Anthrax

From Wikipedia, the free encyclopedia

Anthrax is an acute disease caused by Bacillus anthracis. It affects both humans and other animals. Most forms of the disease are lethal. There are effective vaccines against anthrax, and some forms of the disease respond well to antibiotic treatment.
Like many other members of the genus Bacillus, Bacillus anthracis can form dormant spores that are able to survive in harsh conditions for extremely long periods of time—even decades or centuries.^[1] Such spores can be found on all continents, even Antarctica.^[2] When spores are inhaled, ingested, or come into contact with a skin lesion on a host they may reactivate and multiply rapidly.
Anthrax commonly infects wild and domesticated herbivorous mammals which ingest or inhale the spores while grazing. Ingestion is thought to be the most common route by which herbivores contract anthrax. Carnivores living in the same environment may become infected by consuming infected animals. Diseased animals can spread anthrax to humans, either by direct contact (e.g. inoculation of infected blood to broken skin) or consumption of diseased animals' flesh.
Anthrax spores can be produced in vitro and used as a biological weapon. Anthrax does not spread directly from one infected animal or person to another; it is spread by spores. These spores can be transported by clothing or shoes. The dead body of an animal that died of anthrax can also be a source of anthrax spores.

Overview

Anthrax is one of the oldest recorded diseases of grazing animals such as sheep and cattle and is believed to be the Sixth Plague mentioned in the Book of Exodus in the Bible.^[3] Anthrax is also mentioned by Homer (in The Iliad), Virgil (Georgics), and Hippocrates. Infection of humans can result from contact with infected animal hides, fur, wool ("Woolsorter's disease"), leather or contaminated soil. Anthrax is now fairly rare in humans, although it still regularly occurs in ruminants, such as cattle, sheep, goats, camels, wild buffalo, and antelopes, in hind-gut fermenters such as zebras and rhinos, and in other wildlife such as elephants and lions in certain endemic areas of the world.
Bacillus anthracis bacteria spores are soil-borne and because of their long lifetime, they are still present globally and at animal burial sites of anthrax-killed animals for many decades; spores have been known to have reinfected animals over 70 years after burial sites of anthrax-infected animals were disturbed.^[4]
Until the twentieth century, anthrax infections killed hundreds and thousands of animals and people each year in Europe, Asia, Africa, Australia, and Southern Vietnam, specifically in the concentration camps during WWI, and North America.^[5] French scientist Louis Pasteur developed the first effective vaccine for anthrax in 1881.^[6][7][8] Thanks to over a century of animal vaccination programs, sterilization of raw animal waste materials and anthrax eradication programs in North America, Australia, New Zealand, Russia, Europe and parts of Africa and Asia, anthrax infection is now relatively rare in domestic animals with normally only a few dozen cases reported every year. Anthrax is even rarer in dogs and cats: there had only ever been one documented case in dogs in the USA by 2001, although the disease affects livestock.^[9] Anthrax typically does not cause disease in carnivores and scavengers, even when these animals consume anthrax-infected carcasses. Anthrax outbreaks do occur in some wild animal populations with some regularity.^[10] The disease is more common in developing countries without widespread veterinary or human public health programs.
There are 89 known strains of anthrax. The virulent Ames strain, which had been used in the 2001 anthrax attacks in the United States, has received the most news coverage of any anthrax outbreak. However, the Vollum strain, developed but never used as a biological weapon during the Second World War, is much more dangerous. The Vollum (also incorrectly referred to as Vellum) strain was isolated in 1935 from a cow in Oxfordshire, UK. This is the same strain that was used during the Gruinard bioweapons trials. A variation of Vollum known as "Vollum 1B" was used during the 1960s in the US and UK bioweapon programs. Vollum 1B is widely believed^[11] to have been isolated from William A. Boyles, a 46-year-old scientist at the U.S. Army Biological Warfare Laboratories at Camp (later Fort) Detrick (precursor to USAMRIID) who died in 1951 after being accidentally infected with the Vollum strain. The Sterne strain, named after the Trieste-born immunologist Max Sterne, is an attenuated strain used as a vaccine.

Description of the bacterium

Bacillus anthracis is a rod-shaped, Gram-positive, aerobic bacterium, about 1 by 9 micrometers in size. It was shown to cause disease by Robert Koch in 1877.^[8] The bacterium normally rests in endospore form in the soil, and can survive for up to decades in this state. Herbivores are often infected whilst grazing or browsing, especially when eating rough, irritant or spiky vegetation: the vegetation has been hypothesized to cause wounds within the gastrointestinal tract permitting entry of the bacterial endo-spores into the tissues, though this has not been proven. Once ingested or placed in an open cut, the bacterium begins multiplying inside the animal or human and typically kills the host within a few days or weeks. The endo-spores germinate at the site of entry into the tissues and then spread via the circulation to the lymphatics, where the bacteria multiply. It is the production of two powerful exo-toxins (edema toxin and lethal toxin) by the bacteria that causes death. Veterinarians can often tell a possible anthrax-induced death by its sudden occurrence, and by the dark, non-clotting blood that oozes from the body orifices. Most anthrax bacteria inside the body after death are out-competed and destroyed by anaerobic bacteria within minutes to hours post-mortem. However, anthrax vegetative bacteria that escape the body via oozing blood or through the opening of the carcass may form hardy spores. One spore forms per one vegetative bacterium. The triggers for spore formation are not yet known, though oxygen tension and lack of nutrients may play roles. Once formed, these spores are very hard to eradicate.
The infection of herbivores (and occasionally humans) via the inhalational route normally proceeds as follows: once the spores are inhaled, they are transported through the air passages into the tiny air sacs (alveoli) in the lungs. The spores are then picked up by scavenger cells (macrophages) in the lungs and are transported through small vessels (lymphatics) to the lymph nodes in the central chest cavity (mediastinum). Damage caused by the anthrax spores and bacilli to the central chest cavity can cause chest pain and difficulty breathing. Once in the lymph nodes, the spores germinate into active bacilli which multiply and eventually burst the macrophages, releasing many more bacilli into the bloodstream to be transferred to the entire body. Once in the blood stream these bacilli release three substances: lethal factor, edema factor and protective antigen. Protective antigen combines with these other two factors to form lethal toxin and edema toxin, respectively. These toxins are the primary agents of tissue destruction, bleeding, and death of the host. If antibiotics are administered too late, even if the antibiotics eradicate the bacteria, some hosts will still die. This is because the toxins produced by the bacilli remain in their system at lethal dose levels.
In order to enter the cells, the edema and lethal factors use another protein produced by B. anthracis, protective antigen. Edema factor inactivates neutrophils (a type of phagocytic cell) so that they cannot phagocytose bacteria. Historically, it was believed that lethal factor caused macrophages to make TNF-alpha and interleukin 1, beta (IL1B), both normal components of the immune system used to induce an inflammatory reaction, ultimately leading to septic shock and death. However, recent evidence indicates that anthrax also targets endothelial cells (cells that lines serous cavities, lymph vessels, and blood vessels), causing vascular leakage of fluid and cells, and ultimately hypovolemic shock (low blood volume), and septic shock.

Bacillus anthracis virulence factors

 The lethality of the anthrax disease owes itself to the bacterium's two principle virulence factors: (i) the poly-D-glutamic acid capsule, which protects the bacterium from phagocytosis by host neutrophils, and (ii) the tripartite protein toxin, called anthrax toxin. Anthrax toxin is a mixture of three protein components: (i) protective antigen (PA), (ii) edema factor (EF), and (iii) lethal factor (LF). PA plus LF produces lethal toxin, and PA plus EF produces edema toxin. These toxins cause death and tissue swelling (edema), respectively.

Exposure

Occupational exposure to infected animals or their products (such as skin, wool, and meat) is the usual pathway of exposure for humans. Workers who are exposed to dead animals and animal products are at the highest risk, especially in countries where anthrax is more common. Anthrax in livestock grazing on open range where they mix with wild animals still occasionally occurs in the United States and elsewhere. Many workers who deal with wool and animal hides are routinely exposed to low levels of anthrax spores but most exposures are not sufficient to develop anthrax infections. Presumably, the body's natural defenses can destroy low levels of exposure. These people usually contract cutaneous anthrax if they catch anything. Historically, the most dangerous form of inhalational anthrax was called Woolsorters' disease because it was an occupational hazard for people who sorted wool. Today this form of infection is extremely rare, as almost no infected animals remain. The last fatal case of natural inhalational anthrax in the United States occurred in California in 1976, when a home weaver died after working with infected wool imported from Pakistan. The autopsy was done at UCLA hospital. To minimize the chance of spreading the disease, the deceased was transported to UCLA in a sealed plastic body bag within a sealed metal container.^[12]
In November 2008, a drum maker in the United Kingdom who worked with untreated animal skins died from anthrax.^[13] In December 2009 The New Hampshire Department of Health and Human Services confirmed a case of gastrointestinal anthrax in an adult female. The CDC is currently investigating the source and the possibility that it was contracted from an African drum recently used by the woman.^[14] Gastrointestinal anthrax is exceedingly rare in the United States, with only one case on record, reported in 1942, according to the Centers for Disease Control and Prevention.^[15] Also in December 2009 an outbreak of anthrax occurred amongst drug addicts in Glasgow, Scotland, resulting in six deaths from contaminated heroin.^[16]

Mode of infection

Anthrax can enter the human body through the intestines (ingestion), lungs (inhalation), or skin (cutaneous) and causes distinct clinical symptoms based on its site of entry. An infected human will generally be quarantined. However, anthrax does not usually spread from an infected human to a noninfected human. But if the disease is fatal the person's body and its mass of anthrax bacilli becomes a potential source of infection to others and special precautions should be used to prevent further contamination. Inhalational anthrax, if left untreated until obvious symptoms occur, may be fatal.
Anthrax can be contracted in laboratory accidents or by handling infected animals or their wool or hides. It has also been used in biological warfare agents and by terrorists to intentionally infect humans, as occurred in, for example, the 2001 anthrax attacks.

Pulmonary (pneumonic, respiratory, or inhalational) anthrax

Respiratory infection in humans initially presents with cold or flu-like symptoms for several days, followed by severe (and often fatal) respiratory collapse. Historical mortality was 92%, but when treated early (seen in the 2001 anthrax attacks) observed mortality was 45%.^[17] Illness progressing to the fulminant phase has a 97% mortality regardless of treatment.
A lethal infection is reported to result from inhalation of about 10,000–20,000 spores, though this dose varies amongst host species.^[18] Like all diseases there is probably a wide variation to susceptibility with evidence that some people may die from much lower exposures; there is little documented evidence to verify the exact or average number of spores needed for infection. Inhalational anthrax is also known as woolsorters' or ragpickers' disease as these professions were more susceptible to the disease due to their exposure to infected animal products. Other practices associated with exposure include the slicing up of animal horns for the manufacture of buttons, the handling of hair bristles used for the manufacturing of brushes, and the handling of animal skins. Whether these animal skins came from animals that died of the disease or from animals that had simply laid on ground that had spores on it is unknown. This mode of infection is used as a bioweapon.

Gastrointestinal (gastroenteric) anthrax

Gastrointestinal infection in humans is most often caused by eating anthrax-infected meat and is characterized by serious gastrointestinal difficulty, vomiting of blood, severe diarrhea, acute inflammation of the intestinal tract, and loss of appetite. Some lesions have been found in the intestines and in the mouth and throat. After the bacteria invades the bowel system, it spreads through the bloodstream throughout the body, making even more toxins on the way. Gastrointestinal infections can be treated but usually result in fatality rates of 25% to 60%, depending upon how soon treatment commences.^[19]

Cutaneous (skin) anthrax

Cutaneous (on the skin) anthrax infection in humans shows up as a boil-like skin lesion that eventually forms an ulcer with a black center (eschar). The black eschar often shows up as a large, painless necrotic ulcer (beginning as an irritating and itchy skin lesion or blister that is dark and usually concentrated as a black dot, somewhat resembling bread mold) at the site of infection. Cutaneous infections generally form within the site of spore penetration between 2 and 5 days after exposure. Unlike bruises or most other lesions, cutaneous anthrax infections normally do not cause pain.^[19]
Cutaneous anthrax is rarely fatal if treated,^[17] but without treatment about 20% of cutaneous skin infection cases progress to toxemia and death.

Laboratory Identification of Bacillus anthracis

Other than Gram Stain of specimens, there are no specific direct identification techniques for identification of Bacillus sp. in clinical material. These organisms are Gram positive but with age can be Gram variable to Gram negative. A specific feature of Bacillus sp. that makes it unique from other aerobic microorganisms is its ability to produce spores. Although, spores are not always evident on a Gram stain of this organism, the presence of spores confirms that the organism is of the genus Bacillus.
All Bacillus sp. grow well on 5% Sheep blood agar and other routine culture media. PLET (polymyxin-lysozyme-EDTA-thallous acetate) can be used to isolate B.anthracis from contaminated specimens and bicarbonate agar is used as an identification method to induce capsule formation.
Bacillus sp. will usually grow within 24 hours of incubation at 35 degrees C, in ambient air (room temperature) or in 5% CO₂. If bicarbonate agar is used for identification then the media must be incubated in 5% CO₂.
B.anthracis appears as medium-large, gray, flat, irregular with swirling projections, often referred to as "medusa head" appearance, and is non-hemolytic on 5% sheep blood agar. It is non-motile, is susceptible to penicillin and produces a wide zone of lecithinase on egg yolk agar. Confirmatory testing to identify B.anthracis includes gamma bacteriophage testing, indirect hemagglutination and enzyme linked immunosorbent assay to detect antibodies. ^[20]

Anthrax vaccines

An anthrax vaccine licensed by the U.S. Food and Drug Administration (FDA) and produced from one non-virulent strain of the anthrax bacterium, is manufactured by BioPort Corporation, subsidiary of Emergent BioSolutions. The trade name is BioThrax, although it is commonly called Anthrax Vaccine Adsorbed (AVA). It is administered in a six-dose primary series at 0, 2, 4 weeks and 6, 12, 18 months; annual booster injections are required thereafter to maintain immunity.
Unlike NATO countries, the Soviets developed and used a live spore anthrax vaccine, known as the STI vaccine, produced in Tbilisi, Georgia. Its serious side effects restrict use to healthy adults.^[24]

Site cleanup

Anthrax spores can survive for long periods of time in the environment after release. Methods for cleaning anthrax-contaminated sites commonly use oxidizing agents such as peroxides, ethylene oxide, Sandia Foam,^[25] chlorine dioxide (used in Hart Senate office building), and liquid bleach products containing sodium hypochlorite. These agents slowly destroy bacterial spores. A bleach solution for treating hard surfaces has been approved by the EPA.^[26] Bleach and vinegar must not be combined together directly, as doing so could produce chlorine gas. Rather some water must first be added to the bleach (e.g., two cups water to one cup of bleach), then vinegar (e.g., one cup), and then the rest of the water (e.g., six cups). The pH of the solution should be tested with a paper test strip; and treated surfaces must remain in contact with the bleach solution for 60 minutes (repeated applications will be necessary to keep the surfaces wet).
Chlorine dioxide has emerged as the preferred biocide against anthrax-contaminated sites, having been employed in the treatment of numerous government buildings over the past decade. Its chief drawback is the need for in situ processes to have the reactant on demand.
To speed the process, trace amounts of a non-toxic catalyst composed of iron and tetro-amido macrocyclic ligands are combined with sodium carbonate and bicarbonate and converted into a spray. The spray formula is applied to an infested area and is followed by another spray containing tert-Butyl hydroperoxide.^[27]
Using the catalyst method, a complete destruction of all anthrax spores can be achieved in under 30 minutes.^[27] A standard catalyst-free spray destroys fewer than half the spores in the same amount of time. They can be heated, exposed to the harshest chemicals, and they do not easily die.
Cleanups at a Senate office building, several contaminated postal facilities and other U.S. government and private office buildings showed that decontamination is possible, but it is time-consuming and costly. Clearing the Senate office building of anthrax spores cost $27 million, according to the Government Accountability Office. Cleaning the Brentwood postal facility outside Washington cost $130 million and took 26 months. Since then newer and less costly methods have been developed.^[28]
Clean up of anthrax-contaminated areas on ranches and in the wild is much more problematic. Carcasses may be burned, though it often takes up to three days to burn a large carcass and this is not feasible in areas with little wood. Carcasses may also be buried, though the burying of large animals deeply enough to prevent resurfacing of spores requires much manpower and expensive tools. Carcasses have been soaked in formaldehyde to kill spores, though this has environmental contamination issues. Block burning of vegetation in large areas enclosing an anthrax outbreak has been tried; this, while environmentally destructive, causes healthy animals to move away from an area with carcasses in search of fresh graze and browse. Some wildlife workers have experimented with covering fresh anthrax carcasses with shadecloth and heavy objects. This prevents some scavengers from opening the carcasses, thus allowing the putrefactive bacteria within the carcass to kill the vegetative B. anthracis cells and preventing sporulation. This method also has drawbacks, as scavengers such as hyenas are capable of infiltrating almost any exclosure. The occurrence of previously dormant anthrax, stirred up from below the ground surface by wind movement in a drought-stricken region with depleted grazing and browsing, may be seen as a form of natural culling and a first step in rehabilitation of the area.