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Anthrax

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ABOUT THE MICROBE

Anthrax is an acute infectious disease caused by the spore-forming, rod-shaped bacterium Bacillus anthracis. Predominantly a cause of livestock disease, B. anthracis forms durable spores that can lie dormant in the soil for years. Once eaten by a grazing animal, the spores are activated and the bacteria reproduce. After the bacteria spread, they typically kill the infected animal and return to the soil or water once again as spores.

The bacterium's destructive properties are due largely to toxins, which consist of three proteins: protective antigen, edema factor, and lethal factor.

  • Protective antigen (PA) binds to select cells of an infected person or animal and forms a channel that permits edema factor and lethal factor to enter those cells.

  • Edema factor (EF), once inside the cell, causes fluid to accumulate at the site of infection. EF can contribute to a fatal buildup of fluid in the cavity surrounding the lungs. It also can inhibit some of the body's immune functions.

  • Lethal factor (LF), once inside the cell, disrupts a key molecular switch that regulates the cell's functions. LF can kill infected cells or prevent them from working properly.

ABOUT THE DISEASE

People rarely contract anthrax from healthy animals. Contact with infected livestock or their products such as leather and wool does, however, cause a limited number of anthrax cases throughout the world. In the United States, only 236 anthrax cases were reported between 1955 and 1999, an average of about five per year. Most of those cases were occupational exposures in people who work with animal carcasses or products. The treatable cutaneous (skin) form of the disease is most common. Worldwide incidence is unknown, but anthrax occurs more frequently in developing countries, especially those without strong veterinary public health programs. Anthrax is not transmitted from person to person.

Human anthrax occurs primarily in three forms: cutaneous, gastrointestinal, and inhalation.

  • Cutaneous anthrax occurs when the bacteria, usually from infected animal products, enter a break in the skin. The skin reddens and swells, much like an insect bite, then develops a painless blackened lesion or ulcer that may form a brown scab. If left untreated, the infection can spread through the body. Cutaneous anthrax is the most common form of the diseases and responds well to antibiotics. It is rarely fatal if treated before it becomes invasive.

  • Gastrointestinal anthrax may arise when a person eats contaminated food. The infection often causes fever accompanied by gastrointestinal problems such as vomiting, abdominal pain, diarrhea, or loss of appetite. In some cases, lesions may form in the nose and throat instead of the lower digestive tract. In both cases, gastrointestinal anthrax can spread through the body and is often fatal if not treated immediately. This form of anthrax, however, is not known to have occurred in the United States.

  • Inhalation anthrax, sometimes called respiratory or pulmonary anthrax, occurs when the bacterial spores are inhaled. The early symptoms resemble those of a common cold or sore throat. The spores travel from the lungs to immune cells called macrophages in the nearby lymph nodes. There they begin to reproduce and secrete their toxins, causing severe breathing problems and shock. Treatment is difficult once the bacteria have reached that stage, and death often ensues. Naturally occurring inhalation anthrax is rare. Prior to the bioterrorist attack of 2001, the last known case of inhalation anthrax in the United States occurred in 1976 in a California craftsman who apparently contracted the infection from contaminated, imported yarn.

TREATMENT AND PREVENTION

Antibiotics

Several different antibiotics kill B. anthracis as it reproduces within people and animals. If diagnosed early, anthrax can be treated. Unfortunately, infected people often confuse early symptoms with more common infections and do not seek medical help until severe symptoms appear. At that point the destructive anthrax toxins, which are not affected by antibiotics, have risen to high levels, making treatment difficult. Although cutaneous anthrax has telltale signs and symptoms making diagnosis easy, early stage gastrointestinal and inhalation anthrax are more likely to be mistaken for common maladies.

Vaccine

An anthrax vaccine is licensed for limited use. The vaccine is currently used to protect members of the military and individuals most at risk for occupational exposure to the bacteria, such as abattoir workers, veterinarians, laboratory workers, and livestock handlers. The vaccine consists of filtered proteins and other components of a weakened B. anthracis strain adsorbed to aluminum hydroxide. PA is the major component of the vaccine that provides protection against infection. The vaccine contains no whole bacteria.

Health experts currently do not recommend the vaccine for general use by the public due to the rarity of anthrax and the potential for adverse side effects. Researchers have not determined the safety and efficacy of the vaccine in children, the elderly, and people with weakened immune systems. In addition, the recommended vaccination schedule is 6 doses given over an 18-month period, so the vaccine would likely offer little protection in response to a bioterrorist attack. For these reasons, a new anthrax vaccine is needed.

NIAID BASIC RESEARCH

Several biologic factors contribute to B. anthracis's ability to cause disease. By uncovering the molecular pathways that enable the bacterium to form spores, survive in people, and cause illness, NIAID hopes to identify new ways to diagnose, prevent, and treat anthrax.

Toxin Biology

Scientists are studying the anthrax toxins to learn how to block their production or action. Recently, NIAID grantees determined the three-dimensional structure of the LF protein as it attaches to its target inside cells. The research showed for the first time that LF uses a long groove on its side to latch onto that target. At the same time, another group of researchers identified a protein receptor on the surface of host cells to which PA attaches. Using a specific fragment of that receptor protein, the researchers were able to block the attachment of PA, thereby preventing formation of the PA channel and inhibiting the toxic effects of LF and EF in test-tube experiments. Other investigators have engineered mutant, inactive PAs that prevent bacteria-produced PAs from forming the channel. The studies of PA and LF should enable researchers to develop small molecules that can be used as therapeutics to treat anthrax by inhibiting its toxins.

The Anthrax Bacterium Genome

The instructions that dictate how a microbe works are encoded within its genes. Bacteria often contain genes at two locations. The bacterial chromosome is a long stretch of DNA that houses most of those genes, but smaller loops of DNA called plasmids also carry genes that can be exchanged between different bacteria. Because plasmids often contain genes for toxins and antibiotic resistance, knowing their DNA sequence is important.

In B. anthracis, the genes for PA, LF, and EF are found on plasmids that have already been sequenced. In addition, researchers recently reported the complete chromosomal DNA sequence of two B. anthracis isolates, including the bacterium that infected a Florida victim of the recent anthrax attack. Genome sequencing of more than a dozen other B. anthracis strains and related bacteria has already begun.

By comparing the DNA blueprints of different B. anthracis strains, researchers hope to learn why some strains are more virulent than others. Small variations among the genomes of different strains may also help investigators pinpoint the origin of an anthrax outbreak. Knowing the genetic fingerprint of B. anthracis might lead to gene-based detection mechanisms that can alert scientists to the bacteria in the environment or allow rapid diagnosis of anthrax in infected people. Variations between strains might also point to differences in antibiotic susceptibility, permitting doctors to immediately determine the appropriate treatment.

DNA sequencing also opens the door to functional genomics, in which the B. anthracis genome will be analyzed to determine the function of each of its genes and how they interact with each other or with host-cell components to cause disease. Genes are the instructions for making proteins, which in turn build components of the cell or carry out its biochemical processes. Knowing the sequence of B. anthracis genes therefore helps scientists discover key bacterial proteins that can then be targeted by new drugs or vaccines.

Spore Biology

B. anthracis spores are essentially dormant and therefore must wake up, or germinate, to become reproductive, disease-causing bacteria. Researchers are therefore studying the germination process to learn more about the signals that cause spores to become active once inside an animal. Efforts are underway to develop models of spore germination in laboratory animals; scientists hope those models will enable discoveries leading to drugs that block the germination process.

Host Immunity

People who contract anthrax produce antibodies to PA, and similar antibodies appear to protect animals from infection. Recent studies also suggest that some animals can produce antibodies to components of B. anthracis spores. Those antibodies, when studied in a test tube, prevent spores from germinating and increase their uptake by the immune system's microbe-eating cells. It therefore might be possible to develop a vaccine that can be given after exposure to fight both the reproductive form of B. anthracis and any spores that may linger in the lungs following antibiotic treatment.

As part of NIAID's strategic plan, researchers will study how both the innate and adaptive immune responses are triggered by a B. anthracis infection. The adaptive immune response consists of B cells and T cells which specifically recognize components of the anthrax bacterium. The innate immune system, however, responds more generally to a wide range of microbial invaders and likely plays a key role in the body's front-line defenses. Scientists will conduct studies of how those two arms of the immune system act to counter infection, including how B. anthracis spore germination affects individual immune responses.

NIAID THERAPEUTICS RESEARCH

Following the recent discoveries of how PA and LF interact with their cellular targets, researchers are screening thousands of small molecules in hopes of finding a compound that is practical for use as an anti-anthrax drug. In addition, NIAID is working with the Food and Drug Administration (FDA), Centers for Disease Control and Prevention (CDC), and Department of Defense (DoD) to accelerate testing of collections of compounds for their effectiveness against inhalation anthrax. Many of those compounds have already been approved by FDA for other indications and therefore could quickly be approved for use in treating anthrax should they prove effective.

NIAID is seeking new drugs that attack B. anthracis at many levels. These include agents that prevent the bacterium from attaching to cells, compounds that inhibit spore germination, and inhibitors that block the activity of key enzymes such as anthrax lethal factor. The Institute will also develop the capacity to synthesize promising anti-anthrax compounds in sufficient purity and quantity for preclinical testing.

NIAID VACCINE RESEARCH

Researchers are working on new, improved anthrax vaccines that may be more easily given to a diverse population. NIAID is collaborating with DoD to develop a next-generation vaccine based on a laboratory-produced, or recombinant, PA variant. Antibodies to PA also appear to recognize some components of the bacterial spore, making PA-based vaccines promising candidates for broad protection against anthrax. The Institute will supervise phase I and phase II trials of the recombinant PA vaccine in different formulations.

To help move potential vaccines into clinical testing, NIAID will develop the infrastructure to produce pilot lots of promising candidates and expand the Institute's testing capacity. To assist in its vaccine research efforts, NIAID will establish a centralized immunology laboratory to assess the efficacy of different vaccine candidates.

NIAID DIAGNOSTICS RESEARCH

Research is underway to develop improved techniques for spotting B. anthracis in the environment and diagnosing it in infected individuals. A key part of that research is the functional genomic analysis of the bacterium, which should lead to new genetic markers for sensitive and rapid identification. Genomic analysis will also reveal differences in individual B. anthracis strains that may affect how those bacteria cause disease or respond to treatment.

ANTHRAX AND BIOTERRORISM

CDC has classified B. anthracis as a Category A agent. Those agents are considered the highest threat to national security due to their ease of transmission, high rate of death or serious illness, and potential for causing public panic.

In October 2001, anthrax spores were sent through the U.S. mail and caused 18 confirmed cases of anthrax (11 inhalation, 7 cutaneous). Five individuals with inhalation anthrax died; none of the cutaneous cases was fatal.



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