41]
Tuberculosis, or "consumption" as it was commonly known, caused the most widespread public concern in the 19th and early 20th centuries as an endemic disease of the urban poor. In 1815, one in four deaths in England was of consumption; by 1918 one in six deaths in France were still caused by TB. In the 20th century, tuberculosis killed an estimated 100 million people.[83] After the establishment in the 1880s that the disease was contagious, TB was made a notifiable disease in Britain; there were campaigns to stop spitting in public places, and the infected poor were "encouraged" to enter sanatoria that resembled prisons; the sanatoria for the middle and upper classes offered excellent care and constant medical attention.[80] Whatever the purported benefits of the fresh air and labor in the sanatoria, even under the best conditions, 50% of those who entered were dead within five years (1916).[80]
Public health campaigns tried to halt the spread of TBThe promotion of Christmas Seals began in Denmark during 1904 as a way to raise money for tuberculosis programs. It expanded to the United States and Canada in 1907–08 to help the National Tuberculosis Association (later called the American Lung Association).
In the United States, concern about the spread of tuberculosis played a role in the movement to prohibit public spitting except into spittoons.
In Europe, deaths from TB fell from 500 out of 100,000 in 1850 to 50 out of 100,000 by 1950. Improvements in public health were reducing tuberculosis even before the arrival of antibiotics, although the disease remained a significant threat to public health, such that when the Medical Research Council was formed in Britain in 1913 its initial focus was tuberculosis research.[84]
It was not until 1946 with the development of the antibiotic streptomycin that effective treatment and cure became possible. Prior to the introduction of this drug, the only treatment besides sanatoria were surgical interventions, including the pneumothorax technique—collapsing an infected lung to "rest" it and allow lesions to heal—a technique that was of little benefit and was largely discontinued by the 1950s.[85] The emergence of multidrug-resistant TB has again introduced surgery as part of the treatment for these infections. Here, surgical removal of chest cavities will reduce the number of bacteria in the lungs, as well as increasing the exposure of the remaining bacteria to drugs in the bloodstream, and is therefore thought to increase the effectiveness of the chemotherapy.[86]
Hopes that the disease could be completely eliminated have been dashed since the rise of drug-resistant strains in the 1980s. For example, tuberculosis cases in Britain, numbering around 117,000 in 1913, had fallen to around 5,000 in 1987, but cases rose again, reaching 6,300 in 2000 and 7,600 cases in 2005.[87] Due to the elimination of public health facilities in New York and the emergence of HIV, there was a resurgence in the late 1980s.[88] The number of those failing to complete their course of drugs is high. NY had to cope with more than 20,000 "unnecessary" TB-patients with multidrug-resistant strains (resistant to, at least, both Rifampin and Isoniazid). The resurgence of tuberculosis resulted in the declaration of a global health emergency by the World Health Organization in 1993.[89]
[edit] Infection of other animals
Main article: Mycobacterium bovis
Tuberculosis can be carried by mammals; domesticated species, such as cats and dogs, are generally free of tuberculosis, but wild animals may be carriers. In some places, regulations aiming to prevent the spread of TB restrict the ownership of novelty pets; for example, the U.S. state of California forbids the ownership of pet gerbils.[90]
Mycobacterium bovis causes TB in cattle. An effort to eradicate bovine tuberculosis from the cattle and deer herds of New Zealand is underway. It has been found that herd infection is more likely in areas where infected vector species such as Australian brush-tailed possums come into contact with domestic livestock at farm/bush borders.[91] Controlling the vectors through possum eradication and monitoring the level of disease in livestock herds through regular surveillance are seen as a "two-pronged" approach to ridding New Zealand of the disease.
In the Republic of Ireland and the United Kingdom, badgers have been identified as one vector species for the transmission of bovine tuberculosis. As a result, governments have come under pressure from some quarters, primarily dairy farmers, to mount an active campaign of eradication of badgers in certain areas with the purpose of reducing the incidence of bovine TB. The effectiveness of culling on the incidence of TB in cattle is a contentious issue, with proponents and opponents citing their own studies to support their position.[92][93][94] For instance, a study by an Independent Study Group on badger culling reported on 18 June 2007 that it was unlikely to be effective and would only make a “modest difference” to the spread of TB and that "badger culling cannot meaningfully contribute to the future control of cattle TB"; in contrast, another report concluded that this policy would have a significant impact.[95] On July 4th 2008, the UK government decided against a proposed random culling policy
