History of the Organism
According to the National Institute of Allergy and Infectious Diseases (NIAID), evidence of Mycobacterium tuberculosis (Mtb) has been found in ancient Egyptian mummies (par. 3). Tuberculosis (TB) was also a prevalent disease in the ancient Roman and Greek civilizations. Overcrowding in 17th-century cities made TB a major public health threat. Significant 19th-century scientific advances, including Koch’s discovery of the causative agent of TB (Tuberculosis Mycobacterium) in 1882, laid the groundwork for TB diagnosis and drug development (NIAID par. 8). The 1921 development of the Bacille Calmette-Guerin (BCG) vaccine was a breakthrough in the efforts to combat TB prevalence rates. However, in recent years, drug-resistant TB strains have emerged, presenting a new threat to public health.
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Habits and Sites in the Body where it is found
Mtb is a bacillus with an acid-fast cell wall that retains the carbon fuchsin stain even after an acid-alcohol treatment (Bauman 154). It is a non-motile bacillus with a size range of between 3.0µm and 0.4µm. Mtb is an obligate aerobe with a slow growth rate in its human host. Its cell wall is rich in mycolic acid and lipids, which make it impermeable to most substances, including common antimicrobials (Bauman 161). The microorganism primarily occurs in the lungs of infected individuals but can infect the “kidneys, spine, and brain” (Centers for Disease Control and Prevention (CDC) par. 6). Most Mtb infections are latent, but immune-compromised people can develop active TB symptoms.
Worldwide Distribution of the Disease
TB affects people on a worldwide scale with an estimated 2 billion people being infected with Mtb (NIAID par. 2). Geographically, World Health Organization (WHO) identifies Africa as the worst affected region with “280 cases per 100,000 people” being reported in 2013 (par.12). TB is also common in parts of Asia and the Western Pacific areas. According to NIAID, in the U.S., 3.6 TB cases per a population of a hundred thousand were reported in 2010; this was a 3.1% drop from the previous year (par. 6). However, the cases were higher among foreign-born persons than among the natives. Other countries showing a sustained reduction in TB prevalence include China, Brazil, and Cambodia.
How the Organism gets to the Host
Mtb is an airborne microorganism that enters its human host through inhaled air. An individual with pulmonary tuberculosis releases these bacteria into the air during coughing, talking, sneezing, or singing (CDC par. 4). Inhaling this air in poorly ventilated conditions introduces Mtb into the lungs, which is the main route of entry of the bacteria into the body. Public Health Agency of Canada (PHAC) states that a cough containing 3000 microns of Mtb is the infectious dose (par. 2). However, Mtb cannot be transmitted through activities such as handshaking, kissing, or sharing of food (CDC par. 6). Proper ventilation is essential in reducing the spread of the microorganism.
Vectors and Reservoirs Involved
The main reservoirs of Mtb are “infected humans and animals” (PHAC par. 17). Up to a third of the human population harbors the Mtb bacterium, but only shows latent TB, which is symptomatic. Besides humans, other hosts of Mtb include animals such as primates (monkeys), cattle, sheep, pets, and goats. Diseased animals, therefore, act as reservoirs of the bacteria. They spread the microorganisms to humans through “aerosols, fomites, and bites” (PHAC par. 8). Mtb has no known transmission vector.
The Parts of the Body it affects
Mtb primarily affects the lungs of its human host. Once inhaled, the bacterium finds its way into the lungs where it multiplies in number, which destroys the lung tissue (CDC par. 3). However, in most cases, Mtb remains dormant for a long time in the lungs without causing TB symptoms. Extra-pulmonary Mtb infection can affect other body organs, including the brain, spinal cord, and kidneys (CDC par. 8). Damage to these organs causes diseases such as meningitis, pulmonary lesions, pleuritis, and pericarditis.
How it contaminates and colonizes the Host
Mtb gains entry into the lungs when a person inhales air contaminated with the bacteria. If there is no immediate immune response, Mtb multiplies and enters ‘inactivated’ macrophages of the alveolar spaces. It then suppresses the acidification process essential for lysosomal enzyme activity and in this way avoids digestion by phagosomes (PHAC). This immune avoidance mechanism allows Mtb to multiply and grow in the alveolar macrophages without inhibition. However, the immune activation of macrophages allows the body to get rid of the bacteria. At a high bacteria load, the immune response produces cytokines that have an inflammatory effect on the lung tissue (PHAC). Mtb infection is more likely to develop into TB in immune-compromised individuals (HIV patients) than in healthy people.
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CDC identifies two kinds of prognostic tests for new Mtb infections, namely, “the TB blood test and the tuberculin skin test” (par. 6). The skin test entails a subcutaneous injection of a substance called tuberculin on the arm. The development of swelling in this area after two to three days is an indication that a reaction occurred. A positive skin test indicates latent Mtb infection. In contrast, TB blood tests determine an individual’s immune response to Mtb infection using interferon-gamma release assays. These tests determine how well a person’s immunity responds to exposure to TB bacteria.
Active TB has high mortality rates among all age groups. WHO reports that, out of the nine million people, who contracted TB in 2013, 1.5 million of them died (par. 5). The mortality rates are higher in low-income countries than in developed ones. Over 95% of deaths caused by TB globally take place in developing countries, where it ranks as one of the top killers of females between the ages of 15 and 44 (WHO par. 6). HIV-positive persons are at a greater risk of dying from TB than non-infected ones. In 2013, 360,000 HIV-infected persons succumbed to TB.
Decline or Increase in Prevalence
The prevalence of TB varies widely depending on the geographical region. A large proportion of TB cases reported in 2013 were from parts of Asia, Africa, and the Pacific regions, representing 56 percent of all the TB infections recorded worldwide (WHO par. 4). TB prevalence is still high in Africa with over a million new cases being reported in 2013 among HIV-infected persons. Overall, China, Cambodia, and Brazil top the list of countries where the prevalence of TB has declined significantly over the last two decades.
A Re-emerging Disease
TB is a re-emerging infectious disease. Its prevalence globally declined significantly with the increased use of antibiotics in TB treatment. However, more recently, antibiotic-resistant strains of Mtb have evolved into multidrug resistant (MDR) TB strains that show resistance to first-line antibiotics, such as rifampicin and isoniazid (NIAID par. 7). In contrast, extensively drug-resistant TB or XDR TB is resistant to both first- and second-line antibiotic therapy. The inappropriate antibiotic treatment causes Mtb to evolve into resistant TB strains. Globally, MDR TB cases are increasing with over 650,000 and 480,000 infections being reported in 2010 and 2013 respectively (WHO par. 21). Even with antibiotic therapy, six out of ten MDR TB patients die from the disease.
Children and infants can be immunized against TB using the Bacille Calmette-Guerin (BCG) vaccine. It is the only available vaccine against Mtb strains. The vaccine contains attenuated Mycobacterium Bovis strains. It is effective against TB in children, but less effective against Mtb in adults (PHAC par. 6). In children, BCG immunization prevents TB-related meningitis. Current vaccine research focuses on attenuated and recombinant Mtb strains.
Medicines Prescribed to Treat the Disease
The treatment of active TB disease involves a course of antibiotic medicines that lasts up to six months. Four first-line antibiotics, namely, “isoniazid, oral rifampicin, oral Pyrazinamide, and oral Ethambutol” are prescribed for susceptible Mtb strain (PHAC par. 14). The treatment prescribed depends on the susceptibility level of the Mtb strain, which is determined using isolated bacterial cultures. MDR TB, which is resistant to these four drugs, is treated with second-line antibiotics like “amikacin, kanamycin, or capreomycin” (NIAID par. 11). Normally, a daily dose of three drugs lasting up to three years is prescribed to patients with MDR TB.
My Advice to a Patient Infected with Mtb
I would advise a person infected with Mtb to register with a health care facility to receive appropriate treatment and care. He or she should undergo testing (skin or blood tests) to help treat Mtb infection at its asymptomatic stage before developing into the TB disease and becoming infectious. In addition, testing helps prevent the spread of the bacteria to other people interacting with an infected individual (WHO par. 9). I would also advise the patient to complete his or her course of the prescribed drugs to avoid contracting MDR TB. Treating MDR TB would require a combination of many antibiotics taken over an extended period.
Mutations and Resistance
Drug-resistant Mtb strains arise when there is an inappropriate TB treatment. The failure to finish treatment and the prescription of the wrong dose or low-quality drug can lead to drug resistance. Drug-resistant phenotypes include MDR TB and XDR TB. Mutations confer Mtb its resistance to available antibiotics, making it difficult to treat the disease. The genetic variants are responsible for the resistance to first- and second-line antibiotics.
The Drugs Mtb is Resistant to
The Mtb causing MDR TB is not susceptible to two of the best available antibiotics, namely, isoniazid and rifampin (CDC par. 5). The two drugs are prescribed to treat active TB infections. In contrast, susceptible strains are sensitive to first-line drugs like “isoniazid, Ethambutol, rifampin, and Pyrazinamide” (PHAC par. 13). On the other hand, XDR TB is highly resistant to both first- and second- group antibiotics. Additionally, it is not sensitive to isoniazid or rifampin. It also exhibits resistance to fluoroquinolones and other TB medicines such as capreomycin, kanamycin, and amikacin (CDC par. 4). As such, the available treatments are less effective against XDR TB.
Factors Contributing to Drug Resistance
One of the causes of drug resistance is the inappropriate use of anti-TB medicines (CDC par. 5). Failure to finish a treatment course, wrong drug prescription, inadequate dosage, low-quality drugs, lack of drugs, and incorrect timing during drug administration are the leading causes of drug resistance by Mtb bacteria (CDC par. 7). Infected individuals, who fail to adhere to the treatment instructions or take drugs as instructed are at risk of developing MDR TB. Recurrent TB infections can also lead to MDR TB in recovering patients.
Bauman, Robert. Microbiology with Diseases by Body System, New York: Person Education, 2014. Print.
Centers for Disease Control and Prevention. Tuberculosis. 2015. Web.
National Institute of Allergy and Infectious Diseases. Tuberculosis (TB). 2015. Web.
Public Health Agency of Canada. Mycobacterium Tuberculosis Complex. 2015. Web.
World Health Organization. Fact Sheets: Tuberculosis. 2015. Web.
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